半椭圆表面裂纹前缘应力强度因子的计算

在断裂力学中,如何求取应力强度因子一直是一个重要的课题.该文通过MSC.Marc提供的断裂力学模块,采用三维J积分法计算含有半椭圆表面裂纹前缘应力强度因子.首先通过MSC.Marc.Mentat建立特定裂纹体有限元模型,假设裂纹前缘处在平面应变状态下.由MSC.Marc计算出裂纹前沿的J积分,再由J积分计算出裂纹前缘的应力强度因子值.最后将计算结果与经验公式得到的结果进行了比较.仿真结果表明,通过MSC.Marc采用三维J积分法计算的应力强度因子具有较高的准确性和可靠性.     

基于概率断裂力学的管道疲劳寿命分析

采用奇异单元模拟裂纹尖端应力场的奇异性,计算裂纹尖端的应力强度因子和张开应力.以概率论为基础,结合确定性疲劳断裂力学估算方法,考虑参数的不确定性和随机性,应用蒙特卡洛模拟法分析管道的疲劳寿命.结果表明：通过J积分计算得到的裂纹尖端张开应力与计算得到的管道工作应力基本相等.采用蒙特卡洛模拟法进行的一定可靠度和置信度下的疲劳寿命预测能反映评定参数的不确定性,较传统的断裂力学计算结果更安全.     

基于等效结构应力的铝合金地铁车体疲劳寿命预测

为评估某铝合金地铁车辆的疲劳寿命,采用美国ASME标准中的网格不敏感的主S-N曲线法对该车焊缝进行疲劳寿命预测.用HyperMesh对车体进行有限元建模,并对焊缝处网格细化;用ANSYS计算焊缝处应力;运用自主开发的FE-Weld软件对其进行等效结构应力的计算和疲劳寿命的预测;对疲劳寿命不符合设计要求的结构进行改进和优化,改进后结构的疲劳寿命符合设计要求.网格不敏感的主S-N曲线法具有重要工程应用推广价值.     

基于主S-N曲线法的焊接结构疲劳寿命预测系统开发和关键技术

根据ASME标准的计算公式,开发基于主S-N曲线法的焊接结构疲劳寿命预测软件,其核心是采用网格不敏感结构应力及一条主S-N曲线的方法计算焊接结构的疲劳寿命.系统开发选用面向对象的VC++语言和OpenGL开发库;采用模块化程序设计的思想将系统划分为有限元定义模块、焊缝定义及结构应力计算模块、载荷谱管理及雨流计数模块、主S-N数据管理及疲劳计算模块等4个核心模块;开发专用的接口程序和三维交互界面.该软件系统已在我国轨道交通制造行业中得到良好应用.以某货车车体关键焊缝疲劳寿命计算为典型实例,用软件系统进行车体疲劳寿命评估,计算结果表明该货车焊缝满足设计要求,同时也证明该软件系统的实用性和高效性.     

大型矿车用减速机齿轮系统的疲劳分析

提出一种基于"放大因子"、采用二维平面单元模拟三维实体动态工况的计算疲劳的方法,对某矿车减速机齿轮系统进行疲劳分析,得到太阳轮、行星轮和扭力管在不同抗拉强度下的疲劳寿命.结果表明:太阳轮和扭力管最易发生疲劳破坏;齿轮寿命对抗拉强度的依赖性较敏感;得出的齿轮系统热处理后的抗拉强度建议值能为设计提供参考,且该方法能大大提高计算效率.     

基于主S-N曲线法的地铁制动箱焊接疲劳分析

为研究地铁车辆制动箱焊接接头的疲劳寿命,根据实际结构建立4节点壳单元有限元模型,给出搭接焊和T型焊的焊缝建模方法.在3种振动工况下,运用主S N曲线法计算焊缝的等效结构应力和对应损伤比.结果表明：该地铁车辆制动箱焊接结构设计合理可靠;通过与实体单元模型计算结果进行对比证明壳单元模拟焊缝的合理性;在不同尺寸单元下对比2种疲劳评估方法,结果表明名义应力法预测疲劳寿命的准确性较低.     

基于名义应力法的弹性链型接触网疲劳寿命预测

针对高速铁路弹性链型接触网进行接触线疲劳寿命预测分析.利用ANSYS,采用直接积分法对弓网耦合系统进行动态仿真,得到接触线的应力时程;运用雨流计数法得到离散的应力循环,采用应力修正算法得到平均应力为零的疲劳应力谱;通过简化方法估算获得材料S-N曲线,从而计算得到疲劳破坏次数;最终运用线性累积损伤理论预测接触线的疲劳寿命.对比分析接触线不同位置的疲劳寿命值,结果表明:接触线每跨疲劳寿命趋势一致;每跨在吊弦处和定位点处疲劳寿命较低,其中寿命最低值出现在左侧第一根吊弦处,疲劳寿命最低值为20 a左右.结果可为高速铁路弹性链型接触网接触线的实际施工维护和更换周期的确定提供参考.     

