**About the Editor**

#### **Jinyang Xu**

Jinyang Xu is an Associate Professor and a Doctoral Supervisor of Mechanical Engineering at Shanghai Jiao Tong University, China. He received his Ph.D. (2016) in Mechanical Engineering from Arts et Metiers ParisTech, France. His research interests focus on composites machining, ´ numerical modeling, micro/nano cutting, and surface texturing. He has published over 100 articles in highly-ranked JCR-referenced journals as a first/corresponding author, and edited 2 monographs, 5 book chapters, 6 int. conf. proceedings & 8 special issues. He now serves as the Editor-in-Chief of Journal of Coating Science and Technology (JCST), International Journal of Precision Technology (IJPTech), and International Journal of Product Sound Quality (IJPSQ). He is an Associate Editor of Proc. Inst. Mech. Eng. Pt. E - J. Process. Mech. Eng. (SCIE/EI), Simulation - Transactions of the Society for Modeling and Simulation International (SCIE/EI), and Frontiers in Materials (SCIE/EI). Presently, he is an Editorial Board Member of 7 SCI journals, including Green Materials, Journal of Superhard Materials, International Journal of Aerospace Engineering, Coatings, Lubricants, etc. He was honored with the prestigious IAAM Scientist Medal of the year 2020, and awarded the Shanghai Pujiang Scholar by the Shanghai Municipality in 2017. He is a fellow of IAAM and also a senior member of ASME and SCS.

## *Editorial* **Manufacturing of Fibrous Composites for Engineering Applications**

**Jinyang Xu**

State Key Laboratory of Mechanical System and Vibration, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; xujinyang@sjtu.edu.cn

Fibrous composites are advanced engineering materials featuring the impregnation of fiber phase with a polymer matrix base to yield enhanced properties. They can be categorized in terms of the fiber type, such as aramid-fiber-reinforced polymer (AFRP), carbon-fiber-reinforced polymer (CFRP), glass-fiber-reinforced polymer (GFRP), Kevlarfiber-reinforced polymer (KFRP), etc. They can also be classified into continuous and discontinuous fiber composites. In general, discontinuous fiber composites show better isotropic behavior than continuous fiber composites, as short fibers can enhance the matrix in every direction [1]. The fibrous composites are often fabricated with a polymer matrix, which can be either thermoset polymers or thermoplastic polymers. The thermoset polymers have the property of becoming permanently hard and rigid when heated or cured. The thermoplastic polymers entail the linear chain of the molecular structures; thus, they can be recycled and reused when heated [2–4].

Since fibrous composites show higher specific mechanical and physical properties than conventional alloys and steels [1,5], they are very attractive in modern aerospace industries [6–9]. For instance, CFRP composites are often employed in the aircraft wing boxes, horizontal and vertical stabilizers, and wing panels [10]. GFRP composites are mainly used in the fairings, storage room doors, landing gear doors, and passenger compartments [10,11]. Reports indicate that most international aircraft manufacturers, including Airbus and Boeing, are seeking the use of fibrous composites to fabricate large load-carrying structures favoring the energy saving of new-generation airplanes. Despite the net shapes of molded fibrous composites, secondary manufacturing operations are still required to create the target shapes and desired quality [8,9,12,13]. However, fibrous composites are extremely difficult to machine, as they possess rather poor machinability compared to conventional alloys and steels. The term "machinability" signifies the degree of difficulty of cutting a workpiece material with qualified quality. The poor machinability of fibrous composites may arise from the inherent anisotropic behaviors of the fiber/matrix system and the heterogeneous architecture of the composites. The most-used machining operations for shaping fibrous composites mainly include trimming, turning, milling, and drilling. Since the reinforcing fibers and the matrix base show completely different properties, machining of these composites has posed huge challenges to the current manufacturing community. The specific issues associated with the cutting of fibrous composites are difficult chip removal, poor surface quality, and rapid tool wear.

Chip separation mode is a critical procedure determining the eventual cutting responses of workpiece materials. However, the cutting mechanisms of fibrous composites are much complicated than those of conventional metallic alloys, as the composite separation modes are fiber-orientation dependent. Many studies have attributed the fiberorientation-dependent chip removal to the effects of the fiber layup on the properties of chips under specific cutting loads. Consequently, producing consistent surface finish of cut composites is very challenging. The fundamental chip removal modes encountered in composite machining mainly include shear-induced fracture, bending-induced fracture, and fiber/matrix interfacial debonding, depending significantly on the varying fiber cutting

**Citation:** Xu, J. Manufacturing of Fibrous Composites for Engineering Applications. *J. Compos. Sci.* **2022**, *6*, 187. https://doi.org/10.3390/ jcs6070187

Received: 22 June 2022 Accepted: 23 June 2022 Published: 24 June 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

angle [14–16]. Additionally, cutting forces and machining temperatures are also firmly related to the variation of the fiber cutting angle during the machining of fibrous composites. Normally, lower magnitudes of cutting forces/temperatures can be produced under the along fiber cutting relation, in which the fiber cutting angle is acute. In contrast, chip separation occurring under the against fiber cutting relation is often considered unfavorable for producing desired surface quality for fibrous composites. Therefore, selecting a proper fiber-cutting angle plays a crucial role in the high-quality machining of fibrous composites.

Poor surface quality is another critical issue associated with fibrous composite machining. Since the removal mechanisms of fibers and matrix are different and change continuously with the fiber layup, even under the action of the identical cutting edges, serious defects involving delamination, burrs, tearing, surface cavities, and glass transition failure easily occur, which can greatly deteriorate the quality of machined composite parts [17]. Additionally, the aforementioned damage not only reduces the surface finish and assembly tolerance, but also affects the fatigue strength of cut holes, accounting for a large proportion of part rejections [18–21]. Moreover, delamination and tearing are often noted as the most critical failures of fibrous composites, and should be carefully suppressed during the machining operations as they cannot be repaired once they occur. Functionally designed special tools are feasible for minimizing the generation of various cutting-induced damages for fibrous composites. The use of optimized cutting parameters can also benefit the improvement of the surface quality of cut fibrous composites. To deal with cutting-induced damages, future endeavours must optimize the process parameters, develop functional cutting tools and use advanced machining techniques for fibrous composites.

Rapid tool wear is a crucial issue when cutting fibrous composites, as hard fibers induce severe abrasions/modifications onto the tool surfaces and lead to the blunting of tool edges. The primary wear mode frequently encountered in the machining of fibrous composites is abrasion wear in the form of cutting-edge rounding (CER) [22–25]. Additionally, CER has become a significant indicator for the assessment of tool wear, particularly for drilling fibrous composites, which provides a more accurate quantification than the conventionally used wear-width indicator [1]. As the composite chips are often separated within a small tool–chip interface, crater wear shows no ability to take place. Progressive abrasion wear often results in undesired consequences when machining fibrous composites, such as increased cutting forces, excessive machining temperatures, deteriorated surface quality, and reduced tool life. Moreover, the dominant tool failure in machining fibrous composites is edge chipping or coating peeling during the chip removal process. Developing superior tool coating materials and optimizing tool geometries would be a feasible solution to the tool wear issues for fibrous composites.

In the end, with the rapid development of manufacturing technologies, various types of advanced machining methods have been developed in recent years, providing new prospects for achieving damage-free machining of fibrous composites. For instance, helical milling, variable feed drilling, and ultrasonic vibration-assisted machining have emerged and have been extensively used in the machining of fibrous composites, which yield outstanding performances in suppressing cutting-induced damages. Moreover, intelligent manufacturing technology, which has become a hot research focus in both academia and industry, can be applied to composites machining to realize the in-situ detection and high-precision control of online machining status. Then, high-efficiency and high-quality machining of fibrous composites can be accomplished.

**Funding:** The work was supported by the National Natural Science Foundation of China (Grant No. 52175425) and the 9th Sino-Hungarian Intergovernmental Scientific and Technological Cooperation Project (Grant No. 2021-07).

**Conflicts of Interest:** The author declares no conflict of interest.

#### **References**


**H. S. Ashrith 1,\*, T. P. Jeevan <sup>1</sup> and Jinyang Xu 2,\***


**Abstract:** This review focuses on the fabrication and mechanical characterization of fibrous composites for engineering applications. Fibrous composites are materials composed of two or more distinct phases, with fibers embedded in a matrix. The properties of these materials depend on the properties of both the fibers and the matrix, as well as the way they are combined and fabricated. The various fabrication methods, along with the process parameters, used to manufacture synthetic and natural fibrous composites for engineering applications, including hand lay-up, compression molding, resin transfer molding, additive manufacturing, etc., are discussed. The mechanical characterization of fibrous composites, including their strength, stiffness, and toughness of both synthetic and natural fibrous composites are discussed. The advantages and disadvantages of fiber reinforcement are discussed, along with their influence on the resulting mechanical characteristics of the composites. It can be observed that the mechanical properties of fibrous composites can be tailored by controlling various factors, such as the fiber orientation, fiber volume fraction, and matrix type. Although fibrous composites offer significant advantages, several challenges hinder their widespread use in engineering applications. These challenges include high manufacturing costs, limited design guidelines, and difficulties in predicting their mechanical behavior under various loading conditions. Therefore, despite their unique properties, these challenges must be overcome for fibrous composites to realize their full potential as high-performance materials.

**Keywords:** FRPC; manufacturing techniques; additive manufacturing; mechanical characterization

#### **1. Introduction**

Fibrous composites are a highly valued group of materials due to their exceptional mechanical characteristics, lightness, and versatility in various engineering applications. These materials possess anisotropic qualities, meaning that their properties vary based on the direction of loading. Fibrous composites are fabricated by combining two or more materials, which include matrix and reinforcing fibers, which possess varying mechanical properties. The matrix material serves to connect the fibers and transfer loads between them, while the fibers offer strength and stiffness to the composite. Various materials, such as glass, carbon, aramid, and natural fibers, can be used as reinforcements in composite fabrication [1–5]. Fibrous composites offer a significant advantage in terms of their high strength-to-weight ratio, making them highly desirable for applications where weight reduction is very crucial. The aerospace industry, for instance, has extensively utilized carbon fiber reinforced polymer (CFRP) composites for their exceptional strength and stiffness properties [6,7]. According to Khan et al. [8] using CFRP composites in aircraft wings resulted in a 15% decrease in weight and a 30% improvement in fuel efficiency in comparison to conventional metallic materials. Besides the aerospace sector, the automotive industry has also made extensive use of fibrous composites to reduce weight and enhance

**Citation:** Ashrith, H.S.; Jeevan, T.P.; Xu, J. A Review on the Fabrication and Mechanical Characterization of Fibrous Composites for Engineering Applications. *J. Compos. Sci.* **2023**, *7*, 252. https://doi.org/10.3390/ jcs7060252

Academic Editor: Francesco Tornabene

Received: 25 May 2023 Revised: 6 June 2023 Accepted: 15 June 2023 Published: 18 June 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

fuel efficiency. The lightweight and high-strength properties of fibrous composites make them appealing for use in sporting goods applications. The use of CFRP composites in bicycle frames resulted in a 30% weight reduction and a 20% increase in stiffness compared to traditional aluminum frames [9]. Kim et al. [10] examined the application of glass fiber reinforced polymer (GFRP) composites for automotive door inner panels and concluded that these composites resulted in a 36% reduction in weight relative to traditional steel panels, without any compromise in mechanical properties. Stepanova and Korzhikova-Vlakh [11] investigated the use of cellulose nanofibers to reinforce polylactic acid (PLA) composites and found that the resulting composites exhibited improved tensile strength and modulus. Researchers are currently exploring new methods for enhancing the properties of fibrous composites.

Despite their benefits, the cost of fibrous composites remains high, limiting their use in some applications. Furthermore, these materials can be damaged by impact or other forms of loading, which can decrease their reliability and durability over time. Fibrous composites can be fabricated by employing different techniques, such as hand lay-up (HL), filament winding (FW), pultrusion, resin transfer molding (RTM), and additive manufacturing (AM), each having their own benefits and drawbacks. The choice of technique depends on the desired properties of the composite material. Various investigations have been performed on the fabrication of fibrous composites to enhance their mechanical properties and reduce the cost of production. Some studies have explored the usage of flax and jute fibers as a cost-effective and sustainable alternative to synthetic fibers [12,13], while others have looked at improving the bonding between the matrix and fibers to enhance the mechanical performance of the composite material [14].

The mechanical characterization of fibrous composites is critical for evaluating their performance and potential use in engineering applications. Mechanical characterization involves the measurement and analysis of the properties, such as strength, stiffness, toughness, and fatigue resistance [15,16]. Understanding this behavior is necessary to access and optimize the performances of fibrous composites under different loading conditions. Various techniques are used to evaluate the mechanical performance of fibrous composites, including tensile, compressive, shear, and bending tests. These tests are usually performed under varying loads to determine the mechanical characteristics of composites in different directions. Additionally, non-destructive assessment techniques, for example ultrasonic testing, X-ray tomography, and thermography, have also been developed to assess the defects and internal structure of composites. Extensive research has been carried out to address the challenges for improving the performance of fibrous composites and their applications. While several detailed reviews are available on CFRP composites, GFRP composites, or natural fiber-reinforced polymer (NFRP) composites, no single review covers all types of fibrous composites and their detailed fabrication processes. Therefore, this paper presents a comprehensive review of the manufacturing techniques of fibrous composites, including a mechanical characterization and factors influencing mechanical properties.

#### **2. Fabrication Methods of Fibrous Composites**

The fabrication of fiber-reinforced polymer composites (FRPCs) is a complex phenomenon and includes several techniques. FRPC fabrication techniques can be broadly classified into conventional, automated, and additive manufacturing techniques. The fabrication technique employed primarily depends on the size and application of the composites. Among the available techniques, conventional technologies, such as hand lay-up, injection molding, compression molding, resin transfer molding, and filament winding, are widely used to fabricate the FRPC [1]. This section offers a brief description of the commonly used techniques in fabricating fibrous composites made of synthetic and natural fibers for engineering applications.

Hand lay-up is the commonly employed technique in fabricating FRPCs. It involves laying the fiber reinforcements on the mold followed by pouring a thermosetting resin to wet the reinforcements. The resin is spread uniformly on the laid reinforcements using a brush or a roller followed by laying the next layer and repeating the process until the desired thickness is obtained. Normally, a silicone demolding agent is applied to the mold surface before laying reinforcements to facilitate easy removal of the composites after curing [17]. Figure 1a illustrates the HL technique employed for composite fabrication.

