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

本课题从理论和实验两方面研究了相变钢在高温多轴应力状态下的响应，研制了一种新型的双轴加载装置，该装置能在高温下对试样分别施加拉伸和扭转载荷，并能使试样快速冷却，观察到马氏体和贝氏体相变。通过实验获得了SCM440钢的相变-温度曲线，发现相变温度不仅与应力状态有关，而且与冷却速度有关。在拉伸和扭转两种应力状态下都存在相变塑性，且相变塑性取决于应力状态，本文还根据内状态变量理论，提出了某些钢在相变过程中的热弹塑性本构模型，将各相的体积分数作为内状态变量。导出了内变量演化方程的显式形式。相变塑性系数也确定在建议的框架内。连续冷却转变（C-C-T）图、等温时间-温度-转变（T-T-T）图和等温温度-转变图的计算结果表明了相变的应力依赖性，并与SCM440钢的实验数据进行了比较，还介绍了该理论在局部加热板弯曲中的应用。应力，位移和温度分布的有限元技术进行了数值计算，并与相应的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.