At present, the THERMOELASTIC-PLASTIC model in the thermal stress model ofis widely used. The model does not directly consider the viscous effect. It regards the material as elastic before yield and plastic after yield. The elastic modulus and yield stress are functions of temperature, and when the temperature of sand casting approaches the melting point, the elastic modulus and yield stress become zero. Therefore, some basic assumptions made in the numerical simulation of stress field are as follows:
1) The material is composed of continuous medium;
2) Geometric equations are regarded as linear equations;
3) Treat all materials as isotropic;
4) The nonlinearity of sand casting material is treated as a bilinear model.
The constitutive relationship of materials is a mathematical formula to describe the mechanical behavior of media based on the mechanical properties of materials observed in experiments and experience and under specific theoretical assumptions. In the process of numerical simulation, the relationship between material stress and strain is a very important parameter in material mechanics parameters.