Numerical analysis for prediction of fatigue crack opening level

Finite element analysis (FEA) is the most popular numerical method to simulate plasticity-induced fatigue crack closure and can predict fatigue crack closure behavior. Finite element analysis under plane stress state using 4-node isoparametric elements is performed to investigate the detailed closure behavior of fatigue cracks and the numerical results are compared with experimental results. The mesh of constant size elements on the crack surface can not correctly predict the opening level for fatigue crack as shown in the previous works. The crack opening behavior for the size mesh with a linear change shows almost flat stress level after a crack tip has passed by the monotonic plastic zone. The prediction of crack opening level presents a good agreement with published experimental data regardless of stress ratios, which are using the mesh of the elements that are in proportion to the reversed plastic zone size considering the opening stress intensity factors. Numerical interpolation results of finite element analysis can precisely predict the crack opening level. This method shows a good agreement with the experimental data regardless of the stress ratios and kinds of materials.     

Numerical Investigation into the Effect of Contact Geometry on Fretting Fatigue Crack Propagation Lifetime

Fretting fatigue is a phenomenon in which two contact surfaces undergo a small relative oscillatory motion due to cyclic loading. There is a need to analyze the effects of contact geometry on crack propagation under fretting fatigue conditions. In this investigation, a finite element modeling method was used to study the effects of different contact geometries along with crack–contact interaction on crack propagation lifetime. Different contacts geometries—that is, cylindrical on flat and flat on flat—along with different contact span widths were analyzed. In addition, the effects of different contact spans on stress distribution at the contact interface were investigated. The computed crack propagation life was compared with experimental results. It was found that the crack initiated near the contact trailing edge for all contact geometries, which agreed with experimental observations. In terms of crack propagation for different contact spans, the fretting fatigue life for a two-based cylindrical pad was shorter than that for a two-based flat pad. By increasing the contact span width for both flat and cylindrical pads, the crack propagation lifetime increased. A comparison between the experimental and numerical results demonstrated a difference of about 18% in crack propagation lifetime.     

Fatigue crack propagation behavior in STS304 under mixed-mode loading

The use of fracture mechanics has traditionally concentrated on crack growth under an opening mechanism. However, many service failures occur from cracks subjected to mixed-mode loading. Hence, it is necessary to evaluate the fatigue behavior under mixed-mode loading. Under mixed-mode loading, not only the fatigue crack propagation rate is of importance, but also the crack propagation direction. In modified range 0.3≤a/W≤0.5, the stress intensity factors (SIFs) of mode I and mode II for the compact tension shear (CTS) specimen were calculated by using elastic finite element analysis. The propagation behavior of the fatigue cracks of cold rolled stainless steels (STS304) under mixed-mode conditions was evaluated by using K_I and K_II(SIFs of mode I and mode II). The maximum tangential stress (MTS) criterion and stress intensity factor were applied to predict the crack propagation direction and the propagation behavior of fatigue cracks.     

基于FRANC2D的疲劳裂纹扩展数值模拟

介绍了二维断裂分析有限元软件FRANC2D(Fracture Analysis Code in 2 Dimensions)疲劳裂纹扩展的基本原理以及一般计算过程,为了验证FRANC2D在疲劳裂纹扩展数值模拟方面的准确性,对含中心裂纹试样平板进行疲劳试验,并将试验结果和软件中模拟的结果进行对比,同时进一步说明其方法.     

