Stress analysis of multi-bolted joints for FRP pultruded composite structures

In the last decades, FRP composites have been widely used for constructing entire civil structures. One of the challenging issues for building with pultruded FRP composites is understanding the behaviour of bolted joints. In this paper, the results of a numerical analysis performed on different types of bolted composite joints with different geometry and subjected to tensile loads are reported. The aim of this study is to examine the distribution of shear stresses among the different bolts by varying the number of rows of bolts as well as the number of bolts per row. The study also considers the presence of variable diameter washers and their influence on the bearing stresses of composites with different fibre orientations. For verification of the validity of the analytical models, numerical results are compared to experimental results reported elsewhere. The results of this study showed that in multi-bolt joints, the load is not distributed equally due to varying bolt position, bolt-hole clearance, bolt-torque or tightening of the bolt, friction between member plates and at washer-plate interface. The results also indicated that in the presence of washers, the stress distributions in the fibre direction, varying fibre inclinations, are decreasing for each value of washer pressure.     

Progressive damage modeling for large deformation loading of composite structures

A nonlinear constitutive model for large deformation loading at different strain rate condition was developed to represent tensile progressive damage of the nonlinear large deformation rate dependent behavior of polymer-based composite materials. The material was characterized by using off-axis composite specimens at different strain rates. A new failure criterion was proposed for the analysis of different loading directions and strain rates. Based on a method of combining the nonlinear constitutive theory and the proposed failure criterion for different strain rates, the progressive damage behavior of large deformation composites was represented. The strength of the material was also successfully represented with a single material constant.     

Progressive damage analysis and testing of composite laminates with fiber waves

In this work, the Progressive Damage Analysis (PDA) of composite laminates with waves was developed, experimentally validated, and discussed. PDA using Continuum Damage Modeling (CDM) and Discrete Damage Modeling (DDM) was conducted. In CDM, the material continuum constitutive properties are updated to incorporate the influence of progressive damage. In DDM, the actual damage is modeled, consistent with the progressive damage model analysis and observations. A commercial finite element code ABAQUS was used for all of the analysis with specialty user subroutines for the CDM and DDM. The laminate wave parameters (wavelength and amplitude) were determined from a statistical analysis of as-manufactured laminates from failed composite wind turbine blades. Laminates with waves under tension and compression loading were considered to create a benchmark set of tests for laminates and waves, and to provide an unambiguous comparison between CDM and DDM for this type of defect. Both methods (CDM and DDM) are compared and contrasted with experimental data. It is important to note that no assumed damage (such as a crack or other discontinuity) was necessary for the analysis. The failure mode and progressive damage is a consequence of the analysis. Correlations are found with each, and the pros and cons are evaluated and discussed. Better correlations were found with DDM, but accounting for nonlinear shear in the stress–strain response using CDM in the analysis provided numerical stability and the best experimental/analytical correlations.     

Reliability analysis of nonlinear laminated composite plate structures

A procedure for the reliability analysis of laminated composite plate structures subjected to large deflections under random static loads is presented. The nonlinear analysis of laminated composite plate structures is achieved via a corotational total Lagrangian finite element formulation which is based on the von Karman assumption and first order shear deformation theory. This formulation is applicable for the nonlinear analysis of plate structures with large rotations but moderate deformation and thus accurate enough to predict the behavior of the structures at the point of failure. The reliability assessment of laminated composite plate structures with random strength subjected to random loads is approached by the determination of limit state surfaces in load space. The limit space surfaces are obtained by performing a series of first ply failure analyses following different load paths in load space using the proposed nonlinear structural analysis technique and an appropriate failure criterion. A numerical technique is then proposed to evaluate the reliability of the plate structures. Examples of the reliability analyses of laminated plates with different layer orientations subject to random loads are given for illustration.     

复合材料层合板阶梯形挖补胶接修理渐进损伤分析

建立了复合材料层合板阶梯形挖补胶接修理构型的渐进损伤分析三维有限元模型, 同时考虑了复合材料母板、 补片和胶层的损伤扩展以及它们之间的相互影响。层合板采用含正交各向异性损伤的连续介质损伤力学(CDM)本构方程进行描述, 材料积分点处的损伤状态采用二阶张量形式的内部状态变量表征。胶层采用含各向同性损伤的CDM本构方程进行描述, 材料积分点处的损伤状态采用常数形式的损伤变量表示。计算结果与试验数据符合较好, 说明该模型可较好预测挖补胶接修理的复合材料层合板拉伸强度及其失效模式。     

复合材料层合板胶接贴补修理渐进损伤分析

建立了复合材料层合板胶接贴补修理构型渐进损伤分析的三维有限元模型, 其中层合板和胶层分别采用正交各向异性损伤和各向同性损伤的连续介质损伤力学模型, 整个分析过程中同时考虑层合板和胶层的损伤形成和扩展以及它们之间的相互影响, 单向压缩载荷作用下的层合板贴补修理构型的试验数据验证了该模型的有效性, 采用该模型分析了不同的贴补修理参数对修补强度的影响。 结果表明: 当层合板补片较薄时, 补片损伤是导致修补结构失效的主要原因; 当补片较厚时, 胶层失效是导致修补结构失效的主要原因, 此时补片厚度增加并不能显著增大修补结构的极限强度。在复合材料贴补修理时需要对补片和胶层进行详细优化设计。      

