Mechanical Behavior Of Unidirectional Composites Research Articles

A theoretical framework underlying an accelerated testing method and its application to composites under constant strain rates and fatigue loading

Abstract A theoretical framework that underlies an accelerated testing method for unidirectional (UD) composites is proposed in this article, based on the continuum damage mechanics and time–temperature superposition. By the way of demonstration, it has been applied to problems involving constant strain rates and fatigue loading conditions. The damage evolution law for the matrix of composites was constructed using the Weibull distribution of defects, which will develop into cracks as a result of deformation. In conjunction with the Wiechert model, the theoretical framework formulated here is capable of capturing the mechanical behaviour of UD composites. Demonstration and initial verification of this acceleration method were carried out through experiments. This work can provide theoretical and technical support for the durability verification and the evaluation of the high cycle fatigue performance of composite structures.

Mechanical behavior of unidirectional composites according different failure criteria

This work is about study of the mechanical behaviour of unidirectional Kevlar / Epoxy composite laminates according to different failure criteria. Varying strength parameters values, makes it possible to compare the ultimate mechanical characteristics obtained by the criteria of Tsai-Hill, Norris, Fisher, Ashkenazi and Tsai-Wu. The epoxy matrix of the material in question is reinforced with up to 60% of its volume by aramid fibers. The stack of four layers composing the arbitrarily oriented and alternating [+q/-q]S materials results in balanced symmetrical laminates. The laminate is subjected to uniaxial tensile membrane forces. Estimate of their ultimate strengths and the plotting of the failure envelope constitute the principal axis of this study. Using the theory of maximum stress, we can determine the various modes of damage of the composite. The different components of the deformation are presented for different orientations of fibers.

Prediction and validation of the transverse mechanical behavior of unidirectional composites considering interfacial debonding through convolutional neural networks

In this work, we propose a prediction model of the transverse mechanical behavior of unidirectional (UD) composites containing complex microstructure with the help of a convolutional neural network (CNN). For this prediction, a total of 900 representative volume elements (RVE) samples were generated by constructing 300 RVEs for each V f of 40%, 50%, and 60% with the random sequential expansion (RSE) algorithm. The stress-strain (S–S) curves in terms of transverse elastic modulus , transverse tensile strength , and toughness considering interphase debonding were obtained by a finite element (FE) simulation with the RVE samples. After converting FE models with 900 RVE samples to corresponding microstructural binary images , CNN modeling was employed to construct a prediction model on the microstructural images. To demonstrate the performance of the proposed CNN model, we predicted the transverse mechanical behavior in terms of the S–S curves on various test datasets . Prediction accuracy was verified in terms of the loss functions and the error of the S–S curve. The prediction results were in excellent agreement with the test datasets, and the transverse mechanical behavior was quickly predicted for other microstructures. This confirmed that the proposed CNN model is simple and powerful and can efficiently clarify the relationship between the microstructure and transverse mechanical behavior of UD composites. • A simple and powerful data-driven CNN model is developed to predict the transverse mechanical behavior. • Interfacial debonding of UD composites and elastic-plastic behavior of matrix in composite materials is considered. • A convergence study of the CNN model is conducted according to the image resolution and the number of RVE samples. • The predicted nonlinear stress-strain curves showed excellent agreement with those of FE simulation.

Fragmentation model for the tensile response of unidirectional composites based on the critical number of fiber breaks and the correction of the fiber-matrix interfacial strength.

Abstract A fragmentation model based on global load sharing (GLS) theory is developed to obtain stress-strain curves that describe the mechanical behavior of unidirectional composites. The model is named CNB+τ* because it is based on the Critical Number of Breaks model (CNB) and on the correction of the fiber matrix interfacial strength, τ*. Model allows both obtaining the ultimate tensile strength of CFRP and GFRP composites, and correcting the σ vs e curve to match its peak point with the predicted strength, which is more accurate than the one obtained by previous GLS-based models. Our model is used to classify the mechanical response of the material according to the energetic contributions of two phenomena up to the failure: intact fibers (IF) and fragmentation (FM). Additionally, the influence of fiber content, Vf, on the tensile strength, σU, failure strain, eU, and total strain energy, UT, is analyzed by means of novel mechanical-performance maps obtained by the model. The maps show a dissimilar behavior of σU, eU and UT with Vf between GFRP and CFRP composites. The low influence of Vf on the percent energetic contributions of IF and FM zones, as well as the larger energetic contribution of the FM zone, are common conclusions that can be addressed for both kinds of composites.

