Fatigue life estimation under random loading using the energy criterion

A procedure for estimating the useful life of a component for a given (admissable) probability of fatigue fracture origination under random loading is presented. The method uses material constants obtained from the S/N and cyclic stress/strain curves, standard deviation and probability density distribution of the loading process and a macroblock of harmonic cycles obtained by applying the rainflow cycle counting method to the random loading process. Theoretical and experimental lives are found to exhibit good agreement.     

高温合金材料时间相关热机械疲劳寿命预测技术

本文在变温非线性运动强化规律所描述的高温合金材料热机械疲劳应力-应变循环特性的基础上,重点讨论了应变控制的时间相关热机械疲劳寿命预测技术。对于温度循环的影响,采用由应变能密度表示的损伤参数,并且引入了温度损伤系数。对于循环时间的影响,引入了蠕变─疲劳相互作用的损伤机制,采用韧性耗散损伤模型。在确定模型的一些参数时,采用等温力学试验和疲劳试验的数据,把等温疲劳研究成果推广到变温疲劳分析领域。     

Improvement of life prediction in AISI H11 tool steel by integration of thermo-mechanical fatigue and creep damage models

In hot forging operations, the die surfaces and the nearest surface layers of the die undergo mechanical and thermal cycles which significantly influence their service life. The real thermal and mechanical cycles have been previously investigated in forging plants by measurements and numerical simulation, and a reasonable variation window of process parameters has been determined. A new simulative test applied to AISI H11 hot working die steel has been used to generate failure data in conditions similar to those of the forging dies, but under a more controlled and economical method. Fracture surfaces of specimens for different tests observed by scanning electron microscopy (SEM) indicate that both thermo-mechanical fatigue (TMF) and creep phenomena can be considered to be main damage mechanisms and their contribution to the failure differs as testing conditions vary. As a result of the experiments, the failure is affected by both thermo-mechanical cycle and resting time at high temperature. Therefore, the authors developed a new lifetime prediction model obtained by combining the damage evolution laws, at each cycle, for pure creep and pure TMF. This combination was based on the linear accumulation rule. The damage evolution law for pure creep is obtained by modifying Rabotnov's law in order to suit the case of actual hot forging cycles, where temperature and stress vary widely. The damage evolution law for pure TMF is based on a generalization of the Wöhler–Miner law. This law is modified in order to take into account the presence of thermal cycle and thermal gradient. Comparison between the experimental cycles to failure and the predicted ones was performed using tests excluded in the determination of the coefficients. The conclusion was that the accuracy of prediction appears to be quite good and that the linear accumulation and interaction of TMF and creep is confirmed.     

Predicting damage and failure under low cycle fatigue in a 9Cr steel

Experimental data have been generated and finite element models developed to examine the low cycle fatigue (LCF) life of a 9Cr (FB2) steel. A novel approach, employing a local ductile damage initiation and failure model, using the hysteresis total stress–strain energy concept combined with element removal, has been employed to predict the failure in the experimental tests. The 9Cr steel was found to exhibit both cyclic softening and nonlinear kinematic hardening behaviour. The finite element analysis of the material's cyclic loading was based on a nonlinear kinematic hardening criterion using the Chaboche constitutive equations. The models’ parameters were calibrated using the experimental test data available. The cyclic softening model in conjunction with the progressive damage evolution model successfully predicted the deformation behaviour and failure times of the experimental tests for the 9Cr steels performed.     

Damage evolution with growing cyclic creep and life prediction of MDYB‐3 PMMA

The influence of cyclic creep accumulation rate on the damage evolution of MDYB‐3 polymethyl methacrylate (PMMA) was experimentally investigated. Fatigue tests were carried out at four stress levels by stress control mode. The steady cyclic creep accumulation stage was observed occupying a substantial proportion of all specimens fatigue processes. Cyclic creep strain growth speed and relaxed modulus degradation rate were deduced as two important indicators for describing the damage evolution characteristics. Linear evolution relations of cyclic creep strain and modulus degradation with cycle times were retrieved from different terms of hysteresis loops. A preliminary model was proposed to be able to estimate the damage extent at different cyclic stress levels. The life predictions by the proposed model were compared with the experiment results and the classical power S–N model, which were demonstrated as a good estimation for the fatigue life. It is feasible to make durability evaluations by the characteristics of steady cyclic creep for multiaxis directed PMMA material.     

