Tensile Strength Of Unidirectional Composites Research Articles

Reducing stress concentrations in static and fatigue tensile tests on unidirectional composite materials: A review

The longitudinal tensile strength is an important basic property of unidirectional (UD) composites or plies that often governs failure in multidirectional laminates. The standard tensile testing methods almost always result in sample failure near the grip. Finite element analyses revealed the presence of multiaxial stress concentrations, including longitudinal, transverse, and shear stresses in the tabbed section of the standards’ recommended design. There are still unknowns and uncertainties about the causes of these stress concentrations and ways to eliminate them. A major challenge is obtaining acceptable failure within the gauge section and yielding the highest tensile strength of UD composites. This paper reviews the different methods for performing quasi-static and fatigue tensile tests on UD composites. The primary sources of stress concentrations and the parameters that affect them are reviewed using the available experimental and modeling investigations. We survey the effects of the different specimen and end tab designs as well as test setups on quasi-static and fatigue loading. Specific proposals are made for each of the discussed parameters for more reliable results.

Crystal structures of polyamide 6 at the interphase and around carbon fiber and mechanical properties of their composites

This study reports the effect of the polyamide 6 (PA6) crystal structure at the interphase and around the carbon fiber (CF) on the mechanical properties of CF-reinforced PA6 composites. The transverse tensile properties of the unidirectional composites and the four-point bending properties of the quasi-isotropic composites were investigated using three types of CFs. Raman spectra of PA6 were measured to characterize the crystal structures of PA6 at the interphase and around the CFs. The results showed that higher α-crystal (the most thermodynamically stable crystal) content of PA6 at the interphase compared to β-crystal (mesophase) and γ-crystal (thermodynamically metastable) contributed to greater transverse tensile strength of unidirectional composites. Furthermore, it was demonstrated that the flexural strength of the quasi-isotropic composites increased in the case of small variations in the distribution of each crystal structure (α-, β-, and γ-crystal structures) around the CFs. This study provides a framework for the PA6 crystals around CFs to reflect the superior properties of CFs in PA6/CF composites.

Machine learning-assisted modelling of stress concentration factor of unidirectional fiber composites for predicting their tensile strength

Significant variations in the tensile strength of unidirectional (UD) fiber-reinforced composites are frequently observed due to randomness in the fiber arrays. Herein, we propose a novel method for predicting tensile strength capable of quantifying uncertainty based on a new recurrence relation for fiber fracture propagation and a determination algorithm for the fracture sequence for random fiber arrays (RFAs). We performed finite element simulations, calculating the stress concentration factor (SCF) for UD composites with various RFAs. Then, we trained an artificial neural network with the obtained SCF data and used it to predict the SCF for composites with an arbitrary RFA. The tensile strength of UD composites was predicted over a range of values, demonstrating that accuracy was superior to conventional prediction methods.

An efficient micromechanics-based finite element model for failure analysis of laminated composites

Accurately predicting ultimate strength of composite laminate is still a great challenge, especially only based on the constituent properties. In this paper, the micromechanics Bridging Model and systematic failure criteria for the constituent materials with emphasis on matrix failures subjected to three dimensional (3D) loads are compiled into a UMAT subroutine for ABAQUS to analyze failures of a laminated composite structure. The main purpose of this research is to greatly reduce calculation costs for loading conditions of in-plane tension or compression by introducing a simplified 3D finite element model based on the Pipes-Pagano theory. In all calculation cases, only the constituent properties together with the transverse tensile strength of unidirectional composite are needed as input data. Later in this paper, sufficient illustrations of 81 different composite laminates composed of 14 material systems are analyzed to confirm the practicability of this model.

Development of the Optimal Shape and Reinforcement Structure of the Specimen for Adequate Determination of the Tensile Strength of Unidirectional Composites

Development of the Optimal Shape and Reinforcement Structure of the Specimen for Adequate Determination of the Tensile Strength of Unidirectional Composites

Predicting strength of a unidirectional composite containing interface crack

In this paper, both analytical Bridging Model and numerical GMC (Generalized Method of Cells) and HFGMC (High-Fidelity Generalized Method of Cells) are applied to analyze failures of a unidirectional (UD) composite with a cracked fiber and matrix interface. Interconnecting the internal stresses in the matrix with those in the fiber by a bridging tensor, the difference between Bridging Model and GMC/HFGMC is in the bridging tensors and the method of determining them. The stress concentration factors (SCFs) of the matrix in the composite before and after the cracked interface are calculated upon the different bridging tensors. Based on the true stresses resulted from the different bridging tensors and the SCFs, off-axial tensile strengths of UD composites are predicted. A total number of 9 different prediction schemes are made. The predicted results are compared with the experimental data of carbon, aramid, and flax fiber composites prepared and measured by ourselves. It is found that the predictions based on the Bridging Model equations provide overall the best agreement with the experiments.

