QT800-2材料应变疲劳特性研究

对QT800－2材料进行了低循环疲劳试验研究，在控制余应变幅的条件下，测定了循环应力－应变曲线，给出了循环应力－应变关系式，对ε-Nf曲线分别采用均值，中值，对数均值进行了处理，得到其应变疲劳性能。     

车轮应变疲劳性能及损伤演变规律研究

试验分析了Φ９１５喂线车轮的低周疲劳性能，测定了循环稳定和单调拉伸应力－应变曲线，给出了应变－寿命及应力－寿命关系，讨论了循环软化循环硬化特性，导出应变疲劳损伤演变方程和寿命估算方程，还列表给出车轮钢的全部应变疲劳参数和与之对应的静态参数，可供设计选材、寿命估算时使用。     

具有应力集中的钢结构的低周疲劳分析

根据材料静力参数，估算其疲劳参数，利用实验加以修正，获得了循环σ－ε曲线和εｕ－２Ｎｆ曲线，进而把非线性有限元法和局部应力应变分析结合在一起计算两个试件疲劳寿命，同时，针对这两个试件进行了低周疲劳实验研究，理论结果和实验结果作了比较，从而证明了本文提出了分析方法是行之有效的。     

基于振动时效仿真的流变塑性模型

振动时效是一种局部循环塑性现象,当循环载荷与材料内部残余应力叠加超过材料的局部屈服强度时就会发生残余应力释放。采用流变塑性模型对振动时效进行仿真,分析了振动时效过程中应力幅、应变幅、振动频率、振动周期和材料屈服应力等对振动时效的影响。结果表明,振动时效中应力释放很大程度上取决于应力幅或应变幅,振动频率和材料参数也是关键因素,而振动周期或时间对振动时效没有很大影响。将此振动时效模型应用于7075铝合金试样机械载荷下的应力松弛实验,所得结果与仿真较一致。     

确定铝合金P—S—N曲线的一种简便方法

根据作者提出的应力疲劳公式，建立了确定铝合金Ｐ－Ｓ－Ｎ曲线的简便方法，如果循环应力下材料的疲劳发布已知，且与高循环应力相比，试件在低循环应力下个有更长的疲劳寿命和更同的寿命分散度，则有可能用本文的方法确定材料的疲劳强度分布和Ｐ－Ｓ－Ｎ曲线表达式。     

SS304不锈钢循环变形行为温度效应的实验研究

对SS304不锈钢进行了系统的室温、高温单轴和非比例多轴应变控制循环和应力控制循环行为的实验研究。重点讨论了温度变化对材料应变循环特性和棘轮行为的影响；同时也讨论了在同一温度、不同加载形式条件下材料的应变循环特性和循环棘轮行为特征。结果表明，SS304不锈钢的循环变形行为具有明显的温度效应，并且在400～600℃范围内，材料会体现出显著的动态应变时效特性。     

7050-T7451铝合金低周疲劳平均应力松弛规律

为了研究平均应变对7050-T7451铝合金低周疲劳力学行为的影响,开展了不同应变比(R=-1、-0.06、0.06和0.5)下的室温恒幅低周疲劳试验。结果表明:在对称循环应变下,材料总体表现为循环软化特征;而在非对称循环应变下,材料表现为初始硬化后的循环稳定行为。非对称循环应变导致了材料出现与应变幅相关的平均应力松弛现象。采用Landgraf模型和非线性Maxwell模型分别研究了7050-T7451铝合金的平均应力松弛规律。结果表明:Maxwell模型能够较准确地描述材料的平均应力循环松弛特征,而Landgraf模型更适用于低应变幅下的平均应力松弛描述。     

应变疲劳循环软化机制的观察,确证及定量估测

通过透射电镜观察和循环应变加时效的试验方法，对淬火回火态地低碳合金钢的循环软件机制进行了研究。     

玻璃陶瓷疲劳断裂的宏观规律研究

对K2O-MgO－AlO3-B2O3-SiO2-F多元系统的可切削微晶玻璃的疲劳行为进行了研究。结果表明：材料在静载荷、动载荷、循环载荷作用下的应力腐蚀指数n分别为69.54、35．2、18.18。应力腐蚀指数n的大小强烈依赖于加载方式，并能作为材料抵抗裂纹扩展的某种抗力指标，循环载荷和动载荷对材料造成了附加损伤，致使在相同应力水平下材料寿命明显下降。     

