表面机械研磨处理对纯铜棘轮行为的影响:宏微观试验与本构模拟

目的 通过金属表面纳米化试验机制备出梯度结构纯铜,提升纯铜材料的疲劳寿命,并揭示其背后的机理。方法 通过系统的宏观力学性能测试、微观组织表征以及本构模拟探究了表面机械研磨处理(Surface Mechanical Attrition Treatment,SMAT)对T2纯铜棘轮行为的影响。结果 循环变形试验结果表明SMAT纯铜样品的循环失效圈数明显多于未处理纯铜样品的循环圈数,且SMAT纯铜样品在循环过程中的累积塑性变形明显小于未处理纯铜样品的累积塑性变形,即棘轮应变明显小于未处理纯铜样品的棘轮应变。电子背散射衍射(Electron Back Scattered Diffraction,EBSD)和X射线衍射分析(X-Ray Diffraction,XRD)表征发现:经过SMAT后,材料的晶粒尺寸均呈现由处理表面到材料芯部逐渐减小的梯度分布。且SMAT时间越长,样品的总位错密度越大。此外,基于应变梯度塑性理论模型对SMAT前后纯铜的单拉及循环变形响应进行了有限元模拟,模拟结果显示累积塑性应变沿深度方向(SMAT冲击方向)呈梯度分布,最大几何必需位错密度以及最大等效应力均出现在模型的次表层。同时,当模拟的循环圈数相同时,代表SMAT样品的梯度结构模型的棘轮应变明显低于代表未处理样品的均匀模型的棘轮应变。结论 循环变形试验结果表明SMAT对于T2纯铜的棘轮应变有抑制作用,有限元模拟进一步揭示了SMAT对于棘轮应变的抑制效应以及背后的机理。     

45钢在应变循环与棘轮变形下的随动硬化演化实验研究

对调质处理的45碳钢进行了不同应变幅值的对称应变和具有平均应变控制下的屈服面半径和背应力演化分析,及具有不同平均应力的低应力、较高应力幅值控制下的屈服面半径和背应力演化分析．研究表明：屈服面半径在给定应变幅值下随循环周次的增加而变小,且随循环塑性应变幅值增大而减小,在单调加载时增大;循环背应力幅值随单调应变而减小,随循环塑性应变幅值增大而增大．     

SS304不锈钢在室温单轴循环加载下的时相关棘轮行为

对SS304不锈钢在室温单轴循环加载下的时相关棘轮行为和时相关应变循环特性进行了实验研究．讨论了材料在不同加载速率、不同保持时间和不同加载波形下的循环软／硬化行为和棘轮行为特征．结果表明：即使在室温下,SS304不锈钢也表现出明显的时相关效应;材料的循环变形行为,特别是棘轮行为明显地依赖于加载速率和保持时间以及加载波形的形状．研究得到了一些有助于时相关循环变形行为本构描述的结论．     

304不锈钢的单轴棘轮变形与失效行为研究

对304不锈钢在室温下进行了单轴应变控制下的应变棘轮变形与失效以及低周疲劳试验研究,系统地揭示了材料在循环过程中的材料变形与失效行为。研究表明：材料的应变棘轮变形与失效既不同于单轴拉伸,也不同于相同应变幅值下的对称应变循环加载时的变形与失效,而是强烈地依赖于应变幅值与每一循环周次在最大拉应变处的应变增加量。观察到了一些有意义的结果,     

Effect of thermal ratcheting on the mechanical properties of Teflon and fiber based gasket materials

This paper discusses the effect of thermal ratcheting on the material properties of expand polytetrafluoroethylene (ePTFE), virgin polytetrafluoroethylene (vPTFE), and compressed nonasbestos fiber (CNA) gasket materials. Comparison between the creep strain at constant temperature and when subjected to thermal ratcheting show a 7.7 and 28% increase in the creep strain of ePTFE and vPTFE, respectively. In addition, thermal ratcheting produces a substantial reduction of creep modulus of these materials. The CNA material does not exhibit significant change in creep strain or in creep modulus with thermal ratcheting. However, all three gasket materials show a momentous raise in the creep strain when the material temperature is reduced. On declining the gasket temperature from 212 to 100 °F at the end of 20th thermal cycle, the materials—ePTFE, vPTFE, and CNA exhibit 27, 48, and 15% increase in creep strain value, respectively. The percentage of thickness reduction raises with the increase of cyclic temperature and with increase of creep pre-exposure time, except for CNA where only a small variation is observed. The coefficient of thermal expansion of both PTFE materials shows a significant change due to cyclic temperature and initial creep exposure. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47265.     