Effect of graphene oxide surface treatment on the interfacial adhesion and the tensile performance of flax epoxy composites

The high stiffness and damping properties of flax fibres promote the integration of biocomposites in structural applications. However, the strength of flax/epoxy composites is still limited compared to glass/epoxy composites. Graphene oxide (GO) has proved to be a promising building block for nanocomposites due to its high toughness, stiffness and tunable interfacial interactions with polymers. This study aims to understand the potential of GO-based surface treatment of flax fibres to modify the interfacial adhesion and tensile performance of flax fibre/epoxy composites. GO-modification improves the interfacial shear strength of elementary flax fibre/epoxy by 43%. The interfacial improvement is also established by the 40% higher transverse bending strength compared to untreated flax/epoxy composites. The tensile moduli of GO-modified flax/epoxy composites are on average 2 GPa higher than for untreated flax fibre/epoxy composites in all strain ranges. The quasi-static longitudinal tensile strength of unidirectional composites is not affected by GO-modification.

Computational evaluation of the effects of void on the transverse tensile strengths of unidirectional composites considering thermal residual stress

To analyze the effects of void on transverse tensile properties of composites considering thermal residual stress (TRS), finite element models with different distribution patterns and shapes for voids are established. Two different void modeling methods, with voids explicitly established and with voids modeled within elements, are compared first. For the method with voids explicitly established, circular, elliptical and arbitrary voids are considered. To reveal the effects of TRS, two different TRS analysis methods are evaluated. Results show that for the models without considering voids and TRS, the interface debonding is the dominant failure mechanism. After considering the voids, the effects of matrix damage on the crack initiation become more important. There are more variations in tensile strengths of models with voids explicitly established than those from the models with voids modeled within elements. It is also found that higher TRS values are obtained when the change of matrix properties with temperature is considered. After considering the TRS, the matrix damage contributes more to the crack initiation and the interface debonding is inhibited. Besides, the TRS results in thermal residual matrix plasticity around the void, which contributes to the premature initiation of crack around the void.

Calculation of Tensile Strength of Unidirectional Composites Based on Probability Theory and Monte Carlo Method

In this paper, Monte Carlo method combined with the random critical-core probability model is proposed to calculate the longitudinal tensile strength of unidirectional composites. This method considers two-dimensional distribution of fibers on the cross-section, while the theoretical analysis method only takes the linear distribution of fibers into account. Using the weakest link principle, the failure probability and average strength of the unidirectional composites are obtained. The results show that the calculated values of longitudinal tensile strength of T300/5208 composite and unidirectional C/C composites agree well with the experimental results.

Effect of Fiber Length on the Tensile Strength of Unidirectional Composites

Effect of Fiber Length on the Tensile Strength of Unidirectional Composites

Analytical model for predicting the tensile strength of unidirectional composites based on the density of fiber breaks

Abstract While analytical fiber fragmentation models following the global load-sharing (GLS) assumption efficiently reproduce the stress strain curves of unidirectional composites loaded in the direction of the reinforcement when the number of breaks is moderate, they completely fail to predict tensile strength. In this paper, we propose that failure takes place when a critical density of breaks, which depends entirely on the constituent properties, is reached. Therefore, we rewrite classic GLS fragmentation models in terms of the linear density of breaks. The critical number of breaks for a set of glass and carbon reinforced polymer composites is extracted from published experimental data and fitted to an empirical law, with good predictive capability. Our approach complements GLS fragmentation models because it identifies the ultimate stress, from which the stress-strain curve given by the model becomes unrealistic.