Micromechanical modeling of the mechanical behavior of unidirectional composites – A comparative study

Laminated fiber reinforced composites are increasingly being used in various load-bearing applications including for instance aerospace and wind energy power industries. Understanding the mechanical behavior of a unidirectional lamina as the basic part of a laminated composite is of great interest from a design perspective and also for the analysis of composite structures. At the micro-level, mechanical behavior of a lamina including stiffness and strength is studied based on the mechanical properties of individual constituents. This study focuses on micromechanical approaches such as strength of materials, elasticity, semi-empirical, and numerical methods for the prediction of the stiffness and strength of a unidirectional lamina. To assess the accuracy of these models, results of each model are compared against experimental results available in the literature for unidirectional composites with different fiber volume fractions. Moreover, recent studies on micromechanical modeling of unidirectional composites are reviewed.

Mechanical behavior of unidirectional composites with hexagonal and uneven distribution of fibers in the transverse cross-section

Uneven distribution of fibers can adversely affect the mechanical behavior of unidirectional composites. A micromechanical model based on finite element analysis is presented to evaluate elastic and strength properties of such composites under normal loading. Analysis starts with identification of micro unit cells/micro repetitive unit cells and/or representative volume elements. Because of uneven distribution/random distribution of the fibers, fiber volume fraction can be different for different micro unit cells present at different locations in the transverse cross-section of the unidirectional composite. Configuration of the micro unit cell is worked out at the outset considering the fiber distribution having the hexagonal arrangement, but with different volume fraction at different locations. For such micro unit cells, elastic and strength properties are obtained based on finite element analysis starting with the elastic and strength properties of fiber and matrix. With the properties obtained for different micro unit cells, elastic and strength properties of the unidirectional composite with micro unit cells having hexagonal arrangement of fibers with different volume fraction at different locations in the transverse cross-section are determined. Further, elastic and strength properties are evaluated for micro unit cells with uneven distribution of fibers in the transverse cross-section.

Viscoelastic experimental characterization of flax/epoxy composites

This work presents an experimental characterization of the mechanical behavior of unidirectional composites based on flax fibers and (bio-)epoxy resins. The study focuses on the viscoelastic and the pseudo-plastic response on the composites. Specimens have been produced through a LRTM process and tested in pure monotonic tension, and cyclic tension. The effect of the strain rate has been also investigated. This is found to have a strong influence due to the very high creep sensitivity of the material. Finally, intra-laminate variability of the mechanical properties has been also investigated. Results show that the mechanical properties of flax-based composites might show either good repeatability or high dispersion, based on the supplier quality of the rough materials. However, results show a defined transition point between E1 and E2, and a strain-based failure criterion.

Micromechanical modelling of the progressive failure in unidirectional composites reinforced with bamboo fibres

A multiscale approach based upon the micromechanical analysis of multiple damage events at the fibre/matrix scale is developed to investigate the mechanical behaviour of unidirectional composites subjected to tensile loading. The composites are reinforced with bamboo fibres under conditions of fibre breakage and matrix transverse cracking, by considering longitudinal splitting derived from the weak interfacial bonding between fibre and matrix. Due to the statistical nature, an improved Weibull model is employed to describe fibre strength distributions affected by the diameter change in the fibres. A simulation scheme coupled with the Monte Carlo method is performed to characterise the progressive failure in the composite. Compared with experimental results, the validity of the model for composite strength prediction is examined as well.

Numerical Analysis on the Influence of Interface on the Mechanical Behavior of Unidirectional Fiber Composites under Different Loads

On the smallest structural scale in the multi-scale structure composites, namely fiber scale, a numerical model was proposed for the analysis on the mechanical properties of unidirectional composites through the representative unit cell (RUC). The progressive method was used to simulate the failure behavior of fiber and matrix, and the debonding between fiber and matrix was characterized by the cohesive zone model (CZM). The failure strength of the unidirectional composite was predicted, and the influence of the interfacial strength on the mechanical behavior of unidirectional composite was discussed. It is shown that fiber dominates the failure strength of the material under the longitudinal load, whereas under the transverse load interfacial properties play an important role in the mechanical behavior of the material. The increase of the interfacial strength can significantly improve the capability of transverse compression and shear resistance.

