An investigation was carried out to examine the influence of overload plastic zone size on stress corrosion crack growth behavior for a low alloy steel. The investigation also examines the influence of initial stress intensity factor on stress corrosion crack growth behavior. Compact tension specimens with TL orientation prepared from SAE 4140 steel in the hardened and tempered condition were used. Two pertinent environments were chosen: deionized water at room temperature (23°C) and 3.0% NaCl solution at room temperature (23°C).
The specimens were initially precracked in fatigue and loaded to different levels of stress intensity factors. Crack growth behavior of these specimens was studied by keeping them submerged in deionized water and 3.0% NaCl solution to establish a base line. To examine the influence of overload plastic zone size on stress corrosion crack growth behavior, the precracked specimens were overloaded to various load levels (i.e., 5, 10, 25, and 50% more than their initial stress intensity level [Ki]) and then unloaded. The crack growth behavior of these specimens was then monitored in a similar fashion after reloading to the original initial stress intensity level. The influence of initial stress intensity level on the incubation period for the growth of the crack and on the stress corrosion crack growth rate was also examined.
The results of the present investigation demonstrate that the incubation period for the growth of stress corrosion cracking increases with an increase in overload plastic zone size. The crack growth rate at a constant initial stress intensity level was found to be lower in salt water solution than in deionized water. The incubation period for the growth of stress corrosion cracking was found to decrease with an increase in initial stress intensity level.
Once the crack has grown beyond the introduced overload plastic zone, the crack growth rate was found to resume its normal rate. However, for the similar overload plastic zone size, the reduction of the effective crack growth rate was found to be greater in deionized water than in salt solution. This reduction in effective crack growth rate in overloaded specimens can be attributed to the presence of residual compressive stresses in the crack tip region in overloaded specimens.