Elastic-Plastic Constitutive Modeling and Experimental Observation of Some Steels for Phase Transformation Process under Multi-axial Stresses State

本文对相变钢在高温下多轴应力状态下的响应进行了理论和实验研究。研制了新型双轴加载装置，该装置可在高温下分别对试样施加拉伸和扭转载荷，并可快速冷却试样，观察到马氏体和贝氏体的转变。实验得到了SCM440钢的膨胀温度图，发现相变温度与外加应力状态和冷却速率有关。还发现在拉伸和扭转应力下均发生转变塑性，并且取决于应力状态。采用内状态变量理论，将各相的体积分数作为内状态变量，建立了钢在相变过程中热弹塑性行为的本构模型。导出了内变量演化方程的显式形式。在该框架内还确定了相变塑性系数。对SCM440钢的连续冷却相变（C-C-T）图、等温时间-温度相变（T-T-T）图和膨胀-温度图的计算结果进行了分析，得出了相变的应力依赖性，并与实验数据进行了比较。本文还介绍了该理论在局部加热板弯曲情况下的应用。采用有限元方法对应力、位移和温度分布进行了数值计算，并与实验结果进行了比较。

In this project the response of phase-transforming steels under multi-axial stress state at elevated temperature is studied both theoretically and experimentally.The new biaxial loading equipment is developed, where tensile and torsional loads can be applied to the specimen independently at elevated temperature and the specimen can be cooled rapidly so that martensite and bainite transformations are observed. The dilatation-temperature diagram is obtained experimentally for SCM440 steel and it is found that the transformation temperature depends on both applied stress state and cooling rate. It is also fount that transformation plasticity is observed to occur for both tensile and torsional stresses and depends on stress state.The constitutive model for thermo-elastic-plastic behavior of some steels during phase transformation process is also proposed in terms of internal state variable theory, where the volume fraction of each phase is considered as an internal state variable. The explicit form of the evolutional equations for internal variables is derived. The coefficient of transformation plasticity is also identified within the proposed framework. Calculated results of the continuous cooling transformation (C-C-T) diagram and the isothermal time-temperature-transformation (T-T-T) diagram, together with the dilatation-temperature diagram show the stress dependency of phase transformation and are compared with experimental data for SCM440 steel.Applications of the theory to the cases of bending of locally heated plate are also presented. Stress, displacement and temperature distributions are numerically calculated by a finite element technique and compared with corresponding experimental results.