Creep-ratchetting of Heat Resisting Steels at Elevated Temperature in Multiaxial Stress State

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Creep-ratchetting behavior at elevated tempareture on two types of heat resisting steels: 2 1/4Cr-1Mo steel at 600゜c and SUS304 stainless steel at 650゜c, has been experimentally examined in multiaxial stress state. In the mechanical retchetting tests under combined steady axial stress and cyclic shear straining, remarkable axial strain accumulation is found on both steels. The axial strain accumulation becomes larger with decreasing cyclic shear strain rate. In the creep test along circular stress path on 2 1/4Cr-1Mo steel, larger creep strain is observed than in uniaxial static creep tests. in the stress-controlled creep-ratchetting tests on SUS304, both in uniaxial and multiaxial stress states, the behavior is strongly affected by the stress ratio.To evalate the accuracy of the existing inelastic constitutive equations, the numerical simulations to the above-mentioned creep-ratchetting tests have been coducted by the equations. The superposition model and two of the unified models: the Chaboche model and the Miller model, were used for the simulation. The unified model gives better results for some of the tests, e. g., the creep along circular stress path, since they account the cree-plasticity interaction. However, the determination of material constants in the unified models are more difficult than that in the superposition model. All of these models can hardly express accurately the creep-ratchetting behavior guantitatively, and the further improvement is required.Creep-ratechetting behavior at room temperature has also been investigated on SUS304 and a carbon steel both in uniaxial and multiaxial stress state. The phenomenological constitutive models for the creep-ratchetting have been presented.The FEM program has been developed based on the unified constitutive equation presented by Chaboche. The program was checked through the numerical tests of s uniaxial tension and a tension of notched plate. The adaptive time-step has been discussed.