Prediction of uniaxial creep rupture and crack growth using a micro/macro constraint criteria model for various steels

Abstract

Stress/strain measured data which have inherent scatter are used to develop predictive models in creep failure assessment under uniaxial and multiaxial conditions. Models developed for this purpose also need to address the multiaxial stress state effect when creep damage dominates. A unifying approach to predict creep damage and rupture under uniaxial/multiaxial and crack growth conditions is presented in this paper by deriving a multiscale based constraint criterion. The model identifies global constraint due to geometry and a microstructural time-dependent constraint arising from local creep diffusional processes occurring at the sub grain level. The concept assumes that at very short times the initial upper shelf material tensile strength and global plasticity controls the creep damage failure and at long term diffusion/dislocation control creep response develop local constraint depending on the time-dependent anisotropic microstructural conditions. Using creep data for two different steels, P91 and 316H at different temperatures it is established that the material yield strength in the short term and a measure of creep failure strain at the creep secondary/tertiary transition region described at the limits by the Monkman-Grant failure strain, are the important variables in both the uniaxial and multiaxial failure processes. It is also shown that the model is consistent with the established NSW crack growth model which bounds the plane stress/strain cracking rates in fracture mechanics geometries and cracked components.