典型构件疲劳微裂纹全寿命预测方法研究

基于损伤力学理论建立金属疲劳裂纹损伤模型,结合有限元法预估疲劳微裂纹萌生及扩展全寿命。引入附加载荷法,实现了对刚度矩阵的连续计算,应用Matlab编程计算,通过对单个单元的损伤计算,得到了单元从无损到破坏过程中等效应力的变化,进而给出疲劳裂纹损伤模型,并对不同规格金属试件的疲劳寿命预估。将计算结果与试验结果进行对比,得到的相对误差在接受范围内,验证了该模型的准确性,对于研究金属试件微观损伤以及疲劳寿命预估具有重要的参考意义。     

塞斯纳172R型飞机刹车活塞杆疲劳断裂有限元分析

机轮刹车系统决定了飞机的着陆安全,是民用飞机地面减速关键的机构。以塞斯纳172R型飞机刹车活塞杆为研究对象,结合实际运行经验分析活塞杆受力情况,并制定载荷谱;基于材料力学和疲劳断裂力学等相关理论,利用CATIA软件建立刹车活塞杆三维实体模型,导入ANSYS软件中对其结构进行有限元分析,计算得到活塞杆的应力云图,确定应力集中(危险)位置。并应用疲劳耐久性分析软件FE-SAFE对活塞杆进行疲劳分析,确定该部件的安全使用寿命,为维护时间间隔的修订提供理论依据。     

声发射技术在疲劳裂纹检测中的应用

主要讨论了声发射技术应用于疲劳裂纹的检测.根据疲劳裂纹的信号特征及测试场合,选用合适的疲劳裂纹检测方法,结合板簧疲劳裂纹试验机上的实验分析,得出声发射技术应用于疲劳裂纹检测的优势.     

Finite Element Analysis of Localized Plasticity in Al 2024-T3 Subjected to Fretting Fatigue

Fretting fatigue is a combination of two complex mechanical phenomena, namely, fretting and fatigue. Fretting appears between components that are subjected to small relative oscillatory motion. Once these components undergo cyclic fatigue load at the same time, fretting fatigue occurs. Fretting fatigue is an important issue in aerospace structural design. Many studies have investigated fretting fatigue behavior; however, the majority have assumed elastic deformation and very few have considered the effect of plasticity. The main goal of this study is to monitor the effect of different fretting fatigue primary variables on localized plasticity in an aluminum alloy (Al 2024-T3) test specimen. In order to extract the stress distribution at the contact interface under elasto-plastic conditions, a modified finite element contact model was used. The contact model was verified through comparison with an elastic analytical solution. Then, a bilinear elasto-plastic isotropic hardening model with a von Mises yield surface was implemented to simulate the material behavior of the aluminum alloy. The effect of different fretting fatigue primary variables, such as axial stress, contact geometry, and coefficient of friction, on localized plasticity was investigated. Finally, the relationship between the location of maximum localized plasticity and Ruiz fretting damage parameter with the crack initiation site is discussed.     

A New Statistical Model of Contact Fatigue

A detailed derivation of a new statistical model of contact fatigue life followed by its qualitative and quantitative analysis are presented. The model is based on contact and fracture mechanics and statistical treatment of the initial distribution of material defect. The model assumptions and their validation as well as the model properties are discussed. A parametric study of the model is performed. A generalization of the model for the case of stochastic residual stress or other contact parameters is proposed. Some analytical formulas for calculation of contact fatigue are proposed and analyzed. The validation of the model and its applicability to calculation of bearing fatigue life and some particular data are considered. A reflection of the quality of bearing manufacturing process on the contact fatigue model is discussed.     

汽车钢圈轮辐裂纹成因分析及优化设计

钢圈对汽车行驶的安全性起着重要的作用。某新型钢圈在弯曲疲劳试验中,在未达到设计使用寿命要求时,轮辐表面就出现了裂纹。为找出裂纹形成原因,在有限元软件中建立钢圈弯曲疲劳试验有限元模型,并分析了钢圈的应力状态及其对疲劳强度的影响。在控制应力幅和最大切应力不超过许用值的条件下,采用正交试验的方法优化轮辐横截面的几何参数。改进后钢圈的疲劳寿命分析表明,借助正交试验可有效改进钢圈的结构,从而增加钢圈的疲劳寿命。     

