*Review* **A Short Review of Recent Progress in Improving the Fracture Toughness of FRP Composites Using Short Fibers**

**Yongan Wang 1, Zhenxing Wang <sup>1</sup> and Lvtao Zhu 1,2,\***


**\*** Correspondence: zhult@zstu.edu.cn; Tel.: +86-15900-706-015

**Abstract:** Fiber-reinforced plastic (FRP) composite laminates have excellent mechanical properties, corrosion resistance, and designability and thus are widely used in various engineering fields. However, their interlayer performance is relatively poor due to no fiber reinforcement between the laminate layers. These layers are adhered through resin bonding only, caused during their usage, which results in composite material delamination damage, thus, limiting its performance. In response, researchers have conducted numerous studies on how to improve the interlaminate properties of laminates through component and structural modifications of the composites and interlaminate toughening. Short fiber toughening is a simple and effective way to solve this problem. This paper reviews the latest research progress in short fiber interlaminate toughening and short fiber modified resin toughening, analyzes the mechanisms of short fiber toughening is fiber bridging, fiber debonding, fracture, and the toughening mechanisms specific to different fibers. This review paper also discusses the current problems encountered in short fiber toughening and provides an outlook on the future development direction for short fiber toughening to provide a reference for subsequent research on short fiber toughened composites.

**Keywords:** FRP; delamination damage; interlaminar fracture toughness (ILFT); short fiber toughening

#### **1. Introduction**

Fiber-reinforced plastic (FRP) composite laminates are widely used in the aerospace field [1], automotive industry [2], wind power generation [3], and civil infrastructure [4] because of their lightweight and relatively high strength characteristics. However, as the laminates are bonded by resin only in the thickness direction, they are prone to cracking and eventually delamination under the action of external forces [5]. Delamination damage limits the development of composite materials as it seriously affects their safety performance and service life. Much work has been carried out to suppress delamination and increase damage tolerance, including Z-directional toughening (3D braiding [6–9], Z-pinning [10], laminate stitching [11], and 3D integrated woven spacer sandwich composites [12,13]), matrix modification [14], and interlaminate toughening [15]. Z-directional toughening, which introduces a separate yarn system in the Z-direction of the composite, has been shown to significantly improve interlaminar fracture toughness [16], but this may come at the cost of compromising the in-plane properties and is difficult to operate. Interlayer toughening and resin modification requires only minor changes to conventional processes for toughening purposes but use materials such as nanofilms and nanoparticles, which require high costs.

Short fibers are a variety of sources, excellent performance, and inexpensive materials that can be added to laminate plies alone for interlaminar toughening or mixed in resin to modify the resin matrix, which can simply and effectively improve the interlaminar strength of composites. This paper presents the research in recent years on two aspects of short fiber interlaminate toughening and short fiber resin modification.

**Citation:** Wang, Y.; Wang, Z.; Zhu, L. A Short Review of Recent Progress in Improving the Fracture Toughness of FRP Composites Using Short Fibers. *Sustainability* **2022**, *14*, 6215. https://doi.org/10.3390/su14106215

Academic Editor: Jorge de Brito

Received: 6 April 2022 Accepted: 30 April 2022 Published: 20 May 2022

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

**Copyright:** © 2022 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/).

#### **2. Interlaminar Fracture Toughness Characterization and Testing of Composite Laminates**

According to the different forms of load, the composite laminate delamination mainly has the types: opening mode (mode I), sliding mode (mode II), and tearing mode (mode III). The three forms of delamination, as shown in Figure 1, of which mode III delamination is less studied, account for a negligible fraction [17,18]. Interlaminar fracture toughness (ILFT) is a quantitative measure of the ability of composite laminates to resist crack extension and an estimate of material toughness. The stress-based stress intensity factor *K* [19] and the strain-based strain energy release rate *G* [20] are often used to characterize delamination extension. Since composites are materials with anisotropic mechanical properties, the calculation of the stress intensity factor *K* for interlaminar cracks is particularly complicated, so most of the studies are conducted using the strain energy release rate *G* as the control parameter for delamination expansion. The strain energy release rate is the strain energy released per unit area of crack growth of the deformed object, and its critical value is called the critical strain energy release rate, expressed by the symbol *GC*, which indicates the energy required for crack expansion per unit area; generally, there is the mode I (*GIC*) and mode II (*GIIC*) [21]. From the ASTM standard, it is known that the mode I and mode II ILFT of composites is mainly obtained by the double cantilever beam (DCB) test [22] and end notched flexure (ENF) test [23], with the test setup diagram in Figure 2. The ILSS testing [24] is a popular method for characterizing the interfacial strength of composite materials. Composites are very sensitive to low-velocity impact loading in practical applications, so the low-velocity impact test [25] and compression after impact (CAI) test [26] are also often used together to characterize the resistance of composites to delamination when subjected to low-velocity impacts.

**Figure 1.** Modes of fracture [18].

**Figure 2.** Schematic diagrams of (**a**) double cantilever beam (DCB) test and (**b**) end notch flexure (ENF) test.

#### **3. Short Fiber Toughening**

#### *3.1. Short Fiber Interlayer Toughening*

In the 1990s, Sohn and Hu covered the surface of carbon fiber prepreg with chopped aramid fibers that accounted for 0.4% to 0.8% of the weight of the laminate and then cured it according to the conventional process. It was found that the addition of short fibers could produce fiber bridging during delamination extension to improve the mode I (*GIC*) and mode II (*GIIC*) interlaminar fracture toughness of the laminate by up to more than two times [27]. Since then, more scholars have begun to study short fibers as interlaminar toughening materials to improve the interlaminar fracture toughness of composites [28–32]. Liu et al. investigated the effect of three basic factors on the interlaminar fracture toughness of CF/EP composites using three-factor four-level orthogonal tests, as in Table 1, which are short fiber type, length, and density [33]. The results showed that the effects of each factor on the ILFT were in the order of fiber type, fiber density, and fiber length. The addition of Kevlar fiber most significantly enhanced *GIC* and *GIIC*; with the increase of short fiber density, *GIC* showed a positive correlation, and *GIIC* showed a decreasing and then increasing trend. Both *GIC* and *GIIC* tend to increase as the length of the staple fiber increases and tend to decrease when the length is too long. This provides some reference for future studies in terms of material selection and dosage, but the authors do not further analyze how length and density affect the interlaminar fracture properties.



