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.     

Composite toughness enhancement with interlaminar reinforcement

An experimental investigation of a newly proposed through-thickness reinforcement approach aimed to increase interlaminar toughness of laminated composites is presented. The approach alters conventional methods of creating three-dimensional fiber-reinforced polymer composites in that the reinforcing element is embedded into the host laminate after it has been cured. The resulting composite is shown to possess the benefits of a uniform surface quality and consolidation of the original unreinforced laminate. This technique was found to be highly effective in suppressing the damage propagation in delamination double-cantilever beam (DCB) test samples under mode I loading conditions. Pullout testing of a single reinforcing element was carried out to understand the bridging mechanics responsible for the improved interlaminar strength of reinforced laminate and stabilization and/or arrest of delamination crack propagation. The mode I interlaminar fracture of reinforced DCB samples was modeled using two-dimensional cohesive finite-element scheme to support interpretation of the experiments.     

缝线增强层合板混合模式弯曲试验的数值模拟

对缝线在复合材料层合板中的桥联作用进行参数分析,结果显示缝线的直径和拉伸强度,缝合角度,层合板厚度和挤压强度均会对缝线的桥联曲线以及断裂能产生影响,甚至改变缝线的破坏模式。以计算所得的桥联曲线作为输入参数,分别用连接器(Connector)和离散内聚力单元(Cohesive单元)的方法建立有限元模型,模拟缝线增强复合材料层合板的混合模式弯曲(MMB,Mix-Mode Bending)试验。两种方法的有限元计算结果具有较好的一致性,且均能够与文献中的试验数据较好吻合。相比离散Cohesive单元模型,Connector模型的计算效率更高,需要的输入参数更少,且建模更为简便。     

Effect of mendable polymer stitch density on the toughening and healing of delamination cracks in carbon–epoxy laminates

This paper presents an investigation into the effect of stitch density on the delamination toughening and self-healing properties of carbon–epoxy laminates. The stitches provide the laminate with the synergistic combination of high mode I interlaminar fracture toughness to resist delamination cracking and healing properties to repair delamination damage. The results show that the fracture toughness of the laminate increased with stitch density, due to higher traction (crack closure) loads exerted by the stitches bridging the delamination. During the healing process these bridging stitches first melt and then flow into the delamination, leading to self-healing with full restoration of the mode I fracture toughness. Furthermore, the stitches were capable of repairing delamination cracks many times larger than the original size of the stitches. The effect of stitch density on the healing process of delamination cracks and restoration of fracture toughness was found to remain approximately the same under multiple repair operations.     

A cohesive zone model for predicting delamination suppression in z-pinned laminates

This paper presents a cohesive zone model based finite element analysis of delamination resistance of z-pin reinforced double cantilever beam (DCB). The main difference between this and existing cohesive zone models is that each z-pin bridging force is governed by a traction-separation law derived from a meso-mechanical model of the pin pullout process, which is independent of the fracture toughness of unreinforced laminate. Therefore, two different traction-separation laws are used: one representing the toughness of unreinforced laminate and the other the enhanced delamination toughness owing to the pin bridging action. This approach can account for the large scale bridging effect and avoid using concentrated pin forces, thus removing the mesh dependency and permitting more accurate analysis solution. Computations were performed using a simplified unit strip model. Predicted delamination growth and load vs. displacement relation are in excellent agreement with the prediction by a complete model, and both models are in good agreement with test measured load vs. displacement relation. For a pinned DCB specimen, the unit strip model can reduce the computing time by 85%.     

含分层复合材料层板的压缩性能

使用商用有限元软件建立了含分层复合材料层板的有限元模型,采用Hashin失效准则对层板内单元进行损伤判断,并编写程序对失效单元进行刚度折减,使用cohesive单元模拟层间区域,并对缺陷区域进行弱化处理,利用应力失效判据和能量释放准则判断层板内起始分层与分层的扩展。对完好以及含分层缺陷复合材料单向层板试验件进行压缩实验研究,实验结果给出了分层位置和尺寸及对材料压缩性能的影响。研究表明,有限元模拟结果与实验结果具有良好的一致性。     

