Boundary element analysis of cracked homogeneous or bi-material structures under thermo-mechanical cycling

We present a sub-domain boundary element procedure to evaluate the failure capacity of cracked homogeneous and bi-material media under cyclic thermo-mechanical loads. The boundary integral equations of uncoupled, time-dependent thermo-elasticity are employed to account for the time-varying nature of the thermal load. If crack closure due to thermal distortion takes place, then the displacement and traction field may affect the heat flux between the crack faces, and the thermal and mechanical parts of the problem will need to be solved repeatedly until thermo-mechanical convergence is achieved. We present results from cases of pure mode-I fracture in homogeneous materials and for interfacial fracture in bi-materials. Our study discusses the influence of crack closure on quasi-static, sub-critical crack extension. Especially in case of interfacial cracks the type of loading, the thermal resistance between the crack faces, and the coefficient of friction are also taken into account. The results suggest that the above parameters may have a severe impact on the predicted failure capacity of cracked structures and should be considered in the evaluation of fatigue life.     

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.     

汽轮机汽缸蠕变-疲劳耦合寿命预测

为探索汽轮机汽缸裂纹产生的原因、带裂纹汽缸的剩余寿命、汽缸延寿等问题,开展蠕变和疲劳交互作用下的汽缸寿命预测。利用有限元计算汽缸在稳态和启停工况下的应力情况。基于蠕变 疲劳耦合理论进行裂纹萌生和扩展的寿命预测,从运行方式和汽缸结构2方面开展优化。研究结果表明：该中压内缸中分面法兰的拐角处存在较大的热应力集中,其寿命损伤大导致裂纹萌生。经过结构修复,机组寿命显著延长。     

Optimal design of machine components using notch correction and plasticity models

Shape optimization computational technology is used in order to maximize life time of notched machine/structural components in low cycle fatigue regime. The present approach is composed of three steps: (i) stress–strain calculation using notch correction and plasticity models, (ii) estimation of critical plane to asses fatigue lives, (iii) formulation of the optimization problem with constraint set on the number of cycles corresponding to crack initiation. The optimal design procedure is a combination of the computer aided geometrical design mathematical methods for the shape definition, the boundary element method used for analysis of the response quantities, assisted by the sequential linear programming method with move limits. Numerical examples display significant increase in the number of cycles corresponding to crack initiation phase in comparison to traditional (regular) notch shapes.     

Finite element analysis of crack closure in plate bending

The effect of crack closure in plate bending is studied using the finite element method. Elastic plates containing through-wall-thickness stationary cracks under transverse pressure loading are considered with different plate thicknesses and boundary conditions, respectively. Crack closure on the compression side is modeled two different ways: line closure and surface closure models. A plate bending element degenerated from a three-dimensional solid element is used to model such crack closure. Effects of crack closure are compared using the line or surface closure model for different plate thicknesses and boundary conditions, respectively.     

正交异性钢桥肋-桥面板焊缝裂纹的三维断裂力学分析

正交异性钢桥的肋-桥面板焊缝处的疲劳裂纹是典型的三维裂纹问题,但是现在普遍采用平面应变二维裂纹模型对其进行断裂力学分析.基于Schwartz-Neuman交替法建立正交异性钢桥肋-桥面板焊缝裂纹的局部三维断裂力学分析模型;评估焊缝处表面裂纹的形状和深度对应力强度因子的影响;采用Paris公式估算等应力幅下焊缝的疲劳寿命.计算结果表明:用平面应变二维裂纹模型进行正交异性钢桥的肋-面板焊缝的断裂力学分析会严重低估其疲劳寿命;采用三维断裂力学模型进行肋-桥面板焊缝裂纹的疲劳寿命分析十分必要.     

Cyclic plasticity models and application in fatigue analysis

An analytical procedure for prediction of the cyclic plasticity effects on both the structural fatigue life to crack initiation and the rate of crack growth is presented. The crack initiation criterion is based on the Coffin-Manson formulae extended for multiaxial stress state and for inclusion of the mean stress effect. This criterion is also applied for the accumulated damage ahead of the existing crack tip which is assumed to be related to the crack growth rate. Three cyclic plasticity models, based on the concept of combination of several yield surfaces, are employed for computing the crack growth rate of a cracked plane stress panel under several cyclic loading conditions.     

