Unidirectional Thermoplastic Composites Research Articles

Tensile and impact properties of thermoplastic fibre hybrid composites based on air textured commingling hybrid yarns composed of carbon/aramid/polyamide 6 and carbon/glass/polyamide 6 filament yarns

This paper reports a comparison of tensile and impact properties between carbon/aramid and carbon/glass fibres reinforced unidirectional thermoplastic hybrid composites. For this purpose, different hybrid composites were produced using commingled hybrid yarns of carbon/aramid/polyamide 6 filament yarns and carbon/glass/polyamide 6 filament yarns with a defined mixing ratio based on air texturing technology. The results show that the density and specific Youngs’ modulus of the composites changes linearly with the proportion of carbon/glass and carbon/aramid fibre in hybrid composites. The specific Young’s modulus decreases linearly with decreasing carbon fibre content in the hybrid composites. On the other hand, the specific tensile strength does not change linearly according to the rule of mixture and a negative hybridization effect can be observerd for both types of hybrid composites. Furthermore, the combination of carbon/aramid and carbon/glass fibres can increase the impact strength by approximately 204% and 218%, respectively, compared to the composite reinforced by CF only. Although the specific tensile strength and specific impact strength of carbon/aramid reinforced hybrid composites are higher than those of CF/GF hybrid composites due to the lower density of carbon/glass hybrid composites, the potential impact strength achievable in hybrid composites is higher with the carbon/glass combination than with the carbon/aramid combination.

Tensile properties of unidirectional thermosetting composites reinforced with ring-spun hybrid yarns including recycled carbon fiber

The study of composites reinforced with aligned recycled carbon fiber (CF) is gaining increasing attention, particularly in light of the circular economy. Hybrid yarns, among other oriented reinforcements, have gained interest from researchers who have already utilized them to produce unidirectional thermoplastic composites. Nevertheless, the evaluation of unidirectional thermosetting composites reinforced with hybrid yarns is still lacking. This article presents their production, leveraging a recently developed process, and mechanical characterization. The results show that the amount of recycled CF and the number of draw frame doublings within the hybrid yarns affect the tensile properties of the composite, while the thermoplastic fiber has no influence, as also suggested by the main effect plots. Two-way interactions exhibit different behaviors depending on the tensile property considered. Overall, the composite material with the best tensile properties was that reinforced with a ring-spun hybrid yarn consisting of 70% recycled CF, 30% polyamide, and five draw frame doublings.

Off-Axis Tension Behaviour of Unidirectional PEEK/AS4 Thermoplastic Composites

An experimental method for non-standard off-axis tension tests of unidirectional composites is developed. A new oblique end-tab is designed to eliminate stress concentration and in-plane bending moment induced by off-axis tension loading. Finite element analysis and experiments on Polyetheretherketone (PEEK)/AS4 unidirectional thermoplastic composites (CFRTP) were conducted to evaluate the effectiveness of the proposed testing method. Simulation and test results demonstrate that the use of oblique end-tabs eradicates stress concentration and bending movements. The digital image correlation (DIC) method was used to help investigate the full-field tension/shear coupling deformation response of the off-axis specimen. Test results show significant nonlinear behaviour and inhomogeneous strain distribution under tension/shear combined stresses. A fractographic study was carried out to study the damage mechanisms under a tension/shear combined stress state. Specimens with 30°, 45° and 60° off-axis angles, fail in tension/shear mixed failure mode. Fracture surface morphology indicates that matrix plastic deformation and ductile drawing under tension/shear coupled stress state induced the nonlinear stress-strain response.

Longitudinal failure mechanisms and crack resistance curves of unidirectional thermoplastic composites

This paper presents a study on the longitudinal fracture toughness, and associated crack resistance curves, of thermoplastic-based composite materials. Double-edge notched (DEN) specimens loaded in tension and compression are used to relate the size-effect laws with the crack resistance curves of unidirectional carbon-fibre reinforced PEKK. The results are corroborated with SEM and fractographic images to provide a better overview of the main failure mechanisms involved during the failure process. Moreover, tensile tests interrupted at 90% of the estimated failure load are CT-scanned to highlight the early stages of the crack propagation. This investigation confirms the suitability of the size-effect method for characterizing thermoplastic composites and it stands out as a first qualitative and quantitative analysis of their characteristic intra-laminar failure mechanisms.