Aramid fibers (AF) have the characteristics of high toughness, high elongation at break, and easy fibrillation on the surface [34]. Under the action of fiber bridging, the deformation and longitudinal tear of the fiber itself can absorb a lot of energy, which plays a very good role in toughening effect [35]. Wang et al. studied the interfacial toughness of CFRP reinforced by short aramid fibers with different lengths and areal densities based on the double cantilever beam test [36]. The results showed that the short aramid fibers with a length of 3 mm and an areal density of 24 g/m2 had the best toughening effect, the ultimate load, and the mode I interlaminar fracture toughness can be increased by 292.87% and 168.59%, respectively. To ensure the feasibility and effectiveness of aramid fiber interlaminar toughening CFRP, they adopted wettability treatment technology with resin pre-coating to treat short aramid fiber tissues (6 mm, 24 g/m2), and the peak load and *GIC* of CFRP were increased by 112%, and 160%, respectively. The pre-coating contributed 37% and 56% improvement, respectively [37]. This explains that the toughening ability of short Kevlar fibers of 3 mm is stronger than 6 mm at the same surface density. This provides a simple way to improve the interfacial compatibility of fibers and resins and also demonstrates that modification of short fibers can further improve the toughening effect.

To enable the short fibers to be evenly dispersed between the layers, Hu et al. impregnated chopped Kevlar fibers in the resin and uniformly coated them on the mid-plane of the laminate. The laminate structure is shown in Figure 3. The effect of chopped Kevlar fiber toughening on the low-speed impact performance of CFRP through drop-weight impact test and compression after impact (CAI) test was studied [38]. The test results showed that the damage resistance of the toughened laminate did not show significant advantages at lower impact energies, and the effect of short fiber toughening was shown only at higher energy impacts. However, the CAI strength and damage strain values increased at all energy levels because the pullout of the chopped fibers consumed part of the energy in delamination expansion under compressive loading, slowing down the yielding process and increasing the CAI strength. The authors considered that short Kevlar fiber toughening is very effective in increasing the damage tolerance when the damage is dominated by mode I fracture toughness.

**Figure 3.** Structure of composite laminate with Kevlar-fiber toughening. The sandwich structure was adopted, with a carbon fiber prepreg sheet at the top and bottom and a Kevlar-fiber layer in the middle [38].

It is believed that adding short fibers to the interlayer is only beneficial to post-impact properties. Still, the interfacial staggering leads to a decrease in the flexural properties of CFRP. Yuan et al. placed an ultra-thin nonwoven short aramid fiber veil (UTNWSAFV) at the interface of all layers of CFRP laminates and found that not only the impact resistance of CFRP could be improved, but also the flexural strength and modulus were improved accordingly [39]. They prepared a toughened prepreg by interweaving the unbonded UTNWSAFV to the surface of the carbon fiber layer in the prepreg production process [40]. Impact tests and compression after impact tests showed that adding UTNWSAFV can effectively suppress out-of-plane deformation and layered expansion. The CAI failure pattern of the laminate with SAFV was changed from delamination damage to shear damage, as seen in Figure 4. The addition of short fibers to the prepreg production process to create short fiber toughened prepreg is a promising technology that focuses on how the short fibers are added to the prepreg in large quantities and uniformly, as this has a major impact on the stability of the composite properties.

Carbon fibers (CF) have high strength and modulus and are often used to make composite materials. Due to their excellent properties, chopped carbon fibers are also used as interlaminar toughening materials to improve the fracture toughness of composites. Xu et al. compared the interlaminar fracture toughness and bending properties of CF/EP composites toughened with two lengths of chopped carbon fibers [41]. It showed that the shorter (0.8 mm) carbon fiber performed better than the longer (4.3 mm) carbon fiber in improving mode I fracture toughness. The longer carbon fiber contributed more to mode II fracture toughness, which was achieved without compromising the bending mechanical properties. Short carbon fibers easily form a three-dimensional interwoven network structure in the epoxy resin of the middle layer of the laminate, and this structure can play a better role in obstructing cracks, as shown in Figure 5. Moreover, due to the addition of short carbon fibers, the electrical conductivity in the thickness direction of the carbon fiber laminates has been improved. According to this conclusion, different lengths

of staple fibers can be selected for selective toughening, depending on the form of loading the laminate is subjected to.

**Figure 4.** CAI test failure morphology of CF9 and CF9-SAF8 (X-ray μCT) [40].

**Figure 5.** The crack propagation path in the mid-layer of laminates during Mode I DCB tests: (**a**,**b**) pure; (**c**,**d**) SCFs (7.8 mg/cm2) and (**e**,**f**) LCFs (7.8 mg/cm2) [41].

To improve the lightning strike resistance of composite materials, Liu et al. bonded chopped nickel-coat carbon fibers (NiCF) together with adhesive to make a NiCF veil, which was used as a multifunctional toughening layer for CFRP [42]. It is shown that the multifunctional NiCF veil can not only improve the interlayer fracture toughness of CFRP, which can increase the *GIC* by 74.75% and *GIIC* by 36.22%, but also improve the electrical conductivity in the thickness direction by 220.49%. Its toughening mechanisms include debonding and fracture of NiCF, as well as flaking and splitting of the nickel coating. The above study shows that the rupture and debonding of the coating absorbs additional energy to improve the interlaminar fracture toughness of the composite without losing the interfacial compatibility of the short fibers and the resin. Du et al. used CNT grafted short carbon fibers as interlaminar toughening materials for CF/EP composites, and the type I interlaminar fracture energy of CFRP could be increased by 125% when the density of CNT-SCF was 10 g/m<sup>2</sup> [43]. Of particular note, he investigated the crack damage sensing ability of toughened CFRP composites using a simple electrical response method using an experimental setupand the results showed that the change in resistance increased monotonically with cracking, which provides a new method for measuring crack extension.