Constitutive Model for a Fiber Tow Bridging a Delamination Crack

A simple model is introduced for a fibrous tow, stitch, or rod that bridges a delamination crack in a laminate. The model is introduced for mode II delamination cracks but is intended for general mixed-mode cracks. Modeling is guided by prior observations and measurements on laminates reinforced through the thickness by stitches or short rods. Salient phenomena include shear deformation of the bridging tow, its debonding from and sliding relative to the surrounding laminate, and its sideways displacement through the laminate. The tow is represented as a beam that can shear and extend axially. Its axial displacement relative to the laminate is resisted over its debonded periphery by friction. The forces associated with its sideways displacement are estimated by regarding it as a punch being driven through a plastic medium (the laminate). Thus the mechanics of the whole problem are reduced to a set of one-dimensional equations. The distinction between continuous stitches and discontinuous rod reinforcement consists of a boundary condition. With realistic values assigned to undetermined parameters, experimental data for stitches are reproduced over the whole range of displacements up to ultimate failure of the stitch. The model generates a bridging traction law that can be used for optimal design of through-thickness reinforcement for damage tolerance in a wide variety of structures.     

Effect of initial crack length on the measured bridging law of unidirectional E-glass/epoxy double cantilever beam specimens

In this paper, the effect of initial delamination length is experimentally investigated on obtaining the mode I bridging law of unidirectional E-glass/epoxy double cantilever beam (DCB) specimens manufactured by hand layup method. To this end, an experimental test set-up is established for accurate measurement of crack tip opening displacement (CTOD) using digital image processing method. DCB tests are performed for three different delamination lengths and the corresponding bridging laws are calculated using J-integral approach. Results showed that the maximum bridging stress, the shape of bridging law and energy dissipation in bridging zone are slightly affected by changing initial crack length. In other words, the measured bridging law acts independent of initial delamination length. Therefore, the obtained bridging law can be used with the cohesive elements available in the commercial finite element software to simulate the delamination propagation behavior in unidirectional DCB specimens.     

Cohesive modeling of delamination in Z-pin reinforced composite laminates

The mode I interlaminar fracture in Z-pin reinforced composite laminates is modeled using a cohesive volumetric finite element (CVFE) scheme. The test configuration used in this study is a Z-pin reinforced double cantilever beam specimen. A bilinear rate-independent but damage-dependent cohesive traction–separation law is adopted to model the fracture of the unreinforced composite and discrete nonlinear spring elements to represent the effect of the Z-pins. The delamination toughness and failure strength of the Z-pin reinforced composites are determined by a detailed comparison study of the numerical modeling results with experimental data. To further reduce the computational effort, we introduce an equivalent distributed cohesive model as a substitute for the discrete nonlinear spring representation of the Z-pins. The cohesive model is implemented on various test problems with varying failure parameters and for varying spatial Z-pin reinforcement configurations showing good agreement with the experimental results.     

Predicting mode-II delamination suppression in z-pinned laminates

A finite element model for predicting delamination resistance of z-pin reinforced laminates under the mode-II load condition is presented. End notched flexure specimen is simulated using a cohesive zone model. The main difference of this approach to previously published cohesive zone models is that the individual bridging force exerted by z-pin is governed by a specific traction-separation law derived from a unit-cell model of single pin failure process, which is independent of the fracture toughness of the unreinforced laminate. Therefore, two separate traction-separation laws are employed; one represents unreinforced laminate properties and the other for the enhanced delamination toughness owing to the pin bridging action. This approach can account for the so-called large scale bridging effect and avoid using concentrated pin forces in numerical models, thus removing the mesh-size dependency and permitting more accurate and reliable computational solutions.     

Experimental investigation of bridging law for single stitch fibre using Interlaminar tension test

In this study, a novel Interlaminar tension test (ITT) method was performed to experimentally investigate the bridging and fracture process of a single stitch fibre used to improve the delamination strength of composite laminates. Kevlar-29, of various thread thicknesses (44, 66, 88 and 132 tex), was used as the through-thickness stitch fibre in the ITT experiments. Key empirical force and displacement parameters, which governed the stitch fibre bridging law, were characterised and identified. Relationships of such parameters with thread thicknesses were determined. Fibre fracture load and fibre fracture energy are found to increase with increasing thread thickness. Frictional pull-out force greatly depends on the type of stitch fracture modes, which can be grouped into three categories. This paper aims to provide better physical understanding of the mechanics and mechanisms of stitch fibre fracture. By correlating critical stitch fracture parameters with stitch fibre thicknesses, the results expect to provide useful reference, which is essential and important for accurate stitch computational modeling and strength prediction of composites using stitching as the interlaminar reinforcement technique.     

