Creep-fatigue interaction has been identified as a possible degradation mechanism leading to failure of power plant components such as thick sections header pipe etc. During startup/shutdown processes, severe thermal gradient sets in across the thickness between the inside and outside of the component. Fluctuation of demand also results in alternation of such operation results in thermal gradient. Both the above mentioned situation leads to hold time fatigue kind of situation. P92 (9Cr-1Mo-2W) martensitic steel is widely used for the manufacturing of header pipes and is subjected to creep-fatigue loading condition. The aim behind the present study is to verify an existing unified viscoplasticity constitutive model, proposed by Choboche. This model includes combined isotropic hardening and kinematic hardening with a viscoplastic flow rule for time-dependent effects. For this study isothermal, uniaxial, fully reversed, strain controlled low cycle fatigue and stress relaxation fatigue tests at various hold time duration at maximum/minimum strain amplitude were conducted on P92 material at 600¢ªC. The P92 material in the present study has been recognized as a cyclic softening material Here the initial value of material constants associated with Chaboche model has been determined from the first cycle stress-strain data, the maximum stress evolution during tests and the stress relaxation data. Then, the initial constants need to be optimized using a least-squares optimization algorithm in order to improve the general fit of the model to experimental data.
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