Ma et al. produced five different structures of short glass fiber interlayer toughened GF/EP composites by the VARTM process, as shown in Figure 6 [44]. The experimental results showed that the interlaminar tearing performance of the short glass fiber toughened composites was significantly enhanced, and the toughening effect was most evident for structures B and C, and the shorter the glass fibers, the better the toughening result. This research indicates that in addition to fiber type, length, and content, the arrangement of short fibers between the layers should also be considered, which is an important factor affecting the mechanical properties of toughened composites.

**Figure 6.** Five kinds of reinforcement structure (**A**): two layers of plain glass fiber fabric without short fibers; (**B**,**C**): short fibers are randomly distributed between the layers; (**D**): two layers of fabric containing short fibers; (**E**): one layer of fabric containing short fibers and one layer of plain glass fiber fabric) [44].

While environmental problems are a growing concern in today's society, natural fibers have the advantages of sustainability, biodegradability, and lower cost, which allows them to replace some synthetic fibers in the manufacture of composite materials [45–54]. It is worth establishing that short natural fibers are a potential interlayer toughening material. Jeong et al. covered short silk fiber within two resin films, placed them in the middle of CFRP prepreg for normal curing, and evaluated mode I and mode II fracture toughness by DCB and ENF tests [55]. The test results showed that the addition of silk was useful in improving the interlaminar fracture toughness of the laminate. The mode I interlaminar fracture toughness was 59% higher compared with the untoughened laminate, the main factors for the increase being the adhesion force on the silk fibers to the epoxy matrix and

the relatively high tensile strength of the silk fibers; mode II interlaminar fracture toughness increased by 44%, mainly because the bridging and fracture of the silk fibers inhibited the crack expansion. If spun silk can be processed and appropriately used, it will not only improve the mechanical properties of the composite material but also avoid the waste of raw materials.

The feasibility of interlaminar toughening of CF/EP composites with short ramie fiber (SRF) was demonstrated by Mo et al. [56]. The SRF layer effectively hindered the crack expansion in the matrix and significantly improved the interlaminar fracture toughness of the composite laminates. The toughening mechanism of SRF can be analyzed from the type II interlaminar fracture microscopic morphology shown in Figure 7: firstly, the fracture surface becomes rougher, which increases the surface area of the fracture surface of the composite material and requires more energy to be consumed during crack propagation; secondly, the SRF bridging in the composite absorbs the energy of the crack tip, which effectively hinders the crack propagation in the matrix; furthermore, the SRF pulling and splitting from the matrix also needs to absorb a certain amount of energy. At the same time, the tensile and bending properties of the composites have been raised to a certain extent. They also treated SRF with surface coupling agents, which proved to be effective in improving the fracture toughness of the composites.

**Figure 7.** Mode II interlaminar fracture morphology of CF/EP composite laminate specimens without and with SRF [56].

Zheng et al. compared the toughening effect of short flax fiber, short carbon fiber, and short Kevlar fiber interlayer toughened CF/EP composites, and the test results showed the toughening effect of short flax fiber was between short carbon fiber and short Kevlar fiber [35]. The reason is that the multi-layer level damage of the fiber itself occurs during the crack extension, which absorbs additional energy and further improves the toughening effect. Li et al. investigated the impact of chopped flax yarns (CFY) on the mode I interlaminar fracture toughness of flax fiber/epoxy composites. The experimental results showed that the mode I fracture toughness of the composites increased with the addition of CFY [57]. The CFY layer hindered the crack expansion and led to more energy consumption in the delamination process, but excessive yarn length and content direct unstable crack extension, weakening the toughening effect. When the yarn length and content are moderate (10 mm, 20 g/m2), the toughening effect is the best, and the *GIC* is

increased by 31%. Zhang et al. proposed a multiscale cellulose fiber interlayer consisting of chopped flax fibers (CFF) and cellulose nanofibers (CNF) to improve the interlayer fracture toughness of CF/EP composites [58]. The results showed that the addition of cellulose fiber interlayer could significantly improve the mode II interlaminar fracture toughness due to the synergistic effect of CFF and CNF, and the toughening mechanism can be seen in Figure 8, which includes fiber bridging, fiber pullout, fiber fracture, and the fibrillation of flax fiber. The existence of CNF can improve the interfacial compatibility between CFF and epoxy resin matrix, which is conducive to the fibrillation of flax fibers and hinders the propagation of interlaminar cracks. Obviously, flax fibers, due to their multi-layered structure, can absorb more energy, which is unique to toughening unlike other fibers mechanism. The price of flax fiber compared to aramid, carbon fiber, and others is lower, but there is a very good toughening effect, which is a cost-effective toughening material.

**Figure 8.** (**a**–**d**) Fracture morphologies and (**e**) toughening mechanism of CNF/FF-interleaved CFRPs [58].

Numerous experiments have demonstrated that short fiber interlaminate toughening is an effective method to improve the interlaminate fracture toughness of composite laminates. Still, many unknown influences on the short fiber interlaminate toughening technique can be affected by factors such as the thickness, dispersibility, geometry, and final cure shape of the short fiber layup and fiber type length and surface density. Instead of limiting themselves to adding short fibers to the layers by simple hand-spreading, researchers have developed methods such as making nonwovens, muslins, films, and blending in resins, all of which are designed to eliminate the effects of uneven distribution of short fibers between the layers [59]. If the effects of these factors can be balanced and allow for the control and manipulation of delamination damage, this will achieve the goal of preparing higher performance, more damage-resistant composites.

Natural fibers are also starting to be used in composite interlayer toughening because of their high-cost performance, but the natural fibers are not as good as the synthetic fibers, which limits their large-scale promotion. The hybrid short fiber interlayer toughening is a method that should be considered for interlayer toughening, which can reduce the cost and make up for the shortage of single-type short fiber toughening.

#### *3.2. Short Fiber Modified Resin Toughening*

Another way to improve the toughness of FRP is to modify the resin matrix, and it has been shown that nanoparticles are very good toughening agents, such as nano titanium dioxide [60], nano clay [61], multi-walled carbon nanotubes [62], etc. Nanoparticles have strong chemical activity and can fully bond with epoxy resin to form an ideal interface to blunt cracks and hinder crack expansion from achieving toughening while improving modulus and heat resistance. However, nanomaterials are easy to agglomerate in the resin matrix, unfavorable to operation and processing, and nanoparticles are usually relatively high in cost, which limits their application to some extent [63].

