Effect of stitch density and stitch thread thickness on compression after impact strength and response of stitched composites

In this paper, the damage failure and behaviour of stitched composites under compression after impact (CAI) loading are experimentally investigated. This study focuses on the effect of stitch density and stitch thread thickness on the CAI strength and response of laminated composites reinforced by through-thickness stitching. Experimental findings show that stitched composites have higher CAI failure load and displacement, which corresponds to higher energy absorption during CAI damage, mainly attributed to greater energy consumption by stitch fibre rupture. The coupling relationships between CAI strength, impact energy, stitch density and stitch thread thickness are also revealed. It is understood that the effectiveness of stitching has high dependency on the applied impact energy. At low impact energy range, CAI strength is found to be solely dependent on stitch density, showing no influence of stitch thread thickness. It is however observed that stitch fibre bridging is rendered ineffective in moderately stitched laminates during compressive failure, as local buckling occurs between stitch threads, resulting in unstitched and moderately stitched laminates have similar CAI strength. The CAI strength of densely stitched laminates is much higher due to effective stitch fibre bridging and numerous stitch thread breakages. At high impact energy level, CAI strength is discovered to be intimately related to both stitch density and stitch thread thickness. Since CAI failure initiates from impact-induced delamination area, stitch fibre bridging is considerable for all specimens due to the relatively large delamination area present. Stitch threads effectively bridge the delaminated area, inhibit local buckling and suppress delamination propagation, thus leading to increased CAI strength for laminates stitched with higher stitch density and larger stitch thread thickness. Fracture mechanisms and crack bridging phenomenon, elucidated by X-ray radiography are also presented and discussed. This study reveals novel understanding on the effectiveness of stitch parameters for improving impact tolerance of stitched composites.     

含分层损伤缝合复合材料层板的剩余压缩强度

基于渐进损伤方法,研究了含单脱层缝合复合材料层板在压缩载荷下的剩余强度。通过商用软件ABAQUS建立了含单脱层缝合复合材料层板剩余压缩强度计算模型,考虑了子层屈曲和分层扩展对剩余强度的影响。通过UMAT子程序实现了层板失效、层间失效和缝线失效的模拟。通过嵌入式杆单元结构模拟了缝线桥联作用及失效。采用Hashin准则及刚度折减法对纤维拉压、基体拉压失效进行了模拟。通过渐进损伤分析,揭示了缝合情况下含单脱层复合材料层板的失效机理,讨论了缝合参数对剩余压缩强度的影响。所预测的破坏模式和剩余强度结果与实验能较好地吻合。分析表明缝合可以明显提高含分层损伤复合材料层板的子层屈曲载荷,抑制分层扩展,并提高层板的剩余压缩强度。     

含任意分层缝纫正交层合板压缩屈曲分析

建立了分析含任意形状分层的缝纫增强复合材料层合板压缩屈曲问题的连续分析模型。该模型允许缝纫层合板含有一个或多个形状不同的分层。分析结果表明,缝纫针脚在分层区域的分布、缝纫密度和缝线的等效刚度系数对分层子板的压缩屈曲应变均有较大影响。      

Finite element model for compression after impact behaviour of stitched composites

A finite element (FE) model using coupling continuum shell elements and cohesive elements is proposed to simulate the compression after impact (CAI) behaviour and predict the CAI strength of stitched composites. Continuum shell elements with Hashin failure criterion exhibit the composite laminate damage behaviour; whilst cohesive elements using traction-separation law characterise the laminate interfaces. Impact-induced delamination is explicitly modelled by reducing material properties of damaged cohesive elements. Computational results have demonstrated the trend of increasing CAI strength with decreasing impact-induced delamination area. Spring elements are introduced into the model to represent through-thickness stitch thread in the composite laminates. Results in this study validate experimental finding that CAI strength is improved when stitching is incorporated into the composite structure. The proposed FE model reveals good CAI strength predictions and indicates good agreement with experimental results, making it a valuable tool for CAI strength prediction of stitched composites.     

Damage resistance and damage tolerance of hybrid carbon-glass laminates

The influence of impact energy and stacking sequence on the damage resistance and Compression After Impact (CAI) strength of Carbon and Glass Fibre Reinforced Plastic (CFRP and GFRP respectively) hybrid laminates is investigated. CAI tests demonstrate that, in comparison to fully CFRP laminates, hybrid laminates show increases in structural efficiency of up to 51% for laminates subject to a 12J impact and 41% for those subject to an 18J impact. Laminates displaying the highest stresses at failure are those that exploit stacking sequences and GFRP content to prevent delaminations from forming close to the outer surface of the laminate during impact. This favourable damage morphology inhibits both sublaminate-buckling-driven delamination propagation and anti-symmetric laminate buckling failures.     