Triphasic ratcheting strain prediction of materials over stress cycles

This study intends to formulate ratcheting strain evolution in steel alloys of 42CrMo, 20CS, SA333 Gr. 6 C‐Mn and OFHC copper over uniaxial stress cycles. Stages of ratcheting deformation were related to stress cycles, lifespan, mechanical properties and amplitude and mean stress components by means of linear and nonlinear functions. Terms of mechanical properties in the ratcheting formulation enabled to characterize ratcheting response of various materials over life cycles. These terms were further employed to interpret the influence of softening/hardening response of materials on ratcheting deformation. Ratcheting data for 42CrMo, 20CS, SA333 steels and OFHC copper reported in the literature were employed to evaluate the proposed ratcheting formulation. The predicted ratcheting strain values based on the proposed equation were found in good agreements as compared with the experimental data.     

核安全一级主管道疲劳校核

本文对某核电厂主管道疲劳及热棘轮进行了独立校核。校核采用基于RCC-M标准的ROCOCO软件,比较了RCC-M标准与ASME标准在核安全一级管道疲劳评价方面的差异。对比的主要方面包括疲劳设计的计算范围界定、一次加二次应力强度的计算方法、弹塑性修正系数的计算、动态载荷叠加方法等。通过对ROCOCO中与ASME标准不一致的算法进行修正,得到主管道冷段壁厚65 mm和55mm的疲劳使用系数和热棘轮设计裕量。结果表明:某核电厂主管道最小壁厚不能小于55 mm,55mm壁厚的热棘轮设计值达到许用值的95%。     

Uniaxial ratcheting and low-cycle fatigue failure behaviors of AZ91D magnesium alloy under cyclic tension deformation

Uniaxial ratcheting and low-cycle fatigue failure behaviors of hot-rolled AZ91D magnesium alloy were studied by uniaxial cyclic stress-controlling tension deformation experiments. The effects of stress amplitude, mean stress and stress rate on the uniaxial ratcheting response and fatigue life of the hot-rolled AZ91D magnesium alloy were analyzed. Results show that (1) the ratcheting strain and ratcheting strain rate of the hot-rolled AZ91D magnesium alloy both increase with the increase of stress amplitude or mean stress; (2) increasing stress rate will decrease the ratcheting strain and ratcheting strain rate of the hot-rolled AZ91D magnesium alloy; (3) the increase of stress amplitude and mean stress can both reduce the fatigue life of the hot-rolled AZ91D magnesium alloy, while the fatigue life will be prolonged with the increase of stress rate.     

Ratcheting assessment of materials based on the modified Armstrong–Frederick hardening rule at various uniaxial stress levels

The present study evaluates ratcheting response of materials by means of the Armstrong–Frederick (A–F) hardening rule, the modified A–F rule (Bower's model), and further modifications of the hardening rule based on new introduced coefficients. The implemented modifications on the A–F‐based hardening rule aims to address stages of ratcheting over stress cycles. The modified hardening rule predicts the ratcheting strain rate decay over stage I and the constant rate of strain accumulation during stage II. The modified hardening rule consisted of the coefficients of the hardening rule controlling stress–strain hysteresis loops generated over stress cycles during ratcheting process (Bower's modification on A–F rule) plus the coefficients controlling rates over stages of materials ratcheting deformation. Stress–strain‐dependent coefficients in the modified rule are responsible to compromise overprediction of ratcheting of A–F during stage I and the premature plastic shakedown beyond stage I induced by Bower's model. Ratcheting strain rate coefficients improved the hardening rule capability to calibrate and control the rate of ratcheting in stages I and II and enabled the modified hardening rule to predict ratcheting strain over a prolonged domain of stress cycles. The modified hardening rule was employed to assess ratcheting response of 304, 42CrMo, 316L steel and copper samples under uniaxial loading conditions. The predicted ratcheting values based on the modified hardening rule and the experimental ratcheting strains were found in good agreements.     

T225NG钛合金的单轴棘轮行为研究

基于单轴常温与高温循环棘轮实验，研究了T225NG钛合金棘轮饱和、演化和破坏的一些特性，发展了一套描述高温棘轮循环的单轴饱和棘轮本构关系的方法，新方法克服了传统棘轮变形二元应力控制论带来的难题，描述方程结构简洁，建模简便、精度高，适合工程应用．研究表明：在常温应力循环下，当均值应力为半峰值应力时，T225NG钛合金会伴随棘轮饱和产生疲劳破坏，而当均值应力低于半峰值应力时，棘轮应变因局部损伤使材料较快达到疲劳破坏而不能达到饱和态。此外，常温应力循环下，单轴试样的低周疲劳寿命与幅值应力之间的关系符合幂律。