Effects of Fiber Arrangement on Mechanical Behavior of Unidirectional Composites

Micromechanical approaches are employed to investigate the influence of different fiber arrangement on the mechanical behavior of unidirectional composites (UD) under various loading conditions. A micromechanical model with a random fiber array is generated and used in a finite element analysis together with two frequently used representative volume elements (RVE), or unit cell models of square and hexagonal arrays. The algorithm for generating the random fiber array is verified by comparing the comprehensive performance of a unit cell based on our random array and that of a unit cell based on a real fiber distribution in the UD cross-section. Performance of the random and regular fiber arrays is also evaluated through frequency distributions of stress invariants in matrix and tractions at the fiber—matrix interface due to various loading types. The effects of different loading angles on the overall response of regular arrays to various loading conditions are investigated thoroughly. Finally, the Weibull distribution of the maximum normal interfacial traction in random array is compared with the cumulative probability distribution of transverse strength data acquired from experiment, and good agreement is achieved.

유연수지를 기지재료로 하는 복합재료의 비선형거동 예측

본 논문에서는 유연수지 복합재료에 대하여 기하학적 비선형해석을 수행하였다. 실제 랜덤한 섬유배열을 사각배열과 육각배열로 가정하고 각각에 대해 단위구조를 정의하였다. 다양한 하중상태를 수치적으로 모사하여 단위구조해석을 통해 전체 구조물의 응력-변형률 선도를 예측하였고 이로부터 등가물성치를 계산하였다. 해석시 유연수지의 초탄성 성질을 정의하기 위해 Mooney-Rivlin모델을 사용하였다. 계산결과, 유연수지 복합재료 구조물은 변형률 증가에 따라 비선형의 응력-변형률 관계를 보였다. 비선형성은 횡방향 하중 상태에서 더욱 두드러지게 나타났으며, 이 경우 복합재 단면의 섬유배열 형태에 따라 상당한 차이를 보여주었다. In this paper, mechanical behavior of unidirectional composites with flexible matrix was predicted by geometrical non-linear finite element analysis. Two typical idealized unit cells of square and hexagonal fiber arrays were modeled and these were subjected to different loadings. The stress-strain behavior of composites was predicted from which the effective properties were calculated. The hyperelasticity of polyurethane matrix was considered using Mooney-Rivlin model. In result, the stress-strain behavior of flexible composites shows non-linearity, especially it is remarkable under transverse normal and shear loading conditions. In this cases, there are great difference between square and hexagonal fiber array models.

A 3D shear-lag model considering micro-damage and statistical strength prediction of unidirectional fiber-reinforced composites

A new numerical model is proposed for simulating the mechanical behavior of unidirectional composites which is based on a three-dimensional (3D) shear-lag model. The 3D shear-lag model considers the micro-damage phenomena of interfacial debonding and interfacial yielding. In order to confirm the validity of the model, the calculated stress concentration is compared with the HVD model (Hedgepeth JM, Dyke P. Local stress concentrations in imperfect filamentary composite materials. J Comp Mater 1967;1:294–309) in the appropriate limit. Monte Carlo simulations with the present shear-lag model were then conducted to obtain the ultimate tensile strength (UTS) as a function of fiber strength and interfacial properties. The damage progression and formation of clusters versus the type of interfacial damage, and the size-scaling of the tensile strengths, are carefully examined. Coupled with a size-scaling analysis, model predictions for tensile strength show good agreement with experiment.

Acoustic emission in carbon fibre-reinforced plastic materials

The mechanical behaviour of unidirectional (UD) and 2D-laminate Carbon Fibre-Reinforced Plastic composites was investigated under 4-point bending and tensile loads at room temperature. The mechanical behaviour of UD composites was first studied. The mechanical behaviour of a quasi-isotropic laminate was subsequently investigated under the same loading conditions and correlated to that of the UD plies. The damage and the failure mechanisms were identified by microscope observations on both UD and laminate composites. Two families of damage mode were identified, involving interfibre cracks and inter-layer delamination cracks homogeneously distributed within the composite. The fracture process is assigned to the propagation of the macrocrack in the failure envelope through a large intra and inter-layer delamination process interrupted by fibres/bundles fracture. Acoustic emission measurement was performed during the tests. An approach using a combination of acoustic waveform parameters as indicators of the physical damage is discussed.

Evaluation of dynamic mechanical properties of unidirectional composites in relation to their morphology

AbstractWe were interested in the dynamic mechanical behavior of unidirectional composites from E‐glass fibers and an epoxy matrix with several glass fiber contents ranging from 43% to 68%. A method is proposed for the determination of the viscoelastic response of such reinforced material, based on successive applications of the three‐phase self‐consistent analysis proposed by Christensen and Lo. When we apply this model in consecutive steps, we can take into account some morphological features including nonuniform arrangement of the fibers in the transverse plane with clustering or connecting arrangements as well as particular strengthening effects of the polymer in the interphase.