TA5钛合金大厚板低周疲劳特性研究

本文给出了ＴＡ５钛合金的低周疲劳性能试验数据。研究了主冷凝器在循环载荷下，材料循环硬化或软化特性、应力与应变曲线以及应变与寿命的关系，并预示了低周疲劳寿命。此外，对疲劳断口进行了分析对比。     

Assessment of fatigue response of thermally aged reduced activation ferritic-martensitic steel based on finite element analysis

Abstract

In the present investigation, effect of thermal ageing on low cycle fatigue (LCF) behaviour of Reduced Activation Ferritic Martensitic steel has been assessed by finite element analysis. The steel was thermally aged at 873 K for 3000 hour. Low cycle fatigue tests were carried out on both the as-received and thermally aged material at strain rate of 3×10^?3 s^?1 at 823 K, over strain amplitudes in the range of ± 0.25 to ± 0.8%. Continuous cyclic softening till final failure, except for initial few cycles especially at relatively lower strain amplitudes, was observed in both the material conditions. Thermal ageing resulted in marginally higher cyclic stress response accompanied by lower fatigue life. The differences in fatigue responses have been attributed to the coarsening of precipitates on thermal ageing. Finite element analysis has been carried out considering combined isotropic and kinematic hardening as material model to estimate the effect of thermal ageing on the response of material under LCF loading. Thermal ageing was found to decrease both the isotropic and kinematic hardening with appreciable effect on isotropic hardening. The predicted cyclic stress response and hysteresis loops were found to be in good agreement with the experimental data. The LCF life of the steel has been estimated based on the hysteresis energy approach.     

Effects of temperature on the low cycle fatigue behaviour of nitrogen alloyed type 316L stainless steel

Strain-controlled low cycle fatigue tests have been conducted in air between 298–873 K to ascertain the influence of temperature on LCF behaviour of nitrogen-alloyed type 316L stainless steel. A strain amplitude of ± 0.60% and a symmetrical triangular waveform at a constant strain rate of 3 × 10⁻³ s⁻¹ were employed for all tests. Crack initiation and propagation modes were evaluated, and the deformation and damage mechanisms which influence the cyclic stress response and fatigue life identified. The cyclic stress response at all temperatures was characterized by an initial hardening to the maximum stress, followed by gradual softening prior to attaining saturation. Temperature dependence of fatigue life showed a maximum in the intermediate temperature range. The drastic reduction in fatigue life at elevated temperatures has been ascribed primarily to the combined influence of dynamic strain ageing effects and oxidation-enhanced crack initiation, while the lower life at room temperature is attributed to detrimental effects associated with deformation-induced martensite.     

考虑循环塑性修正的薄片材料低周疲劳试验方法

对N18锆合金和SAPH440板材采用薄片漏斗试样进行了室温下的等幅低周疲劳试验,基于局部应力、应变相等的疲劳损伤等效原理,建立了用于估算N18和SAPH440材料疲劳寿命的Manson-Coffin模型和循环应力-应变方程。对于符合Masing律的材料,提出了一种获得循环本构关系的新方法,用转子材料B65A-S在300℃高温下的低周疲劳试验进行了验证。通过基于标定循环本构关系的有限元分析,建立了两种材料薄片漏斗试样平均应力到漏斗根部轴向应力、测试应变到漏斗根部轴向应变的转换模型。完成了SAPH440材料等直圆棒试样的低周疲劳试验,证明了采用薄片漏斗试样的低周疲劳测试新方法具有较高的精度。     

Low cycle fatigue behaviour of a 1Cr-Mo-V steel with bainite and ferrite microstructure

A 1Cr-Mo-V turbine rotor steel forging, heat treated to obtain a bainite-20% ferrite microstructure, has been investigated for low cycle fatigue behaviour at room temperature and 535°C. In addition to establishing life expectancy curves, cyclic stress response and cyclic stress/strain curves were derived and the influence of time-dependent effects on high temperature low cycle fatigue life was also determined by introducing varying hold times at the peak tensile strain level of the fatigue cycle. The life expectancy curve obtained is comparable to that reported for the bainitic structure in this steel. Introduction of a dwell period of 5 min is found to reduce the low cycle fatigue life by a factor of 1.6.     

