A service-aged P91 steel was used to perform an experimental programme of cyclic mechanical testing in the temperature range of 400°C to 600°C, under isothermal conditions, using both saw-tooth and dwell (inclusion of a constant strain dwell period at the maximum (tensile) strain within the cycle) waveforms. The results of this testing were used to identify the material constants for a modified Chaboche, unified visco-plasticity model, which can deal with rate-dependant cyclic effects, such as combined isotropic and kinematic hardening, and time-dependent effects, such as creep, associated with visco-plasticity. The model has been modified in order that the two-stage (non-linear primary and linear secondary) softening which occurs within the cyclic response of the service-aged P91 material is accounted for and accurately predicted. The characterisation of the cyclic visco-plasticity behaviour of the service-aged P91 material at 500°C is presented and compared to experimental stress-strain loops, cyclic softening and creep relaxation, obtained from the cyclic isothermal tests.

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