Progressive damage analysis as a design tool for composite bonded joints

This paper discusses the application of progressive damage analysis (PDA) methods as a design tool. Two case studies are presented in which the effects of changing design features on the strength of bonded composite joints are evaluated. It is shown that the trends of parametric evaluations performed with full-featured PDA models can be unintuitive and the trends can be opposite to those obtained with traditional design criteria. The joint configurations that were tested exhibit multiple damage modes, requiring several different PDA tools to accurately predict the structural peak loads. For damage tolerant structures that exhibit complex sequences of multiple failure mechanisms, traditional failure prediction tools are insufficient. Parametric PDA models encompassing a bonded joint specimen's design space have the potential to reveal unintuitive and advantageous design changes.     

Progressive damage modelling of SMC composite materials

The present work deals with the modelling of damage behaviour for sheet moulding compound (SMC) composite materials using a finite element analysis package. Specifically, a comparison is made between the results obtained experimentally for a three-point bending test, and those obtained from numerical simulation using a material model already implemented. The simulation has been performed for the material models available within the PAM-CRASH software. The simulation results are compared and validated with respect to experimentation.     

Geometrically nonlinear composite beam structures: design sensitivity analysis

The purpose of this paper is to develop a finite element model for optimal design of composite laminated thin-walled beam structures, with geometrically nonlinear behavior, including post-critical behavior. A continuation paper will be presented with design optimization applications of this model. The structural deformation is described by an updated Lagrangean formulation. The structural response is determined by a displacement controlled continuation method. A two-node Hermitean beam element is used. The beams are made from an assembly of flat-layered laminated composite panels. Beam cross-section mass and stiffness property matrices are presented.

Design sensitivities are imbedded into the finite element modeling and assembled in order to perform the structural design sensitivity analysis. The adjoint structure method is used. The lamina orientation and the laminate thickness are selected as the design variables. Displacement, failure index, critical load and natural frequency are considered as performance measures. The critical load constraint calculated as the limit point of the nonlinear response is also considered, but a new method is proposed, replacing it by a displacement constraint.     

Homogenization and damage for composite structures

After presenting shortly the main results of the homogenization theory for periodic media, we give two applications related to damage evaluation and simulation for composite materials: (i) simulation of the evolution of damage by fibre rupture in a unidirectional composite, involving parameters defined on the microscale; and (ii) prediction of debonding near an unloaded boundary in a stratified structure, using boundary layer asymptotic expansions.     

基于渐进损伤分析的复合材料螺栓连接强度包线法研究

针对中国民机采用T800级复合材料这一新材料体系而基础数据匮乏的现状,采用渐进损伤分析(PDA)替代试验以显著降低研究周期和成本。综合渐进损伤方法和工程算法各自的优点,提出以渐进损伤分析替代应力集中减缓因子(SCRFs)测定试验,进而建立强度包线,并进行多钉连接强度预测的数值策略。为验证该数值策略的可行性,针对典型铺层应力集中减缓因子,测定试样,并开展渐进损伤分析,获得了试验件强度预测值来计算应力集中减缓因子,采用旁路载荷修正的强度包线法,绘制了典型铺层复合材料多钉连接旁路载荷修正强度包线,预测多钉连接的失效载荷,并与试验结果进行对比。结果表明:采用该数值策略预测的强度包线、多钉连接的失效载荷和失效模式均与试验结果吻合良好,证明了该数值策略的可行性。     

三维复合材料层合板渐进损伤非线性分析模型

为有效反映复合材料层合板层间相互作用和材料损伤非线性,建立了中等尺度的三维复合材料层合板渐进损伤分析模型。非线性渐进损伤分析过程包括应力求解、材料损伤失效判据及材料性能退化方案3个方面。讨论了损伤材料性能退化方案,引入与材料损伤模式相对应的损伤变量表征材料点的损伤状态,材料的刚度矩阵按损伤变量退化。基于该模型可成功预测复合材料层合板损伤起始、扩展直至最终失效的整个过程和极限强度。经文献试验数据验证,12种不同铺层顺序层合板的计算强度与试验数据均吻合较好,表明该模型在复合材料层合板极限强度预测上的有效性。     

