A Study of Transient Crack Growth in AISI 4340 Steel in NaCl Solution

Subcritical crack growth in high strength AISI 4340 steel in 3.5% NaCl aqueous solution at 25 C in response to sudden changes in applied stress intensity was studied. The material-environment system was potentiostatically controlled at cathodic (−1.0 VSCE), open circuit (−0.6 VSCE), and anodic (−0.55 VSCE) potentials in order to ascertain the effect of potential on the crack growth transient behavior. Two loading schemes were used. In loading scheme I, the initial crack was stationary, whereas with loading scheme II, the sudden change in crack tip stress intensity, KI, was applied to a crack growing at a steady-state rate. The results obtained from loading scheme I show that the steady-state crack growth rate is achieved only after an incubation period of no crack extension and a transient period of time-dependent crack growth rate. The time taken to achieve the steady-state crack growth rate was found to decrease with increasing applied stress intensity. The results obtained from loading scheme II also demonstrated the existence of incubation and transient periods in most cases. For the case of decreasing applied stress intensity, the time taken to achieve the steady-state crack growth rate was found to increase exponentially with the change in applied stress intensity. However, the steady-state crack growth rate was independent of loading history. The general effect of applied potential is that as the applied potential is made more negative, the time taken to achieve the steady-state crack growth rate is decreased, and the steady-state crack growth rate at a given stress intensity level is increased. The results are explained in terms of the interaction between hydrogen and the stress distribution ahead of the crack tip.