The effect of frequency and temperature on the environmentally assisted fatigue crack gowth below Ksub(ISCC) in steels

The below-Ksub(1SCC) corrosion fatigue crack propagation behaviour of two steels, En56C a martensitic stainless steel, and A533B-1 a low alloy pressure vessel steel, has been studied over a wide range of testing frequency (0.001 - 10 Hz) and temperature (20-900C) in environments, which included vacuum, laboratory air, distilled water and neutral 3% NaCl. In general aqueous solutions were found to cause acceleration of growth rates when compared with air data, with an observed acceleration factor of up to X8. For a given ΔK and using triangular wave-forms, a critical frequency fsub(crit) was found at which maximum environmental attack occurred. Above fsub(crit), the fatigue propagation rate was approximately proportional to fsup(-n) where n = 0.5 for En56C and 0.1 for A533B-1. Below fsub(crit) the fatigue crack propagation rate decreased as the testing frequency was lowered. Environmental enhancement in A533B-1 was primarily controlled by rise time, and not the total cycle time, provided hold periods at Ksub(MAX) were short. The critical frequency or rise time has been shown for En56C at low R ratio to increase with increasing solution temperature. It is suggested that the crack retardations observed below fsub(crit) are due to passivation, crack tip blunting and/or corrosion products leading to a reduction in the effective crack tip alternating stress intensity. The activation enthalpies determined above fsub(crit) are discussed in terms of slip dissolution/hydrogen embrittlement models of stress corrosion. (author)