Use of local approaches to calculate changes in cleavage fracture toughness due to pre-straining and constraint effects

Presented are experimental data and analyses of fracture toughness tests across three different crack tip constraint conditions and two different material conditions – as-received and pre-strained to 5% plastic strain. Analyses are performed using a local approach, based on a plasticity-modified Weibull stress interpreted as a crack driving force for cleavage fracture. Local approach parameters are calibrated using experimental data from the highest and lowest constraint geometries of as-received material. Results suggest one point of practical importance for testing reduction: use of large constraint difference for calibration yields good predictions for cleavage fracture toughness at intermediate constraints; use of small constraint difference for calibration cannot guarantee good predictions for constraints outside the calibration interval. Further, the calibrated local approach is used to predict the fracture toughness of pre-strained material. Pre-straining is found to reduce both the elastic modulus and the proportionality stress of the material, which at the micro-structural scale could be attributed to increased dislocation mobility after unpinning, and in addition to possible damage around second phase particles. Resulting characteristic toughness values are 2-2.5 times smaller than those of as-received material for corresponding constraint conditions, with pronounced diminishing constraint benefit. The degradation of deformation properties changes the initial conditions for Weibull stress calculation. A simple scaling method is proposed, based on flow stress changes, to account for changes in initial conditions. Predictions based on the proposed Weibull stress scaling are shown to be in good agreement with experimental data. This suggests a second point of practical importance for testing reduction: changes in fracture toughness due to load history could be predicted using only tensile test data of aged material and tensile and fracture toughness data from original material at two significantly different constraint conditions. Further experimental evidence is required to support this suggestion.