Experimental investigation of delamination buckling of stitched composite laminates

In the current investigation, effects of through-the-thickness stitching with two different types of aramid threads, Kevlar^® and Twaron^® threads, on the buckling loads of delaminated glass/epoxy composite laminates are studied. Buckling loads are predicted based on the Southwell, Vertical displacement and Membrane strain plot methods by using the experimental data. From the Southwell, Vertical displacement and Membrane strain plot methods it is observed that stitching either by Kevlar^® or Twaron^® threads is effective in improving the buckling strength of glass/epoxy composite laminates when the delamination length is greater than 0.5L, L being the length of the laminate. For long delaminations, Kevlar^® stitched glass/epoxy composite laminate is best in retaining its buckling strength when re-loading is done. Southwell plot method tends to overestimate the buckling loads as the data obtained from this method are influenced by the breakages in the glass/epoxy composite laminate buckling test specimens.     

Numerical analysis of influence factors on low-velocity impact damage of stitched composite laminates

Abstract

A 3D dynamic finite-element model is proposed in this paper to simulate the damage development process of stitched laminates subjected to low-velocity impact. The strain-based Hashin criteria and a sudden degradation scheme are employed to determine the intra-laminar damage initiation and evolution; a mix-mode bilinear constitutive model is adopted to evaluate the inter-laminar delamination damage. The predicted numerical results agree well with the available experimental data, thus validating the proposed damage analysis model. Moreover, the influence factors, including the thickness of laminates, stitching density, diameter of stitching thread and strength of stitching thread, are analyzed and discussed in detail.     

Modelling complex progressive failure in notched composite laminates with varying sizes and stacking sequences

The emergence of advanced computational methods and theoretical models for damage progression in composites has heralded the promise of virtual testing of composite structures with orthotropic lay-ups, complex geometries and multiple material systems. Recent studies have revealed that specimen size and material orthotropy has a major effect on the open hole tension (OHT) strength of composite laminates. The aim of this investigation is develop a progressive failure model for orthotropic composite laminates, employing stepwise discretization of the traction–separation relationship, to predict the effect of specimen size and laminate orthotropy on the OHT strength. The results show that a significant interaction exists between delamination and in-plane damage, so that models without considering delamination would over-predict strength. Furthermore, it is found that the increase in fracture toughness of blocked plies must be incorporated in the model to achieve good correlation with experimental results.     

Experimental study on the stitching reinforcement of composite laminates with a circular hole

Experimental study is carried out on the stitching reinforcement of composite laminates containing a circular hole. First, the tensile strength and stiffness are measured, and their dependence on stitching parameters such as stitching needle span, row spacing, edge distance and stitching type are analyzed. Next, the strain distribution and concentration are investigated analytically and experimentally for different stitching parameters, external load and edge location of the hole. It is shown that the results of stitching reinforcement are quite different for composite laminates with a circular hole, which could provide proper stitching parameters for designers.     

Buckling and Delamination Growth Analysis of Composite Laminates Containing Embedded Delaminations

The objective of this work is to study the post buckling behavior of composite laminates, containing embedded delamination, under uniaxial compression loading. For this purpose, delamination initiation and propagation is modeled using the softening behavior of interface elements. The full layer-wise plate theory is also employed for approximating the displacement field of laminates and the interface elements are considered as a numerical layer between any two adjacent layers which delamination is expected to propagate. A finite element program was developed and the geometric non-linearity in the von karman sense is incorporated to the strain/displacement relations, to obtain the buckling behavior. It will be shown that, the buckling load, delamination growth process and buckling mode of the composite plates depends on the size of delamination and stacking sequence of the laminates.     

