Evaluation of Constraint Effect on Creep Crack Growth by Advanced Creep Modeling and Damage Mechanics

Effects of constraint induced by crack depth and sample geometry on creep crack behavior of high chromium steels was investigated by numerical simulation. An advanced mechanism-based creep model formulation, which accounts for primary and secondary creep stage was used. Here, the transient creep rate is modeled considering the evolution of the internal stress with the activation energy while the steady state creep rate is modelled considering both diffusional and dislocation creep mechanisms. This formulation allows one to predict accurately creep strain accumulation over a wide range of stress and temperature. Model parameters were identified on constant load creep tests and their transferability to the multiaxial state of stress was verified comparing predicted creep life with data obtained on notched bar samples. Continuum damage mechanics was used to predict the occurrence of tertiary creep stage and crack advance. To this purpose, a non-linear damage law, as proposed in Bonora and Esposito [1] was used. The effect of the geometry constrain on creep crack growth was investigated in different sample geometries (C(T), SEN(T), SEN(B), DEN(T) and CCP(T)) for a given crack depth values, and the same biaxiality ratio for SEN(T), SEN(B) and DEN(T). Numerical simulation results were validated by comparison with available experimental data for P91 steels.