考虑三维应力的复合材料层压板疲劳寿命分析

由于层间应力的存在,受面内载荷作用的复合材料层压板实际处于多轴应力状态。构建了由刚性元、弹簧元和二维板元构成的准三维有限元模型,结合单向板在典型应力状态下的疲劳试验结果和疲劳损伤模型,发展了一种考虑三维应力的、预测任意铺层多向层压板疲劳寿命的分析方法,包括应力分析、静力和疲劳累积损伤失效分析及材料性能退化3个主要部分,能够模拟面内和层间损伤产生、发展直至层压板整体破坏的完整过程,并得到疲劳寿命。对2种T300/QY8911多向铺层板进行了实际计算,寿命预测结果与试验结果吻合较好。      

3238A/EW250F复合材料的弯曲疲劳性能

运用三点弯曲加载方式对3238A/EW250F复合材料的弯曲疲劳性能进行了研究。结果表明:该复合材料弯曲疲劳寿命的分散性较小,所有试验点的疲劳寿命都满足90%置信度的中值寿命;所有试样均在有效部位发生破坏,且试样表面出现分层后,还有较长的一段寿命期;试验测得0°方向铺层3238A/EW250F复合材料的条件疲劳极限以动应力表示为σr=73.2 MPa。     

低速冲击下复合材料层板压缩许用值

为评估航空结构中常用的T300级和T800级2种碳纤维/环氧树脂复合材料层压板的冲击后压缩许用值,对2种材料体系下具有不同厚度及铺层的层板进行了低速冲击和冲击后压缩试验;讨论了冲击能量、凹坑深度、损伤面积及冲击后剩余压缩强度等之间的关系,以及厚度、铺层、表面防护等因素对其造成的影响;重点关注了2种材料体系下各组层板的目视勉强可见冲击损伤(BVID)形成条件以及含BVID层板的剩余强度.结果表明:厚度及铺层对层板的凹坑深度-冲击能量关系影响较大,而对冲击后压缩强度-凹坑深度及冲击后压缩破坏应变-凹坑深度关系影响较小,且在相同铺层比例下,BVID对应的冲击能量随厚度近似呈线性增长.X850层板的损伤阻抗性能明显优于CCF300/5228层板的,但二者损伤容限性能相当.加铜网、涂漆等表面处理显著提高了层板的损伤阻抗,但对损伤容限性能影响不大;在损伤不超过BVID时,所有CCF300/5228试件的压缩破坏应变均大于4 000 με,而X850材料体系下压缩破坏应变均在3 000 με之上.     

压缩预应力对复合材料层板抗冲击损伤性能的影响

为确定压缩预应力对复合材料层板抗冲击损伤性能的影响,首先对不同压缩预应力下的碳纤维/双马树脂CCF300/5428层板进行了低速冲击和准静态压痕试验,然后通过热揭层和冲击后压缩试验分别得到了层板分层面积和剩余强度。结果表明:压缩预应力会大幅降低层板的接触刚度和弯曲刚度,从而导致相同冲击能量下层板凹坑深度和背部基体开裂长度增大;对于准静态压痕过程和相同冲击能量下的冲击过程,分层起始载荷和峰值载荷均随压缩预应力的增大而减小;在相同冲击能量下,随着压缩预应力的增大,层板内部分层总面积及冲击能量吸收比不断增大,剩余压缩强度不断降低。因此,压缩预应力会降低复合材料层板的冲击损伤阻抗,对损伤容限性能不利,在对承受压缩载荷结构的试验验证过程中应考虑压缩预应力对抗冲击损伤性能的影响。      

温度对碳纤维平纹布正交层合板拉伸疲劳性能的影响

温度环境可降低复合材料的疲劳性能,在确定复合材料结构寿命时须考虑温度的影响.本试验测量了碳纤维平纹布正交层合板在低温干态(CTD)、常温干态(RTD)和高温干态(ETD)环境下的拉伸疲劳性能,获得了三种环境下复合材料的S-N曲线,分析了温度对复合材料疲劳性能的影响.基于试验结果,建立了温度条件下复合材料疲劳性能有限元分析模型,对复合材料的疲劳寿命进行了估算并分析了其损伤机理.线性拟合结果显示:在106 疲劳寿命下,与RTD环境疲劳最大应力相比,CTD环境疲劳最大应力略有降低,而ETD环境疲劳最大应力下降明显. CTD环境下,试验件的疲劳破坏断口比较齐整,纤维基本在同一纵向位置断裂,断口附近基体基本完好,无分层现象;RTD环境下,试验件断口处也没有明显分层现象;ETD环境下,试验件出现了明显的分层,同时还有纤维拔出,且断口处基体开裂程度严重.有限元分析表明,CTD环境下试验件的疲劳断裂呈现脆断的特征,断裂截面平整,断裂区域窄;RTD与ETD环境下试验件的纤维疲劳断裂损伤的断裂截面不平整,断口不一,断裂区域相对较宽.     

