Corrosion fatigue crack propagation of 304 stainless steel in boiling MgCl2 solution was investigated under trapezoidal cyclic stresses. The crack propagation rate at high stresses obeyed the law of linear summation of fatigue and stress corrosion cracking, but deviated from the law at low stresses because of a significant effect of the environment. The value of the exponent m in the equation dl/dn=AΔKm was considerably smaller in corrosion fatigue than in fatigue in air.
Characteristic fracture patterns appeared in the respective regions in the fracture map, and characteristic striations were observed on the fracture surface in the high stress region. It is suggested that these striations are formed in every stress cycle by the operation of two crack propagation mechanisms, ductile fatigue crack propagation due to slip-off at a crack tip during the stress-increasing period and brittle stress corrosion cracking during the subseqent period of stress-holding. The acceleration of the crack propagation in corrosion fatigue seemed to occur in both of these periods.
A transition phenomenon of the crack propagation was observed when the load was changed from cyclic stress to static stress.