A statistical model of fatigue failure incorporating effects of specimen size and load amplitude on fatigue life

Among many contributing factors, the load range, number of load cycles and specimen geometry (including configuration and size) are three major variables for fatigue failure. Most existing statistical fatigue models deal with only one or two of these three variables. According to the statistical distribution of microcracks with respect to their size and spatial location, a weakest-link probabilistic model for fatigue failure is established to incorporate the combined effect of load range, number of load cycles and specimen size. The model reveals a compound parameter of load range and number of load cycles reminiscent of the empirical formulae of fatigue stress-life curve and its correlation with another compound parameter of cumulative failure probability and specimen size. Four sets of published fatigue test data are adopted to validate the model

EFFECTS OF ENVIRONMENTAL MEDIA ON HIGH CYCLE AND VERY-HIGH-CYCLE FATIGUE BEHAVIORS OF STRUCTURAL STEEL 40Cr

选用40Cr结构钢,分别在空气、水和3.5%NaCl水溶液中进行旋转弯曲疲劳实验,研究环境介质对该结构钢高周和超高周疲劳特性的影响.结果表明,40Cr钢在水环境中的疲劳强度比在空气中明显降低;在3.5%NaCl水溶液环境中的疲劳强度比在水中低.断面观察显示,在水和3.5%NaCl水溶液中,疲劳裂纹多源萌生;在稳态扩展阶段,裂纹沿晶界扩展并存在广泛分布的沿晶二次裂纹

一种基于有限元结果的多轴疲劳寿命矩阵化计算方法

本发明公开了一种基于有限元结果的多轴疲劳寿命矩阵化计算方法，基于全局三维坐标轴对应的应力/应变张量，利用坐标变换和矩阵乘法计算斜面的三维坐标轴对应的应力/应变张量，将应力/应变张量中的正应力/应变数据和剪应力/应变数据进行分类；建立应力/应变存储数组、空间离散坐标变换数组和空间变换应力/应变数组；分别提取空间变换应变数组中的正应变数据和剪应变数据，计算所有节点对应的最大正应变幅值和最大剪应变幅值；基于最大正应变幅值和最大剪应变幅值，以及正应变临界面疲劳寿命计算公式以及剪应变临界面疲劳寿命计算公式，求解所有节点对应的疲劳寿命，本发明采用矩阵运算代替了传统算法中的循环、判断和标量代数运算

Statistical characterization of specimen size and temperature dependence of cleavage fracture toughness of ferritic steels

Based on the numerical results on the non-linear correlation between volume of plastic zone and global loading level represented by stress intensity factor, the dependence of cleavage fracture toughness of ferritic steels on specimen size and temperature is evaluated based on an empirical statistical model. The statistical correlation is supported by two sets of toughness data measured from different sized compact tension specimens at different temperatures. To further improve the accuracy of the statistical model, future work is identified to quantify the variation of volume of plastic deformation zone with loading level and measure the average volume for each microcrack at different temperature. A non-Weibull statistical model of cleavage toughness is proposed. Dependence of cleavage toughness on temperature and specimen size is studied. The model is validated by two sets of cleavage toughness data. Two research topics are identified for model improvement

Fatigue properties and S-N curve estimating of 316L stainless steel prepared by SLM

This paper investigates the fatigue properties of 316L stainless steel manufactured by selective laser melting technology (SLM) under hot-isostatic-pressing (HIP) heat-treatment. The fatigue tests of 316L specimens were carried out under pulse tensile cyclic loading to obtain the fatigue performance. To avoid falling into local optima and improve the convergence speed, the dynamic multiswarm particle swarm optimizer (DMS-PSO) algorithm was first introduced for parameter optimization of the three-parameter Weibull distribution model, and then the S-N curves based on the model were obtained. The S-N curves were highly consistent with the metallographic and fractographic phenomena. Besides, both the S-N curves and fractographic analysis show that the resistance of high-stress fatigue fracture is significantly improved by using HIP heat-treatment

ESSENTIAL CHARACTERISTICS AND INFLUENTIAL FACTORS FOR VERY-HIGH-CYCLE FATIGUE BEHAVIOR OF METALLIC MATERIALS

合金材料在超高周疲劳下具有与低周和高周疲劳不同的裂纹萌生和扩展行为以及不同的S-N曲线特征.材料的强度、循环加载的频率、所处的环境等都显著影响超高周疲劳的特性.本文综述了合金材料超高周疲劳行为的基本特征和影响因素的研究进展

CONSTRAINT ASSESSMENT FOR CRUCIFORM SPECIMENS WITH A SEMI ELLIPTICAL CRACK

A real crack to be assessed in a RPV is generally a shallow crack subjected to biaxial far-field stresses. However, the fracture toughness Ke or Je, which is an important material property for the structural integrity assessment of RPV containing cracks, are usually tested on deep cracked compact tension [C(T)] or single-edged bending [SE(B)] specimens under uniaxial loading. The fracture toughness data do not reflect the realistic biaxial loading state that the cracks are subjected to. Cruciform bending [CR(B)] specimen is therefore developed to simulate the biaxial stress state. In this paper, a series of finite element (FE) simulations of the CR(B) specimens containing different semi-elliptical cracks are conducted, Stress-strain curves of materials of different yield strength and hardening behavior reflecting the variation in the mechanical properties of RPV steels due to aging or temperature change are implemented into the finite element models. The J-A2 theory is applied to analyze the crack tip constraint. The results show that the biaxial effect is material property dependent and affected by load levels

随机缺陷下结构疲劳寿命及其尺寸效应的概率建模与评估

通常球墨铸铁和铝合金制成的工程部件的疲劳寿命由制造过程中产生的制造缺陷所决定。因此,本文基于制造缺陷的尺寸和位置,提出了考虑制造缺陷对疲劳寿命影响的概率模型。具体而言,就是将CT扫描获得的缺陷近似作为圆形裂纹进行裂纹扩展分析,并结合威布尔分布对试件的疲劳寿命建模。基于此,提出一个基于表面初始裂纹损伤机理的表面缺陷修正系数,并利用三组铸铁和铝合金的实验数据对模型进行验证和比较。此外,还探讨了等幅载荷作用下统计尺寸效应对疲劳寿命分布的影响。最后,分别对三种不同尺寸材料的疲劳寿命进行预测,预测的P-S-N图表明所提出模型进行预测的结果与概率分散带吻合一致,具有较好的预测效果