SS304不锈钢高温非比例多轴加载下时相关循环变形行为的实验研究

在973 K下,对SS304不锈钢多轴循环加载下的时相关应变循环特性和时相关棘轮行为进行了实验研究.揭示了材料在不同加载路径、不同加载速率以及不同保持时间下的循环软/硬化特性和棘轮变形行为.结果表明：在高温下,材料的循环软/硬化和棘轮变形均体现出明显的时相关效应,即其循环变形行为不仅强烈依赖于加载速率,而且还明显依赖于保持时间;另外,材料的变形行为在高温下也具有强烈的加载路径依赖性.研究得到了一些有助于后续建立时相关本构模型的结论.     

304不锈钢非比例循环棘轮行为的实验研究

在室温下对３０４不锈钢在非比例循环加载下的棘轮行为进行了实验研究。在圆形、菱形和直线形状的非比例双轴循环加载路径下,研究不同轴向平均应力、等效应力幅值大小及基历史下材料的棘轮行为特征,为了比较,还进行了单轴加载下该材料的循环棘轮行为实验,实验表明,３０４不锈钢的室温非比例循环棘轮行为明显地依赖于加载路径的形状;与单轴循环棘轮行为一样,非比例循环棘轮行为也明显地依赖于轴向平均应力和等效应幅值的大小及     

6061-T6铝合金单轴时间相关循环变形行为

对6061-T6铝合金进行系统的单轴应变循环和应力循环实验，揭示该材料在室温和高温下的循环变形行为，讨论环境温度、加载速率、峰，谷值保持对其应变循环特性及棘轮行为的影响。结果表明，6061-T6铝合金表现出弱的循环软化特性，其棘轮行为不仅依赖于平均应力和应力幅值的大小，还依赖于加载历史。尽管该合金的单拉行为对应变率的变化不敏感，但其循环变形行为却体现出明显的时间相关特性，即；应变循环下，在峰／谷值有保持时的响应应力幅值明显小于没有保持时的值，且随着保持时间的增加，响应应力幅值将进一步减小；应力循环下，在峰值有保持时产生的棘轮应变比没有保持时的值大，且随着峰值保持时间的增加及应力率的降低，棘轮应变明显增大。     

SiCp/6061Al合金复合材料的单轴循环变形行为及其时间相关特性

对T6热处理后的6061Al合金与SiCp／6061Al合金复合材料（14％,21％SiCp）的室温单轴循环变形行为进行了实验研觅讨论了基体合金和其复合材料在不同加载条件下的循环软／硬化行为和棘轮变形特征．实验研究表明：复合材料在宏观层次上体现出与基体相类似的应变循环特性和棘轮变形规律,即复合材料在非对称应力循环下也将产生一定的棘轮变形,并随应力幅值和平均应力的增加而增加;颗粒的引入使复合材料抵抗棘轮变形的能力增强,棘轮变形随颗粒含量升高而下降;复合材料的棘轮行为具有明显的时间相关特性,棘轮应变值依赖于应力率和峰值保持时间．研究得到了一些有助于复合材料循环变形行为本构描述的结论．     

室温下20钢高周次单轴棘轮行为的实验研究

通过对20钢的单轴应变控制循环和应力控制循环实验研究, 揭示了20钢在室温下的循环变形特性, 讨论了材料的循环软/硬化特性和材料的屈服平台以及平均应力、应力幅值和应力比对材料高周次棘轮行为的影响. 研究表明: 20钢表现出弱的、与应变幅值相关的循环硬化特性; 其棘轮行为依赖于平均应力、应力幅值和应力比的大小, 在高应力水平时的高周次循环后期, 棘轮变形会出现<再次增长的现象; 材料的屈服平台对棘轮行为有明显的影响, 在对20钢的棘轮行为进行本构描述时需要加以合理考虑.     

锆－4合金的时间相关低周疲劳

研究了锆－４合金在不同应变速率下的低周疲劳和带应变保持时间相关低周疲劳行为。结果表明：在不同加载方式下，锆－４合金均表现为循环硬化，应变保持和应变速率降低均进一步提高循环饱和应力。应变保持降低材料的疲劳寿命，特别是在高应变幅区域。随着应变速率降低，材料疲劳寿命降低。疲劳变形亚结构观察表明：锆－４合金时间相关疲劳过程中以柱面滑移和晶界滑移方式变形。     

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 在室温下通过对63Sn37Pb进行一系列棘轮实验和单轴拉伸实验，得到加载历史对材料棘轮应变累积速率以及延伸率的影响关系。拉伸棘轮试验和压缩棘轮试验表明，在相同加载条件下，没有加载历史的试件较有加载历史的试件会发生更大的棘轮变形，加载历史在很大程度上抑制棘轮变形的累积速率。此外，没有加载历史的试件断后延伸率要比有加载历史的试件大。热处理可以改善材料的延性，有效的增大材料的延伸率。     

304不锈钢棘轮变形过程中应变诱发马氏体相变行为研究

利用OM, SEM和XRD对单轴非对称应力循环下304不锈钢棘轮变形过程中的微观组织变化进行了实验观察. 结果表明: 304不锈钢棘轮变形过程中,当棘轮应变达到一定值后会 产生应变诱发马氏体相变, 形成板条状 马氏体,并且随循环周次的增加, 形成的应变诱发 马氏体相对量逐渐增加.因马氏体相变而诱发的塑性变形对总的棘轮变形量产生一定的影响,材料的棘轮应变应由两部分组成, 即应力引起的塑性应变和相变诱发的塑性应变.     