On the Through-the-Width Multiple Delamination, and Buckling and Postbuckling Behaviors of Symmetric and Unsymmetric Composite Laminates

Multiple delamination causes severe degradation of the stiffness and strength of composites. Interactions between multiple delamination, and buckling and postbuckling under compressive loads add the complexity of mechanical properties of composites. In this paper, the buckling, postbuckling and through-the-width multiple delamination of symmetric and unsymmetric composite laminates are studied using 3D FEA, and the virtual crack closure technique with two delamination failure criteria: B-K law and power law is used to predict the delamination growth and to calculate the mixed-mode energy release rate. The compressive load-strain curves, load-central deflection curves and multiple delamination process for eight composite specimens with different initial delamination sizes and their distributions as well as two angle-ply configurations 0₄//(±θ)₆//0₄ (θ?=?0° and 45°, and “//” denotes the delaminated interface) are comparatively studied. From numerical results, the unsymmetry decreases the local buckling load and initial delamination load, but does not affect the global buckling load compared with the symmetric laminates. Besides, the unsymmetry affects the unstable delamination and buckling behaviors of composite laminates largely when the initial multiple delamination sizes are relatively small.     

Structurally stitched NCF CFRP laminates. Part 2: Finite element unit cell based prediction of in-plane strength

Based on experimental investigations on structurally stitched non-crimp fabric (NCF) carbon fiber/epoxy laminates under in-plane tension, compression and shear loading [1], a finite element based unit cell model was developed to estimate the in-plane strength of NCF laminates taking into consideration the yarn diameter, the stitching pattern and direction as well as the load type. Depending on these parameters, regions with undisturbed and disturbed fiber orientations leading to resin pockets as well as local changes of the fiber volume fraction are taken into account in the model.The comparison of experimental and numerical results showed that the strength of structurally stitched NCF laminates under in-plane tension, compression or shear loading can be predicted with an acceptable accuracy. The overall mean deviation between simulation and experiment observed was between 8% and 13%.     

Unification of strength scaling between unidirectional,quasi-isotropic,and notched carbon/epoxy laminates

An investigation into size effects and notch sensitivity in quasi-isotropic carbon/epoxy laminates was carried out. The purpose is to draw a complete picture of the strength scaling in unidirectional, quasi-isotropic, and notched carbon/epoxy laminates. A link was established between the strength scaling of the unidirectional and quasi-isotropic laminates. Efforts were made to understand the relationship between unnotched and open-hole strengths. For very small holes, the notched strengths approach the unnotched strength limit. A scaling law based on Weibull statistics was used to predict the unnotched laminate strengths. For very large holes, the same scaling law in conjunction with a detailed 3D ply-by-ply FE analysis with matrix cracks in the 90° plies and delamination cohesive interface elements was used to predict the large notched strengths. A good agreement between the modelling and experimental results was achieved. The effects of 90° matrix cracks on unnotched and notched strengths were also studied.     

Delamination buckling growth in laminated composites using layerwise-interface element

In this paper, a numerical investigation on the buckling of composite laminates containing delamination, under in-plane compressive loads, is presented. For this purpose, delamination propagation is modeled using the softening behavior of interface elements. The full layerwise plate theory is applied for approximating the displacement field of laminates and the interface elements are considered as a numerical layer between any two adjacent layers where the delamination is expected to propagate. A non-linear computer code was developed to handle the numerical procedure of delamination buckling growth in composite laminates using layerwise-interface elements. The load/displacement behavior and the contours of embedded and through-the-width delamination propagation for composite laminates are presented. It is shown that delamination growth can be well predicted using this layerwise-interface elements with decohesive law.     

XFEM simulation of delamination in composite laminates

In this paper, the extended finite element method (XFEM) is extended to simulate delamination problems in composite laminates. A crack-leading model is proposed and implemented in the ABAQUS® to discriminate different delamination morphologies, i.e., the 0°/0° interface in unidirectional laminates and the 0°/90° interface in multidirectional laminates, which accounts for both interlaminar and intralaminar crack propagation. Three typical delamination problems were simulated and verified. The results of single delamination in unidirectional laminates under pure mode I, mode II, and mixed mode I/II correspond well with the analytical solutions. The results of multiple delaminations in unidirectional laminates are in good agreement with experimental data. Finally, using a recently proposed test that characterizes the interaction of delamination and matrix cracks in cross-ply laminates, the present numerical results of the delamination migration caused by the coupled failure mechanisms are consistent with experimental observations.     