The compression molding (CM) technique is employed for sheet molding compounds (SMCs) and bulk molding compounds (BMCs). SMCs and BMCs are semi-cured sheets of thermoset resin reinforced with short fibers or impregnated fibers, inert fillers, catalysts, pigments and stabilizers, release agents, and thickeners. BMCs are stored at a lower temperature to avoid hardening before subjecting them to compression molding [18]. Molding compounds are put in a hot mold and cured by pressing (Figure 1b). The pressure and temperature applied during the process depend on the material type and desired thickness [1].

**Figure 1.** Schematic representation of the (**a**) HL and (**b**) CM process used for composite fabrication [1,19].

Injection molding (IM) is the commonly used method for fabricating polymer composites. The injection molding machine consists of a single screw extruder that melts the granular raw material and forces it into the mold cavity. A schematic representation of the IM technique employed for composite fabrication is represented in Figure 2. IM is a two-stage process. In the first step, the thermoplastic material is mixed with reinforcements in the form of particulates and chopped fibers (synthetic or natural) by compounding to obtain the pellets. In the second step, the pellets are injected into the mold cavity to obtain the components. A water jacket is provided around the mold to maintain the required temperature during the process. IM is a cyclic process, and very little scrap is generated during the process. The size and shape of the component and kind of raw material influence the IM cycle time [20].

**Figure 2.** Schematic representation of the IM process used for composite fabrication [21].

Vacuum-assisted resin transfer molding (VARTM) is depicted in Figure 3. This process utilizes a vacuum bag and half mold to fabricate the composites. Reinforcements in the form of woven fabric or mats are placed on the bottom mold, which in turn is put inside the vacuum bag. The thermosetting resin is then drawn through the reinforcements placed in the vacuum bag to impregnate the fabrics, followed by curing using heating elements placed around the molds. Microwave heating sheets or thermal ovens can also be used to cure the fabricated composites. The process can be performed at higher pressures for fabricating higher-volume fraction composites that exhibit superior mechanical characteristics [22–24].

**Figure 3.** Schematic representation of the VARTM process used for composite fabrication [25].

RTM is a low-temperature and low-pressure process/technique employed for the fabrication of thermoset composites (Figure 4a). It is performed using two rigid closed molds and without the assistance of a vacuum. One of the molds is fixed, and the other can be moved using a hydraulic unit after laying the reinforcements. The curing agent and resin are then injected into the mold cavity under high pressure. Molds are surrounded by heating elements, which helps to cure the fabricated composites. RTM performed using higher pressure reduces the cycle time in the fabrication of intricate shape components [26,27].

FM involves meandering the impregnated fibers or tapes on a rotating mandrel along a prescribed path, such as helical, hoop, or polar (Figure 4b). Thermosetting resins are generally used to impregnate the fibers before winding. Fibers can be used in both wet and dry (pre-impregnated form) conditions. Wet winding involves unwinding the fibers from the roving and passing them through a resin mixture to impregnate them before they are wound on a mandrel with a certain orientation. Whereas the dry process makes use of fibers that have already been impregnated i.e., in semi-cured conditions, eliminating the onsite resin bath. Composites after winding are cured using an oven or autoclave or by using infrared radiation and removed from the mandrel. Higher fiber loading can be achieved using this process, which results in components with superior mechanical properties. This process can be easily automated and is suitable to produce only axisymmetric (cylindrical) components. Additionally, the processing behavior in filament winding is highly reliant on the properties of the resin [28,29].

**Figure 4.** Schematic representation of the (**a**) resin transfer molding and (**b**) filament winding processes used for composite fabrication [1].

An autoclave molding process is a kind of open molding process, where molded parts are kept inside a plastic bag and cured utilizing vacuum, heat, and high-pressure inert gases inside an autoclave (a combination of an oven and pressure chamber). The autoclave molding process is shown in Figure 5. In the first step, prepregs of the desired component are firmly placed on the mold coated with a mold-releasing agent. Cores and inserts can also be used during the process. Later, the whole assembly is covered by a plastic vacuum bag before curing in an autoclave followed by vacuuming out the air resulting in denser and desired void-free components. High pressure is maintained throughout the curing process; however, the vacuum is maintained only during the early stages of the process. A high-temperature thermosetting resin, such as epoxy resin, can be processed using this process. The high cost and limited part size due to the use of autoclave machines and prolonged curing cycles are the limitations of the autoclave process [30–33].

**Figure 5.** Schematic representation of autoclave molding process used for composite fabrication [19].

Pultrusion is the oldest technique to produce fiber-reinforced composites of a constant cross-section, employing closed heated dies. The pultrusion process is illustrated in Figure 6. The shape of the components produced relies on the die design used in the process. It is the simple, efficient, flexible, and most cost-effective technique for fabricating continuous-fiber structural composites with a constant cross-section. A creel for feeding the fiber, a resin reservoir, forming dies, a machined die with a temperature control feature, a puller, and a blade for cutting the product from the continuous system make up a pultrusion machine.

**Figure 6.** Schematic representation of pultrusion process used for composite fabrication [25].

Fiber forms (tape, woven, and/or mat) can be impregnated using both thermosetting and thermoplastic resins. Impregnated fibers are passed through preforming guides to remove excess resin and entrapped air, followed by passing through the heated die for curing. A sawing system is used to cut the cured composites according to the requirement. Resin-bath pultrusion and resin-injection pultrusion are the commonly used pultrusion techniques. High-fiber loading, as compared to the HL, process can be achieved in this process resulting in products with higher stiffness and strength [34–36].

AM belongs to the group of advanced manufacturing techniques where the components are fabricated in a layer-by-layer approach directly from computer-aided drawing without the use of tools or dies [37]. Various AM techniques can be used to fabricate the composite materials without using any molds and by selectively depositing the fibers [38]. Various AM techniques are illustrated in Figures 7 and 8.

**Figure 7.** Schematic representation of stereolithography AM process used for composite fabrication [39].

A selective laser sintering (SLS) process is used to fabricate composites made of thermoplastic powder and short fibers using a low-power laser. Composites with complex geometries can be easily fabricated using the SLS process without using support structures [40]. The stereolithography (SLA) AM process utilizes a photosensitive resin along with a laser to fabricate the components. The photosensitive resin is mixed with particles, short fibers, continuous fibers, and woven fabric during fabrication. The volume fraction of the reinforcements is limited to 20 vol.% since a higher volume fraction of reinforcements may lead to non-uniform dispersion and inferior mechanical properties. Components after printing are cured using a UV light oven [41]. Fused deposition modeling (FDM) is an extrusion-type of AM method where the filament made of thermoplastic material is melted (up to softening temperature), extruded, and deposited using a heated nozzle (Figure 8). Multi-material deposition can also be achieved using this process utilizing multiple nozzles [41].

Continuous fiber fabrication and liquid deposition modeling (LDM) are the two common types of FDM processes. The process utilizes pre-impregnated continuous fibers with a thermoplastic matrix to fabricate the composites [43]. A matrix in liquid form is reinforced with discontinuous reinforcements and whiskers to fabricate the composites in the LDM type of the FDM process [41]. Other less commonly used AM processes to fabricate composites are three-dimensional printing and laminated object manufacturing [1]. Techniques adopted for the fabrication of composites reinforced by various artificial and natural fibers for engineering applications are discussed in detail in the following sections.

**Figure 8.** Schematic representation of (**a**) FDM and (**b**) SLS AM processes used for composite fabrication [42].

#### **3. Fabrication of Commonly Used Structural Composites**

#### *3.1. Aramid Fiber and Its Hybrid Composites*

Aramid fibers (AFs) are slowly replacing glass fibers (GFs) to manufacture composites for engineering applications in the last decade because of their distinctive properties, such as low density, abundant availability, good thermal stability, high toughness, chemical solvent resistance, nonconductive property, impact and abrasion resistance, etc. These fibers exhibit excellent mechanical characteristics compared to steel and GFs on a weightfor-weight ratio. AFs are susceptible to ultraviolet radiation, absorb moisture, and exhibit low compression characteristics. AFs are extensively utilized in the production of insulated composites for military purposes, aircraft components, fire-resistant clothing, and armored vests [44].

AFs in the plain-woven fabric, along with natural kenaf fibers, were used to fabricate hybrid epoxy laminates using the HL technique. External pressure using dead weights (about 10 kg) was used to cure the composite at ambient conditions. The maximum fiber content of AFs in the study was limited to 53.64%. Fabricated composites were evaluated for their ballistic impact performance. Superior ballistic performance was observed in the composites with a lower volume fraction of kenaf fibers. Fabricated hybrid composites can be used for ballistic resistance applications, such as vehicle spall-liners [17]. Hybrid epoxy composites combined with aramid and semi-carbon fibers were also fabricated using the HL technique. Five different types of laminates were fabricated varying the aramid layer from zero to four. Aramid (Kevlar 49) and semi-carbon fibers were used in a twill form. Samples were cured overnight in ambient conditions. The reinforcement volume fraction was limited to 40%. Fabricated composites were evaluated for thermal and mechanical characteristics. Results indicate that with the increasing semi-carbon fiber content in the hybrid composite decreases the tensile strength and elastic and flexural modulus but decreases the rate of burning [45]. Aramid and natural (palm) fibers were successfully reinforced in an epoxy matrix to fabricate hybrid composites employing the HL technique. Reinforcement content was varied from 0 to 15 wt.% and was used in equal ratios (i.e., aramid:palm). Fabricated laminates were evaluated for various mechanical characteristics, namely tensile, flexural, impact, hardness, and water absorption tests. The flexural modulus of the fabricated composite decreases at the highest fiber content of 20 wt.% due to ineffective load transfer from one to another end of the composite. The water absorption rate was found to be highest for the composite reinforced with uncoated natural fibers due to voids [46]. Bio-degradable composites were successfully manufactured using bagasse/epoxy resin and AFs via an HL technique utilizing steel molds. Bagasse fibers were dried in sunlight, followed by chopping in a ball mill and washing using water to remove the pulps. Bagasse fibers were used in 40, 50, and 60 by wt.% of an epoxy matrix, while the aramid fiber weight percent was restricted to 5 wt.%. Laminates were dried at ambient temperature for 24 h and evaluated for dynamic mechanical properties, such as tensile, flexural, and impact strength. Superior mechanical properties were obtained in the composites reinforced with treated fibers [47].

The CM technique was employed to produce hybrid vinyl ester (VE) composites reinforced with coir fiber, coconut shell powder, and AFs (Kevlar 29). Coir fiber and coconut shell powder were used in as-received and surface (alkali and silane)-treated conditions. The AF content varied from 5 to 15 w.t%. The maximum fiber loading, including all types of reinforcement, was limited to 35 wt.%. Laminates were prepared by pressing the calculated wt.% of reinforcements into the mat using two chromium-coated mild steel molds, followed by resin pouring and rolling to remove entrapped air. Later, the mold was closed to cure the laminates using ambient temperature for a day under 1 MPa pressure. Prepared composites were studied for hardness, tensile, and flexural characteristics. Alkali and silane treated fiber-reinforced composites exhibited enhanced mechanical characteristics. The formation of silanol bonds and hydrogen bonds on fiber surfaces due to silane treatment enhances the adhesion between the filler–matrix, resulting in superior properties. Composites reinforced with treated fibers may be used in automobiles for structural applications [48]. Ultrahigh-molecular-weight polyethylene (UHMWPE) is combined with the polymeric filler polytetrafluoroethylene (PTFE) and AFs using the CM technique. UHMWPE in powder form is mixed with reinforcements in the presence of ethanol to obtain a homogeneous mixture followed by heating in an oven at 79 ◦C to get rid of the ethanol. The molding temperature, pressure range, and time were set to 150 ◦C, 100–300 bar, and 5 min, respectively. Out of 5 min, heating was performed for 3 min followed by 2 min of cooling. The AF weight content was varied between 2 and 5 wt.%. Tribological tests were conducted to analyze the wear performance of fabricated composites. Composites manufactured at a higher molding pressure exhibited superior wear characteristics due to enhanced consolidation in the microstructures. Among the analyzed composites, the aramid fiber-reinforced composite demonstrated superior wear resistance [49]. Hygro-thermal degradation and mechanical studies were performed on E-glass woven roving and AF-reinforced hybrid epoxy composites fabricated via the CM technique. The AF content was varied from 9 to 13 wt.% [50]. Polyetherimide hybrid composites were manufactured utilizing woven CFs, GFs, and AFs (Kelvar 29) via a CM technique and evaluated for mechanical properties. Polyetherimide in a granular form was immersed in dichloromethane to fabricate different laminates with varying reinforcement contents. Composites were compression-molded at 400 ◦C and 8 MPa followed by cooling for 4–5 h maintaining the same pressure [51].

Short AFs were reinforced with polypropylene (PP) and thermoplastic elastomer ethylene-propylene-diene using the IM technique and evaluated for mechanical characteristics. AFs (Twaron 1488) used in the study had a diameter and length of 12 μm and 6 mm, respectively. The addition of AFs increases the impact strength of the PP matrix composites; however, a declining trend was observed for the ethylene-propylene-diene matrix composite. AFs display greater affinity towards a PP matrix, resulting in increased impact performance [52]. A polyamide (PA) matrix was reinforced with flakes and longfiber granules obtained from aramid pulp to produce prepregs for the IM technique. First, compounding was performed employing a twin-screw extruder to produce the prepregs. The AF weight was varied from 5 to 20 wt.%. After compounding IM was performed to fabricate the composites. Initial materials during both stages were dried for 4 h at 80 ◦C using an oven. Fabricated samples were evaluated for tensile, flexural impact, and tribological characteristics. The authors successfully demonstrated a unique way of fabricating aramid fiber-reinforced PA prepregs for the IM technique. Fabricated composites showed decent tribological properties, which make them suitable for manufacturing components, such as gear wheels, deflection pulleys, etc., subjected to tribological loads [53]. Alumina

nanoparticle-filled epoxy matrix and AF (Kevlar 49)-reinforced composite laminates were fabricated using RTM. Nanoparticles by 40 wt.% were dispersed in epoxy resin using a SpeedMixerTM at 2500 rpm. Laminates were fabricated using 14 layers of Afs, which was equivalent to 41% by volume of epoxy resin. RTM was performed using 3 bar injection pressure at 50 ◦C. Composites were first cured in the mold at 80 ◦C for 120 min, followed by secondary curing at 150 ◦C for 180 min in a forced convection oven for demolding. Fabricated composites were subjected to a particle burnout technique to measure the nanoparticle distribution. Nanoparticle distribution was found to be non-homogenous within the laminate, and a higher particle concentration was observed next to the entry of the epoxy matrix. Increasing the resin flow tie increases the concentration of nanoparticles in the fabricated laminate [54]. VARTM was employed to fabricate epoxy composites reinforced with carbon nanotubes (CNTs)/AFs (Kevlar-29). The diameters of AFs and CNT were 12 μm and 13 nm, respectively. First, purified CNTs were used to prepare polyethylene oxide nanocomposite films. Next, epoxy was heated to dissolve the nanocomposite film and disperse CNTs into the epoxy resin. Later, AF preforms were soaked in CNT-dispersed epoxy to fabricate the composites. Fabricated composites were evaluated for flexural characteristics. The authors successfully demonstrated a new fabrication technique to produce CNT/AF-reinforced epoxy composites. Incorporating CNTs enhances the flexural performance of hybrid composites [55].

