In-plane Fiber Research Articles

Fractographic study into the effect of drilling damage on bearing mechanisms and performance in Carbon-Fibre epoxy composites

With the widespread adoption of polymer composites in primary structures, understanding and prediction of the performance of composite to metal hybrid joints is now critical to engineering design of transport structures. This work investigated the damage processes associated with bearing failure of such composite joints, for both pristine holes and holes damaged during drilling. An aerospace grade composite was drilled under three different conditions, tested to failure under quasi-static double bearing loading, and then characterised using fractographic techniques. In the pristine condition, the initial damage process was 0° longitudinal splitting tangential to the lateral extents of the hole which then dictated the extent of the subsequent bearing damage development. Beneath the bearing face of the hole inclined lines of in-plane microbuckled fibres had developed whilst beyond the constraint of the washer there was considerable delamination and massive out-of-plane fibre microbuckling. As the degree of drilling damage increased, 0° longitudinal split development was inhibited, and the local pre-existing damage at the periphery of the hole had extended into the bearing damage zone, directly initiating out-of-plane fibre microbuckling. Consequently, the bearing damage zone exhibited irregular distributions of fibre microbuckles, both across the thickness and depth beneath the bearing face of the hole. The observations in this work provide a means to validate predictive models and offer potential routes to improve bearing performance and the tolerance of laminates with drilling damage when under bearing loads.

An experimental approach to reproduce in-plane fiber waviness in thermoplastic composites test coupons using a reverse forming method

In-plane fiber waviness is one of the defects that can occur from the stamp-forming process of thermoplastic composite (TPC) parts. The influence of this defect on the mechanical performance of multidirectional composites is not yet fully understood. The main challenge in determining the influence on mechanical properties lies in reproducing the waviness in test coupons that can subsequently be subjected to testing. This paper describes an experimental approach to reproduce representative in-plane waviness defects, specific for TPC, by reverse-forming of V-shape parts of various bend angles and inner radii. Characterization results show that this method enables the manufacturing of localized in-plane waviness in flat 24-ply quasi-isotropic C/PEEK composites with no voids. Furthermore, laminates having varying levels of maximum waviness angle ([Formula: see text]), between 14° to 64°, were successfully produced in this work. By comparing the [Formula: see text] value with the examples of industrial stamp-formed parts, it can be concluded that the developed coupon manufacturing method can reproduce waviness from TPC part production reasonably well. Finally, all of the produced laminates have defective region lengths smaller than 20 mm, localized within a predefined location which makes them well suited for standard compression test coupons.

Mechanical properties of a novel Tri-directional carbon-flax-aramid fiber reinforced composite

A novel Tri-directional Carbon-Flax-Aramid fiber reinforced composite (3D-CFAFRP) was proposed, manufactured and tested in this study, which was designed based on the idea of better tailoring the strength, stiffness, toughness and weight requirements, adopting three different types of fibers to reinforce the matrix along three orthogonal directions. The in-plane and interlaminar mechanical properties of 3D-CFAFRP composites were studied experimentally, and were compared with those of the carbon fiber orthogonally laminates (2D-CFRP). Results showed that the Mode I and Mode II interlaminar fracture toughness of 3D-CFAFRP composites were enhanced by 6.2 times and 64.77%, respectively. The interlaminar fractured surfaces indicated that the preventing of the crack propagation by the needle aramid yarns and the hierarchical failure modes due to the unique microstructure of the flax fibers contributed to the improved interlaminar fracture toughness. In addition, the tensile strength of 3D-CFAFRP composites was about 33% higher than that of 2D-CFRP composites due to the existence of the needle aramid yarns in the thickness direction. The flexural and compressive strength of 3D-CFAFRP composites were 14.09% and 7.6% lower than those of the 2D-CFRP composites, respectively, which was attributed to resin-rich region, in-plane fibers distortion and damage introduced by stitching. Nevertheless, the specific in-plane strength and modulus of 3D-CFAFRP composites were all higher than or similar to that of 2D-CFRP composites due to its lower density. Furthermore, 3D-CFAFRP composites also possessed lower cost and eco-friendly advantages compared with 2D-CFRP composites.

