The effects of prior loading conditions on stress corrosion cracking (SCC) of high strength low alloy steel AISI 4340 in 3.5 percent NaCl solution environment were investigated with blunt-notched specimens of compact tension type. Without prior loading of notched specimens, the life to crack initiation at the notch root was found to be determined by the quotient of the stress intensity factor divided by the square root of notch tip radius in the range where the radius is larger than a certain value, i.e. the effective radius for a crack. In this range, it can be concluded that the initiation life is characterized by the maximum stress at the notch root. The effect of prior loading on the delay of crack initiation was examined with the specimens having the radius larger than the effective radius. The amount of delay was found to become larger as the pre-loading level increased. The residual stress formed near the notch root due to pre-loading was calculated by elastro-plasticity. A unique relation was established between the initiation life and the actual stress near the notch root which was obtained by subtracting the compressive residual stress from the applied maximum stress near the notch tip. When the K-value was raised in the growth stage, the growth rate immediately became the rate obtained in constant K-tests. On the other hand, when the K-value was partially reduced from K2 to K1, the incubation time was observed before the crack started growing under K1-value. The threshold stress was also found to increase due to prior high loading. The incubation time tin was expressed as a second power function of the difference of ΔK=K2→K1. The relation between tin and ΔK was unaffected by the stress variation sequences tested, i.e. K2→K1, K2→0→K1 and K1→K2→K1. The experimental relation was discussed based on the residual stress distribution due to load variation and the mechanochemical mechanisms of stress corrosion crack growth.
