Search Results (17)

The thermal, thermomechanical, and viscoelastic properties of continuous unidirectional (UD) glass fiber/high-density polyethylene (GF/HDPE) and ultra-high-molecular-weight polyethylene/high-density polyethylene (UHMWPE/HDPE) tapes are characterized in this paper in order to support their use in extreme environments. Unlike prior studies that focus on short-fiber composites or limited thermal conditions, this work examines continuous fiber architectures under five operational environments derived from Army Regulation 70-38, reflecting realistic defense-relevant extremes. Differential scanning calorimetry (DSC) was used to identify melting transitions for GF/HDPE and UHMWPE/HDPE, which guided the selection of test conditions for thermomechanical analysis (TMA) and dynamic mechanical analysis (DMA). TMA revealed anisotropic thermal expansion consistent with fiber orientation, while DMA, via strain sweep, temperature ramp, frequency sweep, and stress relaxation, quantified their temperature- and time-dependent viscoelastic behavior. The frequency-dependent storage modulus highlighted multiple resonant modes, and stress relaxation data were fitted with high accuracy (R² > 0.99) to viscoelastic models, yielding model parameters that can be used for predictive simulations of time-dependent material behavior. A comparative analysis between the two material systems showed that UHMWPE/HDPE offers enhanced unidirectional stiffness and better low-temperature performance. At the same time, GF/HDPE exhibits lower thermal expansion, better transverse stiffness, and greater stability at elevated temperatures. These differences highlight the impact of fiber type on thermal and mechanical responses, informing material selection for applications that require directional load-bearing or dimensional control under thermal cycling. By integrating thermal and viscoelastic characterization across realistic operational profiles, this study provides a foundational dataset for the application of continuous fiber thermoplastic tapes in structural components exposed to harsh thermal and mechanical conditions. Full article

Designing thermoplastic composites for particular uses requires understanding their dynamic mechanical behaviour, which affects how well they operate in practical settings. The Split Hopkinson pressure bar (SHPB) test allows for evaluating these materials’ responses to high strain rates. In this study, an in-situ laser-assisted fibre placement (LAFP) machine has been utilised to produce laminate composites with varied designs, i.e., different angles of layers [0/45/–45/90]_4s, using three types of thermoplastic tapes (UD-CF/PPS, UD-CF/PEEK, and UD-CF/PEKK). Using a servo-hydraulic testing machine and SHPB apparatus, we have examined the dynamic compressive behaviour of thermoplastic laminate composites with various matrices (PPS, PEEK, and PEKK) in in-plane directions and at strain rates of approx. 0.001, 0.1, 10, 800, 1800/s. Experimental results indicate that the type of thermoplastic matrix and strain rate significantly affect how the laminate composites behave. The in-plane compressive strength and modulus increase approximately linearly with the strain rate. According to the fracture of morphological pictures, the main failure mechanism of all three types of specimens is shear failure under in-plane compression loads, which is followed by delamination and burst. Full article

Within this study, the impregnation behavior and resulting mechanical properties of unidirectional flax fiber-reinforced polyamide 11 biocomposites were investigated. Therefore, different grades of bio-based polyamide 11 have been evaluated regarding their eligibility as composite matrix material. The production of the unidirectional flax fiber-reinforced biocomposites was investigated using a continuous film-stacking method. It was found that the flow behavior of the polyamide 11 matrix polymer significantly affected the impregnation quality and the resulting mechanical properties as tested by tensile and bending tests. A lower shear viscosity and stronger shear thinning behavior led to better impregnation, a 15% higher stiffness, and 18% higher strength. This was also analyzed with morphological analysis by scanning electron microscopy. Additionally, the effect of the fiber volume content of the flax fibers on the mechanical properties was tested, showing a positive correlation between the fiber content and the resulting stiffness and strength, leading to an increase of 48% and 55%, respectively. In result, a maximum Young’s modulus of 16.9 GPa and tensile strength of 175 MPa at a fiber volume content of 33% was achieved. Thus, the unidirectional flax fiber-reinforced polyamide 11 biocomposites investigated can be a sustainable construction material for lightweight applications, e.g., in the automotive industry. Full article

