层压复合材料分层扩展分析的虚拟裂纹闭合技术及其应用

提出一种基于虚拟裂纹闭合技术的界面元模型,用以模拟复合材料的分层破坏和预测结构的承载能力。界面元被嵌入在模型分层扩展路径上,计算结构的能量释放率,结合幂指数破坏准则,模拟复合材料的分层扩展。对由于裂尖单元长度不同所带来的分析误差进行了适当的修正,以降低网格粗细变化所带来的不利影响。为了检验该界面元的可靠性,分别将其应用于对双悬臂梁(DCB) 模型、端边切口(ENF) 模型和混合模式弯曲(MMB) 模型的分层扩展分析中。计算结果与解析解基本吻合,从而验证了采用该界面元模拟复合材料分层破坏的可行性。用该方法对3个含有不同初始损伤复合材料T型接头的界面拉脱分层破坏进行数值模拟,计算结果与试验数据吻合良好。      

基于剩余应变的复合材料结构寿命预测

为实现对复合材料结构的寿命预测,对已有的复合材料疲劳寿命预测模型进行了研究,确定了基本的刚度降模型,提出了剩余应变的概念,并将其应用到渐进疲劳损伤方法中,以Abaqus为平台,编写UMAT子程序,实现了对复合材料结构的寿命预测及疲劳损伤扩展分析.针对某碳纤维增强复合材料TS800开展相关试验,试验结果与预测结果吻合较好.研究表明,本文所改进的渐进疲劳损伤方法能较好地完成对复合材料结构的寿命预测.     

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。      

卫星桁架复合材料多连通接头性能的测试与分析

以典型的卫星桁架结构接头为研究对象,利用三维或二维半编织技术和三维有限元分析技术,对碳纤维增强复合材料接头进行了结构设计、试验测试和数值分析。同时对用特殊编织技术制成的复合材料接头进行了拉伸、压缩和弯曲试验,并对实验结果进行有限元分析和处理。结果显示,接头的断裂强度为400MPa,理论预测断裂位置与试验结果吻合。卫星天线桁架接头用复合材料代替某些航天航空材料是可行的。      

复合材料三维损伤模型在大开口结构强度预测中的应用

为了预测复合材料大开口结构的强度及损伤扩展情况,开展了3种不同复合材料三维损伤退化模型(瞬间退化模型、渐进损伤退化模型和连续损伤退化模型)在复合材料大开口结构强度预测中的应用研究,完成了含口框加强件的复合材料大开口结构面内纯剪切试验,并采用3种损伤退化模型对试验件及相关文献算例进行了数值模拟。数值模拟结果与试验结果的对比表明:3种损伤退化模型均可以较为精确地预测复合材料大开口结构的极限强度;与其他模型相比,连续损伤退化模型的预测精度最高、通用性最好且网格依赖度最低。研究结果为复合材料大开口结构力学性能的研究提供了理论指导。     

基于超高周三点弯曲的复合材料试验方法

针对复合材料的超高周疲劳问题,提出了一种基于超声疲劳实验装置对复合材料进行三点弯曲试验的方法;阐述了进行复合材料超高周疲劳试验的必要性;利用ABAQUS软件分析了变幅杆、连接杆及试件的模态,得出设计的该装置及试件的谐振频率满足试验需求;针对试验过程中的温升现象,提出了液氮与压缩空气复合式冷却系统。结果表明,该方法大大缩短了试验周期,提高了试验效率,消除了试样在高频加载过程中的热效应,保证了实验结果的有效性。     