Hybrid UHMWPE composites were fabricated by reinforcing AFs (Kevlar 29) in fabric form for vehicular ballistic protection using the autoclave molding technique. AFs were initially dried using an oven for 4 h at 105 ◦C before autoclaving to eliminate moisture. Ethylene vinyl acetate adhesive films were placed in between AFs and molded using the autoclave technique. The process was performed by varying the pressure from 6 to 24 bar and temperature from 80 to 110 ◦C for 55 min. Composites were subjected to a consolidation process at 24 bar and 110 ◦C. Later, the autoclave temperature was reduced to 30 ◦C along with the pressure. Fabricated composites were evaluated for dynamicmechanical properties and ballistic performance. Good adhesion was obtained between the AFs and matrix resin. The highest mechanical and ballistic performances were exhibited by the composite reinforced with 25 vol.% of AFs. Fabricated composites can be used for fabricating vehicle armors [56]. Graphene oxide-reinforced AF/epoxy composites were successfully fabricated using autoclave forming technology. The graphene oxide content was varied from 0.1 to 0.4 wt.%. After dispersing graphene oxide in the epoxy matrix, the mixture is vacuumed at 45 ◦C for 30 min before composite fabrication. A highpressure spray gun was used to soak the aramid fiber before molding. Autoclave molding was performed at 150 ◦C and 0.4 MPa for 2 h. Fabricated composites were evaluated for their mechanical properties. The addition of graphene oxide significantly enhances tensile characteristics of AF-reinforced composites by inhibiting the stress concentrations introduced by the spaces between the fiber and matrix due to poor adhesion [57].

The FDM additive manufacturing technique was utilized to fabricate AF-reinforced nylon composites. A nylon filament reinforced with 2 wt.% short AFs was fabricated using a twin-screw extruder. The layer thickness, extruder temperature, infill part density, and raster angle varied in the range of 0.2–0.4 mm, 280–300 ◦C, 70–90%, and 0–90 degrees, respectively. A layer thickness of 0.4 mm, extruder temperature of 300 ◦C, infill part density of 90%, and raster angle of 90 degrees were identified as the optimum parameters for printing. Fabricated composites were assessed for tensile, flexural, compression, and impact performance. The layer thickness and raster angle significantly influence the mechanical performance of the printed samples. A uniform distribution of AFs throughout the nylon matrix was confirmed using scanning electron micrography [58]. A Multi Jet Fusion (MJF) AM technique was used to print high-strength AF/PA12 composites. Composites were printed using AF/PA12 composite powder and used in the as-received condition. PA12 powder was combined with short AFs utilizing a mechanical mixer. The AF content varies from 0 to 14 wt.%, and the mean fiber length was at the limit of 0.5–1.2 mm. The energy input, build platform temperature, layer thickness, and recoating speed were considered as

process parameters for printing. Samples were printed using a layer thickness and build platform temperature of 20 μm and 220 ◦C, respectively. After printing, samples were naturally cooled to ambient temperature followed by cleaning using bead blasting [59].

#### *3.2. Carbon Fiber and Its Hybrid Composites*

CFRP composites find their applications in many engineering sectors, such as automobile, aviation, marine industries and civil engineering, wind-turbines, sports equipment, and robotics due to their unique properties, such as excellent specific mechanical properties, lightweight property, high damping ability, good dimensional stability, and resistance to the corrosive environment [60,61]. A carbon fiber can be reinforced with a variety of matrices, such as polymer, metal, ceramic, or carbon [62]. CFRP can be produced using various manufacturing techniques, such as vacuum bagging, CM, FW, HL, or AM techniques [63,64]. Even though CFRP finds many engineering applications, the fabrication costs are exceptionally high and the machining of CFRPs using conventional cutting tools is not economical [65,66].

The HL method was employed to fabricate CFRP using bisphenol-A epoxy matrix and CNTs. CFs were used in the fabric form and procured from Toray Industries Inc., Tokyo, Japan. Fibers were first treated with acetone to remove impurities, followed by treatment with nitric acid to enhance the wettability during processing. CFs were carefully cleaned in deionized water before being vacuum-dried for 12 h at 80 ◦C. Composites were produced via the HL method, followed by pressing at 150 ◦C and 7.4 MPa for 150 min. The CF content was limited to 60 wt.%. Fabricated composites were evaluated for their electrical and mechanical properties [67]. Multi-wall CNTs (MWCNTs) and CF-reinforced epoxy (bisphenol-A) nanocomposites were successfully fabricated using the FW technique. The MWCNT content was varied from 0 to 1 wt.% of the matrix weight. First, MWCNTs were dispersed in an epoxy matrix using a sonication process, followed by ball milling for 2 h at 250 rpm. The mixture was later used to fabricate unidirectional CF-reinforced composites using the FW technique. CF loading was limited to 60 wt.% of the epoxy resin. Composites were first dried at 120 ◦C for 120 min, followed by post-curing at 180 ◦C for 180 min. Fabricated composites were tested to evaluate their fracture toughness and mechanical properties. The crosslink density of the epoxy matrix was significantly enhanced by the presence of amino-functionalized-MWCNTs. This crosslinking between the filler and matrix enhances the interfacial strength of the composite along with the fracture toughness [68].

CM was employed to fabricate phenolic nanocomposites reinforced with CFs and MWCNTs. MWCNTs were first dispersed in phenolic resin by them stirring for 60 min at 500 rpm. CF and MWCNT loading was restricted to 50 wt.% and 2 wt.%. BMC paste was first subjected to CM at 1 bar pressure and 100 ◦C for 30 min. Later BMC paste was subjected to the second stage of CM at 10 bar pressure and 170 ◦C for 5 h. Composites were postcured for 120 min at 200 ◦C. Fabricated composites were tested for thermal and mechanical properties. The flexural and thermal performance of the fabricated composite material was found to increase up to 1 wt.% of MWCNTs and decrease at 2 wt.% of MWCNTs. Micrography results revealed that failure of the pure composite was due to brittle fracture; however, ductile fracture was observed in MWCNT-reinforced composites. CNTs were uniformly dispersed in the phenolic resin without agglomeration [69]. Epoxy (Bisphenol-A) composite was successfully fabricated using CFs in a fabric form and MWCNTs using a CM technique. MWCNTs were ultrasonically dispersed in the epoxy resin before applying them on carbon fabric. The diameter and length of MWCNTs used in the study were in the range of 10–20 nm and 10–30 μm, respectively. The highest MWCNTs and carbon fiber loading were limited to 0.75 and 64 wt.%, respectively. MWCNTs were dispersed in acetone and mixed with epoxy, followed by sonication at 50◦C for 60 min. Later, hardener was added and mixed thoroughly using a magnetic stirrer. Six layers of carbon fabric were stacked in a different orientation to fabricate laminate of a 1.5 mm layer thickness using the CM technique. Fabricated composites were cured using an autoclave under a vacuum and tested for various mechanical and thermal properties. The micro-crack bridging effect

exhibited by nanotubes leads to the enhanced tensile and flexural performance of the composites. Uniform dispersion of MWCNTs was observed at a lower weight fraction; however, agglomeration was witnessed at a higher MWCNT content [70].

RTM was employed to successfully fabricate thick CFRP/epoxy composites. Composites were fabricated using 39 layers of CFs in different orientations. CF loading was limited to 55 wt.% of the matrix resin. The epoxy resin was injected into the molds at an injection speed of 2 l/m and 20 bar. Fabricated composites were tested for evaluating porosity and mechanical characteristics. The authors tried to propose optimum RTM process parameters to fabricate a 20 mm-thick CFRP plate. Performing compaction by means of compressed air enhances the aesthetic aspect and porosity of fabricated composites. Tested composites exhibited comparable mechanical characteristics to those fabricated via autoclave molding [71]. Hybrid epoxy composites reinforced with CFs and GFs were fabricated by employing the vacuum bagging technique. Composites were evaluated for various mechanical properties, such as hardness, tensile strength, and modulus. Bi-directional GFs and CFs in the woven mat form of 200 GSM were used for composite fabrication. The fiber weight percentage was varied from 15 to 60 wt.%. First, the laminates were prepared using the HL technique at ambient temperature, followed by vacuum bagging at 0.1 bar to remove the trapped air. Composites were post-cured in three stages, namely at 50 ◦C for 30 min, at 65 ◦C for 45 min, and at 75 ◦C for 60 min. Enhanced mechanical performance was obtained at a higher wt.% of fiber reinforcements. CF-reinforced composites exhibit enhanced ductile characteristics compared to the composites reinforced with GFs [72].

Hybrid PP composites mixed with short carbon fibers (SCFs) and short glass fibers (SGFs) were successfully fabricated by employing extrusion compounding and IM techniques. Fiber loading in hybrid composite fabrication was restricted to 25 wt.%. A twinscrew extruder was employed to manufacture PP composites using GFs and CFs in the roving form. The extrudates were cooled and pelletized, followed by IM, using a twin-screw injection molding machine at 230 ◦C [73]. PP composites reinforced with short CFs were fabricated using an extrusion and IM process. CF loading was limited to 40 wt.%. The CF diameter and mean length were in the ranges of 7–9 μm and 150 μm, respectively. Six different types of composites were manufactured by varying the reinforcing CF content. Initially, mechanical mixing is performed to mix the constituents, followed by melt-mixing using a twin-screw extruder at 210 ◦C, followed by pelletization. The pellets were injection-molded at 230 ◦C to fabricate the composites. The composites were subjected to mechanical testing to determine the tensile strength and modulus. CFs were randomly oriented with respect to the injection direction. More fiber breakage and agglomeration were noticed at higher SCF loading. The composite exhibits inferior ductility characteristics due to the presence of hollow spaces at fiber ends and lower interfacial strength among the constituents [74].

SCF-reinforced polycarbonate composite material was successfully printed employing the fused filament fabrication (FFF) technique. Initially, filaments of 2.85 mm were fabricated using a single screw extruder at 270 ◦C and a 25 mm/s speed. Constituent materials were dried in a furnace for 120 min at 90 ◦C to eliminate the water content before extruding them. The SCF content was varied from 3 to 10 wt.%. Samples were printed in a ULTIMAKER3 printer by maintaining a bed temperature of 107 ◦C, extrusion temperature of 270 ◦C, infill density of 100%, and layer thickness of 0.2 mm, with varying printing speeds in the range of 25–75 mm/s. Printed samples were subjected to mechanical testing to determine tensile, flexural, compression, and micro-hardness properties [75]. Continuous fiber-reinforced nylon composites were printed employing an FDM-based Markforged Mark One 3D printer. Carbon, glass, and Kevlar fibers were used as reinforcements for printing hybrid composites. A nylon filament of 1.75 mm was stored in a closed box to avoid moisture absorption before printing. The diameter of carbon, Kevlar, and glass fibers was 8 μm, 12 μm, and 10 μm, respectively. Samples were printed in two stages. In the first stage, nylon was printed on a built platform kept at ambient temperature. In the second stage, fibers were oriented as per the desired direction and printed on a nylon layer. Printed composites of glass and Kevlar consisted of 32 layers, while carbon fiber-reinforced

composites consisted of 26 layers. Samples were printed using fibers in concentric and isotropic patterns. Both the matrix and reinforcements were extruded at 263 ◦C using distinct print heads. The printed composites were analyzed for both tension and flexure performances [76]. An SLS additive manufacturing technique was successfully employed to print CFRP composites using PA12 in powder form. CF loading was varied from 30 to 50 wt.%, and the surface of the fibers was modified using an oxidation modification technique to enhance adhesion with the matrix material. The composite powder was prepared using a suitable technique, and the samples were printed using a continuous wave CO2 laser. The following process parameters were employed during printing: laser beam speed—500 mm/s; laser power—22 W; layer thickness of powder—0.1 mm. Manufactured composites were examined for evaluating their mechanical and thermal characteristics [77]. An SLS additive manufacturing technique was employed to print recycled PA12 composites reinforced with milled CFs up to 30 wt.%. Constituent materials were first dried using an oven for 24 h at 60 ◦C, followed by manual mixing using a plastic bag. The mixture was then compounded utilizing a twin-screw extruder maintaining a constant temperature setting and screw speed. Extruded material was pelletized and dried again for a day at 60 ◦C. Filaments were fabricated using a single screw extruder, followed by printing using Ultimaker 2. Samples were printed at a printing speed of 60 mm/s, build platform temperature of 240 ◦C, and layer thickness of 0.2 mm, with 100% fill density. Printed samples were evaluated for mechanical and thermal characteristics [78].

#### *3.3. Glass Fiber and Its Hybrid Composites*

Glass fibers are widely used in the fabrication of structural composite materials because of their high strength, stiffness, and durability. GFs can be used in a variety of forms, such as short fibers, continuous fibers, fabric, or mat forms to fabricate the composites. GFs can be reinforced with many polymers, such as epoxy, phenolic, polyester, vinyl ester, etc. [62]. GF-reinforced composites have several advantages over traditional materials, such as metal and wood. They are lightweight and corrosion-resistant and have high strength-to-weight ratios. They are also highly customizable, as the properties of the composite can be tailored to meet specific application requirements. These materials are used in many applications, namely aerospace, automotive, marine, and construction industries. They are commonly used to make parts, such as body panels, structural components, and insulation [79–81].