保持时间对定向合金DZ125热/机械疲劳断裂行为的影响

对DZ125定向凝固铸造镍基高温合金进行了应变比为-1.0的同相位三角波和同相位梯形波,550℃()1000℃热/机械疲劳实验研究.实验结果表明:在相同应变幅下,同相位三角波载荷情况下的热/机械疲劳寿命比同相位梯形波载荷情况下的热/机械疲劳寿命长.研究了在两种载荷情况下材料的热/机械疲劳循环应力响应行为.试样断口的微观分析表明:在热/机械疲劳过程中,同时存在疲劳、蠕变和氧化损伤;在同相位三角波载荷下,穿晶 沿晶断裂为疲劳断裂的主要特征;在同相位梯形波载荷下,裂纹主要为沿晶萌生与扩展.这是导致在同相位梯形波载荷下疲劳寿命缩短的主要原因.     

Damage Operator‐Based Lifetime Calculation Under Thermomechanical Fatigue and Creep for Application on Uginox F12T EN 1.4512 Exhaust Downpipes

Abstract:  The article focuses on the application of a recently developed damage operator‐based lifetime calculation to a thermomechanically loaded exhaust downpipe. The damage operator approach enabling online continuous damage calculations for isothermal and non‐isothermal loading with mean stress corrections is reviewed. The article also highlights an extension of the strain‐life approach to take into account viscoplastic effects and creep. The transient results from thermal and structural analyses using finite element analyses have been applied to the exhaust downpipe in LMS Virtual.Lab and the damage predicted. Tested exhaust downpipes were then subjected to the same loading conditions as in the calculation, and load cycles were repeated up to the point of failure. Simulated and test results are comparable.     

金属蠕变律及蠕变行为研究

蠕变过程中 ,材料内部状态的不断演化 ,使得材料的蠕变行为发生改变。本文提出考虑损伤和硬化影响的蠕变律。利用该蠕变律讨论了 12 Cr1Mo V钢蠕变行为。分析结果表明 ,ε·c在蠕变过程中始终变化 ,第二阶段仅仅是ε·c相对稳定的阶段 ,其相对稳定程度和持续范围与载荷大小有关。在相同寿命分数下 ,不同应力水平引起的硬化状态也不相同     

老化沥青混合料粘弹性疲劳损伤模型研究

为了较真实反映沥青路面服务期的疲劳损伤特性,从粘弹性材料的基本特性出发,通过本构方程和耗散能的定义构造了耗散能的泛函,定义耗散能为损伤变量,建立基于Burgers模型的老化沥青混合料粘弹性疲劳损伤模型,通过直接拉伸试验确定沥青混合料的粘弹性参数,求出损伤函数、损伤演化方程,提出一种考虑疲劳过程中老化程度对疲劳损伤影响的累积疲劳损伤计算理论与方法,为沥青混合料在不同老化程度下的疲劳寿命预估提供了依据。根据疲劳损伤模型推导出沥青混合料临界损伤度、疲劳寿命计算公式,并对其进行计算比较,精度满足要求,从而验证了粘弹性疲劳损伤模型的合理性。     

加载历史和裂纹闭合对疲劳裂纹扩展行为影响的数值模拟

采用不同应力比条件下的16MnR钢紧凑拉伸试样,设计了三种有限元分析模型,即不考虑加载历史效应的静态裂纹扩展模型,同时考虑加载历史和裂纹闭合的动态裂纹扩展模型以及仅考虑加载历史的伪动态裂纹扩展模型,对疲劳裂纹闭合过程、裂纹尖端的应力-应变迟滞环、疲劳损伤和裂纹扩展速率进行了数值模拟与分析,进而着重探讨了加载历史和裂纹闭合影响疲劳裂纹扩展行为的交互作用机制。结果表明：对于同类分析模型,应力比越大越不容易产生裂纹闭合;而在应力比相同的情况下,加载历史引起的残余压应力对裂纹闭合有明显的促进作用。裂纹闭合效应阻碍了平均应力的松弛,减小了裂纹尖端附近的应力-应变场强度、疲劳损伤和裂纹扩展速率,而加载历史引起的残余压应力则加快了平均应力的松弛和抑制了棘轮效应。与实验结果比较发现,只有同时考虑了裂纹闭合效应和加载历史影响的动态裂纹扩展模型,才能对疲劳裂纹扩展行为进行准确、定量的模拟。     