Short-fiber toughened resins offer a viable, low-cost method for resin modification. Chawla et al. prepared unidirectional glass fiber/epoxy laminates from two different resins, one pure and the other containing 2% volume fraction short glass fiber reinforcement, and performed DCB tests [64]. The experimental results showed that fiber bridging was observed in the laminates with the presence of short fibers and that the toughened laminates increased *GIC* by about 19.6% compared to the untoughened laminates. Imagawa et al. added 0.3 wt% weight content of micro-glass fibers to thermoplastic epoxy resin and thermoset epoxy resin to make CFRP and conducted ENF tests [65]. The results showed that the addition of glass fibers increased fiber bridging to prevent crack extension and improved the *GIIC* of composite regardless of the resin. Fiber bridging was observed in all of these studies, and the toughness of the composites was improved, demonstrating the feasibility of short fiber-modified epoxy resin toughening.

Ravindran et al. prepared CFRP composite laminates by adding CNF, CSF, and CNF/SCF fillers to an epoxy resin matrix, and the experimental results showed that the addition of CNF or/and SCF fillers improved the delamination toughness, impact damage tolerance, and CAI strength of CFRP laminates without reducing their in-plane bending and interlaminar shear strength properties compared to CFRP laminates made with the unmodified epoxy resin matrix [66]. In terms of mode II fracture toughness, the multiscale short fiber toughening effect is better than the sum of CNF and CSF alone. Future research on multiscale toughening needs to focus on the synergistic effect of multiscale toughening, which is similar to hybrid fiber toughening. Dasari et al. investigated the role of short carbon fibers (SCFs) as secondary reinforcement in glass fiber/epoxy composites. They evaluated the effect of SCF concentration on the mechanical properties of GFRP by tensile, flexural, and interlaminar fracture toughness tests [67]. The results showed that when the SCF content was 0.1 wt%, the tensile and flexural strengths of the composites were increased by 29.02% and 16.08%, respectively. In mode I and mode II interlaminar fracture toughness tests, the samples with 0.1 wt% exhibited the highest strain energy release rate, with an increase of 13.49% and 20.45% in *GIC* and *GIIC*, respectively. They prepared glass fiber composites (SGF-GE) by modifying epoxy resin with short glass fibers of 2–5 mm at different concentrations, and the results of three-point bending tests showed that the short fibers as secondary reinforcement could improve the flexural strength and flexural modulus of the composites [68]. Yu et al. mixed short basalt fibers (SBF) in unsaturated polyester (UP) resin and then coated them in the middle of basalt fabric curing molding to make BF/UP composites. The results showed that SBF had an excellent toughening effect on BF/UP, including mode II interlaminar fracture toughness, tensile strength, elongation at break, bending strength, and energy absorption were significantly enhanced [69]. From these studies, it can be seen that the addition of very small amounts of short fibers not only improves the interlaminar fracture toughness of the composite but also the inplane properties to a greater or lesser extent. This is further evidence of the feasibility and superiority of short fiber-modified resin toughening.

The use of recycled fibers to manufacture useful engineered composite components is a current research trend and requirement to protect the environment and save costs. Cholake et al. modified epoxy resin by recycled short-milled carbon fiber, 5 wt% and 10 wt% of SMCF could increase the *GIC* of epoxy matrix by 261% and 692%, respectively. The *GIC* of ground laminate using modified epoxy resin was increased by 50% and 64%, respectively, which proved to be a low-cost and effective method to toughen CFRP [70]. Saravana Kumar et al. investigated the effect of milled glass fibers filler on the interlaminar fracture toughness of GF/EP composites. They showed that the fracture toughness of mode I and mode II could be maximized by adding 5 wt% milled glass fibers by 102% and 175%, respectively, while the bending strength was also enhanced [71]. The interlaminar fracture toughness of composites can be effectively improved by adding recycled short fiber fillers. Still, fiber recycling is a very big topic, and it is of great concern to ensure that the recycled fibers have less property damage that may affect its toughening effect.

Graphene oxide (GO) is commonly used to treat fibers to improve the interfacial properties of composites [72]. Dang et al. used GO-modified short glass fibers to improve the ILSS of GF/EP composites. The experimental results showed that GO significantly increased the surface roughness of SGFs and improved the interfacial adhesion between SGFs and EP, while GO-SGFs could improve the ILSS of GF/EP composites by 18.4% [73]. Nie et al. added short carbon fibers treated with ethanol and concentrated nitric acid to the epoxy matrix for the ILSS improvement of CF/EP composites, and the preparation process of the composites is shown in Figure 9a [74]. The ILSS testing showed that adding oxidized short carbon fibers equivalent to a very low epoxy resin content could effectively improve the ILSS. They also prepared GO-SCF by a simple aqueous solution method, as shown in Figure 9b, and studied the effect of GO-SCF toughening resin on the ILSS of CF/EP composites, which showed that the ILSS of composites containing GO-SCF was higher than that of SCF at the same SCF content because GO increased the bonding force between SCF and epoxy resin, and GO-SCF could increase the ILSS by up to 14.7% [75]. The authors concluded that this method has practical application in improving the interlaminar shear properties of CF/EP composites due to the low cost and easy processability of SCF.

**Figure 9.** Schematic diagrams of preparation and mechanical testing of (**a**) oxidized SCF/CFF/EP composites; (**b**) GO@SCF/CFF/EP composites [74,75].

As stated in the literature [71], until the problem of agglomeration of short fibers in the resin is solved, only a limited amount of short fibers can be added to the resin, and this method cannot be used on a large scale yet.

#### **4. Conclusions**

This paper reviews the enhancement of composite interlaminar fracture toughness and other mechanical properties by incorporating short fibers in the composite preparation. In summary, short fiber interlaminar toughening techniques and short fiber modified resins can effectively improve the interlaminar fracture toughness of composites. Short fiber interlaminate toughening is specifically designed to improve the interlaminate properties of laminates. It is highly targeted to improve the interlaminate properties of laminates; short fiber modified resin toughening can improve the interlaminate properties and improve the in-plane properties. By adding a small number of short fibers, improving the interlaminar fracture toughness of composites without damaging other properties can be achieved. For different toughening methods, the toughening mechanism of short fibers is discussed. The toughening mechanisms are fiber bridging to hinder crack expansion, fiber debonding, and fracture from consuming additional energy and specific to different fibers and methods.