Effect of Ø 0.11 mm Z-pinning on the properties of composite laminates via an ultrasound guided insertion process

This paper develops an ultrasound guided Z-pinning process, which can insert finer Z-pins of Ø 0.11 mm (in comparison with traditional Ø 0.28 mm - 0.50 mm) with minimal in-plane damage and substantially improve the interlaminar performance of Z-pinned composite laminates. The experimental results show that for unidirectional fibre/epoxy laminates reinforced by 0.1 vol% of Ø 0.11 mm carbon Z-pins, the in-plane strength reduction is only 1.31% in tension and 3.46% in compression, respectively, while the Mode-I fracture toughness can be improved by 11 times in comparison with the unpinned specimens. The results can be attributed to the small damage to the microstructures of the pinned area, where long and narrow resin-rich zone with small in-plane fibre waviness and neglectable out of plane fibre crimp are represented. The interlaminar toughening mechanism of fine Z-pinned laminates is elucidated by the bridging law with the establishment the DCB testing model based on the traditional Z-pin reinforcement mechanism.

Effect of in-plane fiber waviness defects on the compressive properties of quasi-isotropic thermoplastic composites

The influence of in-plane fiber waviness defects on the compressive properties of quasi-isotropic (QI) carbon polyether-ether-ketone (C/PEEK) composites was investigated experimentally. Specimens with localized waviness were manufactured using a stamp forming process resulting into laminates featuring multiple wavy plies, i.e. from one to three wavy 0° plies in a 24-ply QI composite and a range of maximum waviness angle between 23° to 60°. No significant influence of the waviness was found on the global laminate stiffness. Compression tests coupled with high-speed camera monitoring were performed to study the failure process. It was confirmed that the waviness defects act as a trigger for the initiation of damage, predominantly by the kinking mechanism, resulting into an early failure and significantly lower ultimate strength than the baseline when loading in 0° direction. Furthermore, it was found that all specimens with waviness and with the same layup have a similar strength, indicating that the maximum waviness angle within the range studied in this work did not significantly influence the ultimate compressive strength. However, the presence of waviness in multiple plies clearly affected the strength. It was found that the ultimate compressive strength decreased proportionally to the percentage of plies oriented in the loading direction that is wavy.

Crush characteristics of flat-plate discontinuous carbon composites

This study presents an investigation of the effect of fiber length, fiber mass content, in-plane fiber distribution, plate thickness, and loading rate on the energy absorption mechanism of discontinuous carbon fiber composite flat-plate specimens utilizing the crush property isolation technique. Similar crushing morphology is obtained for flat-plate testing as for discontinuous carbon fiber tubes. Fiber length and fiber mass content showed only a minor effect on the specific energy absorption (SEA). Results obtained for the in-plane fiber distribution showed to be consistent. SEA-values increased substantially with increased plate thickness. Loading rate dependency could be observed as a decrease in SEA when increasing plate thickness.

Tensile strain induced texture evolution in a Ni–Mo alloy with extremely fine nanotwinned columnar grains

In the present paper, plastic deformation in an electrodeposited nanotwinned columnar grained Ni–Mo film with an extremely fine columnar grain size of 8 ± 1.7 nm and a twin thickness of 1.3 ± 0.5 nm was investigated via uniaxial micro-tensile tests at room temperature. We found significant detwinning and an evident texture evolution from a randomly-oriented in-plane fiber texture to a highly-oriented out-of-plane texture in the alloy upon tensile straining when tensile direction was oriented in parallel to the twinning plane. Interface energy minimization characterized by the substitution of high angle grain boundaries by low angle grain boundaries and incoherent twin boundaries constitutes the driving force of the texture evolution. Tensile plastic deformation can be attributed to the activity of threading dislocations, which not only contributes to a high yield strength of 2.11 GPa, but also results in in-plane lattice rotation with noticeable deformability.

Thickness effect of anisotropic conductive behavior of carbon fiber/polyetheretherketone unidirectional tape

The conductive anisotropic polymer composites (ACPC) have important applications like field emission devices and electronic sensors. Conventional ACPC are difficult to obtain large conductive anisotropy coefficients and present limited mechanical properties. In this paper, novel technology integrated carbon fiber (CF) spread, surface modification with stacking thermoplastic film was used to prepare CF reinforced polyetheretherketone (CF/PEEK) composite unidirectional tape with thicknesses of 0.04 mm and 0.1 mm. CF/PEEK composite unidirectional woven cloth was prepared with PEEK fiber was used as a binder weft, and CF/PEEK composite unidirectional laminate was prepared by thermoforming process. The in-plane and the thickness directional resistivities of unidirectional laminate were measured by digital multimeter and in-plane electron mobility was tested by Hall effect system. The fiber arrangement within in-planes along the fiber direction and the thickness direction of CF/PEEK composite unidirectional laminate were observed with an ultra-depth microscope. The results show that the in-plane (fiber direction to transverse direction) conductivity ratio of ultra-thin CF/PEEK composite unidirectional laminate reaches to 377, while conductive ratio in the transverse direction to the thickness direction is close to 1, indicating that thin CF/PEEK composite unidirectional laminate presents transverse isotropic electric performance. The results of electron migration also indicate in-plane huge anisotropic conductivity. The results are important for CF/PEEK composite to be used in field emission devices and electronic sensors.