单边驱动式摇摆筛偏心轴的应力与疲劳分析

应用PROE软件建立了单边驱动式摇摆筛的三维结构模型,并对其进行动态仿真分析,得到作用在偏心轴上的载荷历程。运用解析法和有限元法对偏心轴应力进行分析,得到其应力分布曲线和应力云图,分析结果表明,偏心轴上的危险部位出现在中间轴承座支撑处,最大应力值为35 MPa。运用局部应力应变法对偏心轴疲劳寿命进行分析,得到单边驱动式摇摆筛偏心轴的疲劳寿命为0.98a,通过分析能够有效预防事故的发生。该研究成果为偏心轴的设计和改进提供了可靠依据。     

摩擦因数对钢丝绳应力和疲劳寿命的影响分析

以6×36+WS结构钢丝绳为研究对象,在SolidWorks中建立了其三维模型,并导入ABAQUS中,通过定义接触对、设置边界条件,建立了钢丝绳有限元模型。在相同的轴向载荷下,仿真分析了不同摩擦因数对钢丝绳应力分布和疲劳寿命的影响。结果表明,钢丝绳在拉伸状态下,绳股外侧钢丝应力较大,且绳股与绳股接触处钢丝应力最大;随着摩擦因数的增大,钢丝绳应力逐渐增大,疲劳寿命逐渐减小,钢丝绳在应力最大处疲劳寿命最小。     

实测数据驱动的挖掘机工作装置疲劳寿命预测

以某型挖掘机为研究对象,建立其工作装置刚柔耦合动力学模型,基于实测油缸位移数据驱动该模型,得到其主要性能参数和典型工况危险部位应力.根据强度理论、动力学仿真结果和工程经验,分析挖掘机动臂和斗杆的易开裂部位,得到典型焊缝高危点,并通过实测应力应变数据进行验证.以刚柔耦合动力学仿真所得的铰点载荷作为输入,利用nCode疲劳分析软件仿真预测挖掘机动臂和斗杆的疲劳寿命.结果表明,实测数据驱动的刚柔耦合动力学仿真可以准确获取挖掘机实际挖掘过程的动力学特性,基于该仿真模型提取铰点载荷并用于预测疲劳寿命的方法切实可行.     

Fracture mechanics based fatigue analysis of steel bridge decks by two-level cracked models

An alternative fatigue assessment is introduced for orthotropic highway bridge decks, based on fracture mechanics. Crack growth simulation is carried out by the numerical integration of the Paris formula, using K factors obtained from two-level cracked models of the bridge deck. Prior to the simulation, unit wheels are applied to the 3D-shell model of the deck, and the translations of characteristic points are transferred to the 2D plane strain or plane stress finite element sub-models of fatigue-sensitive areas. The K factor is computed by numerical extrapolation, introducing cracks in the sub-models. Fixed values of input parameters (initial- and critical crack length, crack growth rate data, and the error of K factor computation) are used for single simulations as well as parametric studies for sensitivity analysis. Reliability assessment is performed by repeated crack growth analysis, using histograms to generate input parameters by Monte-Carlo simulation at the start of each “virtual experiment”.     

Three-dimensional J-integral calculations of part-through surface crack problems

In the field of nonlinear fracture mechanics, the J-integral is well known to play a significant role in predicting crack propagation and fracture strength. In this paper, we develop the three dimensional form of the J-integral using Eshelby's energy momentum tensor on any closed surface surrounding a crack front. Twenty-node isoparametric hexahedra elements in ADINA are used to calculate stress and strain distributions and Gaussian surface integration is used to evaluate J-integral values. Using the above method, we calculate the stress intensity factors for part-through surface cracks and verify the accuracy of these results.     

Damage tolerance based design optimisation of a fuel flow vent hole in an aircraft structure