Study of TBM cutterhead fatigue crack propagation life based on multi-degree of freedom coupling system dynamics

Cutterhead is the core component of TBM tunneling equipment, which endures strong, multi-point distributed impact loads when the TBM tunnels, owing to the extreme surrounding rock environment of high hardness, high temperature and high quartz content. For this reason, the cutterhead works in an extremely severe vibration environment, which leads to engineering fault by a large area crack damage before the service life. Hence, the study on life prediction of TBM cutterhead under the impact loads is a core part of cutterhead design. This paper combines with the technology of system dynamics, linear elastic fracture mechanics and cumulative theory of fatigue damage, for the first time, proposes a method of fatigue crack propagation life prediction for the large and complex structures. In this paper, the TBM cutterhead of an actual project is taken as an example, to predict the fatigue crack propagation life of cutterhead piece and analyze the influences of plate thicknesses on fatigue life, then a new improved scheme of cutterhead structure is presented. The results show that the fatigue crack propagation life of actual cutterhead is 26.6 km, which is able to meet the requirement of 20 km service life. Moreover, the upper cover plate thickness has the greatest influence on cutterhead fatigue crack propagation life, with the thickness increasing 10%, the life increases nearly by 1.24 times. Then, the other influencing factors are as follows: thickness of the main support plate, thickness of the annular support plate and thickness of the support plate, whereas the influence of the lower cover plate thickness on fatigue life is minimal. Furthermore, the plate thickness limit sizes meeting the life requirement are obtained, and a new structure modified scheme of cutterhead is proposed. Compared with the original scheme, the new cutterhead scheme meets the requirements of structural strength and service life with 8.08% weight decrease, which achieves life determination design and lightweight design. The proposed method of fatigue crack propagation life prediction is feasible in the design and application stage of TBM cutterhead, besides, it is flexible enough and can also be applied in damage strength assessment, dynamic parameters optimization and establishment of nondestructive inspection cycle for the other large and complex structure, and takes on stronger project value and generality.     

Computer prediction of misaligned welded joints

Experimental tests have shown that misalignment can reduce the fatigue strength of welded joints significantly. Further theoretical investigations are also required to understand the behavior of these joints. In the present study, boundary element technique together with the hypersingular boundary integral equation are employed to estimate the notch stress concentration factors at the surface. These are then used to predict the fatigue life. The addressed variable is the root crack length, or partial penetration depth, which can be accurately modeled by boundary element method. It is shown that there is a good correlation between the previous experimental results and the present theoretical results.     

CAE在同步辐射装置挡光元件设计中的应用

针对第三代同步辐射装置挡光元件承受极高热负载、设计难度大的问题,综述CAE在其设计中的应用情况:借助有限元分析可以获得挡光元件关键部件吸收体的最高温度和最大应力这两个最主要设计参数;通过计算流体力学(Computational Fluid Dynamics,CFD)软件模拟可以获得吸收体中冷却管道的对流换热与流动阻力特性参数;采用热弹塑性有限元分析可以获得用于低周疲劳寿命预估的吸收体热应力应变迟滞循环.下一步工作将围绕当今研究的核心问题——基于低周疲劳的设计准则展开,包括同步辐射高热负载作用下低周疲劳裂纹的起裂寿命和扩展寿命预测,以及吸收体倾角、表面光滑度、冷却管道排布等对寿命的影响.这些研究均需要充分利用CAE的强大分析功能.     

Fatigue-life estimation of functionally graded materials using XFEM

The present work deals with the fatigue crack growth simulation of alloy/ceramic functionally graded materials (FGMs) using extended finite element method (XFEM). Various cases of FGM containing multiple inhomogeneities/discontinuities along with either a major edge or a center crack are taken for the purpose of simulation. The fatigue life of the FGM plate is calculated using Paris law of fatigue crack growth under cyclic loading. The effect of multiple inhomogeneities/discontinuities (minor cracks, holes/voids, and inclusions) on the fatigue life of cracked FGM plate is studied in detail. These simulations show that the presence of inhomogeneities/discontinuities in the domain significantly influences the fatigue life of the components.     

冷却器在瞬态工况下的温度场与热应力分析

针对冷却器在发电厂寿命期内频繁承受热冲击作用而导致设备出现疲劳破坏问题,用ANSYS建立设备管程结构温度场和应力场分析模型,计算瞬变热冲击工况下结构的热应力,并按ASME标准对设备的疲劳强度进行评定.数值仿真结果可明确冷却器裂纹产生的主要原因和部位,为结构的安全性改进提供参考.     