#### **5. Prospect**

It is not difficult to find that the short fibers are randomly distributed in the composite during the experiment, which brings many uncertainties to the short fiber toughening. If the short fibers can be oriented and arranged to eliminate the uncertainties of random distribution, it may make short fiber toughening stable and applicable.

Moreover, the issue of the source of staple fibers is also an issue worth considering; with the development of a low-carbon economy, natural fibers and recycled fibers have gradually become the hotspot of composite materials. Reasonable use of these fibers as toughening materials can reduce costs on the one hand and reduce environmental damage and waste of resources on the other hand. However, these fibers may have deficiencies in performance compared to synthetic fibers, so hybridization with synthetic fibers should be considered.

Hybrid fiber composites can make up for the shortcomings of single fiber composites, especially the hybrid of synthetic fibers and natural fibers, which is the trend of composite material development [76,77]. There are few research reports on hybrid short fiber toughened composites. Further research is needed to verify the feasibility and effectiveness of hybrid short fiber toughening and explore the mechanism of hybrid toughening.

**Author Contributions:** Conceptualization, L.Z.; supervision of the study, L.Z.; Funding acquisition, L.Z.; investigation, Y.W.; data curation, Y.W.; writing—original draft preparation, Y.W.; writing review and editing, L.Z. and Z.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Zhejiang Provincial Natural Science Foundation of China under Grant No LGG21E050025, the Fundamental Research Funds of Zhejiang Sci-Tech University (Project Number: 20202113-Y), as well as the Fundamental Research Funds of Shaoxing Keqiao Research Institute of Zhejiang Sci-Tech University (Project Number: KYY2021001G).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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

#### **References**

1. Aamir, M.; Tolouei-Rad, M.; Giasin, K.; Nosrati, A. Recent Advances in Drilling of Carbon Fiber-Reinforced Polymers for Aerospace Applications: A Review. *Int. J. Adv. Manuf. Technol.* **2019**, *105*, 2289–2308. [CrossRef]


## *Systematic Review* **Slow Fashion Consumer Behavior: A Literature Review**

**Mariana Domingos 1, Vera Teixeira Vale 2,\* and Silvia Faria <sup>3</sup>**


**Abstract:** This study aims to understand the dimensions associated with the behavior of the Slow Fashion consumer, their values, attitudes, and motivations, as well as to know in depth the literature related to Slow Fashion. The present article is a literature review related to the concept of "Slow Fashion", and follows a qualitative methodology based on an in-depth literature review of 25 papers from the Scopus and Web of Science research databases. For this literature review, a content analysis was initially performed through a bibliometric analysis. Then, a mind map was developed where the five major dimensions related to Slow Fashion consumer behavior were identified: "ethical values", "sustainable consumption", "consumer motivations", "consumer attitudes", and "sustainability awareness". We than related the mind map with the main conclusions of the literature review. The main limitation of this research results from the low number of published papers approaching the research concept. We were, however, able to identify the main concepts associated with the movement of Slow Fashion, thereby contributing to the available information on the variables that impact consumers' purchasing decisions.

**Keywords:** slow fashion; slow fashion consumer; consumer behavior; sustainability

#### **1. Introduction**

The social and ecological impact of the fashion industry has been the subject of much research and considerable documentation in recent years. It is recognized as one of the most polluting industries at all stages of its life cycle [1–3]. At an early stage and until approximately twenty years ago, two collections were produced per year (Spring/Summer and Autumn/Winter); currently, trend cycles have sped up and between 50 and 100 minicollections are placed on the market per year [2]. An example is the Inditex Group, especially the Zara brand, which changes its in-store collection every fifteen days [4].

Nowadays, some companies are increasingly adopting green initiatives such as cultural innovation and social responsibility as a part of their business philosophy and core values. This is mainly due to three reasons: being socially and environmental responsible (one of the main concerns of all markets), gaining competitive advantages, and following consumer trends [5].

In 2017, Europeans bought 6.4 million tons of new clothes; between 1996 and 2012, studies estimate that each person increased by 40% the amount of new clothes they purchased [2], increasing the ecological negative impact of this industry. Actual well-informed clients are concerned and focused on the long-term well-being of people and the planet itself, and thus a new movement has appeared called Slow Fashion [6]. It represents the need to adopt sustainable performance and a change in core values in the fashion industry [1,6]; the challenge is to focus on more durable products and traditional production techniques or design concepts that have no season [6–8], emphasizing quality in order

**Citation:** Domingos, M.; Vale, V.T.; Faria, S. Slow Fashion Consumer Behavior: A Literature Review. *Sustainability* **2022**, *14*, 2860. https:// doi.org/10.3390/su14052860

Academic Editors: Hafeezullah Memon, Xiaoke Jin, Tian Wei, Chengyan Zhu and Stefan Hoffmann

Received: 15 December 2021 Accepted: 21 February 2022 Published: 1 March 2022

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

**Copyright:** © 2022 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/).

to achieve sustainability [9,10]. In other words, the movement encourages brands to embrace a quality-based rather than a time-based business philosophy based around slower production, ethical attitudes, and well-made and long-lasting products.

There are data on the values that drive consumers to invest in Slow Fashion, namely increased awareness of sustainability, the key explanation for the acceptance of this movement. Concern for preserving the environment and guaranteeing the well-being of individuals is leading to a consequent change in consumers' value perceptions and purchasing behavior. Slow Fashion is seeing an increasing support over the last several years as consumers begin to demand higher standards in sustainability and corporate ethics [7,9,10].

Despite several studies proving that social and ecological awareness influence purchasing decisions, it has been discovered that other variables such as style are often related to fashion buying decisions [1,5,11–13]. This leads us to the following question: in which way and how do consumers engage in this movement by adopting more responsible consumption habits and demands?