基于界面单元的复合材料层间损伤分析方法

为了研究复合材料层间损伤, 建立了一种新型零厚度界面单元模型, 可以准确地预测复合材料 Ⅰ 型层间裂纹扩展。模型包括本构关系建立、损伤准则和损伤演化引入, 并在大型商用有限元软件ABAQUS用户单元子程序VUEL中实现, 采用显示积分方法求解, 不存在收敛性问题, 同时允许使用较粗的有限元网格。最后将该模型应用于国产碳纤维增强树脂基复合材料(CCF300/5428)双悬臂梁试验(DCB)模拟分析中, 结果表明, 此界面单元模型能够准确模拟复合材料层板 Ⅰ 型裂纹扩展, 为复合材料层间损伤分析提供了一种有效的方法。     

缝合复合材料层板低速冲击损伤数值模拟

建立了缝合复合材料层板在低速冲击载荷下的渐进损伤分析模型。模型中采用空间杆单元模拟缝线的作用;采用三维实体单元模拟缝合层板,通过基于应变描述的Hashin准则,结合相应的材料性能退化方案模拟层板的损伤和演化;采用界面单元模拟层间界面,结合传统的应力失效判据和断裂力学中的应变能释放率准则判断分层的起始和扩展规律。通过对碳800环氧树脂复合材料（T800/5228）层板的数值仿真结果和试验结果相比较,验证了模型的正确性,同时讨论了不同冲击能量下缝合层板的损伤规律。研究结果表明：缝线能够有效地抑制层板的分层损伤扩展;相同冲击能量下缝合与未缝合层板的基体损伤和纤维损伤在厚度分布上相似,缝合层板的损伤都要小于未缝合层板。     

一种改进的内聚力损伤模型在复合材料层合板低速冲击损伤模拟中的应用

针对传统内聚力损伤模型(CZM)无法考虑层内裂纹对界面分层影响的缺点,提出了一种改进的适用于复合材料层合板低速冲击损伤模拟的CZM。通过对界面单元内聚力本构模型中的损伤起始准则进行修正,考虑了界面层相邻铺层内基体、纤维的损伤状态及应力分布对层间强度和分层扩展的影响。基于ABAQUS用户子程序VUMAT,结合本文模型及层合板失效判据,建立了模拟复合材料层合板在低速冲击作用下的渐进损伤过程的有限元模型,计算了不同铺层角度和材料属性的层合板在低速冲击作用下的损伤状态。通过数值模拟与试验结果的对比,验证了本文方法的精度及合理性。     

缝合层合板的I型层间断裂韧性研究

通过缝合的方法改善织物增强复合材料层合板的层间断裂韧性.采用双悬臂梁(DCB)试验测试和研究了缝合层合板的层间断裂韧性与断裂行为.为了评价缝合工艺参数(缝合密度)对层间断裂韧性的影响, 用改进的插入型夹具在实测不同缝合工艺层合板的I型层间断裂韧性值(GIC)的基础上, 分析和阐明了缝合工艺参数(缝合密度)与GIC间的关系; 以提高层合板的平均层间断裂韧性值为目标, 以拉伸和弯曲强度为约束条件优化了缝合工艺; 采用摄影显微镜对分层断裂面进行了观察, 分析和考察了缝合对其它性能的影响.结果表明 改进的插入型夹具可方便地完成缝合层合板的I型层间断裂韧性测试; 缝合后裂纹不连续扩展, 缝合密度对裂纹扩展行为有较大影响; 随着缝合密度的增大, 层间断裂韧性值增大, 但拉伸和弯曲强度降低, 缝合密度存在最佳值.     

