Multiaxial cyclic viscoplasticity model for high temperature fatigue of P91 steel

Abstract

This paper presents a novel multiaxial, cyclic viscoplasticity material model for high temperature low cycle fatigue of P91 power plant steel. The model incorporates mechanisms-based variable strain-rate sensitivity and the key high temperature cyclic deformation phenomena of cyclic softening and non-linear kinematic hardening. The model has been calibrated to accurately represent the cyclic high temperature constitutive behaviour of ‘as received’ P91 steel. Details on the material Jacobian, with the consistent tangent stiffness for finite element implementation, are presented. The multiaxial implementation is applied to a notched specimen under strain-controlled loading at 600°C and a thin walled pipe under representative pressurised thermomechanical fatigue loading conditions. It is shown that the model for variable strain-rate sensitivity of the present paper predicts significantly different Coffin–Manson notch fatigue life compared to the Chaboche power law model. Ratchetting is shown to be a key candidate failure mechanism for next generation thermomechanical power plant loading conditions, for thin walled pressurised pipes.

Keywords:

• Hyperbolic sine constitutive model
• P91 steel
• Thermomechanical fatigue
• Ratchetting
• Strain-rate sensitivity

This publication has emanated from research conducted with the financial support of Science Foundation Ireland under Grant No. SFI/10/IN.1/I3015. The Authors would like to acknowledge the contributions made by the collaborators of the METCAM project, including Mr S. Scully of ESB Energy International, Dr P. Tiernan, and Dr D. Li of the University of Limerick, and Professor T. H. Hyde, Dr C. J. Hyde and Dr W. Sun of the University of Nottingham.