Characterisation of mechanical behaviour of fibre reinforced thermoplastic composites subjected to off-axis compressive loading

This paper presents an experimental and numerical investigation on the mechanisms of damage onset and evolution in unidirectional carbon fibre reinforced thermoplastic (CFRTP) composites subjected to off-axis compressive loadings. A test fixture was designed to prevent buckling, splitting and end collapsing of specimens during test. A series of compression tests were conducted with specimens of various off-axis angles. Different failure mechanisms and nonlinear stress-strain responses were observed and studied. The fracture angles of the tested specimens were evaluated and analysed according to Puck’s theory. The off-axis compression failure envelope based on LaRC05 and Hashin criteria was presented using finite element analysis and compared to experimental results. It is shown that the LaRC05 criterion can provide accurate predictions for matrix cracking failure mode of unidirectional thermoplastic composites controlled by compression-shear combined stresses.

In-Plane Shear Characterization of Unidirectional Fiber Reinforced Thermoplastic Tape Using the Bias Extension Method

Through an improved characterization methodology, this work contributes to better prediction quality in composite forming simulations for unidirectional thermoplastic composites. A better understanding of the forming behavior will aid in the adoption of these lightweight materials in aerospace applications. The bias extension method was implemented and applied to cross-ply laminates from unidirectional carbon fiber reinforced thermoplastic materials to characterize the in-plane shear deformation resistance of the molten material. Two commercially available materials were characterized at three rates and three temperatures. The shear deformation was measured directly on the specimen throughout the test using a video extensometer, avoiding the use of the pin-jointed net assumption to relate deformation to the clamp displacement. In addition, the distribution of shear deformation over the specimen surface was characterized after the test using image analysis. The observed deformation was similar to the typical deformation for woven materials, with some agreement to the pin-jointed net assumptions but also some identified deviations. Localization of shear deformation along the fiber direction of the outer ply was observed to occur at approximately 15° shear angle. The proposed bias extension method directly relates the measured force to the deformation on the specimen, ensuring the characterization of the correct deformation mechanism. This key benefit of the bias extension method solves a common issue found in other characterization methods for in-plane shear on the molten material.

Identification of plasticity-controlled creep and fatigue failure mechanisms in transversely loaded unidirectional thermoplastic composites

In continuous fiber-reinforced thermoplastics, the macroscopic failure mode in transverse long-term failure is dominated by a brittle crack-growth mechanism. Neat thermoplastic matrices, on the other hand, generally display also a plasticity-controlled mechanism in long-term loading at elevated stress levels and/or temperature. This failure mechanism requires a different approach to lifetime prediction than crack growth; hence, it is important to identify it in the long-term performance of composites. In this study, we demonstrate the presence of the plasticity-controlled failure mechanism in long-term failure of transversely loaded unidirectional (UD) thermoplastic composites made of glass/iPP, carbon/PEEK and carbon/PEKK. The main method used is to compare the lifetime in cyclic loading to that in static loading at the same level of maximum stress, where an increase in lifetime is characteristic for plasticity controlled failure, and, vice versa, a decrease is indicative for fatigue crack growth. In addition, the applicability of a lifetime prediction method common to plasticity-controlled failure of neat thermoplastics is evaluated for the composites investigated. The results of this study indicate that the plasticity-controlled failure was present in composites, although the extent to which the effects are present varied depending on the materials investigated. Glass/iPP showed the most explicit evidence of the plasticity-controlled failure over the entire load range experimentally covered. Its long-term failure was delayed with a decrease in the stress ratio and lifetime was predicted well using the principles of plasticity-controlled failure.

