奥氏体不锈钢高温循环棘轮行为的实验研究

对两种不锈钢材料（316L和304）进行了高温应力控制下的系统循环试验。对该类材料在应力循环下的平均应力、应力幅值及其历史对循环蠕变（棘轮效应）的影响进行了分析,同时也分析了环境温度的变化以及先前应变循环对后继应力循环的棘轮行为的影响。研究表明,两种不锈钢材料在高温非对称循环下的单轴棘轮行为基本相同,不但依赖于当前温度和加载状态,而且还依赖于先前加载历史。研究得到了不锈钢材料高温单轴循环棘轮行为的一些有意义的结果。     

循环软化45碳钢和循环硬化304不锈钢的棘轮行为实验研究

对循环软化45碳钢的单轴应力循环下的平均应力、应力幅值以及先前应变循环对棘轮效应的影响进行了实验研究;并对循环硬化的304不锈钢进行了多种非比例循环加载路径下路径形状、路径等效应力幅值、平均应变与平均应力对材料棘轮变形行为的影响实验．发现平均应力和应力幅值及其历史对于材料的棘轮行为都有很大的影响．     

304不锈钢室温单轴循环棘轮行为的粘塑性本构描述

在统一粘塑性循环本构模型的框架下对循环硬化的304不锈钢的单轴棘轮行为进行了本构描述。模型中通过随动硬化背应力演化和各向同性变形阻力演化对304不锈钢在非对称应力循环下的循环附加硬化和循环流动特性进行了分析，同时考虑了加载历史对循环棘轮行为的影响。将模型应用于304不锈钢室温单轴循环棘轮行为及其对加载历史依赖性的描述中，预言结果与实验结果吻合较好。     

退火和调质42CrMo合金钢单轴循环塑性行为的实验研究

在室温下对退火和调质42CrMo合金钢进行了单轴应变控制和应力控制的系统循环实验,并对它们的应变循环和应力循环特性进行比较.揭示和分析了应变幅值、平均应变及其历史对材料应变循环特性的影响以及应力幅值、平均应力及其历史对棘轮行为的影响.讨论了应变循环和应力循环间的交互作用以及不同热处理工艺下材料循环变形行为间的区别.研究发现材料的热处理工艺、平均应力和应力幅值及其历史对材料的棘轮行为都有很大的影响.得到了一些有助于进行合理本构描述的结果.     

SS304不锈钢高温非比例多轴循环变形行为的粘塑性本构描述

在350℃和700℃下SS304不锈钢的非比例多轴循环变形行为进行了系统的实验研究。在此基础上,在统一粘塑性本构理论的框架下改进和发展了一个新的循环本构模型。该模型给出了新的背应力演化方程,引入了非比例度参量,考虑了温度效应和最大塑性应变幅值记忆效应,能够对材料的高温非比例多轴应变循环变形行为和棘轮行为进行统一描述。模型的模拟结果与实验结果比较表明:该模型对SS304不锈钢高温非比例多轴循环变形行为的描述比较合理。     

316L不锈钢随动强化模型改进研究

本文对316L不锈钢进行了单轴与多轴非比例路径下的应力控制棘轮试验,考察了应力幅值、平均应力和加载历程对棘轮特性的影响。同时进行了应变控制循环试验以研究材料的应力松弛特性。试验结果表明轴向棘轮效应在对称剪切荷载下效果明显,同时棘轮应变随应力幅值和平均应力的增加而增加。研究了Chen-Jiao随动强化模型与Jiang-Sehitoglu随动强化模型采用的单轴与多轴参数对背应力分量增量方向的影响,将Chen-Jiao模型中的多轴系数替换为界面饱和率,并在此基础上引入新的参数对塑性模量系数进行修正,计算结果表明修正后的模型能提升应力控制下多轴棘轮的预测精度,并能很好的预测应力松弛现象,表明了新模型的正确性与有效性。     

纯铝在单轴应力循环作用下棘轮行为的试验研究

对纯铝进行了单轴应变控制和应力控制下的系统循环试验。对纯铝应变循环下的循环应变幅值、应变幅值历史、平均应变对循环特性的影响进行了揭示，对纯铝在非对称应力循环下的应力幅值、平均应力及其历史对循环蠕变〈即棘轮〉的影响进行了分析，得到了纯铝单轴循环行为的一些有意义的结果。     

考虑塑性应变记忆恢复的循环棘轮本构模型研究

在统一粘塑性循环本构理论框架下,以Ohno-Abdel-Karim非线性随动硬化模型为基础,建立了一个循环本构模型。模型通过引入塑性应变幅值记忆效应,并在塑性应变记忆项中加入恢复系数,提高了对循环硬化材料单轴棘轮行为的预言能力。将模型应用于316L不锈钢单轴棘轮行为的描述中,模拟不同平均应力、应力幅值下的棘轮应变,均与实验数据吻合较好,证明本文改进的本构模型能合理地描述循环硬化材料的单轴棘轮行为。     

1Cr18Ni9Ti不锈钢多轴非比例棘轮行为及其影响因素研究

对１Ｃｒ１８Ｎｉ９Ｔｉ不锈钢材料进行了两种不同加载路径的多轴非比例应力循环棘轮效应试验，研究了应力路径形状、平均应力及其历史对棘轮效应的影响，揭示了些材料多轴非比例棘轮效应的一些特征及其原因。为建立能较为精确地估计材料在多非比例应力循环下的累积变形的本构模型提供了一定的实验和理论分析的依据。     

