高铁锻钢制动盘热疲劳裂纹扩展仿真及寿命评价

巨大的制动热能导致高铁制动盘面出现热斑或热点,形成沿径向扩展的热疲劳裂纹,严重危及行车安全.通过仿真获得热疲劳裂纹扩展规律,据此制定出合适、经济的检测周期,具有重要意义.基于断裂力学理论,采用扩展有限单元法和自主研发的裂尖网格加密技术,得到热点形成规律,植入半椭圆形裂纹进行热疲劳断裂仿真,得到制动速度为400 km/h...     

Evaluation of stress intensity factors for bi-material interface cracks using displacement jump methods

The aim of the present work is to investigate the numerical modeling of interfacial cracks that may appear at the interface between two isotropic elastic materials. The extended finite element method is employed to analyze brittle and bi-material interfacial fatigue crack growth by computing the mixed mode stress intensity factors (SIF). Three different approaches are introduced to compute the SIFs. In the first one, mixed mode SIF is deduced from the computation of the contour integral as per the classical J-integral method, whereas a displacement method is used to evaluate the SIF by using either one or two displacement jumps located along the crack path in the second and third approaches. The displacement jump method is rather classical for mono-materials, but has to our knowledge not been used up to now for a bi-material. Hence, use of displacement jump for characterizing bi-material cracks constitutes the main contribution of the present study. Several benchmark tests including parametric studies are performed to show the effectiveness of these computational methodologies for SIF considering static and fatigue problems of bi-material structures. It is found that results based on the displacement jump methods are in a very good agreement with those of exact solutions, such as for the J-integral method, but with a larger domain of applicability and a better numerical efficiency (less time consuming and less spurious boundary effect).     

疲劳裂纹形成、扩展统计特性的大子样试验研究

针对疲劳损伤过程的随机性，该文开展了等幅谱大子样疲劳裂纹形成、扩展至破坏全寿命过程的试验研究．在计算确立了裂纹形成、扩展分界工程小裂纹长度的基础上，整理得到了裂纹形成，扩展寿命的统计分布数据．根据断口金相ＳＥＭ观察，进一步定性分析了疲劳条纹分布的相关性特征．     

基于扩展有限元法的混凝土细观断裂破坏过程模拟

扩展有限元法（XFEM）是分析不连续力学问题（特别是断裂问题）的一种有效的数值方法。在常规的有限元位移模式中,基于单位分解的思想加入一个跳跃函数和渐进缝尖位移场来对不连续体附近的节点自由度进行局部加强,从而反映了位移的不连续性。介绍了扩展有限元的基本原理,给出了扩展有限元进行混凝土开裂及裂纹扩展的分析方法,最后采用扩展有限元法模拟了湿筛混凝土单轴拉伸作用下及WinklerL-型混凝土板的细观断裂破坏过程。分析了混凝土裂纹萌生、扩展的过程及破坏形态,数值结果与实验结果吻合良好。研究表明：扩展有限元法通过特定的位移模式,使裂纹两侧不连续位移场的表达独立于网格划分,能有效地模拟混凝土材料细观断裂破坏过程。     

动载下裂纹应力强度因子计算的改进型扩展有限元法

相较于常规扩展有限元法(extended finite element method, XFEM), 改进型扩展有限元法(improved XFEM) 解决了现有方法线性相关与总体刚度矩阵高度病态问题, 在数量级上提升了总体方程的求解效率, 克服了现有方法在动力学问题中的能量正确传递、动态应力强度因子数值震荡、精度低下问题. 本文基于改进型XFEM, 采用Newmark 隐式时间积分算法, 重点研究了动载荷作用下扩展裂纹尖端应力强度因子的求解方法, 与静力学方法相比, 增加了裂纹扩展速度项与惯性项的贡献. 通过数值算例研究了网格单元尺寸、质量矩阵、时间步长、裂尖加强区域、惯性项、扩展速度项及相互作用积分区域J-domain的网格与单元尺寸对动态应力强度因子求解精度的影响, 验证了改进型XFEM计算动态裂纹应力强度因子方法的有效性. 针对文献中具有挑战性的 "I 型半无限长裂纹先稳定后扩展"问题, 改进型XFEM给出目前为止精度最好的动态应力强度因子数值解.      

