CRACK CLOSURE AND STRESS-CORROSION FRACTURE THRESHOLDS IN MILD-STEEL

The resistance of normalized plain carbon mild steel to intergranular stress corrosion cracking (IGSCC) has been characterized from measurements of crack growth rates obtained under conditions of constant stress intensity. The magnitude of the stress intensity threshold (K//l//S//C//C) was increased and the plateau cracking velocity was reduced by subjecting the normalized steel to additional subcritical annealing treatments. The findings were investigated further by fractography and crack geometry studies. Stress corrosion damage in normalized steel was shown experimentally to occur only if the applied stress intensity was above the level at which stress corrosion cracks were closed off by corrosion product deposits on the crack walls. The results were in general agreement with a theoretical description which has parallels with fatigue crack closure models. Additional study results are dicussed.