MORPHOLOGY OF SUB-CRITICAL GROWTH ON STRESS CORROSION CRACKING AND PLASTIC ZONE AT CRACK TIP

Using polished cantilever specimens made of two high strength steels 40CrNiMoA and AISI4330M, the nucleation and propagation on stress corrosion cracking in distilled water have been traced by metallographie shadowing technique. The morphology of sub-critical crack growth has also been observed. It was indicated that the shadow zone, i.e. a plastic deformation area in the front of crack tip, reveals. its different morphologies in various steels. In 40CrNiMoA, the shadow zone grows and joins one another with the crack propagation. In 4330M, it does not join together, but develop along two parallel paths. The catlike shadow zone immediately appeared at the crack tip after loading can be considered as the plastic zone at the crack tip. However, the shadow zone hereafter observed during whole process of the crack propagation seems not to present the plastic zone at the crack, but to represent the deformation induced by. the formation of shear lip and can be related closely to the pop-in. This should be distinguished from the plastic zone at the crack tip in the linear elastic fracture mechanics.