A study developed in Finland in 2009 by Niinimaki [13] on eco-fashion consumption and consumer buying decisions concluded that during a purchase journey, consumers think about aspects related to ethics and the environmental impact of the products. However, at the moment of buying these thoughts tend to decrease. This can be related to individual and personality traits and consequent ethical commitment; it seems that the greater consumer ethical commitment is, the higher the importance of sustainability and the lower the attention towards physical appearance [13]. This leads to more sustainable consumption; in other words, consumers with higher ethical commitment and stronger values place more importance on their ideals than on aspects related to their own identity and/or beauty [13].

The main reasons that seem to lead consumers to engage in sustainable consumption are related to the capacity to find alternatives that minimize the damage caused to the planet without giving up on style, that is, "sustainability as a facilitator of style and sustainable fashion as a source of pleasure and well-being" [12] (p. 12). This complements Szmigin and Carrigan's study [14] that associates sustainable consumption to feelings of personal growth, well-being, and experiential pleasure.

Previous studies on Slow Fashion consumer behaviour have addressed specific issues such as the sales journey and the fashion industry's evolution over time. The consumer and consumption habits are addressed as contexts and often analysed as secondary data. Furthermore, the already extant literature is dispersed due to lack of systematization, and there are no studies that allow for tracing consumer profiles in order to understand how they behave. Thus, in order to be able to profile them in the future, it is necessary to understand in depth the related literature and the dimensions associated with such consumers.

The present study has a two primary aims: (1) to permit understanding of the main dimensions associated with Slow Fashion consumer behavior (e.g., values, attitudes, and motivations); and (2) to better understand in depth the main research pertaining to Slow Fashion.

#### **2. Materials and Methods**

A systematic literature review was carried out on Slow Fashion following a qualitative methodology based on an in-depth literature review.

On 2 March 2021, research was carried out on all the papers published to date on recognized databases, namely, Scopus and Web of Science.

On Scopus, we searched using the keyword "Slow Fashion", limiting the research areas to "business", "social science" and "environmental" and obtaining a total of 49 research documents.

In the Web of Science database, we replicated the survey using "Slow Fashion" as the keyword and defining the following research areas: "business", "green sustainable science technology", "environmental", "management", "environmental studies" and "interdisciplinary social sciences". We found a total of 24 research documents. The chosen areas

were intended to limit the results to slow fashion consumer behavior and its impact on sustainability both as a whole and from a management perspective.

The decision to set a limit on the search was due to our intention to focus as much as possible on the industry under analysis.

A total of 73 articles were obtained, of which 16 were excluded because they appeared in duplicate in both databases; in all, 57 documents were considered (see also supplementary material).

The collected documents were placed in an Excel sheet and sorted by year of publication. The data collected included author(s), title, type of document, the journal where the publication was made, keywords, abstract, objectives, research questions, variables, hypotheses, context, theories addressed, sample, methodology, achieved results, and recommendations for future studies. In several articles it was not possible to complete all fields for reasons of consistency, document context, or access. Through this choice and compilation of data it was possible to create a comprehensive screening of all the considered research papers.

After synthesizing all the information collected from the final 57 papers in Excel, one was excluded for not being in Portuguese or English and 13 were excluded for not allowing full access. We than read the 43 documents that could be fully accessed; it was possible to separate them into "+/− relevant" (13 articles), "relevant" (12 articles) or "not relevant" (18 articles). The 18 articles that were not relevant were excluded for not being related to consumer behavior in the context of Slow Fashion. We carried out a thorough analysis of the 25 documents considered relevant for the study in order to better understand their respective alignment with the topic under analysis. PRISMA Article was used for the analysis and exclusion of the articles [15], as shown in Figure 1.

**Figure 1.** Slow Fashion PRISMA (Source [15]).

Data collected from the papers were descriptively analyzed by year of publication, number of papers per journal, and content. In the content analysis the theories presented in the papers, research methodologies used, recommendations for future research, keywords, and relevance were studied. A bibliometric analysis in VowViewer allowed for keyword analysis and the creation of a word cloud.

#### **3. Results**

#### *3.1. Brief Descriptive Analysis*

2017 was the year with the highest number of documents published in databases (11). Of the eleven documents, four were excluded for not allowing full access; 2019 turned out to be the year with the highest number of publications (7).

Slow Fashion is a relatively recent topic in academic studies, and in fact the first article to address the concept dates back to 2008, only twelve years ago. Figure 2 describes the number of articles per year.

**Figure 2.** Papers: number per year.

Sustainability (Switzerland) was the journal with the highest number of papers (4). Two of the papers found did not clearly identify the journal. Figure 3 summarizes the number of papers per journal that we were able to find and use for content analysis.

**Figure 3.** Number of papers per journal.

#### *3.2. Content Analysis*

The content analysis of the final 25 documents used in this study provides an overview of the literature supporting the concept of "Slow Fashion". The theories presented throughout the papers were divided into seven groups, as they are associated with the same theoretical environment. Table 1 presents the groups of theories and corresponding authors.



The seven groups were assigned the following designations: sustainability, marketing, culture, fashion, human behavior, methods, and business. The name of each group is related to the main topic analysed. However, there are theories that are present in more than one group and that approach other topics as well.

The groups with the highest representation of theories were Human Behavior and Sustainability. Fashion and Business bring together a considerable number of theories as well.

The published papers considered in this research used different methodologies, which are summarized in Table 2.

Two types of methodological analysis were used: qualitative research and quantitative research. In quantitative analyses, surveys were the most used methodology. As for qualitative studies, the most used methodologies were mainly semi-structured interviews along with literature reviews and case studies.

For suggestions for future research, Table 3 summarizes the main recommendations.

People make up organizations, whether customers in general or other stakeholders. Therefore, research needs to be linked to consumer behavior, attitudes, ethics, and demographic characteristics.

When the Slow Fashion movement emerged, a main concern was to understand what should be done from a business perspective to embrace the concept and rethink business models. Initial papers focused on the production of Slow Fashion clothes such as yarn, yarn producers, and craftsmen. These studies attempted to help craftsmen/producers to develop their work, increase their activity, rethink their business models, gain recognition from consumers, and at the same time to understand and properly communicate the growing importance of adopting sustainable fashion practices.