Tool-ply interaction in the formation of waviness during C/PEEK consolidation

It is well established that fiber waviness in continuous fiber-reinforced polymer composites is a defect rather than an aesthetic flaw, as it potentially leads to a severe knockdown of the mechanical properties. Consequently, it is desirable to minimize or prevent fiber waviness during processing. This research explored experimentally the influence of tool material, release media and laminate size on the formation of in-plane fiber waviness during C/PEEK consolidation. The formed waviness was quantified and correlated to the measured tool-ply friction coefficients and thermal coefficients of expansion of the tool plates. This research provides additional evidence that tool shrinkage is the driving force for waviness formation in C/PEEK laminate consolidation. Furthermore, the formation of waviness depends on the balance between the coefficient of thermal expansion of the tool, the tool-ply coefficient of friction and the laminate size.

Kinematic and mechanical response of dry woven fabrics in through-thickness compression: Virtual fiber modeling with mesh overlay technique and experimental validation

The through-thickness compressive behavior of fabric reinforcements is crucial in liquid composite molding manufacturing processes. Predictive simulations of the compressive response are thus necessary to enable a virtual processing workflow. These are complex however, as the compressive behavior of the reinforcement fabrics is non-linear. Altough virtual fiber modeling has proven to be a strong kinematical tool, it cannot predict the compressive response due to the lack of bending stiffness in the virtual fibers. Here, we describe a solution that enables predictive compressive simulations through hybrid virtual fibers. It is based on an overlay mesh-element technique, combining both (i) finite elements that determine the in-plane fiber properties as well as (ii) finite elements that determine out-of-plane fiber bending. Using these hybrid virtual fibers, the through-thickness compression of a twill woven fabric ply is simulated and experimentally validated using both μCT-based as compliance-based measurements. Excellent agreement between simulation and experiment is obtained for the right set of input parameters.

Thermal conductivity of a thick 3D textile composite using an RVE model with specialized thermal periodic boundary conditions

Finite element analysis is performed to virtually measure homogenized thermal conductivity of a thick 3D woven textile composite (T3DWC). Temperature-dependent thermal and mechanical properties of constituents are considered for the measurements over a wide range of temperature. A two-step homogenization approach is adopted here to simplify the analysis at the microscopic level without losing heterogeneity of the material at the macroscopic scale. First-step homogenization is carried out at a tow level using an analytical homogenization scheme. Fiber tows are homogenized and assigned with effective elastic and thermal properties. The solid tows are then implemented into a representative volume element considering the unique in-plane periodic fiber architecture of the thick composite material. Due to the unique in-plane periodicity, conventional periodic boundary conditions for thermal and mechanical loading conditions are reformulated. Anisotropic thermal conductivity of T3DWC is obtained from the second-step homogenization based on virtual thermal tests performed at ambient to elevated temperatures.

The Effect of Fiber Waviness on the Residual Stress State and Its Prediction by the Hole Drilling Method in Fiber Metal Laminates: A Global-Local Finite Element Analysis

In this paper, fiber waviness, as one of the most frequently occurring defects in fiber reinforced composites, is numerically investigated with regard to the formation of residual stresses in fiber metal laminates. Furthermore, the prediction of the residual stress state in the thickness direction by means of the simulated hole drilling method is studied. To this regard, a global-local finite element analysis based on the submodel technique is presented. The submodel technique essentially consists of two governing steps: In the first step, a global model is first utilized to calculate and analyze the residual stress distribution and deformation in the intrinsically joined hybrid structure. Effective cure-dependent thermo–elastic properties predicted by a numerical homogenization procedure were used to simulate the curing-process and analyze the residual stresses state. However, the dimension of the intrinsically manufactured hybrid plate is large compared to the diameter of the drilled hole (2 mm), so that a local model is necessary, which provides only a geometric partial portion of the global model. The local model takes the global stress state into account and is subsequently used to simulate the incremental hole drilling method with a refined mesh discretization. The production-related fiber waviness is modeled by an element-wise orientation approximating a sinus function. In order to validate the global-local modeling approach, a comparison between numerical results and experimental data from literature is presented. The comparison between global residual stress state (global model) and the simulated hole drilling method (local model) is used to assess the applicability and reliability of the hole drilling method in case of fiber waviness. It is found that an in-plane fiber waviness leads to a rather low variance of residual stresses over thickness. In case of an out-of-plane fiber waviness, oscillating residual stress fields occur over the entire thickness along the fiber direction. Moreover, the current limits of the incremental hole drilling method could be pointed out by the presented investigations. It is seen that the simulated results of the incremental hole drilling method are sensitive to waviness, even if the amplitude-wavelength-ratio is small. Without further adjustment of the calibration coefficients the oscillating stress and strain fields lead, in particular fiber waviness in thickness direction, to unreliable predictions. For the experimental application it can be concluded that the specimens have to be carefully examined with regard to fiber waviness.

