Research Of Constitutive Mode On Power Plant High Temperature Materials Subjected To Ratchetting-creep Interaction

With the development of industrial technology, higher steam temperature and higher press are required in power plants, nuclear power plants and chemical industry. For example, in ultra-supercritical steam turbine, the temperature has reached 600℃and the pressure has reached 25MPa. Running at such a high parameters, the failure of metal material would inevitable occur because of the fatigue and the creep behavior. When these two kinds of behavior take place simultaneity, ratchetting deformation is produced which is a greater destructive behavior. For its non-zero mean stress, the accumulated plastic strain is continuous increased until the structure was demaged. In conventional power plants and nuclear power plants, the ratchetting analysis is required to be considered on strength design of pressure vessels. the mechanical properties of high-temperature materials is one of the reasons which limit the parameters of ultra-supercritical steam turbine to be enhanced. The research of the material constitutive model is a improtant part of the field of mechanical properties. A good constitutive model has great significance for life assessment.At present, for most unified viscoplastic constitutive model (such as the Ohno-Wang, Chaboche, etc.),when the stress is keepped in low zone, the viscous stress will fall rapidly to zero which result the plastic strain rate fall to zero, so this model have no reasonable description of the creep behavior. To this phenomenon, the Norton creep criteria was introduced to plastic strain of constitutive model, and creep was coupled with fatigue by hardening equations.Norton creep equation can only describe the first and second stage, but not the third stage. According to strain equivalence principle proposed by Lemaitre, the creep damage variable was introduce to the constitutive model, and the equivalent stress was applied to the unified viscoplastic constitutive equation. The new model can have more reasonable description of ratchetting and creep interaction.The parameter optimization method using particle swarm algorithm and finite element was Proposed. The finite element analysis module was developed using object-oriented C++ language, compared with some FEA software coded with Fortran language, the program has a good scalability. The optimization method can directly obtaine parameter by using experimental data as input data, which can reduce much workload compared with the traditional method of parameter determination.The porcess of finite element implementation has two main technical difficulties which are the implicit stress integration and the consistent stiffness matrix's derivation,which were realized in this paper with the creep criteria and demage variable coupled, and new model was successfully embedded into the the finite element software ANSYS. Pipe samples subjected internal pressure and axial cyclic loading were simulated, results show that the new constitutive has a good agrement with experiment data in description the interaction of ratcheting creep.