A STATISTICAL INVESTIGATION OF FATIGUE CRACK INITIATION AND GROWTH PROCESS BASED UPON A LARGE SAMPLE SIZE EXPERIMENT

After finishing 102 replicate constant amplitude crack initiation and growth tests onLyl2-CZ aluminum alloy plate,a statistical investigation of the fatigue crack initiation and growthprocess is conducted in this paper.According to the post-mortem fractographic examination by scan-ning electron microscopy (SEM),some qualitative observations of the spacial correlation among fa-tigue striations are developed to reveal the statistical nature of material intrinsic inhomogeneity duringthe crack growth process.From the test data,an engineering division between crack initiation andgrowth is defined as the upper limit of small crack.The distributions of crack initiation life N.,growthlife N,and the statistical characteristics of crack growth rate da/dN are also investigated.It is hopedthat the work will provide a solid test basis for the study of probabilistic fatigue,probabilistic fracturemechanics,fatigue reliability and its engineering applications.     

SGBEM modeling of fatigue crack growth in particulate composites

The symmetric-Galerkin boundary element method (SGBEM) has previously been employed to model 2-D crack growth in particulate composites under quasi-static loading conditions. In this paper, an initial attempt is made in extending the simulation technique to analyze the interaction between a growing crack and clusters of perfectly bonded particles in a brittle matrix under cyclic loading conditions. To this end, linear elastic fracture mechanics and no hysteresis are assumed. Of particular interest is the role clusters of inclusions play on the fatigue life of particulate composites. The simulations employ a fatigue crack growth prediction tool based upon the SGBEM for multiregions, a modified quarter-point crack-tip element, the displacement correlation technique for evaluating stress intensity factors, a Paris law for fatigue crack growth rates, and the maximum principal stress criterion for crack-growth direction. The numerical results suggest that this fatigue crack growth prediction tool is as robust as the quasi-static crack growth prediction tool previously developed. The simulations also show a complex interplay between a propagating crack and an inclusion cluster of different densities when it comes to predicting the fatigue life of particulate composites with various volume fractions.     

考虑T-stress的非正态随机参数疲劳裂纹扩展寿命的可靠性分析

基于断裂力学的疲劳裂纹扩展寿命问题的研究常常将裂纹尖端应力展开项的高次项忽略,引起了裂纹扩展模拟的误差,本文考虑高次项T-stress对裂纹扩展角的影响,对裂纹扩展过程做了数值模拟,结果显示相同裂纹扩展长度下,考虑T-stress会延长裂纹扩展寿命。文章首先采用修正的Paris-Erdogan 公式计算了两端承受均布拉伸载荷的边缘斜裂纹板的疲劳裂纹扩展寿命,裂纹扩展方向采用两参数修正的最大拉应力准则。由于结构尺寸,材料特性和载荷等因素具有不确定性,导致疲劳裂纹扩展过程带有一定的随机性,本文以材料属性和载荷为随机变量,在随机有限元法的基础上,结合计算可靠度的四阶矩法,Edgeworth级数展开方法,提出随机参数服从任意分布时的结构疲劳裂纹扩展寿命可靠度的计算方法。分析了参数为非正态分布时的平板裂纹扩展寿命可靠度随裂纹扩展的变化过程。本文方法可预测工程中板裂纹的扩展行为,以及预测裂纹板的可靠度。     

An engineering predictive approach of fatigue crack growth behavior: The case of the lug-type joint

The present attempt proposes a predictive approach of the fatigue crack growth (FCG) behavior of a lug-type joint used in an aeronautic context. The crack tip residual stress distribution and material dispersions are considered. The developed approach was implemented by coupling the Extended Finite Element Method (XFEM), the Residual Corrected Stress Intensity Factor (RC-SIF), developed by the authors, and the Monte Carlo simulation (MCS) method. The Lemaitre–Chaboche model, developed upon the ABAQUS commercial code, was considered for characterizing material behavior. The developed approach treats FCG life by considering the stochastic behavior of material parameters and the crack tip residual stress field during propagation. Comparing with experimental data, the proposed approach exhibits a good ability in evaluating the FCG reliability of a cracked lug-type joint subjected to different loading conditions. The iso-probabilistic P–a–N curves can be used as an efficient tool for ensuring the safety behavior of cracked components.     

