Simulation and experimental studies of the repaired composite laminate

Firstly the compress experiment is undertaken to investigate the efficiency of repaired panels in this paper, and then modeling of the mechanical behavior of the repaired composite panel under compressive static load is conducted by using of the finite element method. The effect of geometric non‐linearity on the stress‐strain response is considered in the numeric analysis. Fatherly, the user material subroutine (UMAT) is integrated with the ABAQUS package with the geometric non‐linearity effect for studying the damage initiation and its progression in the composite structure, and quadrilateral, linear, thick shell elements (S8R) are adopted. Finally, the predicted strain distribution, damage evolution and strength of the laminate are compared with the test results.     

复合材料层合板开口翻边补强试验和数值研究

对复合材料开口补强层合板进行压缩试验研究。对复合材料开口非补强和开口补强层合板建立有限元模型,采用三维8节点厚壳单元 ( S8R),分别利用非线性分析和线性分析方法对复合材料的应力-应变响应进行模拟。通过试验数据与数值结果对比发现,几何非线性分析方法较线弹性分析方法能更好地符合试验结果。通过编制材料子程序 (UMAT) 来预测材料的损伤萌生和扩展,预测的应力-应变响应、损伤扩展、失效强度基本与试验结果一致。      

Nonlinear analysis of bonded composite patch repair of cracked aluminum panels

Analyses of adhesively bonded composite patches to repair cracked structures have been the focus of many studies. Most of these studies investigated the damage tolerance of the repaired structure by using linear analysis. This study involves nonlinear analysis of the adhesively bonded composite patch to investigate its effects on the damage tolerance of the repaired structure. The nonlinear analysis utilizes the three-layer technique which includes geometric nonlinearity to account for large displacements of the repaired structure and also material nonlinearity of the adhesive. The three-layer technique uses two-dimensional finite element analysis with Mindlin plate elements to model the cracked plate, adhesive and composite patch. The effects of geometric nonlinearity on the damage tolerance of the cracked plate is investigated by computing the stress intensity factor and fatigue growth rate of the crack in the plate. The adhesive is modeled as a nonlinear material to characterize debond behavior. The elastic-plastic analysis of the adhesive utilizes the extended Drucker-Prager model. A detailed discussion on the effects of nonlinear analysis for a bonded composite patch repair of a cracked aluminum panel is presented in this paper.     

基于损伤演化的共混/填充高聚物体系本构关系研究进展

建立复合材料的本构关系是进行材料设计的基础性工作。针对共混/填充高聚物体系,总结了微损伤演化的基体规律,讨论了在考虑微损伤演化的情况下,材料体系的宏观本构关系研究的思路与主要研究成果。本文作者指出,在共混/填充高聚物体系的破坏过程中,还有很多重要机制需要进一步研究。例如,微孔洞成核和串通汇合过程的分析、夹杂之间的相互作用和基体的非线性对损伤演化的影响以及应变率和温度对损伤的影响等都需要进一步分析。     

Whole-field strain analysis and damage assessment of adhesively bonded patch repair of CFRP laminates using 3D-DIC and FEA

The problem of damage evolution in composite structures, the way it propagates, performance and behavior is of paramount importance in utilizing them for structural applications. In the present work, an experimental study is carried out using digital image correlation (DIC) technique to analyze the behavior of adhesively bonded patch repair of carbon/epoxy unidirectional composite laminates under tensile loading. The damaged panel is repaired with both double and single sided circular patch made of same parent material. Damage initiation and propagation in notched and repaired panel as well as patch debonding is studied using 3D-DIC. Also a 3-D finite element analysis is carried out and obtained strain values are compared with the experimental prediction. They are found to be in good agreement.     

Postbuckling analysis of composite panels with imperfection damage

In order to promote the efficient use of composite materials in civil engineering infrastructure, effort is being directed at the development of design criteria for composite structures. Insofar as design with regard to buckling of composite shells is concerned, it is well known that a key step is to investigate the influence of initial geometric imperfection. At present, imperfection sensitivity study of composite shells has not been explored in detail. Thus, the objective of this paper is to present the formulation used in developing a composite shell element and to validate the element from the composite curved panel. The non-linear formulation of the shell element is based on the updated Lagrangian method. The shell element is capable of small strain and large displacement analysis with finite rotations. In order to remove the rigid body rotation, a co-rotational method is used. Subsequently the postbuckling analyses from the modeling of the curved panel with initial imperfection damage are performed to investigate the effect of initial geometric imperfection shape and amplitude. The results are used to estimate imperfection sensitivity for such panels.     

Homogenised non-linear soft inclusion for simulation of impact damage in composite structures

This paper presents the development of a homogenised non-linear soft inclusion which captures the geometric and material non-linearity of impact damage zone loaded in tension and compression. The homogenised non-linear soft inclusion can present a conservative worst case damage zone or use experimental data to mimic the behaviour of a particular damage zone in a simple and computationally efficient way that can be used as a structural design tool for composite structures subjected to impact. The development of the non-linear soft inclusion, implemented in an ABAQUS/Explicit VUMAT, is presented at element and coupon level. The non-linear soft inclusion is validated against experimental coupon data and produces a conservative worst case estimate in all cases investigated.     