非比例循环塑性内时本构关系研究

在Valanis的内时本构理论的框架中,引入内结构张量以反映由于非比例加载而引起金属材料的附加等向强化及异向强化效应,同时提出材料强化程度的度量采用沿路径法线方向的塑性应变分量来描述.这些考虑的有效性已经通过用所建模型对304不锈钢材料在一些典型非比例循环加载路径下的响应进行的理论预测得到了验证;将该模型应用于U71Mn材料室温单轴棘轮行为描述中,结果显示内结构张量的引入不仅能较好地反映应变控制下的非比例附加效应,而且也能较好地反映应力控制下塑性应变的累积及变化率.     

Uniaxial and non-proportionally multiaxial ratcheting of SS304 stainless steel at room temperature: experiments and simulations

The uniaxial and non-proportionally multiaxial ratcheting behaviors of SS304 stainless steel at room temperature were initially researched by experiment and then were theoretically described by a cyclic constitutive model in the framework of unified visco-plasticity. The effects of cyclic stress amplitude, mean stress, and their histories on the ratcheting were experimentally investigated under uniaxial and different multiaxial loading paths. The shapes of non-proportional loading paths were linear, circular, elliptical and rhombic, respectively. In the constitutive model, the rate-dependent behavior of the material was reflected by a viscous term; the cyclic flow and cyclic hardening behaviors of the material under asymmetrical stress-controlled cycling were reflected by the evolution rules of kinematic hardening back stress and isotropic deforming resistance, respectively. The effect of loading history on the ratcheting was also considered by introducing two fading memorization functions for maximum inelastic strain amplitude and isotropic deformation resistance, respectively, into the constitutive model. The effect of multiaxial loading path on the ratcheting was reflected by a non-proportional factor defined in this work. The predicting ability of the developed model was proved to be good by comparing the simulations with corresponding experiments.     

Uniaxial ratcheting and failure behaviors of two steels

The strain cyclic characteristics, ratcheting and failure behaviors of 25CDV4.11 steel and SS304 stainless steel were experimentally studied under uniaxial cyclic tests and at room temperature. The cyclic hardening/softening features of the materials were first observed under uniaxial strain cycling; and then the ratcheting and failure behaviors of the materials were researched in detail under cyclic stressing. The effects of stress amplitude and mean stress on the ratcheting and failure were discussed under uniaxial asymmetrical stress cycling. It is concluded that the ratcheting and failure behaviors of the materials depend greatly on the cyclic softening/hardening features of the materials and the stress values of cyclic loading. Some conclusions useful to understand the fatigue failure of the materials presented under asymmetrical cyclic stressing are obtained.     

Uniaxial ratcheting of SS304 stainless steel at high temperatures: visco-plastic constitutive model

The uniaxial ratcheting of SS304 stainless steel at high temperatures (300, 600 and 700 °C) were analyzed experimentally, and described by a cyclic constitutive visco-plasticity model. The rate dependence of the material was accounted for by introducing a viscous term. The cyclic hardening and cyclic flow behavior of the material under asymmetrical stress-controlled cycling were described by the evolution rules of kinematic hardening back stress and isotropic deforming resistance. Under the isothermal condition, temperature effect was included by terms involving temperature in the evolution equations of isotropic deforming resistance. The effect of load history on ratcheting was also considered by introducing a fading memory function of the maximum inelastic strain amplitude and isotropic deformation resistance. After the material constants were determined from the experimental data, the uniaxial ratcheting of SS304 stainless steel was numerically simulated and compared with the corresponding experimental results at high temperatures. The predicted results agree well with the experimental ones.     

An experimental study on the ratcheting behavior of pure aluminium under uniaxial cyclic stressing

An experimental study was carried out for the cyclic properties of pure aluminium subjected to uniaxial cyclic straining and stressing. For a material of pure aluminium the effects of the cyclic strain amplitude history and mean strain on the cyclic deformation behavior were investigated, and the influences of stress amplitude, mean stress and their histories on cyclic creep (i. e., ratcheting) were analyzed. It is shown that either uniaxial cyclic property under cyclic straining or ratcheting behavior under asymmetric uniaxial loading depends not only on the current loading, but also on the previous loading history. Some significant results were obtained.Financial support from NFSC is acknowledged.     

Cyclic plastic deformation and cyclic hardening/softening behavior in 304LN stainless steel

Low cycle fatigue experiments have been conducted on 304LN stainless steel in ambient air at room temperature. Uniaxial ratcheting behavior has also been studied on this material and in both engineering and true stress controlling modes. It is shown that material’s cyclic hardening/softening behavior in low cycle fatigue and in ratcheting is dependent not only on material but also on the loading condition. Improvement of ratcheting life and mean stress dependent hardening are observed in the presence of mean stress. A method based on the strain energy density (SED) is used to represent cyclic hardening/softening behavior of the material in this work. The decrease of SED with cycles is an indication that the life in low cycle fatigue and in ratcheting is improved. The SED represents the area of the hysteresis loops.