EFFECT OF LOADING TYPE ON FATIGUE CRACK GROWTH BEHAVIOR OF SOLDER JOINT IN ELECTRONIC PACKAGING

Fatigue cracking tests of a solder joint were carried out using in-situ scanning electron microscopy （SEM） technology under tensile and bending cyclic loadings. The method for predicting the fatigue life is provided based on the fatigue crack growth rate of the solder joint. The results show that the effect of the loading type on the fatigue crack growth behavior of a solder joint cannot be ignored. In addition, the finite element analysis results help quantitatively estimate the response relationship between solder joint structures. The fatigue crack initiation life of a solder joint is in good agreement with the fatigue life （N50%） of a totally electronic board with 36 solder joints.     

Two dimensional numerical simulation of crack kinking from an interface under dynamic loading by time domain boundary element method

The hybrid time-domain boundary element method, together with the multi-region technique, is applied to simulate the dynamic process of propagation and/or kinking of an interface crack in a two-dimensional bi-material. The whole bi-material is divided into two regions along the interface. The traditional displacement boundary integral equations are employed with respect to each region. However, when the crack kinks into the matrix material, the non-hypersingular traction boundary integral equations are used with respect to the part of the crack in the matrix. Crack propagation along the interface is numerically modelled by releasing the nodes in the front of the moving crack-tip controlled by the fracture criterion. Kinking of the interface crack is controlled by a criterion developed from the quasi-static one. Once the crack kinks into the matrix, its propagation is modeled by adding new elements of constant length to the moving crack-tip controlled by a criterion extended from the quasi-static maximum circumferential stress. The numerical results of the crack growth trajectory for different material combinations are computed and compared with the corresponding experimental results. Good agreement between numerical and experimental results implies that the present boundary element numerical method can provide an excellent simulation for the dynamic propagation and deflection of an interface crack.     

扩展有限元法(XFEM)及其应用

扩展有限元法(extended finite element method,XFEM)是1999年提出的一种求解不连续力学问题的数值方法, 它继承了常规有限元法(CFEM)的所有优点, 在模拟界面、裂纹生长、复杂流体等不连续问题时特别有效, 短短几年间得到了快速发展与应用. XFEM与CFEM的最根本区别在于, 它所使用的网格与结构内部的几何或物理界面无关, 从而克服了在诸如裂纹尖端等高应力和变形集中区进行高密度网格剖分所带来的困难, 模拟裂纹生长时也无需对网格进行重新剖分.重点介绍XFEM的基本原理、实施步骤及应用实例等, 并进行必要的评述. 单位分解概念保证了XFEM的收敛, 基于此, XFEM通过改进单元的形状函数使之包含问题不连续性的基本成分, 从而放松对网格密度的过分要求. 水平集法是XFEM中常用的确定内部界面位置和跟踪其生长的数值技术, 任何内部界面可用它的零水平集函数表示. 第2和第3节分别简要介绍单位分解法和水平集法;第4节和第5节介绍XFEM的基本思想、详细实施步骤和若干应用实例, 同时修正了以往文献中的一些不妥之处; 最后, 初步展望了该领域尚需进一步研究的课题.      

焊趾表面裂纹的形态发展曲线与疲劳寿命预测

以作者建立的焊地椭圆表面裂纹应力强度因子数据库以及复杂应力场中焊践半随圆表面裂纹前缘应力强度因子分布计算的基本模式法为基础上,给出了复杂应力场中焊践表面表纹在疲劳扩展过程中形态变化规律及寿命的工程分析方法。     

利用间隙元的方法计算和分析疲劳寿命

提出了一个利用结构有限元方法中间隙元的原理来计算和分析疲劳寿命的新的累积损伤模型和方法。该方法利用间隙来模拟裂纹的形成和扩展 ,并提出新的累积损伤理论。不仅可以得到构件的疲劳总寿命 ,而且还可以用于计算裂纹的生成寿命、扩展寿命。同时又描述了裂纹的开裂过程 ,即裂纹的开裂尺寸和开裂方向。这为进行各种疲劳强度设计提供了一个新的思路和方法     