弯曲载荷下碳纤维/双马复合材料湿热特性实验研究

研究了碳纤维/双马复合材料单向层板在三点弯曲载荷作用下70℃水浸的湿热特性,测试分析了层板的吸湿量、玻璃化转变温度、弯曲性能及其破坏模式随湿热时间的变化,通过对比研究弯曲载荷作用和无外载荷条件下层板湿热性能的差异,探讨了应力对复合材料湿热行为的作用规律。结果表明：弯曲载荷作用下试样整体的吸湿规律、剩余弯曲性能等方面与未加载情况相比没有显著差异,但弯曲加载试样吸湿饱和前相同吸湿率时玻璃化转变温度下降更多;弯曲加载试样受压应力部分吸湿速度小于受拉应力部分,同时弯曲断口的扫描电镜照片显示,与未加载情况相比弯曲加载试样受拉应力部分树脂与纤维粘结较弱,说明应力会对复合材料湿热性能产生影响,并且不同方向应力的作用规律不同。     

复合材料层板低速冲击后疲劳性能实验研究

通过对T300/5405复合材料层板进行低速冲击后的压-压疲劳实验,研究含不同冲击损伤层板的压缩性能与其在多级应力水平下的疲劳寿命与损伤扩展,并讨论冲击能量、应力水平、损伤扩展对层板疲劳寿命的影响。结果表明:冲击损伤明显降低层板的剩余强度;在低应水平下,冲击能量越大,含冲击损伤层板的疲劳寿命越小;疲劳实验中损伤经历平稳扩展和快速扩展两个阶段,其中平稳扩展阶段约占总体寿命的80%,快速扩展阶段约占总体寿命的20%,损伤扩展速率随着应力水平降低而减小。     

预应变法调整芳纶－铝层板的残余应力

采用预应变法能有效地调整芳纶-铝层板的层间残余应力[1]。本文分析了3/2铺层的芳纶-铝层板的残余应力与所施加的预应变的关系;并用腐蚀法测定了几种不同的预应变层板中铝层的残余应力;研究了施加预应变对层板的拉伸性能和疲劳性能的影响。      

预应变法调整芳纶－铝层板的残余应力

采用预应变法能有效地调整芳纶-铝层板的层间残余应力[1]。本文分析了3/2铺层的芳纶-铝层板的残余应力与所施加的预应变的关系;并用腐蚀法测定了几种不同的预应变层板中铝层的残余应力;研究了施加预应变对层板的拉伸性能和疲劳性能的影响。     

含孔复合材料层合板孔边的应力集中

针对含孔有限宽复合材料层合板的应力集中问题,提出一种计算孔边应力分布及应力集中因子的方法：先利用经典层板理论,将复合材料层合板化归为各向异性板;再将各向异性板等效转换为一偏轴拉伸的单向纤维层板;最后利用含孔偏轴单向板的孔边应力计算公式来分析一般铺层层合板孔边应力集中情况。根据所推导的计算公式,分析讨论了板宽/孔径比、铺层比例、铺层方式、材料性能参数等因素对孔边应力集中的影响。     

Describing the Flexural Behaviour of Cross-ply Laminates Under Cyclic Fatigue

The objective of this work is to derive modelling of the fatigue behaviour of cross-ply laminates from the experimental results obtained in the case of three-point bending tests. Modelling the fatigue behaviour is based on the stiffness reduction of test specimens. Firstly, experimental results are described using interpolation functions. Then, the characteristic coefficients of these functions are studied as function of the laminate properties and loading conditions. This approach allows to predict the fatigue life of composite laminates while avoiding a large number of fatigue tests. Wöhler curves are used to compare the experimental and analytical results, and a good agreement is found between the results. Next, a simple approach is considered to define a damage parameter. It is based on the analogy between the mechanical behaviour and the fatigue damage evolution of composite laminates during fatigue tests. The developed models are applied to analyse the influence of constituents on the fatigue behaviour and damage development of composite materials under fatigue loading.     