基于率相关三维弹塑性损伤模型的复合材料渐进失效分析

建立了一个同时考虑复合材料非线性力学响应、应变率效应和损伤累积导致材料属性退化的弹塑性三维损伤本构模型。采用改进的塑性力学模型表征材料在动态荷载下的非线性力学行为。为准确预测复合材料在动态荷载下的弹塑性力学响应,引入了率相关放大系数对准静态下的塑性强化函数进行修正。采用“断裂带模型”对已开发的本构模型软化段进行规则化,以减轻有限元分析结果的网格敏感性。采用分区反抛物线插值法对基体损伤初始断裂面角度及纤维扭结/劈裂平面角度进行求解。开发包含数值积分算法的用户材料自定义子程序VUMAT,并嵌于有限元程序ABAQUS V6.14中,对力学行为展现显著非线性力学效应和应变率效应的IM7/8552碳纤维/环氧树脂复合材料层合板进行了渐进失效分析,验证本文提出的材料本构模型的有效性。结果显示,预测结果与已报道的试验结果吻合良好,表明已建立的率相关三维弹塑性损伤本构模型能准确预测此类复合材料层合板的在动态荷载下的力学行为,为复合材料构件及其结构设计提供了一种有效的分析方法。     

Diametral compression of pultruded composite rods

Diametral compression tests were performed on pultruded composite rods comprised of unidirectional glass or carbon fibers in a common matrix. During compression tests, acoustic emission (AE) activity was recorded and images were acquired from the sample for analysis by digital image correlation (DIC). In both composite systems, localized tensile strain developed in the transverse plane under the load platens prior to failure, producing non-linearity in the load–displacement curve and AE signals. In situ SEM diametral compression tests revealed the development of matrix microcracking and debonding in regions of localized strain, perpendicular to the tensile strain direction (parallel to the load axis). Comparison of linear finite element simulations and experimental results showed a deviation from linear elastic behavior in the load displacement curve. The apparent transverse modulus, in plane shear modulus, and transverse tensile strength of the GF rod was greater than that of the CF rod, and fracture surfaces indicated greater fiber/matrix adhesion in the GF system compared to the CF system. A mixed mode fracture surface showed that two failure modes were active – matrix tensile failure and matrix compression failure by shear near the loading edge.     

Application of refined theory in the nonlinear analysis of composite laminated structures

A higher order theory for the analysis of plates and shells is presented. The theory considers transverse shear and normal deformation and is based on hypotheses in which nonlinearly elastic or elastoplastic behavior of the materials is included. The number and sequence of transversely isotropic layers are arbitrary and the layers may differ significantly in their physical and mechanical properties. A discrete-continuous scheme of the finite element method are developed for the implementation of the nonlinear higher order theory. In this scheme, the degrees of freedom of the elements are independent of the number of layers. The results presented illustrate the importance of modelling both transverse shear and normal deformation in the nonlinear analysis of laminated structures.The research presented in this study was supported by a Core Programmes Grant from the Foundation of Research Development of South Africa     

复合材料结构冲击损伤检出概率的试验研究与分析

复合材料结构在服役过程中遇到的损伤主要来自于意外冲击, 会产生永久性的凹坑。为了得到凹坑的检出概率, 采用最大似然估算法对一般目视检测(GVI)和详细目视检测(DET)的两种损伤检出概率模型(POD评估模型, 即累积对数正态模型和对数奇函数模型)中的参数进行评估; 开展详细目视检测的试验研究, 并根据检测数据对POD模型进行了验证。结果表明: 凹坑深度的检出概率曲线更符合累积对数正态模型。绿色板的凹坑比黑色板更容易被检出, 且检测角越大越容易被检出。目视勉强可见损伤(BVID)的门槛值在0.35~0.8 mm之间。     

Progressive damage assessment of centrally notched composite specimens in fatigue

The aim of this study is to assess the residual properties and the corresponding damage states within centrally notched quasi-isotropic [0/−45/+45/90]_S T650/F584 (Hexcel) carbon-fiber/epoxy composites subjected to fatigue loading using Digital Image Correlation (DIC), radiography, and a non-contact vibration measurement technique. Quasi-static tests were performed on virgin samples using DIC to determine the full-field in-plane strains at different applied load levels. Fatigue tests were interrupted during the fatigue lifetimes in order to perform quasi-static tests with DIC measurements. Non-contact vibration measurements were performed to investigate the effect of fatigue damage on residual frequency responses. X-ray computed tomography was used to determine the type, location, and extent of fatigue damage development. The results provide an important step in the validation of DIC and vibration response as a powerful combined non-destructive evaluation tool for monitoring the development of fatigue damage as well as predicting the damage level of notched composite materials.     

层合轧压SMAT纳米化304不锈钢复合材料的渐进损伤模拟

基于表面机械研磨处理技术(SMAT)和温轧工艺,可以加工出具有高强度和理想韧性的层合纳米化结构复合材料。为了研究层合轧压SMAT纳米化304不锈钢的变形行为以及随后的损伤起始与演化过程,采用内聚有限元方法,建立了用于预测该复合材料力学性能的有限元模型。基于这个模型,评估了材料中纳米晶层性质,包括法向内聚强度、切向内聚强度、损伤演化断裂能和体积含量对于材料整体强度和韧性的影响。通过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,同时预测结果表明增加纳米晶层的内聚强度和减小其断裂能都能提高材料的韧性;增加纳米晶层的体积含量,材料的整体韧性降低,但是强度会增加。