不同加载条件下钢筋混凝土构件力学特性及影响规律研究

基于OpenSees中的BeamwithHingesElement单元,采用直接在截面层次上定义线弹性剪切恢复力模型,引入零长度单元利用粘结滑移材料模拟构件末端粘结滑移,考虑P-D效应引起的几何非线性,钢筋材料采用能精确模拟钢筋低周疲劳损伤与开裂现象的Mohle-Kunnath本构模型,分析构件在不同加载历史条件下的力学特性。研究构件在单调加载、非对称循环加载、对称循环加载条件下构件的力学特性,并进一步分析构件在不同循环周数条件下对称、非对称加载时构件的力学特性及钢筋材料应变随位移加载历史的变化特点,分析结果表明,钢筋混凝土框架柱在循环加载条件下的构件刚度退化、强度退化现象明显,构件的损伤程度随着剪跨比的降低和轴压比的增大逐渐加剧,构件随着循环周数的增加损伤程度更明显,钢筋材料应变随着循环周数的增加变化明显,随着轴压比增加和剪跨比的降低,构件刚度退化与强度退化随循环周数的增加更显著,钢筋应变同样有一定程度增加。     

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.     

拉伸载荷下贴补复合材料层合板的渐进损伤分析

基于连续介质损伤力学和粘聚区模型建立了贴补复合材料层合板的渐进损伤分析模型,计算了拉伸载荷下修补结构的极限强度。数值仿真结果和实验结果吻合较好,验证了该模型的有效性。基于建立的模型研究了贴补复合材料层合板的损伤演化过程,并讨论了补片参数对修补结构拉伸性能的影响。研究结果表明:补片参数对贴补复合材料层合板的破坏模式与损伤演化过程有显著影响;不同破坏模式下,补片参数的改变对修补结构极限强度的影响效果不同。研究结果可为复合材料层合板的贴补设计提供部分理论参考。     

单面贴补修理后层合板的拉伸性能

对含孔损伤复合材料层合板单面贴补后进行拉伸试验研究。测量了层合板的应变分布、修理后层合板中心点的离面位移及拉伸强度等, 考察了补片的厚度、大小等因素对修理效果的影响。结果显示, 增加补片的厚度和直径能够提高母板的承拉能力, 但是增加补片的厚度会导致层合板离面位移增大。对无侧边支持的单面贴补层合板进行计算分析时, 必须考虑偏心载荷引起的弯矩的影响。在此基础上, 采用分层损伤判据建立了三维有限元模型, 对单面贴补层合板的破坏机理和拉伸强度进行了计算和分析。结果表明, 修理后层合板的拉伸破坏是由补片或母板内与胶接面相邻的层间分层引起的; 计算结果与试验结果一致。      

Compressive properties of single-filament carbon fibres

Compressive properties of mesophase pitch-based carbon fibres (NT-20, NT-40 and NT-60) were measured using the tensile recoil test and the elastica loop test. The NT-40 fibre with a 400 GPa tensile modulus showed a smaller loop compressive yield strain and a larger recoil compressive strength compared to these values obtained from the longitudinal compression test on its unidirectional composites. Further, the recoil compressive strength of this fibre was higher than that of PAN-based carbon fibre with a corresponding modulus. Under the ideal conditions in the tensile recoil test, the strain energy was conserved before and after recoil, and the initial tensile stress and the recoil compressive stress do not coincide when fibre stress-strain behaviour is non-linear, and the non-linearity in compression and in tension is different. The difference between the composite compressive strength and the recoil compressive strength of NT-40 was quantitatively explained by taking account of the fibre compressive stress-strain non-linear relation. The difference between the loop compressive yield strain and the composite compressive strain to failure was also explained by this non-linearity.     

Identification of crack and disbond fronts in repaired aircraft structural panels with bonded FRP composite patches

Structural health monitoring of fatigue-cracked aircraft structural panels repaired with bonded FRP composite patches for extending the service life of aging aircraft has received wide attention. In this paper a method for identifying the locations and shapes of crack and disbond fronts in aircraft structural panels repaired with bonded FRP composite patches is presented. The identification is performed by minimizing the residual norm between the measured in-plane strain range on a strain measurement plane in the FRP composite patches and the calculated in-plane strain range. Several numerical examples of identification of the locations and shapes of crack and disbond fronts are examined. The effects of the number of strain measurement points, position of the strain measurement plane, and measurement errors of the in-plane strain ranges on the identification results are discussed. The validity of this identification method is verified by comparing the identification results with the exact ones.     