Synergistic effect of cyclic mechanical loading and moisture absorption on the bending fatigue performance of carbon/epoxy composites

This study investigates the effects of hygrothermal condition on the static bending strength, the bending fatigue and the residual bending strength of carbon/epoxy composite laminates. Displacement-controlled three-point bending fatigue tests were conducted on carbon/epoxy composite laminates of immersion for 0, 7 and 14 days, respectively. After 40000 cycles the fatigue test was stopped and the residual properties were measured on the tested specimens. The effects of hygrothermal condition and fatigue on the micrographs of the specimens have been studied by the metallurgical microscope and scanning electron microscope. Experimental results reveal that moisture absorption can accelerate damage propagation of the composite; the accumulation of irreversible structural damage under the cyclic loading leads to a change in the macroscopic mechanical properties of the composites; the bending strength and the residual strength retention decreased with increased immersion time; hygrothermal aging lowered the threshold level for the onset of fatigue.     

纸板厚度方向的力学模型建立

目的理论推导纸板在受到z向压缩和弯曲时的本构模型。方法对纸板进行z向压缩时,用Ramberg-Osgood方程描述加载过程的应力-应变关系。根据纸板的内部结构特点,用多孔弹性材料模型描述卸载过程的应力-应变关系。基于对纸板弯曲时变形过程的分析,利用复合材料层板模型和纤维网络模型描述弯曲时纸板层应力-应变关系。结果该模型能描述纸板在厚度方向上的力学特性,为建立完整的纸板本构建模提供参考,同时为有限元模拟纸板厚度方向变形提供依据。     

非对称/非均衡复合材料层压板弯曲与扭转耦合变形试验方法

对几种铺层的非对称/非均衡层压板在纯弯载荷下的耦合变形进行了模拟分析, 并进行了板宽、 板厚等参数的影响分析, 在此基础上设计了一套四点弯曲柔性支持装置。采用该装置对两种铺层的非对称/非均衡层压板进行了柔性四点弯曲试验, 并与常规四点弯曲试验进行了对比。柔性四点弯曲装置对非对称/非均衡层压板施加弯曲载荷的同时没有限制试件的扭转变形, 而常规四点弯曲几乎完全限制了非对称/非均衡层压板的扭转变形。柔性四点弯曲加载装置基本上达到了设计要求, 可用于非对称/非均衡层压板弯-扭耦合系数的测试。      

飞机整体结构件的“加工变形-疲劳寿命”多目标结构优化方法

在毛坯制造过程中,材料力学性能的非均匀性导致铝合金厚板内产生残余应力,以致在后续的高速切削加工过程中,随着材料的大量去除,残余应力的释放使得整体结构件发生变形,严重影响着整体结构件的尺寸稳定性。因此,研究零件结构与零件变形之间的关系对于实现加工过程的高效化和精密化至关重要。首先,将铝厚板内残余应力的释放合理地等效为外载荷的施加,利用材料力学弯曲变形公式建立铝厚板在厚度方向上加工变形的挠度模型。由实际加工测量可知:加工变形的公式解析值、有限元仿真值与实际测量值吻合得很好。为了进一步分析零件结构与疲劳寿命之间的关系,通过名义应力法对零件的最小疲劳寿命与疲劳载荷下零件的最大应力进行等效以简化分析,对部分具有代表性的结构进行静力分析后将其作为样本进行神经网络拟合,得到了以3个腹板位置为输入、零件最大加工变形及最大疲劳应力为输出的神经网络模型。最后利用神经网络模型构建了一个使得最大加工变形和最大疲劳应力都尽可能小的多目标优化问题,使用遗传算法求解该多目标问题后取得的最优解为:3个腹板与零件底部距离分别为8.868 mm、27.992 mm、28.000 mm,此时零件的最大加工变形为0.088 mm,最小的随机疲劳载荷寿命为4.432×107次。     

不同孔隙率CFRP层合板静态力学性能研究

为了研究孔隙率对织物碳纤维/环氧树脂复合材料层合板静态力学性能的影响规律,分别测量了孔隙率为0.33%至1.50%的CFRP层合板的弯曲强度和层间剪切强度,并进行有限元模拟.在适用于复合材料单向板的改进Hashin失效准则基础上,建立了适用于织物纤维增强复合材料静态力学强度的失效准则.通过引入复合材料基本强度参数预测不同孔隙率CFRP层合板的力学性能,结合刚度突然退化模型,采用ABAQUS软件建立了有限元模型.试验结果表明,随着孔隙率的增加,复合材料层合板的弯曲强度和层间剪切强度均呈下降趋势.有限元模型较为准确地预测了不同孔隙率织物碳纤维/环氧树脂复合材料层合板的弯曲强度和层间剪切强度.     

