CRACK-GROWTH RETARDATION OF SINGLE OVERLOAD FOR A537 STEEL IN A 3.5-PERCENT NACL SOLUTION UNDER CATHODIC POTENTIAL AND FREE CORROSION CONDITION

Single peak overload retardation of corrosion fatigue crack growth und er constant Delta K control was studied for A537 steel in air and in a 3.5% NaCl solution at the applied cathodic potential of 1400 mV(SCE) and in free corrosion condition. The crack closure behaviour was exami ned by the speckle interferometry technique at different stages of fat igue crack propagation. Scanning electron microscopy was used to obser ve the morphologies of fracture surface. The experimental results show ed that, in all conditions, overload retardation occurred and the reta rded propagation distances of crack were longer than the plastic zone size caused by overload at the crack tip. In air or in 3.5% NaCl at th e cathodic potential, delayed retardation occurred after applying a pe ak overload, the crack growth consisted of three stages: crack extensi on due to overload; decelerating in crack growth; and resumption of cr ack growth rate. Immediate retardation occurred when specimens were te sted in a 3.5% NaCl solution in free corrosion conditions. Crack exten sion due to overload was not observed. Under both cathodic polarizatio n and free corrosion condition, the overload retardation phenomena wer e not as obvious as that in air. The retarded propagation distance and the retardation degree were both smaller than those in air. The mecha nisms for overload retardation of corrosion fatigue crack growth under the control of hydrogen embrittlement and anodic dissolution were dis cussed on the basis of analyses of crack closure, residual compressive stress and crack tip blunting. (C) 1998 Elsevier Science Ltd.