Effect of stress ratio on fatigue life of GFRP composites for WT blade

Fatigue life of GFRP (glass-fiber reinforced plastic) composites used in wind turbine rotor blades has been evaluated considering the glass fiber orientations. Three different laminate composites with the respective laminating orientation of 0°, ±45°, and 0°±45° were prepared using vacuum infusion method. Tensile properties and S-N curves for these composites were experimentally determined at room temperature. From the tensile tests, it was found that tensile properties were greatly dependent upon the fiber orientation and the tensile strength of unidirectional composite was the largest and bidirectional (45°) composite was the weakest among three composites. The fatigue properties were determined under constant amplitude load control at different stress ratios, R, of 0.5, 0.1 and −0.2. The properties also show the dependency of stress ratios and fiber orientation. The fatigue life diagrams of these three composite were relatively well presented with the double logarithmic S-N curve. The linear slopes of the respective S-N curves for three composite were not greatly different. The fatigue limits for the composites were evaluated and predicted with linear Goodman and Gerber diagrams.

Effects of fluctuation of fibre orientation on tensile properties of flax sliver-reinforced green composites

Plant-based natural fibres are often used as a reinforcing material for environmentally friendly green composites. Especially, the form of slivers of natural fibres is anticipated for increasing their stiffness and strength. However, the sliver structure has fluctuations in fibre orientation, which decreases their mechanical properties. This paper describes the effects of such fibre orientation fluctuation on tensile properties of fibre-reinforced fully green composites. The composites were reinforced with slivers of high-strength flax fibres, for which a fabrication method called ‘direct method’ was applied. To quantify the morphology of the fibre orientation, fibre orientation angles were measured on fine segments, which were divided into 1mm×1mm squares on a photograph of the whole composite surface. Although it is well-known that tensile strength of unidirectional composites decreases with increasing fibre orientation angle, the tensile strength obtained here did not show any appreciable relation to the statistical properties of measured fibre orientation angles such as average and standard deviation. The concept of two-dimensional (2D) autocorrelation was used in the present study to express the degree of similarity between fibre orientation angles in two different local areas. Results show that, if high 2D autocorrelation coefficients occupy more area on a composite surface, then this composite possesses more regular fibre orientation and tends to exhibit higher tensile strength. This tendency is stronger in the composites close to on-axis alignment, whereas it became weak in the off-axis composites angled more than 15° because of shear fracture.

The random crack core model for predicting the longitudinal tensile strengths of unidirectional composites

A perfect evolvement process of random crack cores is presented and a random crack core model for predicting the longitudinal tensile strengths of unidirectional composites is built in this paper. Based on the crack propagation rules, the numerical relationship of the number of random crack cores, evolvement probability of a random crack core evolving to critical size, and failure probability of a unidirectional composite are deduced. With considering some fibers breaks simultaneously and the influenced-length of the random crack cores increasing with the number of broken fibers, evolvement probability algorithms of a random crack core are developed based on the perfect evolvement process. At last, the longitudinal tensile strengths of unidirectional composites are predicted by the random crack core model, and the result shows that the random crack core model is more accurate than the classical theoretical models.

Why interface testing by single-fibre methods can be misleading

The reasons for our interest in the interface are reviewed, and bond-strength sensitive properties are discussed. It is shown that the four direct methods currently used for interface measurement have problems that make their results inconsistent between methods and inaccurate. The failure process has been monitored in glass/epoxy pull-out specimens with very short fibre free lengths, so that crack propagation is stable. This showed that, close to ultimate failure, a very small length of the interface was carrying nearly all the (very high) load. This apparently high interfacial shear strength can be explained on the basis of the recently documented lack of a shear failure mode for most polymers. All four interface tests are centro-symmetric, or nearly so. In this situation, a matrix that cannot fail in shear, must fail in tension. Yet tensile failure is strongly inhibited by the centro-symmetry. Hence shear failure is forced at the interface itself by a tensile-type process. This involves tensile pulling of the polymer over the high points on the fibre. (No surface is perfectly smooth.) The result is thus unduly influenced by the surface roughness of the fibre and the shrinkage pressure, neither of which are directly related to adhesion. Although great progress has been made with fracture analysis of interface failure, this cannot resolve the problem of the unrealistic centro-symmetry of the tests. It is, therefore, recommended instead that direct tests on composites be examined for relevance and usefulness. The most useful method may well be to measure the off-axis tensile strength of unidirectional composites. This has been shown to be more sensitive to interface properties than both the fragmentation test and the pull-out test. Single-fibre tests and fracture analyses of these tests should be used only for basic research into centro-symmetric failure modes.