A study on the prevention of cracks on the trepan area of motor bearing

Trepan prevents the wear of the inside part of a bearing when the initial shaft rotates. It continuously contacts with the eccentric part of the shaft in rotation and is loaded repeatedly. Therefore, even if an early crack of a trepan part is small, the crack may progress by the repeated load. If the crack progresses, very small chips come out. This is put in the rotor and prevents the rotation of the compressor. There can be leaks in a microgroove and extreme wear can occur due to lack of oil on the surface contact part. Therefore, this study was carried out to compare and investigate the trepan strength and deflection characteristics between trepan locations and dimension changes using the finite element method and search a motor bearing for a model with bigger stiffness of a trepan part and the same deflection.     

Investigation of high cycle and low cycle fatigue interaction on fretting behavior

Fretting-fatigue behavior and damage accumulation under a variable-amplitude cycling load is investigated in a configuration involving a cylindrical indenter in contact with finite width plate. Relative magnitudes of cyclic tangential and bulk loads not only affect the contact conditions, but also their relative positions with respect to each other. Several stick–slip conditions on the contact surface may develop during the application of variable-amplitude fatigue load, and these are secondary and tertiary slips as well as shake-down. Further, residual shear traction develops during the application of cyclic load. The appropriate characterization of fretting-fatigue behavior or life should, therefore, include the complete history of applied cyclic tangential and bulk loads. Furthermore, experiments from a previous study conducted under a variable-amplitude fatigue loading condition are analyzed to characterize the damage accrual from its individual components involving constant-amplitude fatigue load by incorporating the contact mechanics and a multi-axial fatigue critical plane parameter. This analysis shows that there is nonlinear damage accumulation during variable-amplitude fretting-fatigue load.     

温挤压模具冷热疲劳早期失效模拟分析与探讨

生产某壳体零件的模具存在着严重的冷热疲劳早期失效现象,结合生产实际利用DEFORM有限元模拟软件对其温挤压过程进行了模拟.通过对比模拟分析可知得到凸模在工作时不同时刻时的温度值,不同模具预热温度和坯料温度下模具的最高温度分布以及急冷急热温度差范围.为了减缓模具的冷热疲劳,模具预热温度最好控制在200～300℃之间,坯料加热温度控制在800℃内.     

汽车自调臂矩形离合弹簧有限元疲劳分析

汽车自调臂单向离合器中的矩形离合弹簧的疲劳失效,直接关系到自调臂的工作寿命。根据自调臂的工作原理,建立自调臂矩形离合弹簧的机械结构力学模型,并将该结构力学模型导入到ANSYS Workbench中,进行疲劳寿命的有限元仿真分析。仿真分析的矩形离合弹簧经过2.0403×105次循环载荷即发生失效。同时,运用自调臂疲劳试验台对20个该结构型号的自调臂进行疲劳试验,实验结果表明:实验的自调臂疲劳寿命均值为1.9101×105次循环载荷,与有限元分析结果一致,有限元疲劳分析方法正确。     

基于疲劳理论的汽车空气悬架结构件的弯曲强度计算

针对某种型号客车的空气悬架，应用ANSYS软件对弹簧支架进行了有限元分析，计算了弹簧支架的应力、变形特性，在此基础上计算了弹簧支架的疲劳强度。     

16Mn钢焊接件疲劳裂纹扩展的微观特征研究

研究了在循环载荷作用下,16Mn钢焊接件疲劳裂纹的扩展情况.通过光学显微镜观察了裂纹扩展的微观特征,发现了裂纹在焊接结构不同部位的扩展路径方式并分析了显微组织对裂纹扩展的影响.裂纹在母材处为穿晶扩展,在热影响区和焊缝金属中的扩展为穿晶和沿晶混合型.     