GFRP composites were fabricated using both HL and VARTM followed by a comparative analysis of mechanical and thermo-mechanical performance for wind turbine blade applications. Bi-directional E-glass woven fibers were utilized as reinforcements with an epoxy matrix. Specimens with 10 layers of GFs were fabricated using HL followed by curing under a 25 kg weight for 24 h. Similarly, specimens of 10 layers were also fabricated using VARTM at room temperature for a comparative analysis [82]. GFRP hybrid composites were fabricated using an HL and vacuum infusion (VI) technique for marine vessel applications. The outer layer of hybrid composites was fabricated using the HL technique and the interior layers using the VI technique at a different orientation. Epoxy vinyl ester and plain vinyl ester were used as matrix material for VI and HL techniques, respectively. Fabricated samples were evaluated for tensile, compression, and in-plane shear characteristics [83].

Short glass fibers and calcium carbonate-reinforced polyester composites were fabricated from BMC using a CM technique. Calcium carbonate at 55 wt.% was used to first fabricate BMC. Fibers of two different lengths were used, and the mean length of chopped glass fibers was 0.4 mm and 6.4 mm. Composites were fabricated by employing the following process parameters: mold temperature of 149 ◦C; mold pressure of 5.5–6.9 MPa; cure time of 2 min. Fabricated samples were evaluated for impact characteristics using the Charpy test for aerospace applications [84]. Hybrid epoxy composites of glass and jute fibers in woven form were successfully fabricated using the CM technique. The E-glass and jute reinforcement thicknesses were 0.7 and 1 mm, respectively. After CM, first, a curing pressure of 15 kg/cm2 was applied for 10–15 min to enhance wettability, followed by postcuring at room temperature for 24 h. Hybrid composites were evaluated for mechanical characteristics in terms of flexural and impact strength followed by numerical analysis to evaluate the predictive flexural response [85].

Glass fiber-reinforced polyester pipes were manufactured using the FW technique and evaluated for tensile characteristics. The inner layer of the composite was fabricated using a glass fiber mat of 450 GSM using polyester resin on a cylindrical mandrel. After curing the first layer, polyester resin-impregnated continuous E-glass woven fabrics were wounded to fabricate the structural layer over the inner layer. The highest fiber and matrix weight fraction was limited to 79.5% and 34.4%, respectively [86]. Hybrid polyester composites reinforced with woven jute/glass fabric were fabricated using the HL technique. The weights of woven glass and jute fabrics were 360 and 320 GSM, respectively. Laminates were first cured using moderate pressure for 60 min, followed by secondary curing without any pressure for 48 h at room temperature. Six types of laminates were fabricated using 10 layers of reinforcements, and the highest fiber loading was limited to 42 wt.%. Fabricated composites were tested for tensile, flexural, and interlaminar shear characteristics [87].

Vacuum Assisted Resin Injection (VARI) was employed to fabricate an unsaturated polyester composite reinforced with E-glass fabrics. Laminates were manufactured using three layers of glass fabrics, followed by 40 h of curing under ambient conditions. Fabricate composites were examined to evaluate the influence of water immersion on mechanical performance [88]. Glass fiber-reinforced unsaturated polyester resin composites were manufactured using the RTM process. Before the RTM process, the fibers were treated with a coupling agent for 10 min and dried under ambient conditions for 48 h, followed by oven drying under a vacuum for 24 h. Laminates were fabricated using 12 layers and mold was wrapped with a rubber dam to avoid resin leakage. Fibers were impregnated at 40 ◦C under 46 kPa pressure using nitrogen gas. Later, the laminates were cured by increasing the mold temperature to 85 ◦C for 60 min. Fabricated composites were tested for void contents and flexural characteristics [89].

An SLA process was employed to print short and continuous fibers composites reinforced with photo-curable epoxy resin. Samples were printed utilizing a Nobel 1.0 SLA 3D printer manufactured by XYZ Printing, Inc. using a laser wavelength of 405 nm. Reinforcements in the form of glass powder, a chopped glass strand mat, and fiberglass fabric were coated with silane before printing for enhancing the interfacial bonding. Glass powderreinforced composites were fabricated by varying the reinforcement content up to 55 wt.% with a layer thickness of 0.1 mm. Specimens were cured for 10 min using an exposure unit. Good quality specimens were obtained up to 50 wt.% of glass powders. Intricate shapes of good quality were produced using glass powders up to 10 wt.%. A chopped strand mat was used to fabricate short glass fiber-reinforced composites employing the same printing parameters. The reinforcement content was maintained at 1 wt.% of the epoxy resin. Continuous glass fiber specimens were fabricated with a layer thickness of 0.1 mm by submerging the fiberglass fabric in a vat containing a photocurable resin. Specimens were successfully fabricated with a few minute voids of less than 1 mm in size using glass powder and continuous fibers. However, the fabrication of short fiberglass composites was not successful due to non-consistent reinforcement dispersion and the existence of large holes [90].

Three-phase GF-reinforced composites were fabricated using the SLS additive manufacturing technique, followed by infiltration using epoxy, for insulating purposes. GF was surface-treated using silane (KH-550) before reinforcing it with a phenol formaldehyde resin. Phenol formaldehyde was dissolved in ethanol and mixed with GF to prepare powder for the SLS process using the ball milling process. Ball milling was performed at 300 rpm for 60 min and constituents were dried for 24 h at 50 ◦C, followed by crushing and sieving, to obtain the suitable powder material. GF loading was varied from 60 to 80 by vol.% of the matrix resin. Samples were printed using a continuous-wave CO2 laser in an HK S320 SLS system. Samples were printed with the following optimum conditions: build plate temperature of 65 ◦C; laser power of 14 W; scanning speed of 3500 mm/s; layer

thickness of 0.1 mm. Finally, the fabricated samples were infiltrated with epoxy to fill the pores, followed by curing in an oven. Fabricated composites were evaluated for mechanical and electrical properties [91]. An FDM additive manufacturing technique was employed to fabricate an SGF-reinforced acrylonitrile butadiene styrene polymer composite. SGF loading was limited to 30 wt.%. ABS pellets were first dried in an oven for 120 min at 120 ◦C to eliminate moisture. After drying, the ABS pellets were mixed with the required quantity of SGFs and compounded using a twin-screw extruder at 50 rpm and 225 ◦C. Compounded pellets were used to fabricate filaments of 1.75 mm with a twin-screw extruder using a temperature range of 200–230 ◦C. Samples were printed adopting the following printer settings: infill density of 100%; layer thickness of 0.1 mm; build plate temperature of 80 ◦C, and extruder temperature of 240 ◦C. Fabricated samples were tested for mechanical performance evaluations [92].

#### *3.4. Natural Fiber and its Hybrid Composites*

A natural fiber-reinforced polymer composite is a material in which a polymer matrix is reinforced with natural fibers. Natural fibers, such as jute, hemp, flax, sisal, and kenaf, are commonly used for reinforcing the polymer matrix [93]. Natural fibers enhance mechanical properties, such as strength, stiffness, and toughness. Natural fibers have been extensively used in polymer composites in recent years because they are renewable, lowcost, lightweight, and environmentally friendly [94]. Various techniques can be adopted to fabricate these types of composites. NFRP composites have many advantages over traditional composites made with synthetic fibers. They have a lower environmental impact, are biodegradable, and have good mechanical properties. Additionally, the use of natural fibers can reduce the weight of the composite, making it lighter and more fuel-efficient in certain applications. Natural fibers were generally surface-treated to enhance the adhesion with the matrix material before composite fabrication. The resulting composite material can be used for many purposes, such as automotive parts, construction materials, furniture, and packaging. Overall, natural fiber-reinforced polymer composites are a promising material that can offer a sustainable solution for various industries [95,96].

Hybrid epoxy composites reinforced with unidirectional sisal/banana fibers were fabricated using an HL Process. Reinforcements were surface-treated with a sodium hydroxide solution before composite fabrication. The reinforcement content was restricted to 30 wt.% of the epoxy resin. After fabrication, laminates were cured under a light load for 24 h at room temperature. Fabricated composites were drilled and evaluated for delamination characteristics [97]. An HL technique was employed to fabricate hybrid polyester composites using natural flax and synthetic GFs for wind turbine blade applications. Three different types of laminates were fabricated by varying the reinforcement contents. Fabricated composites were examined to estimate the mechanical characteristics and water absorption characteristics [98]. VE composites reinforced with surface-treated coir fibers were fabricated using the HL technique. Coir fibers were derived from the coconut husk procured from a local market. Samples were cured under a constant pressure of 5 MPa for 24 h at room temperature. The mean length and diameter of the fibers were 8 mm and 200 μm, respectively. Fabricated composites were evaluated for tensile and flexural characteristics [99].

The comparative mechanical and thermal characterization was performed on flax fiber/epoxy composites fabricated using HL and CM techniques. Reinforcements were used in mat form without any surface treatments. The reinforcement contents were varied from 24–30 by vol.% of the epoxy resin. HL was performed at room temperature, followed by two stages of curing. Samples were initially cured at 80 ◦C for 4 h and later at 120 ◦C for 2 h. CM was performed by varying the compression pressure in the range of 2–10 MPa and temperature from 80 to 120 ◦C. Superior characteristics were obtained for the compression -molded composites with 27 vol.% of flax fibers at 4 MPa pressure and 100 ◦C temperature [100]. Polylactic acid (PLA)-based bio-composites were fabricated using the CM technique using bamboo, cotton, and flax fibers in non-woven mat form. The mean lengths of bamboo, cotton, and flax fibers were 126, 35, and 80 mm respectively. Constituent materials were first dried at 60 ◦C overnight before compression using a hot press for 15 min at 185 ◦C under 1250 psi pressure. Laminates were cured at 120 ◦C for 30 min using an oven. Fabricated composites were evaluated for mechanical (flexural and impact), thermal, damping, and acoustic characteristics and were to be used in automobile and construction industries. A PLA composite reinforced with cotton and bamboo fibers exhibited better performance than the commercially available material [101].

RTM was employed to successfully fabricate epoxy composites reinforced with unidirectional high-quality flax fibers in mat form. Composites were fabricated using three different volume fractions of the reinforcements. The highest fiber loading was limited to 48 by vol.% of the epoxy resin. Injection pressures of 2 bar and 1 bar were employed to fabricate the composite reinforced with high- and low-volume fractions of reinforcements, respectively. Resin was injected into the closed mold at 50 ◦C, followed by curing at 80 ◦C for 8 h. The prepared composites were tested to evaluate their mechanical properties [102]. Green PLA composites reinforced with bamboo and pineapple leaf fibers were fabricated using the IM technique. Natural fibers were surface treated using three different solutions to enhance the wettability before fabrication. Fiber loading was limited to 10 wt.%, and the mean length of the fibers was 4 mm. First, PLA pellets were dried in an oven at 70 ◦C for 6 h to remove the water content. Later, IM was performed by considering the following process parameters: barrel temperature range, injection pressure, and speed of 125–165 ◦C, 90 bars, and 60 mm/s, respectively, followed by a 25 s cooling cycle. Fabricated composites were evaluated for various mechanical properties, such as tensile, flexural, compression, and shear properties [103].

Bamboo fiber-reinforced PP/PLA composites were successfully fabricated using the FDM additive manufacturing technique. Natural fibers were subjected to various surface treatments to enhance their wettability before printing. First, PLA composites were fabricated using IM by adopting the following conditions: IM temperature range of 180–185 ◦C; injection pressure of 60 MPa; and holding time of 10 s. Next, PP/PLA/bamboo fiber composite filaments were manufactured using a single-screw extruder. Filaments were made of 20 wt.% bamboo fiber, 52.5 wt.% PP, and 22.5 wt.% PLA. Composites were printed using an FDM-based printer using an extrusion temperature of 180–200 ◦C, a printing speed of 40–60 mm/s, and a build plate temperature of 40–60 ◦C. Printed samples were evaluated for mechanical and thermal characteristics [104]. A hybrid AM technique (FDM in combination with shape deposition modeling) was employed to print natural-fiber granulated composites made of sugarcane, jute, ramie, banana, pineapple fiber, and seashell powder. The volume fraction of the reinforcements was fixed at 80 vol%. Fibers were first washed using distilled water, next treated with sodium hydroxide solution, and later dried at 80 ◦C for 180 min to eliminate moisture before milling using a ball miller to the size of 70 to 100 μm. The reinforcements were mixed in the required proportion and converted into a paste using epoxy resin. After the paste was obtained, the composites were printed using a specialized printing head using an FDM printer. Samples were printed by varying the number of layers from 6 to 12; however, the thickness of the composites was limited to 13 mm. Granulated composites were tested for evaluating their various mechanical properties [105]. An SLS additive manufacturing technique was successfully employed to fabricate bamboo flour/co-polyester composites. Bamboo flour contents of 20, 25, and 30 by wt.% were used to fabricate the composites. Bamboo floor and co-polyester powder were mixed in different ratios using a high-speed mixer for 10 min at 700–800 rpm and 50 ◦C to prepare the composite power for SLS. SLS was performed using a CO2 laser with a 10.6 mm wavelength and a scan speed of 2000 mm/s. Printed samples were evaluated for mechanical properties in terms of tensile, flexural, and impact strength [106].

#### **4. Mechanical Characterization of Fibrous Composites**

Mechanical characterization of fibrous composites involves studying the behavior of composite materials under mechanical loading, which can provide insights into the strength, stiffness, and durability of the material. The mechanical properties of the fibers, the matrix material, and the interface between them are important in determining the overall mechanical behavior of the composite. The results of these tests can be used to design and optimize composite materials for specific applications, such as aerospace, automotive, and construction industries. The mechanical characterization of synthetic and natural fibrous composites is discussed in the following section.