Unidirectional (UD) fiber-reinforced thermoplastic tapes provide excellent specific mechanical properties; thus, they are being increasingly used for the targeted local reinforcement of plastic components in lightweight construction applications. An essential step in the production of UD tapes is fiber spreading, the aim of which is to expand fiber rovings from an initial width to a defined final width. Using a test rig under realistic conditions, we systematically investigated the factors that influence fiber spreading by deflection. Carbon-fiber rovings with various numbers of filaments were guided over deflection rods, and roving width before and after spreading was recorded with cameras. A full design of experiments (DoE) plan was set up, in which (i) the number of rods, (ii) rod diameter, (iii) immersion depth of the rod, and (iv) take-off speed of the fiber roving were systematically varied. We statistically evaluated the results of the experiments and found that the main factors that influenced the response variables investigated were number of rods and rod diameter, followed by immersion depth. We also observed that a higher number of filaments in the roving led to more complexity and greater variability. Our results can be used to optimize the spreading configuration in the production of UD tapes. Full article

Steel pipeline systems carry about three-quarters of the world’s oil and gas. Such pipelines need to be coated to prevent corrosion and erosion. An alternative to the current epoxy-based coating, a multi-layered composite coating is developed in this research. The composite coatings were made from carbon fiber-reinforced thermoplastic polymer (CFRTP) material. Uniaxial carbon fiber CF/PPS prepreg tape was utilized, where the PPS (polyphenylene sulfide) is employed as a thermoplastic (TP) matrix. Compression molding was used to manufacture three flat panels, each consisting of seven plies: UD (Unidirectional), Biaxial, and Off-axis. Samples of carbon steel were coated with multi-layered composites. The physical, mechanical, and corrosion-resistant properties of steel-composite coated samples were evaluated. A better and more promising lap-shear strength of about 58 MPa was demonstrated. When compared to the Biaxial and Off-axis samples, the UD assembly had the maximum flexural strength (420 MPa); however, the Biaxial coating has the highest corrosion resistance (445 kΩ·cm²) when compared to the Off-axis and UD coatings. Full article

The thermomechanical and viscoelastic properties of a glass fiber polyethylene terephthalate glycol (GF/PETG) continuous unidirectional (UD) tape were investigated using differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and dynamic mechanical analysis (DMA). This study identified five operational conditions based on the Army Regulation 70-38 Standard. The DSC results revealed a glass transition temperature of 78.0 ± 0.3 °C, guiding the selection of temperatures for TMA and DMA tests. TMA provided the coefficient of thermal expansion in three principal directions, consistent with known values for PETG and GF materials. DMA tests, including strain sweep, temperature ramp, frequency sweep, creep, and stress relaxation, defined the material’s linear viscoelastic region and temperature-dependent properties. The frequency sweep indicated an increased modulus with rising frequency, identifying several natural frequency modes. Creep and stress relaxation tests showed time-dependent behavior, with strain increasing under higher loads and stress decreasing over time for all tested input values. Viscoelastic models fitted to the data yielded R² values of 0.99, demonstrating good agreement. The study successfully measured thermomechanical and viscoelastic properties across various conditions, providing insights into how temperature influences the material’s mechanical response under extreme conditions. Full article

Bending is one of the dominant material deformation mechanisms that occurs during the forming process of unidirectional (UD) thermoplastic tapes. Experimental characterization of the bending behavior at processing temperatures is crucial to obtaining close-to-reality data sets for process analysis or material modeling for process simulation. The main purpose of this study is to characterize to a high degree of accuracy the temperature-dependent bending behavior of single and multi-ply specimens of carbon fiber-reinforced polycarbonate (PC/CF) UD tapes at processing temperatures, which implies a molten state of the thermoplastic matrix. The application of the rotation bending test using a customized fixture may come with systematic deviations in the measured moment that result from a pivot offset or an effective clearance that is unknown under realistic test conditions. The present research analyzes these effects with analytical methods, experimental investigations, and simulations using a finite element model. In this context, a compensation method for the toe-in effect is evaluated. With this approach, we were able to obtain reliable data and characterize the bending resistance within the desired processing window. The data reveal a major drop in bending resistance between 200 °C and 250 °C and a less significant decrease between 250 °C and 300 °C. Analysis of the thickness-normalized bending resistances indicates a non-linear relationship between specimen thickness and measured moment but an increasing shear-dominated characteristic at higher temperatures. Full article