Accumulative damage near crack tip for welded bridge members: fatigue life determination

The behavior of crack growth for the fatigue damage accumulation near tip where damage is most severe is analyzed. Fatigue life is assessed for the welded members of bridges under traffic loading. Two parts are considered. They consist of the development of a fatigue damage accumulation model for welded bridge members and a method for calculating the stress intensity factor that is needed for evaluating the fatigue life of welded bridge members with cracks. Based on the concept of continuum damage accumulation and fatigue and fatigue crack growth relations, results are obtained to describe the relationship between the cracking count rate and the effective stress intensity factor. Crack growth and fatigue life are found for two types of welded members assisted by using fatigue experimental results. The stress intensity factors are modified by correcting for the geometric shape of the welded members in order to reflect the influence of the weldment and geometry. This is accomplished via the stress intensity factor. The calculated and measured fatigue lives were generally in good agreement for the initial cracking conditions of two types of welded members widely used in steel bridges.     

含缺口试件疲劳全寿命预测一种建议的方法

传统的研究含缺口构件的疲劳的方法是将疲劳启裂和疲劳裂纹扩展两个过程完全独立起来,用不同的方法来模拟,相互间并没有定量的关系。本文是基于最新发展的多轴疲劳损伤理论,建立了一种适用于各种载荷条件下的疲劳启裂和裂纹扩展的普适方法。根据从弹塑性分析中得到的应力应变,确定疲劳损伤模型,建立能够预测疲劳启裂、裂纹扩展速率和扩展方向的新方法。整个模拟可以分为两步:弹-塑性应力分析得到材料的应力应变分布;再运用一个通用的疲劳准则预测疲劳裂纹启裂和裂纹扩展。通过对1070号钢含缺口试件的疲劳全寿命预测,得到了与实验非常吻合的模拟结果。     

疲劳裂纹扩展分析的随机方法

本文介绍了近年发展起来的疲劳裂纹扩展分析的随机方法,即疲劳裂纹扩展随机模型建立方法和疲劳裂纹扩展统计分析方法。这一分析方法为金属构件疲劳裂纹扩展寿命及可靠性研究提供了有力的工具。      

疲劳裂纹扩展寿命的可靠性及灵敏度分析

将影响结构疲劳裂纹扩展寿命的不确定因素视为随机变量,用随机有限元和可靠性分析理论,从概率论和数理统计的角度出发对含裂纹平面结构的断裂过程进行了可靠性分析。通过疲劳裂纹扩展寿命可靠度对随机变量的灵敏度分析,可以看出不同因素对疲劳裂纹扩展寿命可靠度影响的差别很大。     

A model for predicting the retardation effect following a single overload

Many engineering components are subjected to variable amplitude loading history. It is well known that retardation in fatigue crack growth occurs due to application of single overloads in a constant amplitude loading block. Many models have been proposed to capture this counter intuitive phenomenon which has resulted in improved understanding of retardation effect following tensile overloads and consequently resulting in better life prediction models. The proposed study is focused on to evaluation of retardation in fatigue life due to application of a single overload. A model for prediction of crack growth and crack growth rate following single overloads is presented. Several modifications to Wheeler’s growth idea are proposed, which incorporate a consideration for effective stress intensity factor, based on Elber’s concept of crack closure, relationship between overload ratio and the Wheeler’s exponent, and fatigue growth rate calculations. The results presented here show that plastic zone interaction following overload and the consideration of crack closure explain retardation effect following a single overload. Correlation between analysis and experimental data obtained from several sources in literature show that the scheme, is robust and provides an insight into the nonlinear aspect of crack growth results. The model has been tested for 2024-T3 aluminum alloy and 6061-T6 aluminum alloy and thorough calibrations performed, established the fidelity of the program.     

A fatigue life prediction methodology for cylindrical shell with an inclined crack under torsion

Considered is the fatigue life prediction of a cylindrical shell with an inclined crack under torsion. The effects of the initial angles of inclination and shell curvatures on fatigue crack growth rate are investigated. The mixed mode fracture parameters are extended to describe the fatigue crack growth behavior. A methodology for fatigue life prediction is suggested.