This paper investigates the design optimisation of a fuel flow vent hole (FFVH) located in the wing pivot fitting (WPF) of an F-111 aircraft assuming a damage tolerance design philosophy. The design of the vent hole shape is undertaken considering the basic durability based design objectives of stress, residual (fracture) strength, and fatigue life. Initially, a stress based optimised shape is determined. Damage tolerance based design optimisation is then undertaken to determine the shape of the cutout so as to maximise its residual strength and fatigue life. For stress optimisation, the problem is analysed using the gradient-less biological algorithm and the gradient-based nonlinear programming methods. The optimum designs predicted by the two fundamentally different optimisation algorithms agree well. The optimum shapes of the vent hole are subsequently determined considering residual strength and fatigue life as the distinct design objectives in the presence of numerous 3D cracks located along the vent hole boundary. A number of crack cases are considered to investigate how the crack size affects the optimal shapes. A semi-analytical method is employed for computation of the stress intensity factors (SIF), and an analytical crack closure model is subsequently used to evaluate the fatigue life. The 3D biological algorithm is used for designing the cutout profiles that optimise residual strength and fatigue life of the component. An improved residual strength/fatigue life (depending on the optimisation objective) is achieved for the optimal designs. The variability in SIF/fatigue life around the cutout boundary is reduced, thereby making the shape more evenly fracture/fatigue critical. The vent hole shapes optimised for stress, residual strength, and fatigue life are different from each other for a given nature and size of the flaws. This emphasises the need to consider residual strength and/or fatigue life as the explicit design objective. The durability based optimal vent hole shapes depend on the initial and final crack sizes. It is also shown that a damage tolerance optimisation additionally produces a reduced weight WPF component, which is highly desirable for aerospace industries. The design space near the ‘optimal’ region is found to be flat. This allows us to achieve a considerable enhancement in fatigue performance without precisely identifying the local/global optimum solution, and also enables us to select a reduced weight ‘near optimal’ design rather than the precise optimal shape.     

Simulation of three-dimensional fatigue crack propagation using enriched finite elements

A three-dimensional methodology for simulation of fatigue crack propagation is presented. The method is leveraged by the use of enriched crack tip elements to compute the mixed-mode stress intensity factors. The crack growth model used and the crack propagation life calculation are also described. As examples, fatigue crack propagation of a mode-I surface crack and crack advancements of mixed-mode surface cracks are simulated. The predicted results showed excellent agreement with experimental data from the literature. Thus, it is concluded that the crack propagation method developed allows efficient and accurate simulation of three-dimensional fatigue crack propagation problems.     

基于主S-N曲线法的T形接头疲劳评估和试验验证

以T形焊接接头为研究对象,将可能发生疲劳破坏的焊趾截面和焊喉截面的节点力转换为相对于中面焊线的等效节点力和弯矩,并利用平衡等效原理转化为线力和线弯矩,基于材料力学公式求出截面的结构应力,解决应力对网格尺寸敏感的难题。从焊缝疲劳破坏的机理出发,基于Paris断裂力学公式推导以等效结构应力为变量的一条主S-N曲线,评估焊缝的疲劳寿命。分析结果发现：采用主S-N曲线法评估的寿命与试验值最接近且破坏位置准确,该主S N曲线法准确性较高。     

Numerical modeling of crack reorientation and link-up

The FRANC3D/BES software system has been used to simulate the reorientation and link-up of hydraulic fractures in three-dimensional (3D) problems. The adopted technique only needs to discretize the body surface and the crack surface. The crack propagation direction is determined using the minimum strain energy criterion. Crack propagation amount is calculated using the mode I stress intensity factor. In hydraulic fracturing, the number of multiple cracks for a given number of perforations depends on the resulting interaction of the cracks. The interaction may be expressed by the fracture stiffness which has been obtained for 3D problems in this paper.     

Topology optimization considering fracture mechanics behaviors at specified locations

As a typical form of material imperfection, cracks generally cannot be avoided and are critical for load bearing capability and integrity of engineering structures. This paper presents a topology optimization method for generating structural layouts that are insensitive/sensitive as required to initial cracks at specified locations. Based on the linear elastic fracture mechanics model (LEFM), the stress intensity of initial cracks in the structure is analyzed by using singularity finite elements positioned at the crack tip to describe the near-tip stress field. In the topology optimization formulation, the J integral, as a criterion for predicting crack opening under certain loading and boundary conditions, is introduced into the objective function to be minimized or maximized. In this context, the adjoint variable sensitivity analysis scheme is derived, which enables the optimization problem to be solved with a gradient-based algorithm. Numerical examples are given to demonstrate effectiveness of the proposed method on generating structures with desired overall stiffness and fracture strength property. This method provides an applicable framework incorporating linear fracture mechanics criteria into topology optimization for conceptual design of crack insensitive or easily detachable structures for particular applications.     

Axisymmetric shell optimization under fracture mechanics and geometric constraint

This paper describes a problem of axisymmetric shell optimization under fracture mechanics and geometric constraints. The shell is made from quasi-brittle materials, and through crack arising is admitted. It is supposed that the shell is loaded by cyclic forces. A crack propagation process related to the stress intensity factor is described by Paris fatigue law. The problem of finding the meridian shape and the thickness distribution (geometric design variables) of the shell having the smallest mass subject to constraints on the cyclic number for fatigue cracks and geometrical constraint on the shell volume is investigated. Special attention is devoted to different possibilities of problem transformation and analytical methods of their solution. Using minimax approach, optimal shapes of the shells and their thickness distributions have been found analytically.