Fused granular fabrication (FGF) is a large format additive manufacturing (LFAM) technology and focuses on cost-effective granulate-based manufacturing by eliminating the need for semifinished filaments. This allows a faster production time and a broader range of usable materials for tailored composites. In this study, the mechanical and morphological properties of FGF test structures made of polyamid 6 reinforced with 40% of short carbon fibers were investigated. For this purpose, FGF test structures with three different parameter settings were produced. The FGF printed structures show generally significant anisotropic mechanical characteristics, caused by the layer-by-layer building process. To enhance the mechanical properties and reduce the anisotropic behavior of FGF structures, continuous unidirectional fiber-reinforced tapes (UD tapes), employing automated tape laying (ATL), were subsequently applied. Thus, a significant improvement in the flexural stiffness and strength of the manufactured FGF structures was observed by hybridization with 60% glass fiber-reinforced polyamide 6 UD tapes. Since the effectiveness of UD-tape reinforcement depends mainly on the quality of the bond between the UD tape and the FGF structure, the surface quality of the FGF structure, the interface morphology, and the tape-laying process parameters were investigated. Full article

Thermoplastic tapes are commonly processed by the rapid and efficient stamp forming process. During this forming process, the individual unidirectional tapes of the composite stack move relative to each other and relative to the surface of the tool while being in contact with the corresponding counterpart. As a result, the material exhibits a certain resistance against this movement, which is generally dependent on velocity, normal pressure, and temperature. Therefore, this work investigates the ply/tool and ply/ply slippage of unidirectional, carbon fiber reinforced polycarbonate tapes and provides an alternative implementation of the experimentally observed slippage using cohesive zone modeling. The backbone of the modeling approach is an experimental data set obtained from pull-through experiments. In comparison to common slippage or friction theories, the force plateau of thermoplastic UD tapes at elevated temperatures is observed after an initial force peak has been overcome. For both configurations, ply/tool and ply/ply, a reduction of the initial force peak was observed for increasing temperature. Furthermore, the resulting plateau force value is at least 36% higher in the ply/ply configuration compared to the ply/tool configuration at 200 °C. The derived cohesive zone model allows for accurate modeling of the initial force peak and the plateau. Full article

The aim of this work was to optimize spot welding of unidirectional tapes made of polycarbonate and carbon fibers. Three studies were performed to investigate the influences of various welding conditions on the quality of the welded spot. First, we used a full factorial experimental design to analyze the influence of temperature and time on the welds’ tensile stress at break. Second, we repeated the experiment with optimized settings and conditions. Finally, we adopted a central composite design (CCD) to investigate the stability of the process. Our results show that temperature had the greatest influence on weld quality. The maximum tensile stress achieved was 23 MPa. Using a relatively high temperature for a short welding time resulted in self-cleaning of the welding head and in a faster and more stable process, and gel permeation chromatography (GPC) confirmed that these conditions caused no additional degradation. Full article

The natural fiber-reinforced thermoplastic tape was produced using a novel process assembly that involves a drawframe and a double belt press. First, the state-of-the-art film-stacking process was modified through the integration of a drawframe to supply the natural fiber preforms for reinforcement, adding thermoplastics films as matrix material and processing them to a unidirectional tape (UD tapes) using a double belt press. Based on that, a new approach was investigated using a commingled sliver containing natural reinforcing and polymer matrix fibers to manufacture UD tapes. This leads to a reduced flow path of the matrix polymer, which is a decisive parameter for production efficiency. To ensure a homogeneous distribution and alignment of the fibers after gilling, the influence of various processing parameters on one another and the resulting UD tape quality were examined. As result, a draft ratio in the range of 10 ± 2, a low linear density (here 12 ktex) and general use of many thin in contrast to fewer heavier slivers is advisable. The differences in impregnation quality and thus the mechanical performances of the UD tapes from both processes were validated using scanning electron microscopy and mechanical testing. It was found that the commingled sliver composite had 10% higher flexural modulus and 34% higher flexural strength compared to the film-stacking-based composites. In conclusion, using commingled sliver could enable the increase in productivity and fiber volume fraction compared to film-stacking-based composites. Full article

Laminated composites with thermoset matrices are already well established in major engineering fields like automotive and aviation. The primary drawbacks of such thermoset-based composites are the high cycle times required during manufacturing and their limited potential for recycling. Providing an alternative to thermoset-based composites, thermoplastic matrix materials gained more and more momentum by addressing these previously mentioned drawbacks. The preferred manufacturing technique for these materials employs fiber-reinforced thermoplastic tapes consolidated and formed together with a compatible substrate. The most critical aspect for all these applications is the stress or load transfer between the thermoplastic tapes and the substrate. If the interface is too weak and fails prior to the substrate or tape, a high amount of theoretical mechanical performance is lost. The presented research investigates the influence of variations in manufacturing parameters, within the industrially relevant process window, on the interface strength of the final composite. The investigated composite material consists of PPGF UD tapes consolidated on a PPGF mat substrate. In particular, the influence of the consolidation parameters of pressure, temperature, and time are of special interest. The results of this work reveal a 400% increase in the measured mean strain energy release rate upon increasing the consolidation time from 60 s to 120 s at a consolidation temperature of 230 °C and a pressure of one bar. In contrast to this, an increase in the consolidation pressure, at constant temperature and time, leads to a minor improvement in the G_C value of 20%. For testing and characterizing the corresponding interface properties, a mandrel peel testing setup was employed. Full article

