Modeling ductile fracture using critical strain locus and softening law for a typical pressure vessel steel

Abstract

This work describes the framework to model ductile damage based on phenomenological stress-modified critical strain criterion (SMCS) coupled with a softening law to predict the onset of ductile initiation and ductile propagation in typical fracture specimens, extracted from a flat plate made of ASTM A285 Gr. C steel. A very detailed and well-illustrated methodology has been developed for the identification of material parameters. Laboratory testing of cylindrical tensile bars and SE(B) specimens, at room temperature, provides necessary and sufficient information to calibrate the numerical parameters in the proposed model. For either geometry, the applied loading is measured by a continuous record of the load (P) and displacement (Δ). After the model parameters have been set, verification studies are carried out for SE(B) specimens having shallow cracks with and without side-grooves. Consequently, parameter transferability, outside small-scale yielding condition between specimens having different crack tip conditions, can be addressed and the constraint influence on the driving force better understood. An additional check is performed by comparing the final crack front profile measured on the fracture surface of the SE(B) specimens with the numerical calculated in the finite element analyses. The phenomenological model adopted herein can reproduce and predict reasonably well the experimental data obtained for specimens with different levels of stress triaxiality (constraint). Overall, it is shown that the SMCS criterion combined with a softening law can be used to study and to predict the influence of stress state on ductile failure initiation and ductile crack growth by identifying nine model parameters through testing notched round bar geometries and SE(B) specimens. The proposed methodology shows great potential as an engineering tool for assessing the integrity of complex structures such as welded pipelines and pressure vessels.