An automated procedure for fatigue crack growth analysis

An automated procedure is developed to determine the influence of a crack detected in a structural element subjected to cyclic combined axial and bending loadings with constant amplitude. A theoretical formulation for fatigue crack growth analysis is presented and the results obtained are compared with the experimental data of the Wöhler curves. The critical crack configurations for which the fatigue failure occurs after a given number of loading cycles can be obtained from the above-mentioned comparison. Finally, an example is shown in order to illustrate the use of a computer program, which translates the proposed procedure.     

航天器电子元器件疲劳寿命分析

为研究空间环境效应对航天器电子设备元器件疲劳寿命的影响,应用Miner累积损伤理论及三带宽技术,根据表贴元器件经历的试验、发射和在轨运行等实际环境进行振动及热疲劳寿命分析．给出振动及热循环疲劳寿命的计算方法,并以某BGA（Ball Grid Array）器件为例进行分析．该方法可为长寿命、高可靠的航天器电子设备抗力学及热设计提供分析途径．     

橡胶材料疲劳断裂特性研究进展

由于橡胶材料的动态疲劳特性对保证橡胶制品使用时的安全性和可靠性具有重要意义,综述机械载荷、环境和橡胶配方等因素对橡胶材料疲劳寿命的影响,总结用疲劳裂纹萌生寿命法和基于断裂力学的疲劳裂纹扩展法预测橡胶材料动态疲劳寿命方法的优缺点,并展望这2种方法的发展趋势.     

A two-parameter model for crack growth simulation by combined FEM–DBEM approach

This paper describes the application of a two-parameters crack growth model, based on the usage of two threshold material parameters (ΔK_th and K_max,th) and on the allowance for residual stresses, introduced at the crack tip by a fatigue load spectrum or by material plastic deformations. The coupled usage of finite element method (FEM) and dual boundary element method (DBEM) is proposed in order to take advantage of the main capabilities of the two methods.The procedure is validated by comparison with available experimental results, in order to assess its capability to predict the retardation phenomena, introduced by a variable load spectrum or by a plastic deformation introduced with a tool on the panel (indentation).In particular two different tests are made: the first test involve a CT specimen undergoing a load spectrum and the second one involve a dented panel undergoing a constant amplitude fatigue load. In both cases a satisfactory numerical–experimental correlation will be proved.The main advantages of the aforementioned procedure are: the simplicity of the crack growth law calibration (few constant amplitude tests are sufficient without the need for any non-physical calibration parameters), and the possibility to simulate residual stress effects on crack propagation with a simplified approach, based on linear elastic fracture mechanics.     

基于主S-N曲线的带止裂槽焊接接头疲劳寿命预测及改进

以带止裂槽焊接接头为研究对象,提取可能发生疲劳失效的焊趾截面、焊喉截面和止裂槽截面的节点力,基于主S N曲线法求解截面的等效结构应力,评估焊缝的疲劳寿命,并提出改进接头的建议。分析发现：止裂槽的存在会显著增加局部位置的应力,导致疲劳寿命明显降低。填充止裂槽并适当增大水平方向焊脚尺寸,可以提高该位置疲劳寿命10倍左右。     

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.     

Prediction of constant amplitude fatigue crack growth life of 2024 T3 Al alloy with R-ratio effect by GP

The objective of this study is to develop a genetic programming (GP) based model to predict constant amplitude fatigue crack propagation life of 2024 T3 aluminum alloys under load ratio effect based on experimental data and to compare the results with earlier proposed ANN model. It is proved that genetic programming can effectively interpret fatigue crack growth rate data and can efficiently model fatigue life of the material system under investigation in comparison to ANN model.     

聚氨酯橡胶摩擦轮有限元及疲劳寿命分析

针对汽车滑板输送机聚氨酯橡胶摩擦轮的开裂问题,对摩擦轮与滑板之间的相互作用进行预压紧力和接触摩擦驱动有限元分析,获得摩擦轮的应力和变形分布等结果. 在获得摩擦轮周向应力分布的基础上,根据疲劳裂纹扩展公式对聚氨酯橡胶部分进行疲劳寿命分析,验证是否满足设计要求. 分析过程和结果能为聚氨酯橡胶摩擦轮的详细设计提供参考.