Simulation and Analysis of Process‐Induced Distortions in Hemispherical Thermostamping for Unidirectional Thermoplastic Composites

This work aims at analyzing process‐induced distortions (PIDs) for unidirectional thermoplastic composites in the thermoforming process and evaluating the effects of fiber reorientations and stamping‐induced stresses. Hemispheres with symmetric and asymmetric lay‐ups are manufactured by the thermostamping process. A simulation method of PIDs is developed and a constitutive model is presented to reflect fiber reorientations in thermostamping. Additionally, stamping‐induced stresses are embedded in the simulation. Good correlation is obtained between the prediction and experimental results. Regarding hemispheres with the symmetric lay‐up [0/90/0/90]S, stamping‐induced stresses play a dominant role in PIDs and determine the deformation trend; fiber reorientations contribute approximately 76% to thermal distortions. Stamping‐induced stresses could be neglected in the PID prediction for the investigated three different asymmetrical lay‐ups, and contributions made by fiber reorientations range from 18% to 28%. Both fiber reorientations and stamping‐induced stresses are significant factors for PIDs for unidirectional thermoplastic composites in the thermoforming process. POLYM. COMPOS., 40:1786–1800, 2019. © 2018 Society of Plastics Engineers

Experimental characterization and analysis of fiber orientations in hemispherical thermostamping for unidirectional thermoplastic composites

Nowadays there is seldom literature concerning in-plane fiber orientation variations in the thickness direction for composites after thermostamping. In present work, cross-section microscopy observations were carried out to characterize fiber orientations at different positions and in different plies for thermoformed unidirectional reinforced thermoplastic composite hemispheres. It is found that there is no fiber angle change in the thickness direction near the vertex; fiber angles of medial plies are lower than that of lateral plies in the middle region; fiber angles decrease from the outside ply to the inside ply on the hemisphere edge. Thermostamping simulations were performed and a constitutive model was applied with fiber reorientations considered. It is shown that embedding fiber reorientations and non-uniform temperature field of the blank into the constitutive model has an important influence on fiber orientations in simulation. The average condition of fiber orientations could be reasonably forecasted at different positions. Simulation shows agreements with experiments on distribution trends of in-plane fiber orientation variations through the thickness near the vertex and on the hemisphere edge. However, simulation failed to describe a distribution feature of in-plane fiber orientation variation through the thickness: fiber angles in medial plies are smaller than that in lateral plies.

A novel tab for tensile testing of unidirectional thermoplastic composites

This study proposes a novel tab for the tensile testing of unidirectional (UD) carbon fiber-reinforced thermoplastic polyamide 6 (CF/PA6) to reduce stress concentration at the tab end. The length of the new tab is identical to that of the UD CF/PA6 specimen to avoid a sudden change in geometry that is typically observed in a conventional tab end. Additionally, three types of UD CF/PA6 laminates from different manufacturers are used to fabricate tensile specimens. Specimens with a new tab and a conventional nontapered tab are tested under quasi-static tension. Finite element simulations of tensile testing are also performed for specimens with two types of tabs. The experimental and simulation results demonstrate that the newly designed tab reduces the stress concentration at the tab end by approximately 10% and improves the estimates of tensile strength for the UD thermoplastic composite laminates.

Compressive failure mechanism and strength of unidirectional thermoplastic composites based on modified kink band model

A modified kink band model for compressive failure in fiber direction of fiber reinforced composite polymers was investigated and established in this study. Practically modifying the previous kink band model by logically applying a quadratic yield function combining transverse tensile and shear stress to the kinking instability, the modified model was able to determine the compressive strength along with the kink band failure angle theoretically. Especially in case of unidirectional carbon fiber reinforced thermoplastic composites, the compressive test procedure the authors have developed and its results revealed that the kink band failed in out-of-plane direction with each different inclined angle depending on temperature. From the correlation of the observed kink band failure angle and the evaluated compressive strength affected by temperature condition, the modified kink band failure formula was validated.

Thermal Modeling in Composite Transmission Laser Welding Process: Light Scattering and Absorption Phenomena Coupling

In previous studies [1, , we have presented a detailed formulation of a macroscopic analytical model of the optical propagation of laser beams in the case of unidirectional thermoplastic composites materials. This analytical model presented a first step which concerns the estimation of the laser beam intensity at the welding interface. It describes the laser light path in scattering transparent composites (first component) by introducing light scattering ratio and scattering standard deviation. The absorption was assumed to be negligible in regard to the scattering effect. In this current paper, in order to describe completely the laser welding process in composite materials, we introduce the absorption phenomenon in the model, in the absorbing material (second component), in order to determine the radiative heat source generated at the welding interface. Finally, we will be able to perform a three dimensional temperature field calculation using a commercial FEM software. In laser welding process, the temperature distribution inside the irradiated materials is essential in order to optimize the process. Experimental measurements will be performed in order to valid the analytical model.

