Progressive failure analysis of 2/2 twill weave fabric composites with moulded-in circular hole

Micromechanical three-dimensional finite element models of 2/2 twill weave T300 carbon/epoxy woven fabric composite panels with moulded-in circular hole are established for stress analysis. In these models, the streamline equation is used as a shape function to simulate the fibre configuration. A progressive failure analysis together with a newly developed ‘maximum notched strength method’ are also proposed to predict the failure modes and notched strengths of the fibre dominated laminate with moulded-in hole. Perforated specimens of different hole sizes are prepared using a special procedure. Tension tests are performed to evaluate the stress–strain and failure characteristics. An increase in tensile strength with increasing hole size is observed within the experimental data range. Numerical results from progressive failure analysis provide good prediction to the failure phenomena of the fractured specimens. The notched strengths from the proposed numerical procedure are slightly higher than the experimental results.     

Numerical and experimental determination of in-plane elastic properties of 2/2 twill weave fabric composites

This paper presents the in-plane elastic properties of 2/2 twill weave, T300 carbon/epoxy, woven fabric composite plates, obtained by both finite element analysis and experiments. A micromechanical, three-dimensional (3D) finite element model of the single layer unit cell of a 2/2 twill weave fabric composite is built, and a homogenization process is implemented. A unit cell is chosen such that it encloses the characteristic periodic repeat pattern in the fabric weave. Detailed geometry together with construction procedures for this new model are developed by using ANSYS Parametric Design Language (APDL). In this respect, the scope for altering the weave and yarn parameters is facilitated. Standard tensile and rail shear tests with modifications are performed for this kind of woven fabric composite. Elastic mechanical properties determined by experiments are presented, and the finite element model is verified. Satisfactory correlation between the predicted and experimental results are obtained.     

Deformation analysis of 2/2 twill weave fabrics

Biaxial tensile loading processes in 2/2 twill weave fabrics are studied. The performed calculations are based on the theory of biaxial deformation of woven materials with account of their geometry and mechanical characteristics of the warp and weft threads/yarns. Numerical procedures are proposed, in order to solve the resolving system of equations which allows for yarn compressibility in transversal directions. The effectiveness of numerical algorithms is demonstrated by comparison of predicted results and the experimental data.     

Progressive damage analysis and strength prediction of 2D plain weave composites

A two-step, multi-scale progressive damage analysis is implemented to study the damage and failure behaviors of 2D plain weave composites under various uniaxial and biaxial loadings. In the progressive damage mode (PDM), a formal-unified 3D Hashin-type criterion is formed to facilitate analysis work and engineering application, with shear nonlinearity considered in the stiffness matrix of yarn. The periodic boundary conditions are developed for the off-axis loading simulations. The simulated stress–strain curves under on-axis uniaxial tension and compression show good agreements with experimental results. The influences of different 3D Hashin-type criteria are subsequently discussed. Moreover, the strength decrease at different off-axis angles and the failure envelopes under on-axis and 45° off-axis biaxial loadings are obtained, with the discussion of different failure characteristics under each loading condition.     

Fatigue strength of woven fabric composites with drilled and moulded-in holes

The fatigue strength of glass-fiber woven-roving composites with a circular hole has been examined. Circular holes of two types, drilled and moulded-in, were considered. Experiments on the fatigue life and damage processes in (0, 90)_s and (45, −45)_s woven-roving composites with hole diameters of 5, 10, 14 and 19 mm were conducted. Experimental results show that laminates with moulded-in circular holes exhibit higher fatigue strength than those with drilled holes as a consequence of the altered mode of failure. Photographs of fatigue failure processes and specimen elongation with life are presented. The failure phenomena of laminates with drilled and moulded-in holes are compared and discussed.     

2D-C/SiC复合材料开孔件拉伸强度有限元计算

对2D-C/SiC复合材料开孔试件最小净截面图像进行观测,获得试件材料内部宏观孔洞的分布形态及密度分布梯度。通过对2D-C/SiC复合材料拉伸应力-应变行为进行非线性拟合,并利用理论模型计算与实验验证相结合的方法得到了材料密度与其拉伸模量和强度的关系,描述了不同密度2D-C/SiC复合材料的拉伸应力-应变行为。在此基础上,将制备工艺造成的试件材料密度分布的非均匀性和材料拉伸应力-应变行为的非线性引入到有限元模型中,进行开孔试件拉伸剩余强度模拟计算,预测结果与实验结果吻合较好。     

An experimental investigation into the tensile strength scaling of notched composites