基于力电响应的压电悬臂梁疲劳寿命预测EI北大核心CSCD

针对压电振动能量收集器的疲劳问题,旨在研究基于力电响应的压电悬臂梁振动疲劳寿命预测方法。结合压电悬臂梁振动疲劳试验测得的试验结果,以剩余刚度理论和“两段式”疲劳寿命分析模型为基础提出一种基于谐振频率的压电悬臂梁疲劳寿命预测方法;进一步分析振动疲劳过程中谐振频率变化与负载功率变化的关系模型,提出一种基于负载功率的压电悬臂梁疲劳寿命预测方法。结果表明:基于谐振频率的压电悬臂梁疲劳寿命预测方法的平均误差在10%~20%之间,其误差范围控制在1.5倍误差线以内;而基于负载功率的压电悬臂梁疲劳寿命预测方法的平均误差集中于25%左右,误差范围均控制在2倍误差线以内且属于偏安全的预测效果。     

三维正交机织复合材料低周弯曲疲劳力学性能有限元模拟

为了深入理解三维正交机织复合材料（3DOWC）疲劳性能,改进材料抗疲劳设计,结合三维正交机织复合材料试样经纱方向准静态三点弯曲及60%应力水平下的三点弯曲疲劳实验与ABAQUS有限元软件,构建了全尺寸三维实体模型,研究了三维正交机织复合材料在低周循环载荷下的弯曲疲劳性能,经分析得到循环加载下模型应力分布情况和疲劳损伤形态。结果表明：经纱为材料最重要的承载部件,中间加载区域为材料应力集中区,损伤主要位于应力集中区的Z纱通道处的经纱上,随着循环增加,逐渐在中心加载区域的上部和下部形成三角形损伤区域,该研究在复合材料设计与优化中具有指导意义。     

钢筋混凝土梁受弯破坏及尺寸效应的细观模拟分析

钢筋混凝土构件破坏的尺寸效应取决于混凝土材料的非均质性以及钢筋/混凝土相互作用。该文借助混凝土细观结构特征,基于非线性弹簧单元来描述钢筋与混凝土之间的相互作用,建立了钢筋混凝土梁破坏行为模拟的三维细观数值模型。在模拟结果与试验结果吻合良好的基础上,拓展模拟了更大尺寸梁的弯曲大变形破坏行为,并分析了单调及循环加载模式对不同尺寸悬臂梁受弯破坏及名义抗弯强度影响规律。模拟结果分析表明：1)该文工况下钢筋混凝土悬臂梁的弯曲破坏存在尺寸效应,弯曲强度随梁深增大而减小; 2)循环加载下,混凝土、钢筋以及两者间的粘结性能由于低周疲劳而使得梁的弯曲破坏呈现出脆性特征; 3)相比于单调加载,循环加载条件下,悬臂梁的破坏具有更强的脆性,名义抗弯强度尺寸效应更明显。     

考虑平均应变的低周疲劳寿命本征损伤耗散预测方法

金属构件疲劳破坏是工业中常见的破坏形式。为了提高构件疲劳寿命的预测精度，针对低周疲劳载荷下平均应变对疲劳寿命的影响，基于连续介质损伤力学及其不可逆热力学框架，并引入Ramberg-Osgood循环本构模型，以等本征损伤耗散功作为等寿命条件，建立了一种考虑平均应变的低周疲劳寿命预测模型。为对比验证新建模型的有效性和先进性，采用新建模型、修正的Ohji模型、Sandor模型和Wei-Wong模型分别对叠加平均应变的45钢和2124-T851铝合金的低周疲劳寿命进行了预测，并与对应的试验结果进行对比。结果表明：新建模型的预测结果与试验结果吻合较好，其预测效果优于现有模型。基于本征损伤耗散理论的疲劳寿命预测方法为金属材料疲劳寿命的预测提供了新思路。     

基于模态频率的缺口梁疲劳裂纹扩展寿命预测

从试验出发,研究了含V型缺口悬臂梁在循环载荷作用下的疲劳裂纹扩展特性及其模态频率变化规律,分析了模态频率与裂纹扩展增量间的关联性。将裂纹扩展增量作为损伤参量,建立了基于模态频率下降率与损伤参量的关系。基于损伤力学,建立了裂纹损伤与循环加载次数的演化模型。结合模态频率下降率与裂纹损伤参量的关系,提出了一种基于模态频率下降率的缺口梁疲劳裂纹扩展寿命预测方法,实现了基于当前裂纹损伤和对应循环次数的疲劳裂纹扩展剩余寿命预测。结果表明,模态频率下降率对缺口梁的疲劳裂纹扩展寿命敏感,该方法预测的疲劳裂纹扩展寿命与实测的疲劳裂纹扩展寿命基本吻合。     

