A Weibull Stress Approach Incorporating the Coupling Effect of Constraint and Plastic Strain in Cleavage Fracture Toughness Predictions

This work describes a micromechanics methodology based upon a local failure criterion incorporating the strong effects of plastic strain on cleavage fracture coupled with statistics of microcracks. A central objective is to gain some understanding on the role of plastic strain on cleavage fracture by means of a probabilistic fracture parameter and how it contributes to the cleavage failure probability. A parameter analysis is conducted to assess the general effects of plastic strain on fracture toughness correlations for conventional SE(B) specimens with varying crack size over specimen width ratios. Another objetive is to evaluate the effectiveness of the modified Weibull stress (σ̃_w) model to correct effects of constraint loss in PCVN specimens which serve to determine the indexing temperature, T₀, based on the Master Curve methodology. Fracture toughness testing conducted on an A285 Grade C pressure vessel steel provides the cleavage fracture resistance (J_c) data needed to estimate T₀. Very detailed non-linear finite element analyses for 3-D models of plane-sided SE(B) and PCVN specimens provide the evolution of near-tip stress field with increased macroscopic load (in terms of the J-integral) to define the relationship between σ̃_w and J. For the tested material, the Weibull stress methodology yields estimates for the reference temperature, T₀, from small fracture specimens which are in good agreement with the corresponding estimates derived from testing of much larger crack configurations.