#### *4.1. Synthetic Fiber-Reinforced Polymer Composites*

Jaiswal et al. [107] examined the effect of AF content and its orientation on the mechanical performance of the composites, such as tensile strength, modulus, and elongation at break. The results showed that increasing the AF content improved the mechanical properties of the composites, with a maximum tensile strength achieved at a 50:50 AF-topolypropylene fiber ratio. Furthermore, the study found that increasing the fiber orientation angle resulted in decreased tensile strength and elongation at break but an increased modulus. Kandekar and Talikoti [108] investigated the effect of the number and position of AF strips on the torsional strength of the reinforced concrete beams. The experimental results indicated that the use of AF strips can significantly improve the torsional strength and stiffness of the reinforced concrete beams. Furthermore, the study found that placing the AF strips at the critical sections of the beam can improve the torsional behavior of the reinforced concrete beams more effectively than placing them at other locations. The addition of treated coir fiber/coconut shell powder and AFs improved the mechanical properties of the VE composites, with the highest mechanical properties achieved at a 10 wt.% fiber content. The study found that the addition of AFs had a more significant effect on the tensile and impact strength of the composites, while the addition of treated coir fiber/coconut shell powder had a more significant effect on the flexural strength [48]. Yahaya et al. [17] investigated the ballistic impact properties of woven kenaf-aramid fiber hybrid composites. The results showed that the addition of AFs improved the ballistic impact properties of the composites, including increased energy absorption and reduced back-face deformation. The composites with a higher percentage of AFs had better ballistic impact properties but at the cost of decreased tensile strength.

In a study by Badakhsh et al. [67], the effect of grafting CNTs onto carbon fibers (CFs) in CFRP composites was investigated. The study found that the grafting process improved the surface energetics of CFs, leading to better bonding between the fibers and the polymer matrix. As a result, the mechanical properties of the CFRP composites were enhanced. The study also suggested that optimizing the grafting process can achieve a balance among surface energetics, electrical conductivity, and mechanical properties. Tariq et al. [70] investigated the mechanical properties of multi-scale CNT/CF/epoxy composites. The incorporation of CNT into carbon fiber-epoxy composites enhances their mechanical properties by providing additional reinforcement at the nanoscale. The resulting composites exhibit improved stiffness, strength, toughness, and fatigue resistance. Gupta et al. [75] conducted a study to examine the impact of varying the CF content on the mechanical properties of a composite material. The results showed that the addition of CFs to the polycarbonate matrix improved the mechanical properties of the composite. The inclusion of 10 wt.% CFs increased the tensile strength and modulus by 52% and 82%, respectively, when compared to pure polycarbonate. Additionally, the incorporation of CFs also improved the flexural strength and modulus of the composite material. According to microstructure analysis, the CFs were uniformly dispersed throughout the polycarbonate matrix, which contributed to the enhanced mechanical properties.

Huang and Sun [88] investigated the effect of water absorption on the mechanical properties of glass/polyester composites. The results showed that water absorption significantly affected the mechanical properties of the glass/polyester composites. Specifically, the tensile strength, flexural strength, and impact strength of the composites decreased after immersion in water for various durations. The authors attributed this decrease in mechanical properties to the degradation of the resin matrix due to water absorption. The study also

revealed that the extent of degradation was dependent on the duration of water immersion and the type of fiber reinforcement. Ahmed and Vijayarangan [87] studied the mechanical properties of jute and jute-glass fabric-reinforced polyester composites. The results of the study showed that the addition of GFs to the jute fabric improved the mechanical properties of the composites. Specifically, the jute-glass fabric-reinforced polyester composites had higher tensile strength, flexural strength, and interlaminar shear strength compared to the jute-reinforced polyester composites. The microstructure analysis showed that the water absorption caused swelling of the resin matrix, which led to the formation of voids and cracks at the interface between the fiber and the matrix. This resulted in a decrease in the interfacial adhesion and, hence, the mechanical properties of the composites. The influence of fiber length on the mechanical properties of GF-reinforced PA12 composites manufactured using multi-jet fusion printing was investigated by Liu et al. [109]. It was found that using longer fibers resulted in increased porosity in the composite parts. The addition of GFs with an average length of 226 μm significantly improved the ultimate tensile strength and tensile modulus of the composites in the direction of powder bed spreading. Specifically, the improvements were 51% and 326%, respectively, compared to pure PA12 specimens. Li et al. [91] evaluated the mechanical properties of electrical insulating composites made from glass fiber, phenol-formaldehyde resin, and epoxy resin using SLS technology. The results indicated that the bending and tensile strength of the composites increase by 30% and 42.8%, respectively, after being infiltrated with epoxy resin. Additionally, increasing the glass fiber content improves the flexural and tensile strength of the composite. These improved properties make the hybrid composites suitable for use in complex structural electrical insulation devices fabricated using SLS additive manufacturing technology, thereby broadening the materials and applications of this technology.

#### *4.2. Natural Fiber-Reinforced Polymer Composites*

According to research of Thomason [110], the transverse and shear modulus of jute fiber is significantly lower than its longitudinal modulus, leading to lower mechanical performance of NFRP composites compared to GFRP composites, mainly due to weaker mechanical properties in the transverse direction. This weakness can be attributed to the detachment of the outer layer and individual fibers in contact with the matrix during compounding and extrusion, which is not caused by a lack of physical and chemical compatibility between the fiber and matrix. A study by Ouali et al. [111] showed that a high-density polyethylene composite made with a 40% kenaf fiber mat had similar mechanical properties to a composite made with 40% discontinuous glass fiber and HDPE. Both composites had specific tensile strengths and moduli, as well as flexural strengths and moduli. Additionally, another process that involved coating flax fabric with adhesive, sandwiching it with polymer films, and compression molding resulted in a flax/PE composite with remarkable tensile strength and modulus values. According to a study by Couture et al. [112], the mechanical properties of flax/PLA and flax-paper/PLA composites were compared. Results showed that both composites had similar specific tensile properties, 252 MPa cm3/g and 217 MPa cm3/g, respectively, compared to composites made with woven glass fabrics and epoxy. The flax-paper composite also had an exceptionally high impact strength of 600 J/m compared to unreinforced resin of 15 J/m.

A summary of the various investigations performed on fibrous composites fabricated using different manufacturing techniques is presented in Table 1.


**Table 1.** Summary of the investigations performed and manufacturing methods adopted for the fabrication of fibrous composites.


**Table 1.** *Cont.*

#### **5. Conclusions**

This review has provided an overview of the fabrication and mechanical characterization of fibrous composites for engineering applications. Fibrous composites offer a range of desirable properties, such as high strength, stiffness, and low weight, making them attractive for use in various engineering applications. The review highlights the importance of proper fabrication methods and their impact on the mechanical properties of the composites. The following are the critical observations made during the study:


The field of fibrous composites is continuously evolving, and there are several future directions and challenges that need to be addressed for their widespread use in engineering applications.


**Author Contributions:** Writing—original draft preparation, H.S.A. and T.P.J.; writing—review and editing, J.X.; supervision, J.X.; funding acquisition, J.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 52175425) and the Shanghai Industrial Collaborative Innovation Project (Grant No. HCXBCY-2022-040). The work was also funded by the 9th Sino-Hungarian Intergovernmental Scientific and Technological Cooperation Project (Grant No. 2021-07).

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Nomenclature**


#### **References**


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### *Review* **Cool-Clave—An Energy Efficient Autoclave**

**Indraneel R. Chowdhury and John Summerscales \***

School of Engineering, Computing and Mathematics, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK

**\*** Correspondence: j.summerscales@plymouth.ac.uk

**Abstract:** Out-of-autoclave (OOA) manufacturing techniques for composites result in lower fibre volume fractions than for fully compressed laminates. The lower fibre volume fraction produces a higher resin volume fraction, which becomes resin-rich volumes (RRV). Textile reinforcements with clustered fibres and consequent RRV generally have low strength but high in-plane process permeability, whereas the opposite is true for uniformly distributed fibres. The inevitable increase in resin volume fraction of OOA composites often compromises composite performance and leads to relatively higher weight and fuel consumption in transport applications. The retention of autoclave processing is recommended for highest performance when compression press moulding is not appropriate (for example, for complex 3D components). The traditional autoclave processing of composites heats not only the component to be cured but also parasitic air and the vessel insulation. Subject to minor modifications of the pressure vessel, electrically heated tooling could be implemented. This approach would need to balance insulation of the heated tool surface (and any heater blanket on the counter-face) against the quenching effect during the introduction of the pressurised cool air. This process optimisation would significantly reduce energy consumption. Additionally, the laminate on the heated tool could be taken to the end of the dwell period before loading the autoclave, leading to significant reductions in cure cycle times. Components could be cured simultaneously at different temperatures provided that there are sufficient power and control circuits in the autoclave. While autoclave processing has usually involved vacuum-bagged pre-impregnated reinforcements, implementation of the cool-clave technique could also provide a scope for using the pressure vessel to cure vacuum-infused composites.

**Keywords:** autoclave; cool-clave; vacuum; heated tooling; fibre-reinforced composites

#### **1. Introduction**

Composite materials are widely used in industrial applications due to their unique characteristics of high-stiffness-to-weight ratio, excellent durability, chemical resistance, and better recycling potential as compared to metallic components. High performance composites, mainly used in aerospace applications, are produced in the autoclave by applying elevated pressure and temperature [1,2]. However, autoclave processing of composites involves long curing cycle times, expensive tooling, and high energy consumption [3,4]. As a result, there is an interest across the range of composites manufacturing processes for cost reduction with a current focus on out-of-autoclave (OOA) processes [1,5], especially OOA prepreg [6] and resin infusion under flexible tooling [7–9]. The OOA process involves manufacturing composites by applying vacuum and heat outside of the autoclave, but has limitations on the maximum laminate fibre volume fraction due to compressibility characteristics of the reinforcement. As a result, composites manufactured by vacuum-only processes cannot achieve high fibre volume contents, which is a primary requirement in high performance composites for aerospace, automobile, and defence sectors. Compression moulding in a hydraulic press creates limited compaction perpendicular to the line of action of the press. The autoclave is the best process for consolidation of complex three-dimensional components, but suffers from several limitations: (a) pre-impregnated

**Citation:** Chowdhury, I.R.; Summerscales, J. Cool-Clave—An Energy Efficient Autoclave. *J. Compos. Sci.* **2023**, *7*, 82. https://doi.org/ 10.3390/jcs7020082

Academic Editor: Jinyang Xu

Received: 23 January 2023 Revised: 8 February 2023 Accepted: 14 February 2023 Published: 16 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

(prepreg) reinforcements incur a high-cost premium, (b) high energy input for heating and air circulation: flow speeds in the range 1.4–2.1 m/s [5], (c) non-uniform heating of the components in the vessel due to turbulent flow, windward vs. leeward location, flow stagnation and consequent temperature differences, (d) thermal lag due to the tool or consumables between the heat source and the composite, and (e) long cycle times, which may be a "bottleneck" constraint [10,11].

However, with minor modifications of the pressure vessel, there is potential to improve the efficiency of autoclave processes. The traditional autoclave not only heats the composite parts to be cured but also inert gasses and vessel insulation. Significant energy savings can result from using electrically heated tooling to only heat the essential parts of the process (the tool and composite), and by cool-air pressurisation of composites [12]. Further to the potential for significant reductions in energy consumption, the laminate on the heated tool could be taken to the end of the dwell period before loading the autoclave, leading to significant reductions in cycle times. Autoclave loading efficiency could be improved by curing different composite systems simultaneously with the composites brought to their respective curing temperatures before loading the autoclave, provided there are sufficient power and control circuits in the autoclave, which would further enhance process efficiency [12].

This paper critically reviews the technique of using heated tooling in the autoclave to enhance the energy- and cost-efficiency of autoclave process, designated as the 'cool-clave' technique.

#### **2. Autoclave**

Autoclaves have become indispensable equipment to process high-quality polymer composite materials for structural industries, such as aerospace, automotive, and defence sectors [13]. Today, for example, in the aircraft industry, investments in such equipment are strategically important. Autoclaves are now being used to produce very large aircraft components, such as wing and fuselage sections. They can process a wide variety of materials, including thermoset [14] and thermoplastic [15]-based composite parts with varying contours and complex shapes.

The quality requirements of the present high-performance composites for aerospace/ defence industries are indeed more stringent. Additionally, there is an urgent requirement to improve the efficiency and cost-effectiveness of high-performance structural composites, along with ensuring reliable and consistent processing methods. Therefore, it is imperative for autoclave design engineers to take into consideration different governing criteria to address the diverse and complex requirements for developing state-of-the-art autoclave systems. In addition to handling a wide variety of consumables, modern autoclaves must respect health and safety requirements [16] and ensure minimum maintenance costs.

Autoclaves are closed pressure vessels used to manufacture high performance composite components. Uncured composites are moulded in an autoclave typically heated using inert gases, such as carbon dioxide or nitrogen, thus allowing the transfer of heat and pressure to the composite component for consolidation and allowing it to cure firmly and uniformly. The application of pressure for consolidation of composites in an autoclave helps in reducing porosity and voids, retains shape around the mould, and enables better control to maintain a higher fibre volume fraction in composite components [17]. The autoclave process draws many similarities with hot pressing technique; however, the main difference pertains to the way heat and pressure are applied [18]. The autoclave operating parameters, such as temperature and pressure are based on the resin systems used. Generally, epoxy resins require temperatures < 200 ◦C and pressures of 0.7 MPa [17].

Figure 1 shows the internal chamber of the Aeroform autoclave located in the composites manufacturing laboratory at the University of Plymouth. Surrounding the main internal area is a metal inner case which shields the components being cured from twelve electric heating elements positioned at intervals around the chamber's circumference. Behind the heating elements is a layer of thermal insulation, protected by sheet metal. As the autoclave

walls are made up of quality carbon steel, up to 150 mm on some autoclaves [17,18], they act as a heat sink. The insulation is designed to minimise heat transfer from the main chamber to the autoclave walls. The insulation commonly used consists of refractory ceramics or fibreglass insulation. The insulation prevents excess energy loss and is designed to keep the autoclave outer walls down to a maximum temperature of 60 ◦C.