Review of z-pinned composite laminates

This paper reviews published research into polymer composite laminates reinforced in the through-thickness direction with z-pins. Research into the manufacture, microstructure, delamination resistance, damage tolerance, joint strength and mechanical properties of z-pinned composites is described. Benefits of reinforcing composites with z-pins are assessed, including improvements to the delamination toughness, impact damage resistance, post-impact damage tolerance and through-thickness properties. Improvements to the failure strength of bonded and bearing joints due to z-pinning are also examined. The paper also reviews research into the adverse effects of z-pins on the in-plane mechanical properties, which includes reduced elastic modulus, strength and fatigue performance. Mechanisms responsible for the reduction to the in-plane properties are discussed, and techniques to minimise the adverse effect of z-pins are described. The benefits and drawbacks of z-pinning on the interlaminar toughness, damage tolerance and in-plane mechanical properties are compared against other common types of through-thickness reinforcement for composites, such as 3D weaving and stitching. Gaps in our understanding and unresolved research problems with z-pinned composites are identified to provide a road map for future research into these materials.     

低速冲击后复合材料层合板的压缩破坏行为

对缝纫层合板和无缝纫层合板进行低速冲击后压缩破坏实验,以研究低速冲击后层合板的压缩破坏机理。采用C扫描、X射线、热揭层等技术对层合板内的损伤进行测量和对比。结果表明,界面不是很强的碳纤维增强复合材料层合板低速冲击后受压时,层合板非冲击面的子层屈曲及其扩展是导致层合板冲击后压缩强度下降的重要因素,而且子层屈曲主要是沿垂直载荷的方向(90°)扩展;对于准各向同性板,屈曲子层中与母层相邻的铺层的方向一般为90°。层合板的剩余压缩强度与板的冲击损伤面积无直接关系。      

Effect of hydrofluoric acid etching of glass on the performance of organic–inorganic glass laminates

Organic–inorganic glass laminates with polyurethane (PU) as an adhesive interlayer were prepared by a warm-pressing method. The hydrofluoric acid etching of glass surface was performed to investigate its effect on the mechanical behavior of glass laminates. Results show that the acidic etching treatment of glass seldom influences the transparency and haze of glass laminates when the etching time is below 30 min. The bonding strength and fracture stress of glass laminates firstly increase and then decrease with increasing etching time. This could be attributed to the formation of three-dimensional interface of glass laminates. The unique interface structure not only increases the contact area between glass and PU layer, leading to the improvement of interface bonding, but also modifies the stress distribution at the interfaces, which is favorable to prevent the crack propagation and delamination failure of laminates.     

含分层损伤的复合材料格栅(AGS)板的非线性热屈曲分析

采用基于复合材料一阶剪切效应理论的有限元法分别研究了含分层损伤的复合材料层合光板、 单向加筋板和格栅加筋(AGS)板的热屈曲性态。在分析中考虑材料热物理性质与温度相关特性, 同时在分层前缘采用了位移约束条件以保证分层区域的各子板的变形相容要求。3种结构的典型算例分析和结果的比较表明, 复合材料格栅(AGS)板具有很强的抗热屈曲的能力, 但是, 分层损伤将使其临界温度降低, 同时还会导致热屈曲的模态发生改变。本文中提出的方法和所得结论对AGS结构的热承载能力预测和损伤容限设计将具有参考价值。      

Stacking sequence optimization to maximize the buckling load of blade-stiffened panels with strength constraints using the iterative fractal branch and bound method

Laminated carbon fiber reinforced polymer (CFRP) composites have widespread applications in aerospace structures, and thus optimization of the stacking sequences in these composites is indispensable. Here, a fractal branch and bound method (FBB) is proposed for optimizing the stacking sequences. This method requires only low computational costs, and an optimal result can be obtained rapidly by means of the deterministic process. For practical stacking sequence optimizations, more than two laminates have to be optimized, because a practical aerospace structural component usually comprises a panel and stiffeners made from composite laminates. Since the stacking sequences of the skin panel and stiffeners affect the buckling load of the stiffened panel, the optimization of both laminates must be performed simultaneously. In the present study, a new method to implement a strength constraint for the FBB method is proposed for the simultaneous optimization of more than two laminates (such as a panel and stiffeners). Moreover, a quadratic polynomial objective function, which includes lamination parameter variables of the two laminates: the stiffeners and the panel, is adopted. The strength constraint is implemented by means of a response surface. The new method is applied to the buckling load maximization of a blade-stiffened composite panel, in which the strength constraint is demonstrated as a feasibility study. The method successfully obtained optimal stacking sequences with the strength constraint at low computational cost.