Study of flow-induced fiber in-plane deformation during high pressure resin transfer molding

High Pressure Resin Transfer Molding (HP-RTM) is a new variant of composite Resin Transfer Molding (RTM) process that enables a short cycle time and a high composite strength to weight ratio, thus presents a great potential for fabricating automotive structural parts. Due to the high injection pressure, fiber-tow washout is becoming one of the major defects which impact the properties of composite materials. To predict and mitigate the fiber-tow washout problem, approaches of both experimental process optimization and computational prediction are essential. In this paper, an experimental study of fiber-tow washout is undertaken to determine the flow injection limits beyond which the preform deformation can be observed at various fiber volume fractions. A feasibility map is developed for a specific fabric and resin combination. It provides a means to determine the injection rates and fiber volume fractions to fabricate a quality part with minimal in-plane fiber washout due to the hydrodynamically flow-induced force during the HP-RTM process.

Glass/Brass-270 Wire-Mesh Nanosilica Toughened Epoxy Composite: Mechanical, Impact and Fatigue Behaviour

High toughness epoxy resin hybrid composites were prepared using nanosilica, glass fibre and brass wire-mesh. The main aim of this research was to study the effect of adding nanosilica particle along with glass fibre and brass wire-mesh in load-bearing properties of the composite. The in-plane glass fibre, brass wire-mesh and nanosilica were surface-treated using 3-Aminopropyletrimethoxysilane. The composites were prepared using the hand lay-up method. The tensile result shows that the addition of 1 vol.% of nanosilica particle in glass-brass wire mesh reinforced epoxy composite gives improved results than other composites. The fracture toughness results revealed highest fracture toughness of 36.5 MPa $$ \sqrt{\mathbf{m}} $$ and high penetration resistance with energy absorption for composite contains 1.0 vol.% of nanosilica along with glass/brass. The fatigue behaviour of composite shows a higher life count of 43,662. In all properties, the addition of nanosilica modifies the values significantly. This high toughness and impact resistance brass wire-mesh toughened epoxy resin composite could be used in automotive, defence, sports components, domestic appliances and structural body applications.

Investigation into the tensile properties of ISO-401 double-thread chain-stitched glass-fibre composites

Interlaminar stitching has proven to be an effective through-thickness reinforcement technique for laminated composites but a major drawback is the negative effect it has on the in-plane tensile properties. This study aims to optimise stitching for composites to reduce the associated negative effects by introducing a new stitch type to composite laminates. The double-thread chain-stitch (ISO-401) has shown great utility in the textile industry but has not yet been applied as through-thickness reinforcement to composites. In this paper, tensile properties of glass fibre composite laminates stitched with ISO-401 double-thread chain stitch have been examined, as stitching reduces in-plane strength while improving damage tolerance. Three variations of the ISO-401 geometry were employed by varying the stitch junction position and each position was applied at two stitch densities by varying the stitch pitch at 3 mm and 4 mm. The composites were manufactured using E-glass 2 × 2 twill fabric and a Bisphenol-A type epoxy resin, stitching was performed with core-spun tex 24 polyester thread. The experimental data demonstrated the influence of ISO-401 junction position and stitch density on the composite tensile properties. Furthermore, adjustment of the junction position was found to affect the size of stitch-induced resin pockets and severity of in-plane fibre misalignment.

Compression properties of z-pinned carbon-fibre/epoxy laminates reinforced with circumferentially notched z-pins

This paper represents results of the investigations on in-plane compression properties of unidirectional carbon-fibre/epoxy laminates with a three-dimensional z-pin reinforcement with circumferentially notched z-pins. Different notch designs - rectangular, circular, triangular and sinusoidal - were examined to identify the impact on the resulting compression strength and modulus. From the determinations it can be concluded that notching of z-pins has a significant impact on the compression strength, no matter of the notch design. Due to the changes in microstructure caused by the implementation of notches at the z-pin surface, eg partially lower in-plane fibre waviness as well as reduced swelling behaviour, the compression strength values are significantly higher compared to those of laminates reinforced with unnotched pins. With increasing notch depth and decreasing notch distances the compression strength values are rising. In contrast, the Young's modulus of the z-pinned laminate is not affected by notching of the z-pins. An influence of the notch design, depth and distance of circumferential notches on the Young's modulus was not found with the investigation carried out.

