Applicability of local approaches to assessment of cleavage fracture in complex constraint and load history cases

The paper presents results and analysis from an on-going investigation to better understand and quantify the effects of constraint on cleavage fracture under various initial stress and plastic strain conditions. Specifically, new fracture toughness data obtained with three-point bend specimens with crack-to-width ratios 0.4, 0.2 and 0.05 is shown for as-received material and material with 5% initial uniaxial plastic strain. Firstly, analysis with the J-Q approach is used to argue that while pre-strain initial conditions change the fracture toughness, the shape of the failure locus in the J-Q space might remain unaffected. The implication is significant reduction of tests required for assessments taking constraint into account. The potential to use current local approach methods to allow a fracture prediction, which is independent of initial conditions, is then investigated by application of modified Beremin models. It is demonstrated that the local approaches are capable of predicting changes in the apparent toughness across the three constraint levels for the as-received material. It is further shown that accommodation of load history effects, such as the initial plastic strain, requires an advancement of the classical approach, a subject of on-going work.