锆—4合金的时间相关低周疲劳

研究了锆－４合金在不同应变速率下的低周疲劳和带应变保持时间相关低周疲劳行为。结果表明：在不同加载方式下，锆－４合金均表现为循环硬化，应变保持和应变速率降低均进一步提高环孢和应力。应变保持降低材料的疲劳寿命，特别是在高应变幅区域。随着应变速率降低、材料疲劳寿命降低。疲劳变形亚结构观察表明：锆－４合金时间相关疲劳过程中以｛１０１↑－０｝柱面滑移和晶界滑移方式变形。     

描述不锈钢材料单轴棘轮行为的一元参量体系

基于304不锈钢和1Cr18Ni9Ti不锈钢单轴常温与高温应力循环棘轮实验，提出了棘轮应力棘轮门槛值的概念，发展了一套基于棘轮应力一元参量的，描述高温棘轮循环饱和材料的单轴棘轮本构行为和常温演化行为的方法体系，新体系降低了传统二元应力控制棘轮变形认识带来的难度，由于新体系方程结构简洁，建模方法简便，描述实验结果精度高，故适合工程应用。     

304不锈钢在室温下非比例应力和应变循环变形行为实验研究

在室温下对304不锈钢进行了多种长短轴比的椭圆形、菱形等非比例循环加载路径下的应力与应变路径形状、路径的应力与应变幅值、平均应变及其加载历史的影响的系统实验．结果表明，材料的循环塑性行为强烈地依赖于应力与应变路径形状、路径的应力与应变幅值及其加载历史。     

A damage-coupled multi-axial time-dependent low cycle fatigue failure model for SS304 stainless steel at high temperature

Based on the time-dependent strain cyclic characteristics and fatigue behaviors of SS304 stainless steel under multi-axial cyclic loading at 700 ? C, and in the frame of unified visoco-plastic cyclic constitutive model and continuum damage mechanics theory, the damage-coupled multi-axial time-dependent constitutive model and fatigue failure model were proposed. In the model, the evolution equation of damage was introduced in and the time-dependent effects, e.g. holding time, loading rate, were taken into account. The model was applied to the simulation of whole-life cyclic deformation behaviors and prediction of LCF life for SS304 stainless steel in multiaxial time-dependent low cycle fatigue tests. It is shown that the simulated results agree well with experimental ones.     

304不锈钢的高温单轴应变循环与棘轮行为

对３０４不锈钢分别进行了高温单轴奕变控制和应力控制下的系统循环试验,揭示和分析了循环应变幅值,应变幅值历史以及温度历史对循环特性的影响以及应力幅值,平均应力及其历史对循环蠕变（棘轮效应）的影响,研究表明,无论单轴应变循环特性还是非对称单侧应力下的棘轮效应不但依赖于当前温度和加载状态,而且依赖于其加载历史,得到３０４不锈钢高温单轴循环行为的一些有意义的结果。     

VISCO-PLASTIC CONSTITUTIVE MODEL FOR UNIAXIAL AND MULTIAXIAL RATCHETING AT ELEVATED TEMPERATURES

Based on the experimental results of the ratcheting for SS304 stainless steel, a new visco-plastic cyclic constitutive model was established to describe the uniaxial and multiaxial ratcheting of the material at room and elevated temperatures within the framework of unified visco-plasticity. In the model, the temperature dependence of the ratcheting was emphasized, and the dynamic strain aging occurred in the temperature range of 400-600℃ for the material was taken into account particularly. Finally, the prediction capability of the developed model was checked by comparing to the corresponding experimental results.     

Experimental investigation of plastic deformation of stainless steel for cyclic loading at low temperature

Cyclic plastic deformation of stainless steel SUS 304 is experimentally investigated at low temperature of liquid nitrogen (77 K) under various cyclic loading conditions Thin walled tubular specimens are subjected to cyclic loading under constant strain ranges. At low temperature, the material shows remarkable hardening by cyclic loading comparing with cyclic loading at room temperature. The hysteresis curves of stress-strain relations by cyclic loading are saturated by increasing the cycle numbers. The saturation tendency depends on loading direction. The saturated stress values are related with cumulative plastic strain of cyclic loading. The prestraining is given at 77 K by axial and torsional loadings, and subsequent cyclic loading under constant strain range is conducted at 77 K. The cyclic stress-strain curves are saturated by increasing cyclic numbers. At small cyclic numbers, cyclic plastic deformation depends on the prestrain direction. The directional effect of pre-strain on cyclic loading becomes small with increasing number of cycles.     

Ratcheting Behavior of a Non-conventional Stainless Steel and Associated Microstructural Variations

Ratcheting fatigue behavior of a non-conventional stainless steel X12CrMnNiN17-7-5 has been investigated with varying combinations of mean stress (σ_m) and stress amplitude (σ_a) at room temperature using a servo-hydraulic universal testing machine. X-ray diffraction profile analysis has been carried out for assessing possible martensitic phase transformation in the steel subjected to ratcheting deformation. The results indicate that ratcheting strain as well as volume fraction of martensite increases with increasing σ_m and/or σ_a; the phenomenon of strain accumulation is considered to be governed by the associated mechanics of cyclic loading, increased plastic damage as well as martensitic transformation. A correlation between strain produced by ratcheting deformation and martensitic transformation has been established.