LOW CYCLE FATIGUE BEHAVIOUR OF STRUCTURAL STEEL MEMBERS AND CONNECTIONS

Abstract— An experimental study was carried out to investigate the low-cycle fatigue behaviour of steel beam and beam-column members, as well as of steel connections, with particular emphasis on beam-to-column joints. Several full-scale specimens were tested, in a multi-specimen testing program, using constant-amplitude displacement histories, to develop a cumulative-damage model. Variable amplitude tests were carried out in order to verify the validity of the proposed model.
It is shown that the S-N curves given by codes for high-cycle fatigue can be used to interpret the low-cycle fatigue behaviour of steel members and connections. Miner's rule can be used, together with the previously defined S-N curves and with a cycle counting method to define a unified cumulative damage model valid for both high and low cycle fatigue. A general failure criterion is proposed which considers the actual energy dissipation through plastic deformation.     

A two-dimensional in situ fatigue cohesive zone model for crack propagation in composites under cyclic loading

In this paper a two-dimensional fatigue cohesive zone model (CZM) for crack propagation in composites under cyclic loading has been formulated and validated through successful predictions of fatigue crack growth under pure and mixed mode conditions for several different composites. The proposed fatigue CZM assumes simple power-law functions for fatigue damage accumulation of which the damage parameters can be calibrated from simple fatigue tests under pure mode I and mode II conditions. The model relies solely on the in situ cohesive responses for fatigue damage rate calculation, enabling the differentiation of the local elemental load history from the global load history. An effective cycle jump strategy for high-cycle fatigue has also been proposed. It has been demonstrated that once calibrated, the fatigue CZM can predict the Paris laws for the pure modes. Furthermore, it can predict the Paris laws of any mixed-mode conditions without the need of additional empirical parameters. This is of significant practical importance because it leads to greatly reduced experimental needs for mixed mode crack propagation widely observed in composites under cyclic loads.     

受移动质量作用的一类变截面梁横向振动分析

采用半解析法计算了横截面积、横截面的惯性矩沿轴线方向以幂函数规律变化的一类变截面梁的固有频率和振型,通过与有限元法的计算结果比较说明了半解析解的精确性。在此基础上,考虑牵连惯性力、相对惯性力、科氏力和离心惯性力的影响,推导了变截面梁受移动质量作用的横向弯曲振动方程,并采用Newmark法进行数值求解。以某梁式结构受移动质量作用为例,将其简化为横截面积和惯性矩沿轴线按任意次幂函数变化的变截面梁,结果表明：该方法能够正确、有效地处理此类问题。     

斜拉索风致动力疲劳损伤的研究

研究了斜拉索风致振动的疲劳损伤问题。首先,根据 Miner 线性累积损伤原理,给出了结构疲劳损伤的计算公式。其次,根据高周疲劳的特性,我们假设斜拉索的疲劳损伤符合 Basquin 疲劳估算模型。第三,在综合目前疲劳损伤既有研究的基础上,进一步提出评价不同风速和不同风攻角对斜拉索疲劳损伤影响的框架。最后,以松花江斜拉桥中的斜拉索为对象,基于动力时程法计算分析了在不同风速和不同风攻角情况下斜拉索上各节点的应力时程,再使用雨流计数法统计斜拉索的应力-应变循环历史,估算出在不同风速和不同风攻角下斜拉索的疲劳损伤。于是,定量地评价了在不同风速和不同风攻角对斜拉索疲劳损伤的影响。     