Figure 4 shows a word cloud concerning the first keyword of each paper used in this study. The size of the words varies according to the relative frequency of their appearance, which allows us to identify fields directly related to Slow Fashion such as Ethics, Sustainability, and Design.

Considering the 25 documents used, 23 keywords were identified with more than one mention: "Slow Fashion" (7), "sustainability" (3), "design" (2), and "ethics" (2). All other keywords appeared only once. Researchers usually place the central themes of their studies in descending order according to their relevance to the paper.

A bibliometric analysis was performed to map the relationships between keywords. First, the keyword network was examined to identify all possible research fields and associated concepts (Figure 5). A co-occurrence analysis was performed with a minimum of one occurrence per word for a total of 79 keywords from the 25 papers. The size of the circles represents the frequency of keyword appearance; the larger the circle, the greater the occurrence, while the color of an item is related to the cluster where that keyword belongs. The lines between the circles show which keywords were mentioned together in a paper. In Figure 5, the keywords with most prominence are "Slow Fashion", "sustainability", and "fast fashion". The most prominent term, "Slow Fashion", is at the center of this paper as well and is linked to almost all of the other words.



The VOSviewer software allows for joining keywords into groups called "clusters" based on affinity and/or proximity. The VOSviewer program presented 18 clusters for the 79 items.

The two clusters with the higher number of items are shown in red and green. The red cluster presents eight items: clothing, clothing industry, ethic, social responsibility, sufficiency, sustainable consumption, sustainable management, and sustainable production. The green cluster has eight items as well: classic design, consumer perception, eco design, esthetic, green fashion, recycling, Slow Fashion, and sustainability. All other unnamed items were divided into clusters grouping from six to two items each. This analysis allowed us to understand how the keywords in different papers are interconnected. VOSviewer made it possible to identify which keywords were most prominent as well as to create clusters of related items.

The red cluster shows a higher interconnectedness between words; however, the expressions are not the ones that appear most often, such as "Slow Fashion" or "sustainability". These results seem to reveal that at the time of the database research there were still few articles that named these items as keywords, and/or that these two concepts have not yet been related to Slow Fashion.

**Figure 5.** Bibliometric analysis.

This analysis made it possible to understand how the keywords of different papers are interconnected.

#### **4. Discussion**

Nowadays, consumers are increasingly looking for unique products and designs and willing to pay more if they feel that their needs and preferences are met [24]. This search for exclusivity is related to any human being's need to be perceived as different from others, enhance self-esteem and social image, mark a position, and consequently to avoid mass consumption [38–40]. In fashion, this search for uniqueness seems to be related to the style aspect; people prefer to reorient the quality of the products they use bearing in mind certain integral aspects, not only appearance. Clothes that are customized and made of recycled materials reveal a high concern for sustainability. In other words, sustainable consumption is positively associated with environment and social responsibility [19].

Several studies have revealed that the more classic a piece of clothing is, the lesser its temporality, and therefore its alignment with a sustainable perspective. A simple and discreet classical design is usually considered an expression of maturity and sophistication [21], revealing a specific social image in which self-image seems to impact an individual's clothing choice. Self-image may act as a variable leading to the choice of slow or fast fashion [27,41], as slow fashion consumers have a non-modern self-image. They follow their individual style and bet on more classic and simple clothes [24,27]. Promoting the idea that a sustainable consumption leads to a very positive and effective image could be a potential solution for achieving an increase in Slow Fashion.

The Slow Fashion movement shows the importance of sustainability to the fashion industry players along with a change in peoples' values [1,6]; it is important to understand which values have changed, the reasons for this, and which reasons currently guide consumer purchasing decisions.

According to the papers used in this research, two specific generations have been studied, namely, Generation Y and Generation Z. Data show that both generations are currently looking for more mature and timeless clothing, especially females. In different contexts, both generations have similarities in their dressing practice and are moving in the direction of timeless clothing; however, they feel the need to be up to date as well. This arises from an external influence resulting from a location with an eco-conscious culture that leads to the involvement in more sustainable shopping [23].

Environmental sustainability has increasingly emerged as an important issue in the fashion industry. On one hand, the chemicals used in the production and finishing of the goods are recognized as capable of harming humans and the environment; on the other hand, there are a lot of products that end up not being consumed, therefore becoming wasted and or being destroyed by brands. Slow Fashion promotes a change in the consumer mindset that encourages a preference for quality over quantity [42].

The Slow Fashion consumer cares about all the participants in the value chain, bearing in mind both producers and society itself. They care about sustainable companies in sustainable markets [28,42].

Based on the existing literature it is possible to state that culture is crucial to consider when developing a marketing strategy. Cultural values influence how consumers evaluate and respond to marketing strategies and how they decide to buy products and services [43]. In other words, human beings are influenced by the community where they were born and grew up. This impacts their personality traits, beliefs, values, thoughts, and concerns, which result in motivations and later actions. In order to better understand how consumers perceive slow fashion, a mind map was developed (Figure 6).

This mind-map tries to present the top five expressions related to Slow Fashion consumer behavior. Five groups, in pink, reveal the key concepts associated with Slow Fashion: "ethical values", "sustainable consumption", "consumer motivations", "consumer attitudes", and "awareness of sustainability". Each group highlights certain questions that need to be properly answered in order to understand who the Slow Fashion consumer is and create a possible profile.

**Figure 6.** Slow Fashion Mind-Map.

Thus far, in considering consumer attitudes studies have looked at various attributes impacting buying decisions: environmental responsibility, employee welfare, animal welfare, Slow Fashion, costs, and physical and extrinsic attributes. Four of these were found to be important dimensions for ethical fashion perception: fashion employee welfare, Slow Fashion attributes, animal welfare, and environmental responsibility, with only the first two appearing to influence consumer attitudes [25,32].

Employee wellbeing has become a fundamental aspect for society, mainly due to news reporting on unfair earnings and working conditions, child labor, and social inequalities. Consumers are increasingly paying attention to these issues, enjoying brands that adopt working policies that value their employees and respect their rights. Being more concerned with socially responsible practices, consumers express it by preferring to make ethical purchases [42,44–46].

However, consumers do not need to associate Slow Fashion with environmental and employee welfare good practices to perceive ethical clothing; the durability of the goods and the materials used in their production is enough for this consumer perception [32], as well as for animal welfare and environmental responsibility.

