Crack Propagation and Crack Branching in Delayed Failure Under Mixed-Mode I-II Loading

Three-point and four-point bending tests were carried out in water on the high strength Ni-Cr-Mo steel quenched and tempered at 653 K in order to investigate the effect of mixed-mode I-II loading on the crack propagation and crack branching in delayed failure. The crack propagation velocity before crack branching was almost constant and independent of the ratio of mode II to mode I stress intensity factors when it was within 0.3. Both the crack-branching angle and the angle between the direction of the main crack and the bisector of the crack-branching angle increased with increases in the ratio of mode II to mode I stress intensity factors. The above results were explained by a criterion which was introduced by combining the hydrostatic tensile stress and the hydrogen concentration at a crack tip.