疲劳/蠕变复合作用下聚苯乙烯的交互损伤研究

探讨了在疲劳／蠕变复合作用下聚苯乙烯的损伤交互作用,结果表明,在疲劳／蠕变复合作用下聚苯乙烯存在疲劳和蠕变的交互损伤,其断裂寿命比纯疲劳或纯蠕变的断裂寿命低;断裂机制是疲劳循环载荷松动和活化了分子链或链段,从而促进蠕变运动和断裂,并且,疲劳／蠕变的交互损伤程度与温度密切相关。     

[±20°]钢丝/橡胶复合材料的疲劳损伤累积

对 钢丝帘线增强的橡胶复合材料在拉伸循环载荷下的疲劳损伤累积进行了研究。结果表明:在载荷控制的疲劳过程中,材料的周期最大应变发展曲线呈现明显的三阶段规律。帘线端头处基体裂纹的出现是宏观疲劳损伤的初始,损伤的累积表现为裂纹数量增加、帘线/基体脱粘和层间裂纹的扩展。以动蠕变为参量建立了线性疲劳损伤累积模型,该模型能够较好地预报两级加载条件下材料的第二级疲劳寿命。     

金属非对称循环下的匀变速加载问题

由损伤力学理论可知,疲劳是由材料内部的损伤演化导致的,但其损伤演化的机理并不清楚。为此,我们将高分子物理中断裂的分子理论推广应用于金属的疲劳损伤,认为金属的断裂是一个松驰过程,宏观断裂是微观原子键断裂热活化的结果。以Q235钢为例,在CMT5105万能电子试验机上进行拉压非对称循环疲劳试验。从原子键离解的视角出发并结合试验数据,讨论并推算诸多因素影响下匀、变速加载时非对称循环疲劳损伤演化律的具体形式。再将速率作为重点考虑因素提出了新的疲劳损伤演化律,分别讨论了匀、变速情况下损伤演化律的基本形式,并对速率相关参数因子进行了修正。结果表明,新的疲劳损伤演化律形式简单、参数少、应用广泛且与试验结果贴合较好。     

涡轮发动机用陶瓷基复合材料涡轮转子研究进展

连续碳化硅纤维增强碳化硅基复合材料(SiC_f/SiC)因其轻质、耐高温和高损伤容限的优点而成为下一代航空发动机的重要热结构材料。然而,疲劳实验周期长、成本高的缺点严重制约了对复杂细观结构SiC_f/SiC的深入理解及其工程应用。为充分发挥SiC_f/SiC的优势与可调性,实现对结构载荷响应预测并进行优化设计,本文采用疲劳迟滞模型和渐进损伤理论分别对单向、正交和二维编织SiC_f/SiC的疲劳寿命曲线进行了分析。通过对界面剪应力(±20%)、纤维强度(±5%)、纤维威布尔模量(±1%)和纤维体积分数(±5%)进行偏值处理实现了对SiC_f/SiC疲劳寿命的敏感性评价,得到的疲劳寿命曲线上下限能够包络主要实验结果。根据上述分析结果,验证了以损伤参数控制危险估计和保守估计的疲劳寿命曲线拟合方法,并以SiC_f/SiC涡轮叶片模拟结构为例现实了该方法用于实际工程评价分析的可行性。     

An overview of the damage approach of durability modelling at elevated temperature

Lifetime prediction techniques for components working at elevated temperature are revisited. Two damage approaches in which time effects at high temperature are introduced in different ways are discussed in greater detail. First, a creep–fatigue damage model considers the interaction of the two processes during the whole life before macrocrack initiation; and second, a creep–fatigue–oxidation model separates the fatigue life into two periods: during initiation the environment-assisted processes interact with fatigue, although bulk creep damage only interacts during the micropropagation period. The second model is illustrated by its application to a coated single-crystal superalloy used in aerojet turbine blades. Its capabilities are illustrated in a number of isothermal and thermomechanical fatigue tests. Anisotropy effects are also briefly discussed and a special test, introducing cyclic thermal gradients through the wall thickness of a tubular component, demonstrates the predictive capabilities for actual engine conditions.