Cyclic Constitutive Relation and Estimation of Biaxial Low Cycle Fatigue Life at Elevated Temperature

测定了一种奥氏体不锈钢SUS 304和1Cr-1Mo-1/4V时效钢和非时效钢在823K推拉和反向扭转联合载荷作用下的循环双轴本构关系。这三种试验方法是相伴法、增量/减量法(I/D法)和升压法。对1Cr-1Mo-1/4V钢只进行了配对试验。采用I/D法对SUS304钢进行了比例和非比例试验。得到的主要结论如下：1.研究成果。除1Cr-1Mo-1/JV时效钢外，其他三种试验方法的双轴循环本构关系均与Mise等效应力和等效应力有很好的相关性。配对测验中数据的离散性较大。2.相伴试验的循环本构关系不同于I/D试验的循环本构关系。相伴试验的循环硬化指数大于I/D试验的循环硬化指数。与比例试验相比，非比例试验的循环应力幅值较大。通过透射电子显微镜观察和X射线衍射分析，讨论了非比例加载引起应力幅值增大的原因。提出了一种估算非临界性试验应力幅值的简单公式。基于该方程的估算结果与实验结果吻合较好。为了了解本构关系的差异对低周疲劳设计寿命的影响，利用相伴试验和I/D试验得到的循环本构关系，对平板缺口试件进行了有限元分析。两种本构关系的计算结果没有差异，因此试验方法引起的本构关系的差异对设计疲劳寿命的影响不大。

Cyclic biaxial constitute relation was determined for an austenitic stainless steel SUS 304 and 1Cr-1Mo-1/4V aged and non-aged steels in the three test methods under combined push-pull and reversed torsion load at 823 K. The three test methods were the companion method, incremental/decremental method (I/D method) and step-up method. For 1Cr-1Mo-1/4V steel only the companion test was carried out. Proportional and non-proportional tests were carried out for SUS 304 steel in the I/D method. The major conclusions obtained are as follows.1. The biaxial cyclic constitutive relation in the three test methods is well correlated by the Mises equivalent stress and stress except 1Cr-1Mo-1/jV aged steel. The scatter of the data in the companion test is comparatively large. 2. The cyclic constitutive relation in the companion test differs from that in the I/D test. The cyclic hardening exponent in the companion test is larger than that in the I/D test.3. Larger cyclic stress amplitude is observed in the non-proportional test in comparison with the proportional test. The cause of the increase in the stress amplitude due to the non-proportional loading is discussed by TEM observations and X-ray diffraction. A simple equation to estimate the stress amplitude in the non-proportinal test is proposed. The estimated results based on the equation agree well with the experimental results.4. FEM analysis is made for plate notched specimens using the cyclic constitutive relations obtained in the companion and the I/D tests in order to know how the difference in the constitutive relation affects the low cycle fatigue design life. The calculated results using both the constitutive relations do not differ each other, so that the difference in the constitutive relation due to the test method does not have an large effect on the design fatigue life.