**Figure 1.** Internal chamber of 0.67 m working diameter Aeroform autoclave at the University of Plymouth.

The design of autoclave systems is multidisciplinary in nature and encompasses mechanical, process control, and instrumentation engineering. Invariably, the state-of-theart autoclave systems are completely computer-controlled and are semi- or fully automated. The computer controls of these modern autoclaves are required to execute the selected cure cycle by sequentially starting various subsystems, download set values at regular time intervals to the front-end controllers, acquire, store, and archive the data, monitor cure status and faults, generate alarms, and perform the functions of sequential shut down and reporting [19]. Ease of maintenance, fail-safe operation, and reliability are among the key drivers in modern autoclaves. The low cost of ownership also needs to be considered in today's context [17]. In recent years, there has been an increasing demand to enhance the service temperature of high performance structural composite components, invariably leading to higher curing temperature and pressure requirements in the order of 300 to 350 ◦C and up to 1.5 MPa. This necessitates the development of high temperature and highpressure autoclave systems, which presents a new set of challenges such as the handling of massive door and locking systems, temperature uniformity, special material requirements for door and shell flanges, fabrication, transportation, and, most importantly, low cost and maintenance requirements [17,19].

#### *2.1. Autoclave Moulding*

In an autoclave moulding, isostatic pressure is applied to the composite component on a vacuum bagged mould prior to applying heat/vacuum/pressure to compact the composite material. The intimacy of contact between the composite and mould therefore depends on the magnitude of the applied pressure. Pre-impregnated (prepreg) reinforcements cured by autoclave processing are first covered in peel-ply, with bleeders used to draw out excess air and to soak up excess resin while curing (unless the system is intended for zero-bleed), prior to vacuum bagging, as this allows for the manufacturing of composite components with a high-quality surface finish. During autoclave processing, engineers adjust the process by maximising air expulsion and minimising excess resin flow. Curing pressures typically ranges between 3–12 MPa [20,21]. Prepreg reinforcements are generally

used as they offer good processability. The fundamental components of an autoclave moulding process are shown in Figure 2.

**Figure 2.** Fundamental components of an Autoclave process (redrawn from [20]).

Autoclave manufacturing is generally a 3-stage process. In the first stage, the vacuum is applied until the desired temperature is reached for composite consolidation. The ramped heating enables resin viscosity to gradually decrease, as well as allowing for the release of volatiles and air bubbles. The reduced viscosity of the resin simultaneously improves wetting of the fibres and resin flow, which in turn facilities movements of volatiles and air bubbles. In the second stage, after a suitable dwell period, the consolidation pressure is applied. The final stage raises temperature to post-cure the composite, following which the resin viscosity stabilises and the material starts to cure [20]. A typical autoclave moulding bagging mechanism is further illustrated in Figure 3.

**Figure 3.** A typical autoclave moulding bagging mechanism.

In thermoset resins, curing is a chemical cross-linking process that is very crucial for optimum performance of thermoset matrix composites. For thermoplastic matrix composites, autoclave consolidation is achieved at high temperatures and bagging materials, and techniques used for thermoset composites are adapted to suit thermoplastics [22].

Autoclave moulding process comprises the following advantages [23]:


The benefits of the autoclave are that the heat and pressure deliver high performance components for the composite industry, to compete with and replace metal components. Therefore, lightweight components, for example composite motor vehicle parts, can achieve higher efficiencies. In this scenario, engines will require less torque and fuel than when used to move a heavy metal structure. The disadvantages include high capital cost of the machine, and non-flexible and poor heat transfer efficiency. To heat the composite component, it is normal to parasitically heat all the air in the pressure vessel and the thermal insulation of the vessel. Most of the energy that is put into the system is taken up by the heater, therefore creating a higher consumption of energy. Other disadvantages include higher waiting times due to slow ramp rates and longer curing cycles [24].

#### *2.2. Heating and Air-Circulation System in an Autoclave*

Forced gas circulation systems using nitrogen or carbon dioxide are most commonly used in autoclaves. The air circulation system consists of a centrifugal blower and ducting system. The heating elements are placed around the impeller. The centrifugal blower takes in gas axially and discharges it radially to pass over the heating elements at a velocity of 1–2 m/s at ambient conditions. The air circulation system also helps in accelerating the cooling process by removing the gas from the outer surface of the cooling tubes at an increasing rate. Modern autoclaves are mechanised with a flange-mounted blower motor encased within a pressure-tight casing and connected to the rear of the autoclave. This enables the motor rotor, stator, and the mechanical components, for example, bearings to directly encounter the autoclave pressure. Power ratings of a typical autoclave range between 100 to 150 kW [17].

Heating system in autoclaves are either electrically controlled or controlled by indirect gas firing (circulating externally heated or cooled thermic fluid). However, the majority of the autoclaves are electrically heated, as these systems are cleaner and more compatible to modern computer control systems and provide better control of autoclave temperature. The electrical heating requirement in an autoclave is based on the charge and resin system requirements for the cure cycle. For example, a typical 4.5-m diameter × 9-m length autoclave requires a heating capacity of approximately 1 MW. Heating elements, typically ranging between 5–10 kW, are usually manufactured using nichrome/kanthal filament with an outer sheath of steel grouped together in banks and connected in star or delta configuration [17]. The air circulation and heating system of a typical autoclave is illustrated in Figure 4.

**Figure 4.** Schematic of air circulation and heating system in a typical autoclave (redrawn from [17]).

#### *2.3. Pressurization and Cooling System*

The pressurization systems in an autoclave ensure that the required pressurization rate is maintained during autoclave processing of composites. The average pressurization rate in modern autoclaves is 0.2 MPa/min [17]. Many modern autoclaves are nitrogen-pressurized instead of air-pressurized since autoclave cure consumables are highly inflammable in air medium due to the presence of oxygen. The pressurization system in an autoclave consists of a primary compressor, booster compressor, storage tanks, and piping circuitry. The primary compressor takes in air from the atmosphere and pressurizes it to 0.7 MPa. The booster compressor further pressurizes the air to high pressure (typically in the range between 1.7–2.2 MPa) in order to create sufficient pressure differentials to attain the required pressurization rate. In nitrogen-pressurized autoclaves, the nitrogen plant receives the air from the primary compressor at 0.7 MPa pressure and isolates nitrogen from atmospheric air by a process called Pressure Swing Adsorption (PSA) [17], which can produce nitrogen in the order of 99% purity suitable for curing polymeric composite materials. Nitrogen is then pressurized by the booster compressor to meet the required pressurization rate and purged into the chamber [17]. In Figure 5, the cooling and pressurization system used in modern autoclaves is schematically represented.

**Figure 5.** Schematic of cooling and pressurization system in an autoclave (redrawn from [17]).

Autoclave processing requires variable cooling rates based on the resin system used. Other variables affecting the cooling system are (a) temperature difference between the autoclave ambient and cooling medium, (b) flow rate of cooling medium, (c) area of heat transfer, (d) type of flow of cooling medium—parallel or cross flow, (e) conductivity of cooling coil material, and (f) velocity of autoclave medium across heat exchanger. The cooling rate in an autoclave is controlled by varying the flow of cooling fluid. In some autoclaves, both air and water are used as the cooling medium. In autoclaves, a closed loop cooling system is generally employed to prevent excess use of water. Liquid nitrogen is also used for faster cooling. A major challenge in designing cooling system for autoclaves is how fast and effectively the cooling medium can be drained from the autoclave heat exchanger, as any delay in draining the cooling medium from heat exchanger can lead to loss of heat during the heating phase and damage to the heat exchanger tubes. In the water-based cooling system of autoclaves, the simplest way of draining the water is to provide a sump just below the autoclave heat exchanger and then pump back the water to the cooling water sump. This process helps in reducing wastage of water and at the same time also prevents the steam from entering the cooling tower [17].

#### *2.4. Vacuum System*

Figure 6 illustrates the vacuum system of a typical autoclave. The main components of vacuum system in modern autoclaves are vacuum pumps, vacuum reservoirs, buffer tanks, suction, and measurement lines. All modern autoclaves must include an adequate number of vacuum ports and must also have the capacity to maintain different levels of vacuum in different bagging systems simultaneously. Therefore, vacuum pumps and reservoirs in an autoclave must have adequate buffer capacity. For example, vacuum pump with minimum capacity of 7 m3/h is necessary for a bagging area of approximately 1 m2. Generally, a 4.5 m × 9 m autoclave can have approximately 60 measurement and suction lines. Correspondingly, the pump capacity ranges to about 180 m3/h with a reservoir capacity of 6 m3. Vacuum requirement ranges between 667–26,664 × <sup>10</sup>−<sup>6</sup> MPa based on the curing system [17].

**Figure 6.** Schematic of vacuum system in a typical autoclave (redrawn from [17]).

#### *2.5. Loading System*

The loading system in an autoclave facilitates positioning of composite components or moulds to be cured. A loading crab was used to move the loading platform in an autoclave. Generally, autoclaves are installed in a pit with the top surface of the loading platform in flush with the floor for convenience in loading components. A loading bridge is generally deployed to bridge the gap between autoclave door and pit.

#### *2.6. Electrical and Control System*

Electrical and control systems in autoclaves play a key role in ensuring safe operation for reliable processing and curing of composite components. The electrical and control system of autoclaves should be robust enough to be able to provide necessary feedback signals and respond to various commands for processing [25]. Autoclaves are generally built with computer system, serial port servers, power supplies, sensors, and are generally resilient enough to be able to operate even if one or more components fail. Electrical and control systems in autoclaves are in-built with capabilities to be operated in multiple modes of operations, for examples, automatic, semi-automatic, or in manually operational mode. The control system generally consists of PID (proportional integral derivative) controllers, set temperature, and vacuum and pressure levels. PLC (programmable logic controllers) are used to ensure safe interlocking, sequential operation, status, and alarm display. All the components are generally connected to a server and computer-controlled via ethernet links. The communication system used in autoclaves includes RS485, USB, and ethernet connection [26].

#### **3. Cool-Clave Technique**

Autoclave processing is used to manufacture high performance composites, but overall, the process is very expensive and requires high energy consumption. In a traditional autoclave, the process not only heats up the composite to be consolidated but also any parasitic air and vessel insulation requiring high energy usage. Additionally, autoclave processing is normally used to consolidate vacuum-bagged pre-impregnated reinforcements. Prepreg materials require freezer storage, and the separate impregnation stage incurs further costs. The cool-clave technique has the potential to achieve autoclave consolidation of fibre-reinforced composites by enhancing the energy- and cost-efficiency: (a) without heating parasitic materials (hence saving energy), (b) with shorter cycle times (hence increased production rate), (c) without prepreg, i.e., using infused laminates (for lower cost). A schematic of the cool-clave technique with the bagging process is illustrated in Figure 7.

Implementation of the cool-clave technique for autoclave consolidation of composites could generate significant cost and energy savings. The equipment required for the coolclave process consists of:


Prudham and Summerscales [27] investigated the cool-clave processing technique using the autoclave at the University of Plymouth. The autoclave was used as a pressure vessel, without using the heating aspect of the system. The heat, required for curing, was provided by a mould tool with built-in heating elements. Figure 8 demonstrates the heated mould tool used for the cool-clave processing.

For implementation of the cool-clave processing technique, temperature uniformity across the face of the mould tool is crucial [18]. During the initial stages of the cure cycle, dwell periods are often used, where the laminate is held at a temperature lower than the curing temperature for a set period of time. This increase in temperature lowers the viscosity of the resin, allowing it to flow. Together with low pressure, the volatiles trapped within the laminate are forced out, thus reducing voids. If the viscosity of the resin is too high it will not flow, trapping the volatiles within the laminate, or if too low, then the resin will flow too much, creating areas of resin starvation. It is important that the temperature across the face of the mould tool remains relatively uniform to maintain uniform viscosity

of the resin. Therefore, Prudham [18] performed thermal imaging of the mould tool to gain insight into the temperature uniformity across the mould tool surface. Figure 9 shows the thermal imaging of the mould tool surface performed by Prudham [18].

**Figure 8.** Heated mould tool used for cool-clave processing (acquired with permission from [18]).

**Figure 9.** Thermal imaging of mould tool surface (acquired with permission from [18]).

The results indicated discrepancy between temperatures recorded by the mould tools built-in thermocouple and the temperatures recorded during thermal imaging technique. A greater heat loss near to the edges of the mould tool was recorded by thermal imaging with a maximum temperature difference across the main region of the mould tool being in the range from 5 ◦C horizontally and 7 ◦C vertically [18] in the image.

From the thermal imaging results, it was concluded that the mould tool would be suitable for cool-clave processing, demonstrating a convective heat transfer across the surface [18]. Greater heat loss across the edges of the mould tool surface was explained by in-plane anisotropic thermal conductivity of carbon fibre-reinforced epoxy composite used in the study [18,28]. The tests highlighted the need for adequate insulation of the mould tool during the manufacturing stage to prevent greater heat losses as recorded during thermal imaging [18].

A Thermal Insulation Hud (TIH) was constructed to prevent any convective heat loss from the mould tool which would encase the mould tool, allowing vacuum and thermocouple connections but limiting convective heat transfer [18]. One of the main challenges of the cool-clave technique was how to get power to the heated mould tool inside the autoclave. The electrical supply used to power the heated mould tool was provided by a power transformer controlled by an 'IMO' PID controller by using a preexisting access hole on the autoclave chamber, plugged together with a specially made fitting encompassing the electrical wires providing the pressure tight seal required [18]. A schematic of the power supply mechanism for the heated mould tool inside the autoclave is shown in Figure 10.

**Figure 10.** Schematic of the power-supply mechanism for the heated mould tool inside the autoclave chamber (redrawn from [18]).

#### *3.1. Analysis of Energy Consumption between a Traditional Autoclave and Cool-Clave Technique*

The main sources of energy consumption in a traditional autoclave and cool-clave process are compressor, and main controls (for both processes), heating elements in traditional autoclave, and heated mould tool in the cool-clave process. The principal objective behind cool-clave process is to achieve autoclave consolidation of composite laminates but being more energy- and cost-efficient. Prudham and Summerscales [27] quantified the energy consumption in a cool-clave processing technique during a defined cure cycle and compared it with a traditional autoclave process. The energy consumption by control units and compressor was deemed to be approximately equal for the traditional autoclave processing and cool-clave technique, as both processes require the autoclave to run with a pressurised chamber. To make a clear distinction between the energy consumption of both processes, the amount of energy required to provide heat during a defined curing cycle was evaluated [27]. The results demonstrated a 35% reduction in energy requirement for heating the laminate when replacing the vessel heaters with heated tooling [27].