Fatigue life prediction of composite laminates by FEA simulation method

This paper is to simulate the fatigue damage evolution in composite laminates and predict fatigue life of the laminates with different lay-up sequences on the basis of the fatigue characteristics of longitudinal, transverse and in-plane shear directions by finite element analysis (FEA) method. In FEA model, considering the scatter of the material’s properties, each element was assigned with different material’s properties. The stress analysis was carried out in MSC Patran/Nastran, and a modified Hashin’s failure criterion was applied to predict the failure of the elements. A new stiffness degradation model was proposed and applied in the simulation and then a strength degradation model was deduced, which is coupled with the presented stiffness degradation model. The reduced or discounted elastic constants were determined based on the failure mechanism of the laminates and the restrictive conditions of orthotropic property. The fatigue behavior and fatigue life of six kinds of E-glass/epoxy composite laminates with different lay-up sequences were experimentally studied, and the S–N curves and stiffness degradation models in longitudinal, transverse and in-plane shear direction were obtained. These fatigue data were adopted in the simulation to simulate fatigue behavior and estimate life of the laminates. The simulation results, including the fatigue life predicted and the residual stiffness, were coincident with the experimental results well except for the quasi-isotropic laminate for the lack of consideration of the out-of-plane fatigue character in the simulation.     

基于频率变化预测玻璃纤维增强树脂复合材料层合板的剩余疲劳寿命

复合材料结构在疲劳过程中的累积损伤将导致结构刚度下降,并进一步引起结构的动态参数如频率发生衰减。因此,可以将结构疲劳状态与结构频率联系起来,基于频率预测结构的剩余疲劳寿命。本文首先基于复合材料在纵向、横向和面内剪切三个方向的疲劳特性,结合ABAQUS与Umat子程序开发了三维有限元模型模拟复合材料层合板中的疲劳损伤演变,并构建了不同疲劳状态下对应的模态分析模型,由此获得了疲劳过程中的频率衰减曲线。之后,基于疲劳过程的频率变化量训练了人工神经网络,用于预测玻璃纤维增强复合材料层合板的剩余疲劳寿命。特别地,在当前的数值模型中为每个单元分配了符合高斯正态分布的材料属性,以模拟实际情况下复合材料性能的离散性。结果表明,疲劳模型数值模拟结果与已有文献的疲劳实验数据吻合,基于频率变化量训练的人工神经网络可以成功预测玻璃纤维增强复合材料试件的剩余疲劳寿命。      

复合材料层合板低速冲击后压-压疲劳试验研究及寿命预报

进行了复合材料层合板低速冲击和冲击后压-压疲劳试验。在疲劳试验过程中详细测量了损伤扩展情况,获得了损伤扩展规律。将冲击损伤等效为一圆形开孔,应用含椭圆形夹杂的杂交应力单元分析含圆孔有限大板的应力分布,采用特征曲线和点应力判据相结合的方式并通过引入损伤扩展规律建立了含低速冲击损伤复合材料层板压-压疲劳寿命预测模型。通过与试验数据的对比,证明了该模型的有效性。同时,该模型还可预报在疲劳载荷下含冲击损伤层板的剩余压缩强度。     

Qualitative separation of the effect of voids on the bending fatigue performance of hygrothermal conditioned carbon/epoxy composites

This study investigates the effect of voids on the static bending and bending fatigue properties of T300/914 composite laminates that are exposed to room temperature, hygrothermal and drying environment, respectively. Displacement-controlled three-point bending fatigue tests were conducted on specimens, while damage in the composite was continuously recorded with a metallurgical microscope. After 40,000 cycles the fatigue test was stopped and residual properties were measured on the tested specimens. Reduction in material strength was found to depend on the level of the specimen’s void content. The changes in weight and size gradually increased with an increase in porosity from 0.33% to 1.50% in the hygrothermal environment. It is found that the maximum rate of dimensional change occurred in the thickness direction. Both bending strength and fatigue performance were reduced with increasing porosity. The damage evaluation of aged specimens was more severe than non-aged and drying specimens.