Analytical and numerical modeling of light scattering in composite transmission laser welding process

Analytical and numerical models for laser beam scattering in the thermoplastics composites are presented. The numerical model is based on Ray Tracing, an optical method to compute the optical propagation of laser beams in inhomogeneous media with spatially varying dispersion of the refractive index. In this study, only the case of unidirectional thermoplastic composites (UD) is presented. During transmission laser welding process, a divergence of the laser beam is observed in the first part (transparent in the laser wavelength) due to the internal refraction at the microscopic scale of the beam at each matrix-fiber interface. At the macroscopic scale, this phenomenon leads to the light scattering of the laser beam in this heterogeneous media. Under these conditions, a modeling of the propagation of a laser beam appears essential. An analytical model compared to numerical model is proposed, that enables to simulate and to optimize the laser source at the welding interface. This model provides a good estimation of the laser beam intensity profile at the welding interface (radiative heat well defined) and the beam widening. Those steps are necessary to describe the heat source in the laser welding process thermal simulations. The distribution of the heat source at the interface is one of the most important parameter in the description of the laser welding process.

A novel processing technique for thermoplastic manufacturing of unidirectional composites reinforced with jute yarns

Abstract This paper primarily investigates the fabrication process of long-fibre reinforced unidirectional thermoplastic composites made using jute yarns (both untreated and treated). Tubular braiding technique was used to produce an intermediate material called “microbraid yarn” (MBY) with jute yarn as the straightly inserted axial reinforcement fibre and polymer matrix fibre being braided around the reinforcing jute yarns. Microbraid yarns were then wound in a parallel configuration onto a metallic frame and compression molded to fabricate unidirectional composite specimens. In this study, two types of polymeric materials (biodegradable poly(lactic) acid and non-biodegradable homo-polypropylene) were used as matrix fibres. Basic static mechanical properties were evaluated from tensile and 3 point bending tests. Test results were analyzed to investigate the effects of molding temperature and pressure on the mechanical and interfacial behaviour. For the unidirectional jute fibre/poly(lactic) acid (PLA) composites, the results indicated that the molding condition at 175 °C and 2.7 MPa pressure was more suitable to obtain optimized properties. Improved wettability due to proper matrix fusion facilitated thorough impregnation, which contributed positively to the fibre/matrix interfacial interactions leading to effective stress transfer from matrix to fibre and improved reinforcing effects of jute yarns. For the jute/PP unidirectional composites, specimens with only 20% of jute fibre content have shown remarkable improvement in tensile and bending properties when compared to those of the virgin PP specimens. The improvements in the mechanical properties are broadly related to various factors, such as the wettability of resin melts into fibre bundles, interfacial adhesion, orientation and uniform distribution of matrix-fibres and the lack of fibre attrition and attenuation during tubular braiding process.

Viscoplastic Analysis of the Off-axis Rate-dependent Inelastic Behavior of Unidirectional AS4/PEEK

Using the finite element program generator (FEPG), the three-dimensional viscoplasticity model is introduced into this finite element automatic generating system. The corresponding element subroutine is developed, which is verified by comparing the finite element computing results with the experimental ones. With the generated procedure the monotonic tensile stress–strain curves are obtained under the strain rate of 0.1 s 1for the off-axis angles 5, 10, 15, 30, 45, 60, 75, and 90, respectively. The above stress versus strain curves show marked nonlinearity for the off-axis angles in the range 10 < <60. Furthermore, in order to observe the rate-dependent nonlinear stress–strain behavior for unidirectional thermoplastic composites of different off-axis angles, we have acquired the stress–strain curves under the strain rate of 1 10 5, 1 10 3, and 1from which we can conclude that the unidirectional AS4/PEEK composites have distant rate-dependent behavior when the stress extends some values which are different especially for the off-axis angles.

