基于振动疲劳试验的复合材料螺栓连接预紧力松弛特性

在强迫弯曲振动试验的基础上,建立了基于模态参数(共振频率和阻尼比)表征螺栓连接结构动态性能的分析方法和试验测试手段;通过施加不同初始预紧力和激振频率,探究碳纤维/环氧复合材料螺栓连接预紧力松弛的时变行为及其影响因素。结果表明:在10h振动疲劳过程中,螺栓初始预紧力越小,激振频率越大,连接件预紧力松弛程度越大;振动疲劳损伤会导致连接结构刚度衰退、阻尼增加;复合材料螺栓连接松弛受到材料黏弹性以及界面摩擦的共同影响,其中约50%的松弛是由复合材料黏弹性效应引起的。      

复合材料螺栓连接预紧力松弛的温度-时间依存行为

针对碳纤维增强环氧基复合材料机械连接结构耐久性设计中的2个关键问题:黏弹性预紧力松弛的温度-时间依赖行为及其长期性能预测方法进行研究。建立了以蠕变全应变理论为基础的预紧力松弛预测模型。36 h的恒温耐久试验表明:初始预紧力越大,温度越高,连接件预紧力松弛速率越快;复合材料连接件的松弛速率远大于金属连接件;预紧力松弛主要表现为材料蠕变过程。对比短期试验结果表明:本模型能较好地实现对不同温度、预紧力和连接材料的松弛预测,为确立试验数据的外推方法提供了依据。      

复合材料螺栓连接预紧力松弛的改进预测模型

该文研究了复合材料构件螺栓连接预紧力的长期性能预测。在作者前期工作的基础上,建立并完善了基于蠕变全应变理论的螺栓连接预紧力松弛预测模型。主要改进包括各蠕变阶段表达式转换为分段形式和修正时-温转换因子的作用方式两方面。采用时-温等效原理获得的长时加速表征实验数据对模型进行验证。结果表明,本模型能较好地描述预紧力长期特性,且预测效果比现有的Shivakumar-Crews模型及Hook-Norton模型更好,从而证明了该模型的准确性和可靠性。     

FRP 结构螺栓接点的预紧力与应力松弛

应用螺栓-垫圈应力理论对FRP 结构搭接点预紧力进行了研究。理论计算的预紧力与垫圈的尺寸, 在结构试验初期与试验数据相吻合, 但随时间的增长, 由于基体的松弛效应, 预紧力有明显的减小, 然后在某一数值上又趋于稳定, 但重新按原预紧力紧固螺栓后, 1. 5 年的试验结果表明, 接点的应力松驰得到非常明显的抑制, 取得了预定的效果。      

复合材料螺栓连接松弛的弹-黏塑性分析方法

预紧力松弛是影响复合材料螺栓连接结构耐久性的主要原因之一。本文重点讨论导致预紧力松弛的材料蠕变与粗糙表面接触蠕变的相互影响。内容包括建立了以弹-黏塑性理论为基础的复合材料蠕变本构模型,并结合考虑粗糙表面的分形接触理论,将其推广到与时间相关的弹-黏塑性接触问题。数值结果与实验结果对比表明,考虑粗糙表面接触效应时,计算误差从2.87%~4.37%降至0.04%~0.5%,预测准确性有显著提高。表面分形参数D和G的讨论结果表明,接触表面越粗糙,预紧力越容易松弛。这对工程上通过控制表面形貌参数来改善表面接触性质具有指导意义。     

复合材料层合板螺栓连接失效的数值模拟

基于二维有限元模型, 对复合材料层合板螺栓连接的破坏失效问题利用刚度降低的方法来模拟损伤演化过程。在ABAQUS 有限元软件中加入损伤子程序对螺栓连接强度和失效模式进行数值计算。计算结果表明, 该方法能够较好地反映和符合实验结果。对计算结果分析发现, 对于正交铺层, 剪切非线性不明显, 非线性和线性分析得到的计算结果差别比较小; 而对于角铺层, 剪切效应比较显著, 非线性和线性分析两种计算结果差别很大, 而且非线性分析比较适合于角铺层层合板的计算。配合间隙的大小对挤压强度会产生很大的影响, 随着配合间隙的增大, 连接强度大大降低。对于不同的配合间隙, 出现严重损伤的位置不同。间隙小时, 在远离螺栓孔中心线处损伤最大; 间隙大时, 损伤在靠近螺栓孔中心线位置最为严重; 间隙越大, 损伤峰值越高。      

振动工况下螺栓连接自松弛机理研究

为研究振动工况下螺栓连接自松弛机理,利用ANSYS参数化语言建立考虑螺纹的三维螺栓连接有限元模型,用降温法加载预紧力,进行螺栓连接横向振动瞬态分析;研究横向激励幅值、初始预紧力、螺纹啮合面、螺栓头及螺母承压面以及连接物之间结合面的摩擦因数等对螺栓连接自松弛影响。结果表明,横向振动时完全滑移先发生于螺纹啮合面处;横向激励幅值越小、初始预紧力越大、螺纹啮合面及螺栓头、螺母承压面摩擦因数越大,螺栓连接自松弛越不易发生;激励幅值一定时连接物间结合面摩擦因数对自松弛无影响,但摩擦因数越大,发生横向振动所需剪切载荷越大。研究结果对理解螺栓连接自松弛、指导防松设计具有重要意义。     