Fatigue and fracture assessment for reliability in electronics packaging

Modern electronics products relentlessly become more complex, higher in density and speed, and thinner and lighter for greater portability. The package of these products is therefore critical. The reliability of the interconnection of electronics packaging has become a critical issue. In this study, the novel testing methods for electronic packaging are introduced and failure mechanisms of electronic packaging are explained. Electronics packaging is subjected to mechanical vibration and thermal cyclic loads which lead to fatigue crack initiation, propagation and the ultimate fracture of the packaging. A small-sized electromagnetic-type bending cycling tester, a micro-mechanical testing machine, and thermal fatigue testing apparatus were specially developed for the reliability assessment of electronics packaging. The long-term reliability of an electronic component under cyclic bending induced high-cycle fatigue was assessed. The high-cycle bending-fatigue test was performed using an electromagnetic-type testing machine. The time to failure was determined by measuring the changes in resistance. Using the micro-mechanical tester, low cycle fatigues were performed and compared with the results of a finite element analysis to investigate the optimal shape of solder bumps in electronic packaging. Fatigue tests on various lead-free solder materials are discussed. To assess the resistance against thermal loads, pseudo-power cycling method is developed. Thermal fatigue tests of lead-containing and lead-free solder joints of electronic packaging were performed using the pseudo-power cycling tester. The results from the thermal fatigue tests are compared with the mechanical fatigue data in terms of the inelastic energy dissipation per cycle. It was found that the mechanical load has a longer fatigue life than the thermal load at the same inelastic energy dissipation per cycle.     

A stochastic finite element method for fatigue reliability analysis of gear teeth subjected to bending

A stochastic finite element method (SFEM) is developed for accurate structural reliability analysis. Using the second-order three-moment reliability analytical model, this method takes into account such random factors as load, material parameters and especially geometry randomness. The calculation of the bending fatigue strength reliability of a cantilever beam is carried out as a numerical example to verify the present method. Monte-Carlo FEM and SFEM based on the first-order second-moment model are used in the example to compare with the proposed method. By incorporating the fatigue theory of gears, the present method is then used to analyze the bending fatigue strength reliability of a spur gear. The effects of random variables' coefficient of variation and skewness and the gear's correction factor (not random variable) on the gear's reliability are also investigated.     

A high-cycle fatigue accumulation model based on electrical resistance for structural steels

For high-cycle fatigue of metals, the DC electrical resistance is a more sensitive parameter to the initiation of micro-cracks during the irreversible fatigue damage accumulation process. This implies that the electrical resistance is a suitable parameter that can be consistent with the fatigue damage physical mechanism. The relation between the ratio of electrical resistance changes and the cyclic fraction of the fatigue specimen may reasonably represent deterioration in mechanical properties of structural steels during the high-cycle fatigue process. The high-cycle fatigue damage accumulation model based on electrical resistance for structural steels was proposed. The model was verified by some experimental data for three structural steels; normalized 45C steel, 20 Mn steel and 16 Mn steel, and good agreement was obtained. The corresponding fatigue lifetime on the basis of the electrical resistance model was also performed. The results show that the approach to fatigue lifetime prediction and failure based on the electrical resistance is a good non-destructive technique.     

Frequency equation and mode shape formulae for composite Timoshenko beams

Exact expressions for the frequency equation and mode shapes of composite Timoshenko beams with cantilever end conditions are derived in explicit analytical form by using symbolic computation. The effect of material coupling between the bending and torsional modes of deformation together with the effects of shear deformation and rotatory inertia is taken into account when formulating the theory (and thus it applies to a composite Timoshenko beam). The governing differential equations for the composite Timoshenko beam in free vibration are solved analytically for bending displacements, bending rotation and torsional rotations. The application of boundary conditions for displacement and forces for cantilever end condition of the beam yields the frequency equation in determinantal form. The determinant is expanded algebraically, and simplified in an explicit form by extensive use of symbolic computation. The expressions for the mode shapes are also derived in explicit form using symbolic computation. The method is demonstrated by an illustrative example of a composite Timoshenko beam for which some published results are available.