直行程电动阀门执行器支架优化设计与测试

直行程电动阀门执行器广泛应用于各类直线运动阀门的启闭、调节,直行程阀门执行器支架起到连接执行器与阀门的作用.针对某公司直行程阀门执行器支架存在的问题进行优化设计,通过有限元分析对优化前后支架的质量、应力分布、疲劳寿命及安全系数进行对比,结果显示,着重考虑轻量化的优化设计并未降低支架的安全系数及疲劳寿命,反而均有所改善....     

The effect of bridging on fatigue crack growth behavior in Aramid Patched Aluminum Alloy(APAL)

A new hybrid composite (APAL: Aramid Patched Aluminum Alloy), consisting of a 2024-T3 aluminum alloy plate sandwiched between two aramid/epoxy laminate (HK 285/RS 1222), was developed. Fatigue crack growth behavior was examined at stress ratios of R=0.2, 0.5 using the aluminum alloy and two kinds of the APAL with different fiber orientation (0°/90° and 45° for crack direction). The APAL showed superior fatigue crack growth resistance, which may be attributed to the crack bridging effect imposed by the intact fibers in the crack wake. The magnitude of crack bridging was estimated quantitatively and determined by a new technique on basis of compliances of the 2024-T3 aluminum alloy and the APAL specimens. The crack growth rates of the APAL specimens were reduced significantly as comparison to the monolithic aluminum alloy and were not adequately correlated with the conventional stress intensity factor range(ΔK). It was found that the crack growth rate was successfully correlated with the effective stress intensity factor range (ΔK _eff =K _br -K _ct ) allowing for the crack closure and the crack bridging. The relation between da/dN and theΔK _eff was plotted within a narrow scatter band regardless of kind of stress ratio (R=0.2, 0.5) and material (2024-T3 aluminum alloy, APAL 0°/90° and APAL±45°). The result equation was as follow:da/dN=6.45×10⁻⁷(ΔK _eff )^2.4.     

Numerical Investigation on the Effects of Machining-Induced White Layer during Rolling Contact

It is well known that a thin phase-transformed white layer can be formed on component surfaces produced by hard machining. However, it is not clear as to how the white layer affects component performance, for example, in rolling contact fatigue. This study aims to determine the effects of white layer and associated residual stress on rolling contact stresses and strains. It is nearly impossible for an experimental study to identify the effects of white layer alone on rolling contact. Furthermore, small-scale contact stresses and strains (less than 30 μm) of the phase-transformed region are difficult to measure using the current experimental techniques. Therefore, a finite element analysis simulation model of rolling contact incorporating machining-induced surface integrity has been developed in this study. Three cases were investigated to decouple the effects of surface integrity factors: surface with white layer only, surface with residual stress only, and surface with white layer and residual stress. The simulation results show that distinct material properties of the white layer significantly influence the magnitudes and distributions of near-surface stresses and strains instead of those in the subsurface. Furthermore, it can be inferred that the white layer would affect near-surface fatigue damage instead of subsurface fatigue damage. The simulated near-surface fatigue damage mechanisms have been substantiated by the fatigue test data.     

A probabilistic analysis on variability of fatigue crack growth using the markov chain

Understanding the stochastic properties of variability in fatigue crack growth is important to maintaining the reliability and safety of structures. In this study, a stochastic model is proposed to describe crack growth behavior considering the variability of fatigue crack growth rates due to the heterogeneity of material. Fatigue life distribution is then predicted based on this model To construct this model, fatigue tests are conducted on a high strength aluminum alloy 7075 T6 under constant stress intensity factor range control. The variability of fatigue crack growth rates is expressed by random variablesZ and Γ based on the variability of material constantsC andm of the Paris-Erdogan equation. The distribution of fatigue life under constant stress intensity factor ranges is evaluated by the stochastic Markov chain model based on the Paris-Erdogan equation. The merit of the proposed model is that only a small number of tests are required to determine this function, and fatigue life required to reach certain crack length at a given stress intensity factor range can be easily predicted. Department of Mechanical Design and Production Eng.