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。     

Damage Constitutive Equations and its Application to Fiber Reinforced Composites under Transverse Impact

Two continuous field variables, called as continuity tensor and damage variable tensor, are used to describe the anisotropic responses of an elastic-brittle material under transverse impact load. Based on the continuum damage mechanics, anisotropic damage constitutive equations in both full and incremental forms are proposed here. The expressions of effective elastic module tensor, damage variable tensor and damage propagation force tensor are further derived, and the methods for determining the tensors are explained in detail. An example of strain and damage response of a fiber reinforced composite laminated plate under transverse impact load is employed to demonstrate the application of this theory. In the example, the damage variable coupled with geometric large deformation of laminated plate is also considered. The calculating results illustrate the influence of damage on strain field in the impacted laminated plate.     

Blast response of cracked steel box structures repaired with carbon fibre-reinforced polymer composite patch

In this paper the blast resistance of cracked steel structures repaired with fibre-reinforced polymer (FRP) composite patch are investigated. The switch box which has been subjected to blast loading is chosen to study. The steel material is modelled using isotropic hardening model, pertaining to Von Mises yield condition with isotropic strain hardening, and strain rate-dependent dynamic yield stress based on Cowper and Symonds model. Three different cracked structures are chosen to investigate their capability in dissipating the blast loading. To improve the blast resistance, the cracked steel structures are stiffened using carbon fibre-reinforced polymer (CFRP) composite patches. The repaired patches reduce the stress field around the crack as the stress is transferred from the cracked zone to them. This situation prevents the crack from growing and extends the service life of the steel structure. It will be shown that CFRP repairing can significantly increase the blast resistance of cracked steel structures.     

Evaluating the localised through-thickness load transfer and damage initiation in a composite joint using digital image correlation

Experimental analysis of the stresses and strains in a glass fibre polymer composite double butt strap joint using digital image correlation (DIC) is described in the paper. Initially the strain fields through the thickness of the joint are derived from DIC measurements, capturing the effect of the initiation and development of cracks in the joint up to failure. It is demonstrated that the relatively small strains developed in the through-thickness direction are critical in the development of damage in the joint at the geometric discontinuity between the adherends. An experimental methodology is established to perform DIC at the mesoscopic scale, enabling accurate, high spatial resolution analysis of the small through-thickness strains around the discontinuity. The DIC enables the strains to be evaluated, their development monitored and hence establish their contribution to the failure process. To determine the full-field stresses in the joint experimentally derived materials properties are used. From component stress maps principal stresses maps are derived, which clearly show where the damage is initiating and its subsequent growth in the composite adherends until final failure of the joint.     

A study of damage development in a weft knitted fabric reinforced composite. Part 2: Stress–strain and early cyclic behaviour of composite laminates with realistic fabric layups

The mechanical behaviour of knitted fabric composites, consisting of four or five layers of weft knitted glass fabric in a Derakane vinyl ester matrix, has been investigated under simple tensile loading and in cyclic tests at increasing peak strains for a range of loading angles with respect to the wale direction of the knitted fabric. Stress–strain curves are non-linear from very low strains for all loading angles and they exhibit a rapidly reducing tangent modulus with increasing strain. Acoustic emission results and the results of the cyclic loading, together with comparisons with the results on the model specimens in Part 1, suggest the following sequence of damage development: damage develops initially between yarns at the loop cross-over points in the knitted fabric structure, progresses to the development of matrix cracking damage from these initiation sites at weak planes in the composite due to the fabric architecture, and then to the pulling out of loops across matrix cracks. The pulling out of loops across matrix cracks, together with matrix non-linearity, gives rise to a stress–strain curve with a very low tangent modulus at high strains and to hysteresis loops which do not close on unloading and reloading.     

二维C/SiC复合材料准静态和动态拉伸力学性能

采用分离式Hopkinson拉杆装置和电子万能试验机研究了二维C/SiC复合材料在4种应变率（0.001、0.010、90.000和350.000 s-1）下的拉伸力学性能,计算并验证了动态试验中的应力平衡状态;采用SEM分析了复合材料在不同应变率下的破坏断口和失效机制;建立了复合材料包含损伤和应变率相关的本构方程。结果表明:二维C/SiC复合材料的应力-应变曲线都表现出非线性的特征。随着应变率的增加,二维C/SiC复合材料的拉伸强度从204 MPa增加到270 MPa,增加了33%,这表明复合材料的拉伸强度具有较强的应变率敏感性。复合材料在准静态和动态加载下表现出不同的破坏模式是由材料内部界面行为的应变率效应造成的。      

SMA增强复合材料力学特性的细观力学模型

基于细观力学的理论 , 将 SMA增强复合材料视为三相等效系统进行分析 , 即基体相、 奥氏体相和马氏体相 ; 求解了 SMA增强复合材料的有效热膨胀系数和有效相变应变系数的一般表达式 , 该表达式适用于各种形状的纤维增强复合材料。在整个相变和逆相变过程中 , 奥氏体相和马氏体相都表现为弹性特性 , 避免了传统方法的强非线性问题。该模型计算过程简单 , 同时还考虑了奥氏体、 马氏体和基体间的相互作用 , 理论分析更接近实际。并分析了纤维的含量、 形状和尺寸等因素对宏观模量的影响 , 通过比较可知 , 所得结果可靠 , 为 SMA增强复合材料的设计和使用提供理论依据。