Low cycle fatigue behavior of thermo-mechanically treated rebar

The strain and stress controlled low cycle fatigue behavior of thermo-mechanically treated rebar are examined in this current work at room temperature. Severe cyclic softening is observed in all applied strain amplitudes during strain controlled low cycle fatigue. Cyclic softening deteriorates the seismic resistance property of the rebar. Decrease in the cyclic yield stress (linear portion of the hysteresis loop) is responsible for cyclic softening. Cyclic softening results progressive opening up the hysteresis loop during stress controlled low cycle fatigue. It is experimentally observed that irrespective of control mode (stress/strain) and loading conditions, fatigue crack initiates form the transverse rib root and propagate along the same region. Finite element simulation result reveal that stress concentration takes place at the root of the transverse rib and stress triaxiality become higher in the same region. Tensile strain accumulation at the transverse rib root is detected in simulation. Simulation result explains the experimental fact that fatigue crack initiate and propagate along the transverse rib root.     

Effects of Hydrogen on Behaviour of Low Cycle Fatigue of 2.25Cr-1Mo Steel

The effects of hydrogen atoms on behaviour of low cycle fatigue of 2.25Cr-1Mo steel have been investigated in present work. The results indicate that the cyclic softening rate and low cycle fatigue life are respectively increased and reduced remarkably by hydrogen atoms. In addition, hydrogen atoms make the original stress amplitude of low cycle fatigue increase, which is because of the drag effect of hydrogen atoms on the moving dislocations. Analyses using electron microscopy show that hydrogen atoms accelerate crack initiation of low cycle fatigue from inclusion and transfer the source of low cycle fatigue crack from the surface of specimen to the inclusion, which results in the marked decrease of low cycle fatigue life. The increase of cyclic softening rate for hydrogen charged specimen is due to hydrogen atoms accelerating the initiating and growing of microvoids from the secondary phase particles in the steel. The reducing of the drag effect of hydrogen atoms on moving dislocations is also helpful to the increase of the cyclic softening rate.     

EVALUATION OF LOW CYCLE FATIGUE UNDER NON-PROPORTIONAL LOADING

Abstract— A series of low cycle fatigue experiments have been conducted on a 42CrMo steel under tension-torsion loading. Thin-walled tube specimens were used. Low cycle fatigue under various loading paths, including circular and square paths, have been investigated.
The plastic work criterion for low cycle fatigue failure has previously been generally accepted, but it is difficult to calculate stress and strain for complex loading paths, especially for non-proportional loading. This present study suggests a simple method for the calculation of the stable cyclic stress and strain values based on a Modified Endochronic Constitutive Theory (MECT) that redefines an intrinsic time scale. The loading path effect under non-proportional loading is also considered when evaluating fatigue life.
The results show that the plastic work approach using the MECT method in multiaxial fatigue calculations correlates reasonably well the data and is a reflection of loading path dependence.     

Cyclic stress response, strain resistance and fracture behavior of modified 1070 steel

A combined experimental and finite element study of fatigue crack closure in modified 1070 steel has been conducted. In this paper, the material property evaluations required for this study are presented. The monotonic and cyclic stress-strain properties, cyclic stress response, cyclic strain resistance, low cycle fatigue life and fracture behavior are examined. The low cycle fatigue tests were conducted using tension-compression cycling, under total strain amplitude control, over a wide range of strain levels. The material was found to possess medium strength and high ductility; while displaying a strain level dependent combination of cyclic strain softening and hardening behavior. The observed softening behavior is attributed to the rearrangement of dislocations produced by processing, formation of slip bands on the specimen surface and the formation of microcracks. The observed hardening behavior is ascribed to contributions from synergistic influences of dislocation multiplication, dislocation-dislocation interactions and dislocation-microstructural feature interactions. The material followed the strain-life relationships attributed to Basquin and Coffin-Manson. The fracture surfaces of the fatigue specimens showed distinct regions of crack initiation, microscopic-macroscopic crack growth and sudden fracture. The low-cycle fatigue characteristics and fracture behavior are discussed in the light of competing and mutually interactive influences of cyclic strain amplitude, concomitant response stress, intrinsic microstructural effects and dislocation-microstructure interactions during cyclic straining.     

Fatigue life evaluation of tension‐compression asymmetric material using local stress–strain method

In this paper, the low‐cycle fatigue characteristics of cold‐drawn steel were investigated under strain‐controlled uniaxial fatigue load. Cyclic softening was observed throughout fatigue life except for the initial relatively short period which exhibited cyclic hardening. Positive mean stress was found under fully reversed strain loading, indicating that there was a significant cyclic asymmetry. A modified local stress–strain method was proposed to estimate fatigue life of notched tension‐compression asymmetric material. In order to verify this method, fatigue experiments on two kinds of notched specimens with different notch radius were carried out under constant and block load spectrum. It was found that the modified local stress–strain method was more accurate than the traditional ones, the maximum relative error between predicted and experimental fatigue life was less than 6%.