Coupled porous plasticity - Continuum damage mechanics approaches for modelling temperature driven ductile-to-brittle transition fracture in ferritic steels

Following; (a) the observation that micro-void and micro-crack driven failure mechanisms co-exist in metallic alloys and (b) the two damage state variable definition given in Chaboche et al. (2006), two coupled porous plasticity and continuum damage mechanics approaches to assess temperature driven ductile-to-brittle transition fracture in ferritic steels have been developed. Based on hypo-elastic formulation of Gurson-Tvergaard-Needleman (GTN) thermoplasticity to account for ductile failure following void growth, continuum damage mechanics formalism have been coupled in order to account for micro-crack driven brittle fracture. Keeping GTN thermoplasticity as a basis for ductile fracture, Leckie-Hayhurst creep rupture criterion has been modified and proposed to account for brittle damage, thus cleavage, in the first model. The second approach, which is proposed following the motivation that plasticity exists in and below the lower transition region, replaces Leckie-Hayhurst model with plasticity driven damage evolution law of Lemaitre et al. (2000). Unlike commonly used cleavage models such as Ritchie et al. (1973) and Beremin (1983), both of the proposed models have been aimed to take into account blended effects of micro-voids and micro-cracks in order to capture energy dissipation and softening accompanying and prior to brittle fracture. Numerical implementation has been done for ABAQUS/Explicit and uses staggered solution based on plastic flow-damage correction structure, while its validation has been performed modeling Small Punch Fracture Experiments for P91 ferritic steel, published by Turba et al. (2011).