Evaluation of detectability of differential type probe using directional eddy current for fibre waviness in CFRP

This paper describes the detectability of eddy current testing (ECT) using directional eddy current for detection of in-plane fibre waviness in unidirectional carbon fibre reinforced plastic (CFRP) laminate. Three different types of probes, such as circular driving, symmetrical driving and uniform driving probe, were proposed, and the waviness angle was extracted from the contour map of the ECT signal by applying a Canny filter and a Hough transform. By comparing both the waviness angle estimated by ECT and that obtained by an X-ray CT image, the standard deviation (precision) and root mean square error (accuracy) were evaluated to discuss the detectability of these probes. The directional uniform driving probe shows the best detectability and can detect fibre waviness with a waviness angle of more than 2° in unidirectional CFRP. The probe shows a root mean square error of 1.90° and a standard deviation of 4.49° between the actual waviness angle and the angle estimated by ECT. This article is part of the theme issue 'Advanced electromagnetic non-destructive evaluation and smart monitoring'.

Effect of fiber exclusion in uniaxial tensile tests of soft biological tissues

The effect of the exclusion of the compressed fibers in the identification of material parameters from uniaxial tensile tests on two orthogonal strips is investigated. The micro-structurally based constitutive model with two dispersion parameters developed by Holzapfel and his colleagues is utilized in the study. A new exclusion method, based on the coefficient reflecting the percentage of stretched fibers, is proposed. The material parameters are identified by using experimental data from 30 uniaxial tensile tests (5 donors, 6 strips per donor) and a genetic algorithm code that is capable to find the optimal set of parameters. The contraction of the strip width computed by using the hyperelastic model with the identified material parameters is compared to the experimental data for two human aortas (one from literature and one experiment, specific for this study), in order to show the accuracy of the identified model. The complex behavior of the thickness deformation of the strip is also obtained and compared to the experimental data derived from in-plane measurements and the incompressibility condition. Results show that the in-plane fiber exclusion is appropriate for aortic material characterization with uniaxial tensile tests, reducing very significantly the computational cost. At the same time, thickness growth of strips during uniaxial tests is possible, depending on fiber dispersion and orientation.

Fiber reorientation in hybrid helicoidal composites

Naturally occurring biological materials with stiff fibers embedded in a ductile matrix are commonly known to achieve excellent balance between stiffness, strength and ductility. In particular, biological composite materials with helicoidal architecture have been shown to exhibit enhanced damage tolerance and increased impact energy absorption. However, the role of fiber reorientation inside the flexible matrix of helicoid composites on their mechanical behaviors have not yet been extensively investigated. In the present work, we introduce a Discontinuous Fiber Helicoid (DFH) composite inspired by both the helicoid microstructure in the cuticle of mantis shrimp and the nacreous architecture of the red abalone shell. We employ 3D printed specimens, analytical models and finite element models to analyze and quantify in-plane fiber reorientation in helicoid architectures with different geometrical features. We also introduce additional architectures, i.e., single unidirectional lamina and mono-balanced architectures, for comparison purposes. Compared with associated mono-balanced architectures, helicoid architectures exhibit less fiber reorientation values and lower values of strain stiffening. The explanation for this difference is addressed in terms of the measured in-plane deformation, due to uniaxial tensile of the laminae, correlated to lamina misorientation with respect to the loading direction and lay-up sequence.

Influence of Fiber Deviation on Strength of Thin Birch (Betula pendula Roth.) Veneers.

The currently pursued implementation of wood into novel high performance applications such as automotive parts require knowledge about the material behaviour including ultimate strength. Previous research has shown that fiber deviation seems to be the dominating factor influencing the strength of thin veneers. This study aims to further investigate and quantify the influence of fiber deviation in two dimension and different hierarchical levels on the tensile strength of thin birch veneers. The fiber deviation in- and out-of-plane as well as the micro fibril angle were assessed by means of wide-angle X-ray scattering. Tensile strength was determined in laboratory experiments. Results show a high variability for in-plane fiber deviation mainly constituted by knots and other growth influencing factors. Pearson correlations between strength and fiber deviation ranged from −0.594 up to −0.852. Best correlation (r = −0.852) was achieved for maximum in-plane fiber deviation directly followed by a combined angle of in- and out-of-plane fiber deviation (r = −0.846). Based on the results it was shown that fiber deviation in- and out-of-plane is the dominating factor influencing ultimate tensile strength of thin birch veneers. Further research in regard to non-destructive strength prediction is necessary.