The fracture toughness of a low alloy pressure vessel steel in the post yield regime

Abstract

Post-yield effects in a low-alloy pressure-vessel steel have been studied in 3 point bend specimens at −30 °C by varying specimen geometry and crack size, a. Specimen widths, W, varied from 60 to 10 mm, specimen thicknesses, B, from 30 to 5 mm, and a/ W from 0.07 to 0.54. As a/ W decreased, either by decreasing a at fixed W or increasing W at fixed a, post-yield effects became more pronounced and substantial increases in fracture toughness were calculated. In the smallest specimens the effect was sufficient to cause failure at stresses close to the plastic collapse stress. For these specimens, the elastic stress intensity calculated at maximum load, K m, did not vary significantly with a/ W and was close to the LEFM K lc. However, for the larger specimens, K m values up to twice the LEFM K lc value were calculated as a/ W decreased. For specimens showing an instability on the load/displacement curve, there was no evidence of a change of stress state throughout the complete range of specimen sizes, and so it is concluded that extreme care should be taken when using K lc as a material constant for use in the post-yield regime.