The cure cycle in a trditional autoclave requires the component to enter the vessel at time 0 to start the dwell stage. Figure 11a illustrates a traditional autoclave cure cycle. Following thermodynamic analysis based on a traditional autoclave cure cycle, the total energy consumption reported by Prudham and Summerscales [27] was 3620 kJ. Table 1 further illustrates the results demonstrated in the study [27]. The cycle that an autoclave goes through to cure a laminate was split into six phases for the process of analysis. The analysis was carried out assuming the cure cycle of a 12.7 mm thick Cytec Cycom 5216 epoxy prepreg [27].


**Figure 11.** (**a**) Traditional autoclave cure cycle, (**b**) cure cycle for cool-clave processing (redrawn from [29]).


**Table 1.** Estimated energy consumption of a traditional autoclave cure cycle (reproduced from [12,27]).

The total volume of pressurised gas inside the autoclave chamber was estimated to be 1 cubic metre through thermodynamic analysis [27], which was comprised of the cylindrical working space of the autoclave, annular heating channels, and domed ends.

Figure 11b illustrates the cure cycle for cool-clave processing technique [29]. In the proposed cool-clave cure cycle, the component can complete the dwell period and reach full heat prior to entering the vessel. This reduces the time required to be inside the vessel, thus increasing the production rate of components. Additionally, quality is key for acquiring the highest possible safety while minimising costs. Composites are excellent for tailoring for the loads and areas with stress concentrations and can provide tailorable tensile strength [29,30].

With the proposed method, the cycle times in industrial production line can be significantly reduced, which can bring a number of benefits, such as [29]:


Following cool-clave processing using the heated mould tool technique, the maximum energy consumption based on processing of three Cytec Cycom 5216 560 gsm non-crimp glass fibre-reinforced epoxy composites with ~50% fibre volume fractions was estimated to be 2340 kJ, demonstrating a 35% reduction in energy consumption [12,27].

#### *3.2. Consolidating Resin-Infused Laminates inside Autoclave*

Lewin [31] conducted initial experiments as a brief feasibility study to investigate the possibility of consolidating resin-infused laminates inside an autoclave following RIFT II (Resin Infusion under Flexible Tooling) manufacturing outside the autoclave. Laminate (Plate E) properties were compared with laminates manufactured using hand lamination with edge dams to constrain the flow of the infusion resin (Plate A), resin infusion under flexible tooling with a flow medium (RIFT II) and 0.03 MPa (Plate B), 0.06 MPa (Plate C), or 0.09 MPa (Plate D) net pressure [12,31]. Each laminate was manufactured using 270 gsm plain woven glass fabric-infused with IP2 polyester infusion resin (initial viscosity 1600 mPa.s at 25 ◦C, according to the manufacturer's datasheet) and 2% Butanox M50 MEKP catalyst by weight [12]. Laminate characterisation was performed by undertaking (a) resin burn-off for fibre volume fraction (Vf), (b) tensile properties (BS EN ISO 527-4), (c) flexural properties (BS EN ISO 14125 Class III), (d) inter-laminar shear strength (ILSS, BS EN ISO 14130), and (e) surface-breaking voids (SBV) by filling voids with carbon dust, followed by image processing and analysis with ImageJ software. The results are presented in Table 2.


**Table 2.** Results from each laminate characterization tests [31].

The fibre volume fraction, elastic moduli, and tensile and flexural strengths all increased with increasing net pressure during manufacturing. The ILSS decreased with increasing fibre volume fraction. The minimal gain in fibre volume fraction for Panel E was attributed to insufficient volume for resin bleed, with the possibility of a greater increase for an optimised process [12,31].

Stringer [32] identified 7500–16,500 mPa.s as an optimum processing window for application of vacuum for void-free high fibre volume fraction composites manufactured by wet lamination and vacuum bagging techniques. Therefore, with some adaptation of the autoclave pressure vessel, it might be practical to load the vacuum-bagged dry composite into the autoclave, then infuse and cure in situ. Preparation outside the autoclave would permit shorter autoclave cure cycles and better utilisation of the pressure vessel [12].

Improved Autoclave Process for Resin-Infused Laminates

Experiments conducted by Wilkinson [33] with a dry fabric reservoir inside the bag and with both the inlet and outlet pipes clamped gave no significant change in fibre volume fraction (demonstrating only 0.26% increase) when compared to plates cured at ambient pressure. Subsequent tests used no reservoir material with the resin inlet clamped, while the resin outlet was vented to atmosphere during autoclave consolidation, demonstrating an 8.6% increase in fibre volume fraction [12,31,33].

After infusion, plates were subjected to (a) vacuum-bag only pressure, (b) 0.31 MPa pressure in the autoclave, or (c) 0.59 MPa pressure in the autoclave. Further laminates were prepared and pressurised after a dwell period to study the effect of viscosity at the time, and pressure was applied for the four times identified by the viscosity tests. The results are illustrated in Table 3.


**Table 3.** Summary of data obtained from laminates manufactured by outlet pipe vented to air [33].

At constant consolidation pressure, delaying the consolidation resulted in a lower fibre volume fraction in the composite panel. Flexural strength increased with increasing consolidation pressure. Increased viscosity limited the quantity of resin expelled from the laminate, reduced the fibre volume fraction, and resulted in lower mechanical properties [33].

#### **4. Conclusions**

The out-of-autoclave manufacturing technique has limitations over maximum achievable fibre volume fraction in composites due to compressibility characteristics of the reinforcement material. Lower fibre volume fraction inevitably results in increased matrix volume fraction and consequent resin-rich volumes. Fibre clustering and RRV cause reductions in composite strength. Autoclave processing of composites is required to achieve the highest performance composites systems for industrial applications in the aerospace, automotive, and defence sectors.

Energy savings can result from decoupling the heat and pressure during the autoclave processing of composites. The use of electrically heated mould tools could eliminate heating of the parasitic systems (pressure vessel walls, insulation, and heat transfer gasses). The consolidation pressure can then be supplied using cool air.

The adoption of heated tooling in the cool-clave technique to achieve autoclave consolidation of composites can significantly reduce process cycle times, as the composites can be taken to the end of dwell period before loading the pressure vessel.

The use of resin infusion, rather than expensive pre-impregnated reinforcements, removes the need for the separate impregnation stage and elimination of power requirements for freezer storage of prepreg materials. Autoclave loading efficiency could be improved by curing different composite systems simultaneously with the composites brought to their respective curing temperatures before loading the autoclave, which would further enhance process efficiency.

**Author Contributions:** Conceptualization, J.S.; writing—original draft preparation, I.R.C.; writing—review and editing, I.R.C. and J.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** No new data was created.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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## *Article* **On the Machining Temperature and Hole Quality of CFRP Laminates When Using Diamond-Coated Special Drills**

**Jinyang Xu 1,\*, Tieyu Lin <sup>1</sup> and Joao Paulo Davim <sup>2</sup>**


**Abstract:** Carbon fiber reinforced polymers (CFRPs) are attractive engineering materials in the modern aerospace industry, but possess extremely poor machinability because of their inherent anisotropy and heterogeneity. Although substantial research work has been conducted to understand the drilling behavior of CFRPs, some critical aspects related to the machining temperature development and its correlations with the process parameters still need to be addressed. The present paper aims to characterize the temperature variation and evolution during the CFRP drilling using diamond-coated candlestick and step tools. Progression of the composite drilling temperatures was recorded using an infrared thermography camera, and the hole quality was assessed in terms of surface morphologies and hole diameters. The results indicate that the maximum drilling temperature tends to be reached when the drill edges are fully engaged into the composite workpiece. Then it drops sharply as the tool tends to exit the last fiber plies. Lower cutting speeds and lower feed rates are found to favor the reduction of the maximum composite drilling temperature, thus reducing the risk of the matrix glass transition. The candlestick drill promotes lower magnitudes of drilling temperatures, while the step drill yields better surface morphologies and more consistent hole diameters due to the reaming effects of its secondary step edges.

**Keywords:** CFRP composites; drilling process; special drills; machining temperatures; hole quality

#### **1. Introduction**

In recent decades, carbon fiber reinforced polymers (CFRPs) have been receiving immense attention in diverse engineering fields due to their superior properties and unique functionality [1–4]. This can be seen by their widespread applications for fabricating the main load-bearing components in the modern aerospace industries. For instance, CFRPs are extensively used in the wing boxes, horizontal and vertical stabilizers, and wing panels of large commercial aircrafts, such as Airbus A380 and Boeing 787 [4,5]. Generally, CFRPs feature two typical constituents, namely reinforcing fibers and an impregnating matrix, which show completely disparate behaviors [2,6]. The composites are characterized by high specific mechanical/physical properties, being a promising alternative to conventional metallic alloys and steels. Contrary to isotopic materials, CFRP composites generally exhibit a heterogeneous structure and anisotropic behavior, being regarded as a rather difficultto-cut material. Although most CFRP components are fabricated to near-net shapes by molding processes, mechanical machining has become a compulsory operation in order to achieve desired dimensional accuracy and target quality attributes for final composite products [7–10]. However, the inherent anisotropy and heterogeneity of fibrous composites complicate the chip separation process and tend to cause extremely undesirable machining consequences such as severe surface damage, rapid tool wear, increased cutting costs. Meanwhile, the machinability of CFRPs is fiber-orientation dependent, owing to the varying fiber fracture mechanisms associated with the fiber cutting angle. It has been reported by

**Citation:** Xu, J.; Lin, T.; Davim, J.P. On the Machining Temperature and Hole Quality of CFRP Laminates When Using Diamond-Coated Special Drills. *J. Compos. Sci.* **2022**, *6*, 45. https://doi.org/10.3390/ jcs6020045

Academic Editor: Francesco Tornabene

Received: 18 January 2022 Accepted: 29 January 2022 Published: 1 February 2022

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previous investigations that interrelated chip separation modes such as bending-induced fractures, shear-induced fractures, fiber buckling, and interfacial debonding occur for the cutting of fibrous composites [11–14]. These unique characteristics result in the poor machinability of CFRP materials, posing tremendous challenges to modern manufacturing sectors. Among the secondary machining operations, drilling is the most frequently-used operation for the cutting of CFRPs due to the need to create boreholes for assembling different composite parts into final products. It is roughly estimated that the range of the number of holes required by a commercial aircraft is up to 1.5–3 million, while a jet fighter requires as many as 300,000 holes [4,6]. Thus, from the manufacturers' point of view, the drilling process becomes essential in the final acceptance of composite parts. However, it is rather challenging to drill CFRPs with desired hole quality and target dimensional accuracy. This is due to the varying chip removal modes associated with the changeable fiber cutting angle and the high abrasiveness of the reinforcing fibers. Some of the critical issues encountered in CFRP drilling include drilling-induced delamination, glass transition failure of the composite matrix, poor dimensional accuracy, rapid tool wear, etc. Apart from the mechanical force effects, cutting heat and resulting temperature development also play a vital role in the surface integrity of CFRP materials. In particular, high temperatures can be easily accessed in the drilling processes due to the semi-enclosed environment of the chip separation, leading to poor heat dissipation. Excessive drilling temperatures promoted at the tool–composite interface can cause severe degradations of the composite properties, debonding of the fiber-matrix interfaces, and the glass transition of the matrix base [15–18]. Therefore, studies dealing with temperature initiation and progression are very meaningful to achieve active control of the thermal effects for the drilling of CFRP materials.

To address the drilling issues of CFRPs, a large amount of research work has been carried out worldwide by scholars [10,19–35], covering a variety of aspects involving drilling forces, drilling-induced damages, tool wear, etc. Some drilling-induced damage, including delamination, hole dimensional inaccuracy, surface roughness, fiber pullouts, and uncut fibers, have been investigated in the scientific literature [19,20]. For instance, Davim and Reis [21,22] were among the earliest to deal with the drilling behavior of CFRP composite laminates. In their work, the correlations between the cutting velocity and feed rate with the machining power, specific cutting pressure, and delamination factor were established. The authors stated that both the cutting speed and the feed rate positively affected the progression of the delamination factor, and the use of brad spur drills favored the reduction of drilling-induced delamination. Bonnet et al. [24] studied the local feed force and its consequences on the exit hole damage during CFRP drilling. It was found that the fiber cutting modes changed dynamically with the composite sequence, and the local feed forces generated on the hole bottom could be correlated with the delaminating aspects. Su et al. [28] addressed the thrust forces and delamination issues when drilling CFRPs using a tapered drill-reamer and found that the drilling parameters significantly affected the maximum thrust force measured in the drilling stage instead of the reaming stage. Ameur et al. [29] conducted drilling studies on CFRP materials using different types of tool materials. The authors stated that both the drilling thrust force and the delamination factor were primarily influenced by the tool material and the feed rate, while the hole cylindricity errors were mainly affected by the spindle speed. Rawat and Attia [31] studied the wear behavior of carbide tools during the high-speed drilling of CFRP laminates, and observed that chipping and abrasion were the main wear modes controlling the deterioration of carbide drills. Faraz et al. [32] highlighted the wear phenomenon of cutting-edge rounding (CER) for CFRP drilling and introduced the CER value for the quantification of drill wear. Wang et al. [33] investigated the wear progression of coated tools while drilling CFRP laminates. The authors pointed out that the dominant wear type was dulling or blunting of the cutting edge during CFRP drilling. The use of a diamond coating could significantly reduce the edge rounding wear, while the AlTiN coating failed to protect the drill due to its oxidation during machining. A critical review conducted by Ismail et al. [34] offered a clear understanding of the current advances in drilling composite materials, which focused

on the aspects of tool geometries, materials, and parametric designs. Fu et al. [35] and Kubher et al. [36] investigated the temperature characteristics in drilling unidirectional (UD) and multidirectional (MD) CFRPs. Due to the associated temperature effects, utilizing MD CFRPs could result in more difficulties in achieving high drilling qualities than UD CFRPs at certain fiber cutting angles. Through the literature survey, although substantial research work has been conducted to understand the drilling behavior of CFRP composites, most of the studies are focused on the analysis of force-related effects, such as drilling thrust forces, delamination damage, hole quality, and tool wear issues. Even though there are some papers that have already addressed the temperature issues for CFRPs, very limited literature has been reported to deal with the temperature variations in drilling high-strength CFRPs with special drills. Moreover, some critical issues related to the machining temperature development and its correlations with the drilling parameters still need to be carefully addressed. Hence, the current work performed in the paper can supplement expertise and knowledge regarding the drilling temperature issues for highstrength CFRP composites. Its novelty lies in identifying the evolution law of the drilling temperature following the entire composite machining operation and in clarifying the parametric effects on the temperature development. Moreover, a particular focus is placed on the evaluation of different special drills for CFRP drilling and on the quantification of hole geometrical accuracy under varying drilling conditions. The experimental results were discussed with respect to the process parameters used. The paper is intended to offer a better understanding of the thermal behavior of CFRP laminates when subjected to the drilling operation.