Application of thermoplastic fiber-reinforced lightweight composite materials provides a wide range of advantages that are of particular importance for the mobility sector. UD tapes composed of unidirectionally (UD) oriented inorganic fibers embedded in a thermoplastic matrix represent light-weight materials with high tensile strength. This publication addresses recycling aspects of novel UD tape made of a combination of basalt fibers and different PLA (polylactic acid) formulations. The kinetics of enzyme-based separation of polymer from the fiber were investigated. Different types of UD tapes with a thickness of 270–290 µm reinforced with basalt fiber weight ratios ranging between 51 and 63% were incubated at 37 °C in buffer solution (pH 7.4) containing proteinase K. The influence of enzyme concentration, tape weight per incubation tube, proteinase K activators, and tape types on the rate of enzymatic decomposition was investigated. Enzyme activity was measured by analyzing lactate concentration with lactate dehydrogenase and by measuring weight loss of the composite material. The rate of lactate release increased in the first 30 min of incubation and remained stable for at least 90 min. Weight loss of 4% within 4 h was achieved for a tape with 56% (w/w) fiber content. For a sample with a surface area of 3 cm² in a buffer volume of 10 mL, the rate of lactate release as a function of enzyme concentration reached saturation at 300 µg enzyme/mL. With this enzyme concentration, the rate of lactate release increased in a linear manner for tape surface areas between 1 and 5 cm². Four tapes with different PLA types were treated with the enzyme for 17 h. Weight loss ranged between 7 and 24%. Urea at a concentration of 0.5% (w/v) increased lactate release by a factor of 9. Pretreatment of tapes in alkaline medium before enzymatic degradation increased weight loss to 14% compared to 5% without pretreatment. It is concluded that enzymatic PLA hydrolysis from UD tapes is a promising technology for the release of basalt fibers after alkaline pretreatment or for the final cleaning of basalt fibers. Full article

Non-crimp fabrics (NCFs) are increasingly used in industry for manufacturing of composite structures due to a combination of high mechanical properties and excellent manufacturability. As with other composites, in-service damage can be a cause for severe reduction in load-carrying capacity of NCF-reinforced plastics. In this experimental and numerical study, two constitutive material models previously used only for damage prediction in unidirectional (UD) tape and woven fabric-reinforced materials (LS-DYNA’s *MAT_ENHANCED_COMPOSITE_DAMAGE—MAT54 and *MAT_LAMINATED_COMPOSITE_FABRIC—MAT58) were evaluated for simulating transverse crushing of composite parts processed from a non-crimp carbon fabric. For this purpose, UD NCF components of tubular shape were subjected to transverse crushing through a controlled indentation of a metallic cylinder, and results of the experiment were compared with numerical modeling. Considered verification metrics included the observed and the predicted patterns of interlaminar damage, the extent of delamination, as well as the ability of the models to replicate force-displacement response exhibited by the tested specimens. Full article

An efficient way to reduce direct operating costs in aerospace applications is to lower the overall weight. In this context, thermoplastic composites offer a high potential for weight reduction. However, their application requires time and cost-optimized process technologies. Thermoplastic tape laying with subsequent out-of-autoclave consolidation represents such a process technology. Typical process chains consist of several automated steps that can influence the component’s quality. Hence, a cross-process approach is applied to identify relevant process parameters. This paper focuses on minimizing the gaps between parallel-placed tapes and thereby reducing their influence on the laminate’s porosity. A geometrical model is developed and validated to predict the maximum gap sizes for a tape-laying process as a function of process accuracy, material accuracy, and process parameters. Based on this, a methodological approach is presented to minimize the influence of gaps on porosity. It is validated using automated tape laying and a novel low-pressure consolidation process. The findings make an important contribution to understanding the development of porosity along the process chain for the manufacture of thermoplastic composites for aerospace applications. It can be shown that the approach enables the prediction of gap sizes and allows to minimize their influence on porosity. Full article