The crack initiation, propagation and branching characteristics of delayed failure under uniaxial and biaxial load were investigated on the Ni-Cr-Mo steel quenched and tempered at 473 or 673K.
In the three point (uniaxial) bending tests of smooth rectangular specimens, a straight crack initiated and propagated perpendicular to the longitudinal axis of the specimen without revealing crack branching. In the uniaxial tensile tests of precracked rectangular specimens, the crack propagated to unstable fracture without branching. In the biaxial bending tests of smooth disk specimens, the edge of which was supported and the center of which was compressed by a round punch, a Y-shaped crack initiated from the center of specimen, and the branching occurred at the tip of the Y-shaped crack when the stress of specimen was large. These aspects were almost the same both for the steels tempered at 473 and 673K.
For the steel tempered at 473K, the crack initiation time in delayed failure was shorter in the biaxial bending than in the uniaxial bending for the same bending stress.