An extensive experimental program has been performed to investigate the effect of scaling on the tensile strength of notched composites. Hole diameter, ply and laminate thickness, were investigated as the independent variables, whilst keeping constant ratios of hole diameter to width and length, over a scaling range of 8 from the baseline size. In most cases strength decreased as specimen size increased, with a maximum reduction of 64%. However the reverse trend of strength increasing with in-plane dimensions was found for specimens with plies blocked together. As well as the variation in strength, three distinct failure mechanisms were observed: fibre failure with and without extensive matrix damage, and complete gauge section delamination. Despite these differences, similar sub-critical damage mechanisms were seen in all specimens, with the extent of the damage determining the failure stress and mechanism. Damage propagated across the gauge section via delamination at the hole, which was controlled by the ply thickness to hole diameter ratio. This same mechanism can explain both the increasing and decreasing strengths observed. Simple analytical criteria for determining notched strength were found to be accurate for fibre failure in the absence of extensive sub-critical damage, but could not account for those conditions where delamination propagated across the width prior to failure.     

Mechanical analysis of 3D braided composites: a finite element model

The analysis of 3D braided composites is more difficult due to its complex microstructure. A new type of finite element method is developed to predict the effective moduli and the local stress within 3D braided composites under the 3D mechanical loading. To verify the present method, the material properties of undamaged 3D braided composites predicted in this paper are compared with the previous work. To demonstrate this method, some examples are analyzed.     

A hierarchic 3D finite element for laminated composites

In this paper a new 3D multilayer element is presented for analysis of thick‐walled laminated composites. This element uses two steps to calculate the full stress tensor. In the first step the in‐plane stresses are computed from the material law using a displacement approximation, and then the transverse stresses are calculated from the 3D equilibrium equations. Since the 3D equilibrium equations require high‐order interpolation functions, a hierarchic interpolation of displacements is used. The new element is compared with existing ones, e.g. from MSC.MARC. Copyright © 2004 John Wiley & Sons, Ltd.     

SiCf/TC17复合材料界面剪切强度测试与有限元分析

界面强度对钛基复合材料的性能有重要影响。采用纤维顶出实验(push-outtest)对连续SiC纤维增强TC17复合材料的界面剪切强度进行了测试,采用SEM观察了样品的形貌。以纤维/基体完全分离后的摩擦力为出发点,采用有限元方法确定了复合材料成型过程中残余应力的产生温度,并计算了残余应力的分布,比较了顶出实验样品制备前后残余应力的变化情况及样品厚度、体积分数对残余应力分布的影响;采用内聚力模型(CZM)分析了界面的化学结合强度。结果表明:SiCf/TC17复合材料高温成型后的冷却过程中开始产生残余应力的温度为775℃;顶出实验样品制备后界面处生成了残余剪切应力,其大小和分布与样品的体积分数和厚度相关,界面处的残余剪切应力造成了界面剪切强度的测试结果与界面化学结合强度的差异;室温下SiCf/TC17复合材料的界面化学结合强度约为450MPa。     

On the influence of weave structure on pin-loaded strength of orthogonal 3D composites

Orthogonal three-dimensional (3D) carbon fibre fabrics with different weave structures were obtained by varying the yarn spacing and number of carbon filaments per tow in the x-, y- and z-directions during weaving. These weave structures were impregnated with epoxy resin to produce orthogonal 3D carbon/epoxy composites. In addition, one-dimensional (0° and 90° unidirectional) and two-dimensional (cross-ply and plain fabric) laminates were prepared from the same carbon fibres and epoxy resin. Single-hole pin-loaded specimens of each material were tested in tension, and the influences of reinforcement type, weave structure, specimen width-to-hole diameter ratio and edge distance-to-hole diameter ratio evaluated. Various modes of failure were observed in the specimens. The effect of in-plane and out-of-plane fibres on the pin-loaded strength of orthogonal 3D composites is discussed.     

A comparative study of tensile properties of non-crimp 3D orthogonal weave and multi-layer plain weave E-glass composites. Part 2: Comprehensive experimental results

This Part 2 paper presents results of comparative experimental study of progressive damage in 2D and 3D woven glass/epoxy composites under in-plane tensile loading. As Part 1, this Part 2 work is focused on the comparison of in-plane tensile properties of two non-crimp single-ply 3D orthogonal weave E-glass fibre composites on one side and a laminate reinforced with four plies of E-glass plain weave on the other. The damage investigation methodology combines mechanical testing with acoustic emission registration (that provides damage initiation thresholds), progressive cracks observation on transparent samples, full-field surface strain mapping and cracks observation on micrographs, altogether enabling for a thorough characterisation of the local micro- and meso-damage modes of the studied composites. The obtained results demonstrate that the non-crimp 3D orthogonal woven composites have significantly higher in-plane strengths, failure strains and damage initiation thresholds than their 2D woven laminated counterpart. The growth of transverse cracks in the yarns of 3D composites is delayed, and they are less prone to a yarn–matrix interfacial crack formation and propagation. Delaminations developing between the plies of plain weave fabric in the laminate at certain load level never appear in the 3D woven single-ply composites.     