A study that looked at 221 consumers in the US revealed that delivering an exclusive product affects the Slow Fashion customer's perception of value creation. Slow fashion and customized products increase perceived value, which in turn positively affects Slow Fashion purchasing intentions and the acceptance of premium prices [10].

When adopting sustainable consumption habits, the consumer tends to pay attention to transparency in terms of the way brands communicate and support local communities [31].

Regarding Slow Fashion consumer values, several studies have been found. A study developed in Turkey showed that main perceived values in Turkish Slow Fashion consumers are authenticity, locality, and exclusivity, while in Kazakhstan the main values are equity, functionality, locality, and exclusivity. Perceived values in both groups positively affect the intention to buy Slow Fashion products and predispose consumers to pay higher prices [7,24]. Based on US consumer orientation, four target groups of consumers have been identified: (1) a group highly involved in the movement; (2) a conventional group; (3) an exclusivity-oriented group; and (4) a group little-involved in the movement. These groups have been created based on values, purchasing behaviors and demographic aspects [9]. For each group, a distinct marketing strategy should be implemented.

In other research, certain values have been clearly identified: (1) quality versus emotionality; (2) price; and (3) social values. Consumers of Slow Fashion products perceive them as having a higher quality standard, which encourages individuals to consume these types of products, feel good about doing it, and invest in more durable goods [29].

Table 4 shows the information presented in this discussion regarding consumer behavior around Slow Fashion and synthesizes and relates it to the five groups identified in the mind map. These five key concepts seem to be somehow related to the information collected from the articles analyzed. However, further data are needed in order to assertively define the Slow Fashion consumer profile.


**Table 4.** Five categories and outcomes.

#### **5. Conclusions**

According to Freudenreich et al. [1], one way to reduce clothing consumption is to create a structure to offer potential businesses. By developing these offerings, the clothing sector could modernize itself and move away from high-speed and low-price production towards a slower, more responsible, more sustainable business approach [1].

The consequences of the environmental and ethical impacts of this industry are now a concern for brands that encourage their consumers to prioritize longevity and ethical consumption over price and novelty [34].

This paper, after a careful analysis of the current existing bibliography on the topic and considering the gaps that still exist, highlights possible paths for future and complementary work. The focus must be on gathering data that allow a deep understanding of who the Slow Fashion consumer is and which factors influence their adoption of this new global tendency.

Highlighting specific research results, Štefko, R. and Steffek, V. [24], complemented by the studies of Santos [38], Giglio, E.M. [39], Vigneron, F., and Johnson, L.W. [40] show that Slow Fashion consumers look for unique designs which are associated with a taste for exclusivity, which in turn influences the probability of being willing to pay more for a Slow Fashion product and a future purchase.

Several authors, such as Štefko, R. and Steffek, V. [24], Barnes, L. et al. [27], and Yoo, B. and Lee, S.H. [41] argue that such consumers seek to distinguish themselves from others, and pay attention to their social and self-image. Self-image is related to creating a unique style instead of merely being trendy; a different and very-own style seems to increase self-esteem. Classic clothes with a simple and discreet design and longer durability due to the materials used expresses maturity and sophistication. Style orientation seems to be interconnected with purchasing decisions and the individual's main concerns and values. Buyers think more and more about recycling and reuse; they try to make more sustainable purchases, as they are worried about corporate responsibility, the transparency of brands' communication, good environmental practices, and how they support local communities. When buying, the Slow Fashion consumer considers the place where the product is produced and whether it is fair trade, sustainably produced, and organic. Increasingly, Slow Fashion consumers are concerned with buying items that are less driven by fashion trends, and with wearing them for longer. In addition, they care about the impact of the fashion industry on workers and society [19].

The current literature allows us to conclude that there are already identified values; these values plus individual ethics have a positive influence on sustainable consumption. The five values identified by ¸Sener, T. et al. [7], and Stefko, R. and Steffek, V. [24] allow a first characterization of this consumer: (1) authenticity; (2) locality; (3) exclusivity; (4) equity; and (5) functionality. It should be noted that these authors pointed out that their samples should be extended and more research topics should be added.

We recommend that future research use a qualitative and quantitative approach gathering data through questionnaires applied to final consumers, focus groups, or Delphi interviews with well-recognized fashion experts.

The main limitations of this paper are the existence of few published papers on Slow Fashion and even fewer on Slow Fashion consumer behavior.

This paper presents a theoretical contribution, being the first literature review focusing on Slow Fashion Consumer behavior. In the context of sustainability, this paper relates Slow Fashion to companies' need to adopt a sustainable internal and external policy. It provides insights into the associations, values, and key motivations that drive sustainabilityconscious consumers.

From a management perspective, it is important to highlight that understanding Slow Fashion consumer behavior is important for any brand; therefore, being able to understand the motivations, attitudes, beliefs, and values of these consumers, which have already been presented in this study, becomes a high-value asset and should be a priority for companies acting in the fashion industry.

Slow Fashion is a concept that consumers and brands are now beginning to pay attention to. Brands want to understand whether consumers are willing to pay more if they adopt a new business model moving from fast to slow fashion. Brands such as H&M and Mango have created new brands with timeless products, higher quality raw materials, and more responsible supply chains, and Zara is now implementing classical and durable products at their stores, which carry a higher price. How this will be accepted by customers and how to properly communicate demands for more detailed information about Slow Fashion adopters is as yet unknown. It demands further market studies to support such strategic decisions as whether to prioritize new brands, new product lines, prices levels, and policies. The number of consumers who are concerned about sustainability and consider Slow Fashion as a way to achieve sustainable businesses is increasing, especially among young individuals, and the challenge is for companies to answer promptly.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/su14052860/s1, CONSORT 2010 Checklist. Table S1—Theory groups. Table S2—Methodologies. Table S3—Future research field. Table S4—5 categories and outcomes.

**Author Contributions:** M.D. developed research ideas based on an extensive literature review, collected, and analyzed the data, wrote the paper; V.T.V. supervised this investigation; V.T.V. and S.F. provided a thorough review to enhance the overall quality of this paper. 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.

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

#### **References**


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