Tensile failure due to interaction between fibre breaks

A new explanation for tensile strength of unidirectional composites is proposed based on the linking of fibre breaks in adjacent fibres if they are within a critical axial distance. No stress concentrations are included in the model. The probability of obtaining a given number of fibre breaks sufficiently close to interact is estimated by scanning an array of fibres for clusters of broken fibres. Using a simple energy balance approach it is shown that a bundle of failed fibres of a certain critical size can pull out from the rest of the composite by shear failure around the edge of the bundle, and this can be expected to lead to catastrophic fracture. Tensile strengths are obtained which are comparable with experimental data for unidirectional carbon fibre/epoxy, suggesting that it may be the interaction of fibre breaks rather than the stress concentrations associated with the breaks which is critical in determining composite tensile strength.

Tensile Strength of Unidirectional Composites: The Role of Efficiency and Strength of Fiber-Matrix Interface

A micromechanics model has been developed to predict the tensile strength of unidirectional composites. The local stresses are calculated using shear lag analysis, including the effects of interfacial debonding. The tensile strength of the composite is estimated by considering the accumulation of fiber fractures as a function of applied load. A new parameter called ‘efficiency’ of the interface is introduced to account for the effectiveness of load transfer from the matrix to the fiber. A simple scheme is described to estimate this efficiency parameter η using the experimentally measured tensile stiffness in the concentric cylinders model. It is postulated that the interface can be completely characterized by two parameters: interfacial shear strength τi and efficiency of the interface η. Results indicate that the interfacial strength and efficiency can be optimized to maximize the tensile strength of a unidirectional composite. The proposed model was used to predict tensile strength of three different sets of materials that possessed carefully tailored interphase variations. The predicted tensile strengths agree well with the experimental data. The predicted failure modes in these material systems are also consistent with experimental observations.

Micromechanical approach to the tensile strength of unidirectional composites

This paper deals with the strength of dry bundles and unidirectional composites. Tensile tests of dry bundles and unidirectional composites of carbon fibers and alumina fibers are carried out. The strength of a dry bundle is compared with Coleman's theory and it is shown that the strength of a dry bundle can be well predicted by Coleman's theory. As for a unidirectional composite, test data are compared with the Rosen model, the Zweben model, and the modified Zweben model. It has been confirmed that the Rosen model overestimates the strength whereas the Zweben model underestimates it. A modified Zweben model can represent the actual failure pattern, although some subjects are left for future investigation.

Micromechanical approach to the tensile strength of unidirectional composites

This paper deals with the micromechanics of advanced composites. The characteristics of individual fibers, matrix, and the fiber/matrix interface are the most important factors for evaluating fiber-reinforced composite materials from a micromechanical viewpoint. To this purpose, we first developed a testing device with which the load-elongation curve of a monofilament can be obtained. Filament diameter was measured by the aid of laser diffraction. Using these devices, we have tensile-tested several kinds of monofilaments and accumulated data. The strength distribution agreed fairly well with the Weibull distribution. The fiber/matrix interaction was also examined with a single fiber embedded model. By measuring a fragment length, an ineffective length was calculated. This paper will provide the basic data to the second report in which the strength of unidirectional composites is predicted by means of computer simulation.

Evaluation of fiber surfaces treatment and sizing on the shear and transverse tensile strengths of carbon fiber‐reinforced thermoset and thermoplastic matrix composites

AbstractThe mechanical performance of advanced composite materials depends to a large extent on the adhesion between the fiber and matrix. This is especially true for maximizing the strength of unidirectional composites in off‐axis directions. The materials of interest in this study were PAN‐based carbon fibers (XA and A4) used in combination with a thermoset (EPON 828 epoxy) and a thermoplastic (liquid crystal poymer) matrix. The effect of surface treatment and sizing were evaluated by measuring the short‐beam shear (SBS) and transverse flexural (TF) tensile strengths of unidirectional composites. Results indicated that fiber surface treatment improves the shear and trasverse tensile strengths for both thermosetting and thermoplastic matrix/carbon fiber‐reinforced unidirectional composites. A small additional improvement in strengths was observed as the result of sizing treated fibers for the epoxy composites. Scanning electron microscope photomicrographs were used to determine the location of composite failure, relative to the fiber‐matrix interface. Finally, the epoxy composites SBS and TF strengths appear to be limited to the maximum transeverse tensile strength of the epoxy matrix, while the thermoplastic composite SBS and TF strengths are limited by the LCP matrix shear and transverse tensile strengths, respectively.