Experimental measurements of deformed crack tips in different yield-to-tensile ratio steels

Crack opening profiles and crack tip opening displacement (CTOD) in single edge notch bend (SE(B)) specimens are investigated by the observation of unloaded crack tips and silicone rubber replication of loaded crack tips. Measurements in different yield-to-tensile ratio (YR) steels are performed, and CTODs in BS7448 and ASTM E1290 are compared with them. Although CTOD calculated in accordance with BS7448 agrees well with the measured CTOD for a high YR steel, CTOD with BS7448 overestimates the actual CTOD for a low YR steel. On the other hand, the relationship between CTOD calculated in accordance with ASTM E1290 and the measured CTOD is independent of YR, but CTOD with ASTM E1290 gives approximately 15% smaller values than the measured CTOD. The application of numerical simulation to the estimation of crack opening profiles is also investigated, and different crack openings are apparent in different YR steel models. The overestimation of CTOD with BS7448 results from the round shape crack tip blunting in the wide region behind the crack tip for a low YR steel, and this blunting is induced by the plastic strain localization depending on the strain hardening properties.