#### **2. Experimental Procedures**

In the present work, machining studies were conducted on the multidirectional CFRP laminates fabricated by high-strength T700 carbon fibers and FRD-YZR-03 epoxy resin. The main composition and basic mechanical properties of the examined CFRP laminates are summarized in Tables 1 and 2, respectively. The composite plate had a total size of 300 mm (length) × 200 mm (width) × 6.60 mm (thickness), which was fabricated by the hand lay-up molding technology. The drilling experiments were performed on a DMU 70 V CNC machining center following a full factorial design of experiments by using candlestick and step drills. The experimental setup for the drilling tests is shown in Figure 1. The input process parameters consist of three levels for the cutting speed (*Vc* = 40, 80, and 120 m/min) and three levels for the feed rate (*f* = 0.06, 0.09, and 0.12 mm/rev).

**Table 1.** The composition of the used CFRP composite.


**Table 2.** The mechanical properties of the used CFRP composite.


**Figure 1.** The experimental setup for the CFRP drilling.

Both drills were diamond-coated special tools featuring an 8.0 mm diameter, a 30◦ helix angle, and a 90◦ point angle dedicated to delamination suppression and anti-abrasion wear during composite drilling. The detailed morphologies of the candlestick and step drills are shown in Figure 2. The candlestick drill featured three protruding tips, including one centering tip and two peripheral tips, which could significantly reduce the drilling thrust force and ensure the sharp flank cutting edges, whereas the step drill was designed following a step-control scheme involving a first step to create a pilot hole and a secondary step to ream the hole surface to the final diameter. Moreover, the examined step drill featured a ratio of the primary diameter (7.8 mm) to the second diameter (8.0 mm) of 0.975. The small difference between the primary and secondary diameters mainly aimed to let the secondary step edges have a reaming action on the previously cut hole surfaces by the first step drill, as a very small chip removal volume is involved in such step drilling. During the drilling operation, the FLIR A615 infrared thermography camera (IFTC), which featured a working temperature from −20 to 2000 ◦C and an image acquisition frequency from 50 to 200 Hz, was utilized to in-situ record the temperature development under varying drilling conditions. A similar method was applied by Xu et al. [37], which proved that the temperature measurement chain in the current experiment is capable of measuring the changes in the cutting temperature. The temperature resolution of the equipment was less than 0.05 ◦C, which guaranteed the accuracy and reliability of the monitored data. The accurate measurements of the cutting temperature by the thermographic camera were also carefully guaranteed by the calibration of emissivity value parameters set in the software. Additionally, an emissivity value of 0.85 was adopted for the composites drilling, according to the recommendations of the infrared thermography camera manufacturer. Moreover, to make the temperature measurements more reliable, all of the composite holes were drilled with a 1.0 mm distance close to the edge of the workpiece. After the completion of the drilling operations, the hole wall morphologies were characterized using the ZEISS confocal laser scanning microscope (CLSM). Finally, the average diameters at the entrance, middle, and exit sides of CFRP holes were measured using a SOLEX EUA coordinate measuring machine (CMM). The obtained results were correlated with the drill bits and the input process parameters.

**Figure 2.** The morphologies of the used special drills.

#### **3. Results and Discussion**

#### *3.1. Characterization of Drilling Temperatures*

Machining temperature is a characteristic phenomenon of heat accumulation resulting from the tool–work interaction following the material separation process. When dealing with the hole-making processes of CFRP composites, drilling temperature is a critical issue that has to be carefully addressed, as high levels of temperatures can be easily accessed due to the poor heat dissipation of the drill–composite interaction. Additionally, high temperatures are extremely detrimental to the surface integrity and mechanical properties of cut composite holes as they can cause degradation of the composite properties, debonding of the fiber/matrix interface, or even the glass transition failure of the matrix base. Therefore, it is essential to characterize the variation laws of the temperature rise and progression during the machining of CFRP laminates. Figures 3 and 4 show the recorded thermal images of the drilling temperature development under varying feed rates for the candlestick and step drills, respectively, during CFRP machining. It is clear that the temperature progression characteristics can be divided into three stages with respect to the tool–work interaction. At the early stage, the drill edges start to attack the composite laminate, and a large amount of cutting heat is progressively generated through the tool–chip and tool–work interactions. As brittle fracture dominates the chip separation of the carbon/epoxy composites, more cutting heat is likely to accumulate within a very narrow tool–chip interface, resulting in a high temperature rise as the drill tends to penetrate inside the composite. At the drill entrance stage, moderate levels of drilling temperatures are produced for both the candlestick and step drills. Meanwhile, with the ongoing tool advancement, the drill edges are fully engaged in the cutting of the fiber/epoxy material. In such circumstances, peak values of drilling temperatures are identified through the thermal image examinations for both drills used. Due to the heat accumulation effects and the full drill interaction with the composite material, the maximum temperature is reached, which indicates the highest risk of occurrence of the thermally-induced damage onto the internal composite hole walls. In most cases, the candlestick drills are found to produce relatively lower values of drilling temperatures than the step drills, particularly under the full drill–work interaction stage, as depicted in Figures 3 and 4. Additionally, increasing the feed rate tends to elevate the maximum temperatures at the full tool engagement stage, due to the increased amount of frictional heat generated as the feed rate rises. Eventually, when the tool retracts from the composite workpiece, the drilling temperature appears to decrease dramatically because of the effective heat dissipation and air cooling of the tool–work system. It is also worth noting that both the candlestick and step drills yield comparable values of drilling temperatures at the drill retraction stage while machining the CFRP laminates.

**Figure 3.** Thermal images of drilling temperature development when using candlestick drills (*Vc* = 120 m/min).

**Figure 4.** Thermal images of drilling temperature development when using step drills (*Vc* = 120 m/min).

Figure 5 also shows the comparative evolution of the drilling temperatures in terms of the cutting time (*t*) following a complete CFRP drilling process for both types of special tools. It is clear that the drilling temperature signals fluctuate significantly during the composite removal process, which exhibits a rapidly increasing trend at the drill entrance stage when the tool starts to attack the composite specimen. Then, the temperature signals for both drills appear to decrease gradually with the tool advancement as the drill edges start to exit the last composite plies, resulting in reduced frictional heat generation following the chip removal process. The previous investigation done by Fu et al. [35] revealed the complex

drill-exit temperature characteristics for UD and MD CFRPs, and similar temperature changes were identified in their work. The maximum drilling temperatures for both drills seemed to be reached at around *t* = 0.5 s under the tested process parameters. The corresponding feed depth of the time is about 2.8 mm after the calculation using the time, feed rate, and cutting speed. Moreover, the candlestick drill is found to promote lower levels of peak drilling temperatures than the step drill. Figure 6 presents the comparison of the maximum temperatures recorded in the CFRP drilling between the two types of drills. Note that the maximum temperatures denoted herein signify the average value of the highest temperature range during the composite drilling. The results also indicate that the candlestick drills generally yield lower drilling temperatures than the step drills for all of the cutting conditions examined. This is due to the two protruding tips of the candlestick drill along the drill periphery that reduce the frictional heat generation and improve the heat dissipation at the tool–chip interactions. Additionally, the cutting speed definitely shows a positive impact on the progression of drilling temperatures for both tools, except for the abnormal temperature data at *Vc* = 40 m/min for the step drill. The phenomenon is associated with the intensified friction of the tool–composite interaction when the cutting speed increases. Moreover, increasing the feed rate appears to raise the drilling temperatures for the two special drills when machining the CFRP materials.

**Figure 5.** Comparison of drilling temperature development between candlestick and step drills (*Vc* = 120 m/min and *f* = 0.06 mm/rev).

**Figure 6.** Evolution of the maximum drilling temperatures in terms of the process parameters.

#### *3.2. Hole Wall Morphologies*

Machining of fibrous composites differs significantly from the cutting of isotropic metals and steels due to the varying chip separation mechanisms associated with the fiber orientation. The material removal complicates the surface generation of hole walls for the composites under drilling operations. Hole wall morphologies can be considered as one of the most important criteria in assessing the quality attributes of drilled CFRP composites. In general, it is rather difficult to generate smooth surface morphologies, as the material removal mode changes dynamically with the drill rotation during the hole-making process. Figures 7–10 show the topographies of the CFRP hole walls produced by the two types of drills under the fixed cutting conditions (*Vc* = 120 m/min and *f* = 0.09 mm/rev). Figures 7–10 all feature the same fiber orientations. From Figure 7, surface flaws due to interlaminar cracking are noted, which feature deep blue colored zones. The finely-cut composite surfaces mainly exist in areas involving the shear-induced fractures of fiber plies. Additionally, the profiles of four circular arc curves at the A–A, B–B, C–C, and L–L cross-sections are plotted in Figures 7 and 9. It is noted that the surface profiles fluctuate significantly along both the radial and axial directions of the holes, which is due to the inherent variations in the surface of the fibers and the matrix. Additionally, the average surface roughness values (*Ra*) of the selected cross-sections mainly range from 5.00 to 6.29 μm along the hole radial direction, while *Ra* reaches its maximum value toward the hole axial direction at the L–L cross-section. This is due to the significant disparity in fiber orientation between adjacent fiber plies toward the composite thickness direction.

**Figure 7.** CLSM image of a CFRP hole wall cut by the candlestick drill and its cross-sectional profiles (*Vc* = 120 m/min and *f* = 0.09 mm/rev).

With respect to Figure 8, it shows the three-dimensional topographies of cut hole walls at the entrance side. It is evidenced that the cut CFRP hole morphologies feature smooth fiber surfaces containing a certain degree of surface cavities. In contrast, the hole wall morphologies produced by the step drill appear to be much better than those cut by the candlestick drill. As depicted in Figure 9, the *Ra* values of the four selected cross-sections are relatively lower than those gained by the candlestick drill. The phenomenon is due to the reaming effects of the secondary step edges, indicating the superiority of the step tools in achieving a better hole surface finish than the candlestick tools while machining the CFRP laminates. Moreover, the surface defects residing within the composite hole cut by the step drill mainly include surface cavities due to the loss of matrix, resin smearing, and fiber pullout voids, as shown in Figure 10. Note that the previous research carried out by Kubher et al. [36] addressed the evolution of in-situ cutting temperature and machining forces during the conventional drilling of MD CFRP laminates. Similar surface defects, including resin smearing and bending-induced fracture of carbon fibers, could be found in the research. For both drills in the current investigation, no significant evidence of interlaminar delamination at the hole entrance side is identified through the CLSM examination.

**Figure 8.** Topographies of a CFRP hole wall cut by the candlestick drill (*V*c = 120 m/min and *f* = 0.09 mm/rev).

**Figure 9.** CLSM image of a CFRP hole wall cut by the step drill and its cross-sectional profiles (*Vc* = 120 m/min and *f* = 0.09 mm/rev).

**Figure 10.** Topographies of a CFRP hole wall cut by the step drill (*V*c = 120 m/min and *f* = 0.09 mm/rev).

#### *3.3. Hole Diameter*

In drilling CFRP composites, diameter value is an essential criterion for evaluating the hole geometrical accuracy, which determines the assembly performance of the composite parts. In the current work, the average diameters at the entrance, middle, and exit sides of cut CFRP holes were measured and correlated with the process parameters and drill bits used. The obtained results are depicted in Figures 11–13. Both the cutting speed and the feed rate have a significant impact on the variations of the hole diameters, irrespective of the measuring side. Under the lowest speed conditions (*Vc* = 40 m/min), increasing the feed rate tends to enlarge the cut hole diameters, particularly for the step drills (Figure 11). In most cases, undersized holes are generally produced by the two drills when *Vc* = 40 m/min. It is worth noting that the diameters measured at the exit side show the largest value, followed by those measured at the middle and entrance sides, regardless of the drill bits and process parameters used. The phenomenon indicates a wedge-shaped cylindrical surface of cut hole walls from the entrance to the exit side due to the intensified tool vibration arising from the decreased stiffness of remaining fiber plies as the fiber layers become much thinner with the tool advancement in drilling. When the moderate speed is used (*Vc* = 80 m/min), the feed rate fails to show a clear effect on the variations of the hole diameters. In particular, more consistent holes close to the nominal diameter value are promoted by the step drills at the feed rate of 0.09 mm/rev (Figure 12). With respect to the highest speed conditions (*Vc* = 120 m/min), typically, oversized holes are produced by the candlestick drills, and undersized holes are generated by step drills, as shown in Figure 13. Under such conditions, more consistent holes are created by the candlestick drill at the feed rate of 0.09 mm/rev. Finally, increasing the cutting speed seems to enlarge the hole diameters for the candlestick drills, but tends to decrease the hole diameters for the step drills. In general, to produce consistent holes close to the nominal diameter, the highest speed and moderate feed values are suggested for the candlestick drills, while moderate speed and lower feed values are recommended for the step drills.

**Figure 11.** The hole diameter in terms of different feed rates and drill bits (*V*c = 40 m/min).

**Figure 12.** The hole diameter in terms of different feed rates and drill bits (*V*c = 80 m/min).

**Figure 13.** The hole diameter in terms of different feed rates and drill bits (*V*c = 120 m/min).

#### **4. Conclusions**

This paper deals with the drilling behavior of CFRP composite laminates using diamond-coated special drills. Machining studies were conducted following a full factorial design of experiments. The composite machinability was evaluated, focusing on the machining temperature characteristics and hole quality attributes under varying conditions. The work addresses the temperature variations and progressions during the CFRP drilling. An attempt was made to assess the performances of different special drills and to quantify the hole geometrical accuracy in the CFRP drilling. Based on the results acquired, the following conclusions can be drawn.


**Author Contributions:** Conceptualization, J.X.; methodology, J.X.; formal analysis, J.X. and T.L.; investigation, J.X.; resources, J.X.; data curation, J.X. and T.L.; writing—original draft preparation, J.X. and T.L.; writing—review and editing, J.P.D.; supervision, J.P.D.; funding acquisition, J.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (grant no. 51705319).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