缝合层合板的Ⅰ型层间断裂韧性研究

通过缝合的方法改善织物增强复合材料层合板的层间断裂韧性.采用双悬臂梁(DCB)试验测试和研究了缝合层合板的层间断裂韧性与断裂行为.为了评价缝合工艺参数(缝合密度)对层间断裂韧性的影响,用改进的插入型夹具在实测不同缝合工艺层合板的Ⅰ型层间断裂韧性值(G_IC)的基础上,分析和阐明了缝合工艺参数(缝合密度)与G_IC间的关系;以提高层合板的平均层间断裂韧性值为目标,以拉伸和弯曲强度为约束条件优化了缝合工艺;采用摄影显微镜对分层断裂面进行了观察,分析和考察了缝合对其它性能的影响.结果表明:改进的插入型夹具可方便地完成缝合层合板的Ⅰ型层间断裂韧性测试;缝合后裂纹不连续扩展,缝合密度对裂纹扩展行为有较大影响;随着缝合密度的增大,层间断裂韧性值增大,但拉伸和弯曲强度降低,缝合密度存在最佳值.     

低速冲击作用下碳纤维复合材料铺层板的损伤分析

建立了一个有效计算模型, 以分析碳纤维复合材料层合板在低速冲击作用下的层内和层间失效行为。针对铺层板的层内损伤, 在基于应变描述的Hashin 失效准则的基础上, 建立了单层板的逐渐累积损伤分析模型;针对铺层板的脱层损伤, 建立了各向同性脱层损伤模型, 通过结合传统的应力失效准则和断裂力学中的能量释放率准则定义了界面损伤演化规律, 并在潜在产生脱层的区域模拟为粘结接触, 并将脱层损伤模型作为界面的接触行为。该计算模型通过商用有限元软件ABAQUS/ Explicit 的用户子程序实现。使用该计算模型对碳纤维增强环氧树脂复合材料层合板在横向低速冲击作用下的损伤和变形行为进行预测分析。数值仿真的结果与试验结果进行了比较, 取得了满意的结果, 验证了该模型的正确性。      

Failure analysis of quasi-isotropic CFRP laminates under high strain rate compression loading

Dynamic compressive strength of quasi-isotropic fiber composite is investigated experimentally and also numerically simulated. In-plane compression tests at strain rates around 400/s quasi-isotropic laminates were performed using the Split Hopkinson Pressure Bar (SHPB). The material system used was Texipreg^® HS160 REM, comprising high strength unidirectional carbon fiber and epoxy resin. The dynamic strength of quasi-isotropic laminates exhibits a considerable increase when compared to the static values. The finite-element model used ABAQUS™ three-dimensional solid elements C3D8I with 8 nodes and user-defined interface finite elements with 8 nodes [Gonçalves JPM, de Moura MFSF, de Castro PMST, Marques AT. Interface element including point-to-surface constraints for three-dimensional problems with damage propagation. Eng Comp: Int J Comput Aided Eng Software 2000;17(1):28–47; de Moura MFSF, Pereira AB, de Morais AB. Influence of intralaminar cracking on the apparent interlaminar mode I fracture toughness of cross-ply laminates. Fatigue Fract Eng Mater Struct 2004;27(9):759–66.]. These interface elements which connect the three-dimensional solid elements modeling the composite layers, include a cohesive damage model allowing the simulation of delamination initiation and propagation. Hence the present model assumes that the phenomenon of failure under these conditions is mainly dictated by interface delamination. This is supported by experimental tests which showed that all quasi-isotropic laminates split into several almost intact sublaminates. The model compares very well with experimental results, confirming the formulated hypothesis that the internal layer damage does not markedly contribute to the quasi-isotropic laminate failure.     

复合材料层合板低速冲击损伤的有限元模拟

建立了用于预测复合材料层合板在低速冲击作用下损伤的3D有限元模型。采用应变描述的失效判据来判断铺层层内的各类损伤, 如纤维断裂、 纤维挤压、 基体开裂、 基体挤裂, 并结合相应的刚度折减方案对失效单元进行刚度折减。使用界面元模拟层间区域, 结合传统的应力失效判据和断裂力学中的能量释放率准则来定义分层损伤的起始和演化规律, 提出了一种界面元损伤起始强度沿厚度方向的分布函数。通过对数值仿真结果和实验结果的比较, 验证了模型的合理性和准确性。