Fabrication Mechanical Properties of Unidirectional Jute/PP Composites Using Jute Yarns by Film Stacking Method

This paper mainly focuses on the fabrication process of long fibre reinforced unidirectional thermoplastic composites made using both natural (untreated) treated jute yarns. Jute yarns were wound in layers onto a metallic frame. Polypropylene films were inserted between these layers and compression moulded to fabricate unidirectional jute/PP composite specimens. Static mechanical properties were evaluated from tensile three point bending tests. Pre- post-failure examination were carried out on the test specimens using optical scanning electron microscopy to analyse the test results and investigate the correlations between their impregnation state, processing conditions, mechanical performances and fracture morphologies. For the unidirectional jute/PP film-stacked composites, the results indicated that the processing condition at the moulding temperature of 160°C and moulding pressure of 2.0 MPa for 15 min was ideally suited to obtain optimized properties. Improved wettability of resin melts due to complete matrix fusion at this processing condition facilitated thorough impregnation with minimum microstructural imperfections (microvoids) being generated. Jute/PP composites that contained treated jute yarns have shown superiority in tensile bending properties. Jute yarns polished or coated with PVA/PP (polyvinyl alcohol/polypropylene) must have contributed positively to fibre/matrix interfacial interactions leading to matrix to fibre effective stress transfer, thereby improving their reinforcing effects. Tensile strength and modulus of PP resin increased by approximately 285% and 388%, respectively, due to 50 wt% reinforcement by natural jute yarns. Further improvements in strength and modulus were achieved by approximately 14% and 10%, respectively, when treated yarns were used . The maximum bending stress modulus of jute/PP composites containing untreated yarns were approximately 190% and 460% higher than those of the virgin PP materials, and bending properties were improved by further 11% and 23%, respectively, due to coating treatments on the yarn surface.

The mechanical properties of unidirectional thermoplastic composites manufactured by a micro-braiding technique

‘Micro-braided yarn' has been proposed as a means of overcoming the difficulty of manufacturing long-fiber-reinforced thermoplastic composites. Micro-braided yarn is a processing material system that may be used for complex-shaped composites such as textile structural composites. In this study, unidirectional glass-fiber/PA6-Nylon composites manufactured from micro-braided yarns were molded in order to investigate the effect of the micro-braiding structure and processing conditions on impregnation and mechanical properties. The void content was determined in order to examine the quality of impregnation and this was found to decrease with increasing holding time when the pressure was applied to the preform. Axial and transverse bending tests were carried out. The axial-bending strength was well correlated with the void content. However, the transverse bending strength was affected by both the void content and the dispersion of the glass-fiber bundles.

Stress Rupture of Unidirectional High Performance Thermoplastic Composites in End-Loaded Bending at Elevated Temperatures. Part I: Experimental Characterization of the Failure Mode

Macro- and micro-observations of unidirectional thermoplastic composites under end-loaded quasi-static bending have been made. The presence of stable microbuckles has been identified on the compressive side of the specimen. The formation of the microbuckles has been videotaped. Frames of this film are examined and the failure process is identified. The stress rupture of these specimens at elevated temperatures in bending seems to be driven by the formation and propagation of microbuckles. Videotaping from a different angle also gives information on the change of shape of the bent specimen in the oven as a function of time. These unique data form the basis for an understanding of the stress rupture of unidirectional thermoplastic composites in bending that was not previously available.

Quantitative image processing analysis of composite materials

AbstractQuantitative digital image processing has been used to analyze unidirectional thermoplastic composites. The fiber volume fraction is determined by the template matching method and compared with the conventional thresholding method. A combination of skeleton and edge cutting is used to obtain the centers of the fibred in the image. The Voronoi distances are calculated from the result. The Hough transform is applied to the fiber orientation in conjunction with thresholding by the line length in the image. To describe the fiber orientation, characteristic function are defined in the width and thickness directions, which allow a much more accurate and effective description of the fiber distribution and orientation than other methods. Advantages of the template method are that similar results to the threshold method can be obtained as thresholding with a lower magnifications, resulting in an improved statistical sample with a reduced number of photomicro‐graphs.

Cyclic Plasticity in AS4/PEEK Composite Laminates

The cyclic stress-strain behavior in notched and unnotched [±45]3s AS4/ PEEK thermoplastic composite laminates was studied. A plasticity model for cyclic deformation developed for unidirectional thermoplastic composites was used to describe the behavior of each lamina, and the laminate behavior was obtained by using the classical lamination theory. Model predictions of stress-strain curves were compared with ex perimental results.