层合板螺栓连接的渐进失效分析

以三维渐进损伤理论为基础,采用ANSYS的二次开发语言,建立了复合材料层合板螺栓连接模型,在拉伸栽荷下对含孔复合材料板进行数值失效模拟计算．分析各铺层的失效过程和失效形式特点,得到层合板的初始失效栽荷和最终失效栽荷,为复合材料含孔板强化结构的研究提供一定参考．     

A three-dimensional progressive damage model for bolted joints in composite laminates subjected to tensile loading

A three-dimensional progressive damage model was developed to simulate the damage accumulation and predict the residual strength and final failure mode of bolted composite joints under in-plane tensile loading. The parametric study included stress analysis, failure analysis and material property degradation. Stress analysis of the three-dimensional geometry was performed numerically using the finite element code ANSYS with special attention given to the detailed modelling of the area around the bolt in order to account for all damage modes. Failure analysis and degradation of material properties were implemented using a set of stress-based Hashin-type failure criteria and a set of appropriate degradation rules, respectively. In order to validate the finite element model, a comparison of stress distributions with results from analytical models found in the literature was carried out and good agreement was obtained. A parametric study was performed to examine the effect of bolt position and friction upon damage accumulation and residual strength.     

复合材料层合板多钉连接结构累积损伤过程可视化仿真模拟

建立了分析复合材料层合板多钉连接结构的三维有限元模型, 考虑了接触状态非线性和累积损伤过程非线性的影响, 运用ANSYS 中的APDL 编制程序实现了对复合材料层合板机械连接结构整个承载过程的可视化仿真模拟, 同时进行了T300/ Q Y9512 复合材料层合板多钉单剪拉伸试验。结果表明, 在一定几何尺寸下, 复合材料层合板多钉连接结构钉载分配的不均匀性在整个承载过程中并无明显改善。根据本文中提出的累积损伤模型对各孔位变形进行了计算, 计算结果与试验结果吻合较好。从累积损伤过程的仿真结果可以明显看出, 不同几何尺寸的多钉连接结构中各钉孔附近损伤的起始和发展过程具有明显的区别。优化设计结果表明, 不同钉孔处层合板厚度的改变对各钉钉载分配无明显影响, 但孔边法向和切向应力大小和分布均受到严重影响, 对整个多钉连接结构的损伤程度产生很大变化。      

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。     

High cycle fatigue mechanisms of aluminum self‐piercing riveted joints

A study examining the fatigue failure mechanism of self‐piercing riveted (SPR) joints between aluminum alloy 6111‐T4 and 5754‐O is presented in this paper. In particular, the high‐cycle fatigue behavior of the SPR joints in the lap‐shear configuration is characterized. Experimental fatigue testing revealed that failure of SPR joints occurred because of cracks propagating through the sheet thickness at locations away from the rivet. In‐depth postmortem analysis showed that significant fretting wear occurred at the location of the fatigue crack initiation. Energy dispersive X‐ray of the fretting debris revealed the presence of aluminum oxide that is consistent with fretting initiated fatigue damage. High‐fidelity finite element analysis of the SPR process revealed high surface contact pressure at the location of fretting‐initiated fatigue determined by postmortem analysis of failed coupons. Furthermore, fatigue modeling predictions of the number of cycles to failure based on linear elastic fracture mechanics supports the conclusion that fretting‐initiated fatigue occurred at regions of high surface contact pressure and not at locations of nominal high‐stress concentration at the rivet.     

接头局部增强的复合材料层合板螺栓连接试验与数值模拟

采用真空辅助成型工艺（VARI）制备了四种局部增强的复合材料层合板螺栓连接试件，通过试验及数值模拟对其力学性能进行了研究。数值研究中将复合材料层合板连接件的拉伸作为一个准静态问题，运用ABAQUS的显示分析算法及所编写用户材料子程序VUMAT对连接件进行了三维渐进失效模拟，同时在有限元模型中采用内聚力单元模拟了层合板与所设增强层的界面分层失效。数值计算结果与试验结果取得了较好的一致，验证了本文中数值方法的有效性。研究结果表明，不同的局部增强方案对复合材料螺栓连接性能的影响较大，设置[0/90/0/90]_S铺层的内置纤维增强层能显著提高层合板的螺栓连接性能。     

Influence of residual stresses from shot peening on fretting fatigue crack growth

One method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental fretting life results from specimens in a Titanium alloy with and without shot peened surfaces were evaluated numerically. The residual stresses were measured at different depths below the fretting scar and compared to the corresponding residual stress profile of an unfretted surface. Thus, the amount of stress relaxation during fretting tests was estimated. Elastic–plastic finite element computations showed that stress relaxation was locally more significant than that captured in the measurements. Three different numerical fatigue crack growth models were compared. The best agreement between experimental and numerical fatigue lives for both peened and unpeened specimens was achieved with a parametric fatigue growth procedure that took into consideration the growth behaviour along the whole front of a semi‐elliptical surface crack. Furthermore, the improved fretting fatigue life from shot peening was explained by slower crack growth rates in the shallow surface layer with compressive residual stresses from shot peening. The successful life analyses hinged on three important issues: an accurate residual stress profile, a sufficiently small start crack and a valid crack growth model.