考虑界面结合的SiCP/6061Al 复合材料时间相关棘轮行为的三维有限元分析

采用多颗粒三维单胞模型和复合材料细观有限元分析方法,借助先进循环黏塑性本构模型的有限元实现,对SiC颗粒增强6061Al复合材料的室温、高温时间相关单轴棘轮行为进行数值模拟。讨论了颗粒排列方式和界面结合状态的变化对复合材料棘轮行为的影响;同时,分析了复合材料中基体和界面的微观变形特征及其演变规律;最后,选取一组合理的微结构参数,对复合材料的时间相关棘轮行为进行了数值模拟,并通过与已有实验结果的比较,检验了有限元分析的合理性。结果表明：多颗粒代表性体积单元能够反映复合材料更多的微观细节;颗粒排列方式的变化显著影响复合材料的整体棘轮行为;界面结合状态越好,产生的棘轮变形越小;具有合理参数值的弱界面模型给出的时间相关棘轮变形预测结果比完好界面模型的结果更接近实验值。     

Behaviour of composite plates with drilled and moulded hole under tensile load

Drilling of composite material causes damages of different natures. To avoid machining in composite material, it is proposed to include holes while moulding. In the literature, there is few works concern the behaviour analysis of specimens with moulded holes. All the literature works concern a specimens with moulded holes made of woven fabric. Moreover no literature is available on local behaviour of specimens around the hole. In this paper, several mechanical tests are carried out on specimens with quasi-isotropic stacking sequence made from UD prepreg with drilled holes and with moulded holes. To analyse the behaviour of the specimens around the hole, the strain field is measured using three-dimensional Digital Image Correlation technique (DIC). The pictures analysed given by the CCD camera reveal that, the damage mechanisms are different between the plates with drilled holes and those with moulded holes. The SEM observation have shown that, the fibre content and porosity content near the hole is around ≈ 8% higher compared to the values obtained far the hole. Moreover the tensile strength of the plates with moulded holes is 28% higher than with drilled holes.     

Analytical study of strength and failure behaviour of plain weave fabric composites made of twisted yarns

A two-dimensional analytical method is presented for the failure behaviour of plain weave fabric composites made of twisted yarns. The studies have been carried out on laminates with different configurations under on-axis uni-axial tensile loading. The cross-sectional area of the yarn was taken to be elliptical and the yarn path was taken to be sinusoidal. Different stages of failure are considered in the analysis. It has been observed that there is no significant reduction in tensile strength properties of plain weave fabric composites as a result of twisting of yarns. For E-glass yarns, twisting of yarns up to 5°, can facilitate ease of fabrication without significantly compromising the strength properties of the woven fabric composites.     

Micromechanical formulation and 3D finite element modeling of microinstabilities in composites

A 3D micromechanical formulation and a FE-model of fiber micro-buckling in materials with isotropic and transversal isotropic fibers in compression is presented. Three variants of geometrical modeling of the characteristic cell are proposed and compared. An appropriate one is then selected. An eigenvalue analysis of a characteristic cell is performed. The results show that the fiber anisotropy reduces significantly the critical loads and must be taken into account.     

三维多向编织复合材料宏细观力学性能有限元分析研究进展

三维多向编织复合材料综合力学性能优异,有望在航空航天领域作为主承力结构而得到广泛应用。有限元分析(FEA)方法是三维多向编织复合材料宏细观力学性能研究最为经济和有效的分析手段。从细观结构模型、刚度强度性能预测及界面性能分析等方面评述了细观有限元法的研究进展;从接头结构承载性能分析和冲击性能模拟等方面介绍了宏观有限元法的研究概况;分析了目前研究中存在的主要问题并对后续的研究工作进行了展望,为研究者了解该领域的研究现状和发展趋势提供了有价值的参考。     

Comparison of 2D finite element modeling assumptions with results from 3D analysis for composite skin-stiffener debonding

The influence of two-dimensional finite element modeling assumptions on the debonding prediction for skin-stiffener specimens was investigated. Geometrically nonlinear finite element analyses using two-dimensional plane-stress and plane-strain elements as well as three different generalized plane-strain type approaches were performed. The computed skin and flange strains, transverse tensile stresses and energy release rates were compared to results obtained from three-dimensional simulations. The study showed that for strains and energy release rate computations the generalized plane-strain assumptions yielded results closest to the full three-dimensional analysis. For computed transverse tensile stresses the plane-stress assumption gave the best agreement. Based on this study it is recommended that results from plane-stress and plane-strain models be used as upper and lower bounds. The results from generalized plane-strain models fall between the results obtained from plane-stress and plane-strain models. Two-dimensional models may also be used to qualitatively evaluate the stress distribution in a ply and the variation of energy release rates and mixed mode ratios with delamination length. For more accurate predictions, however, a three-dimensional analysis is required.