In the previous paper, the results of constant load delayed fracture tests by using the compact tension specimen of SNCM 8 (AISI 4340) steel were presented and the characteristics of crack growth rate and delayed fracture toughness were discussed.
In this study, constant deflection delayed fracture tests were performed in tap water by using modified wedge-opening (WOL) specimens of SNCM 8 to examine the effects of specimen thickness and strength level on delayed crack growth rate and threshold stress intensity KISCC. Hydrogen was cathodically charged into the specimen before and during the testing. The results obtained are as follows:
(1) The shape of crack front was almost a straight line regardless of the specimen thickness, in contrast with the constant load test results. Thus, there is no effect of the specimen thickness on da/dt-KI curves and the threshold stress intensity factor KISCC.
(2) The material tempered at lower tempering temperatures had a larger value of da/dt than that tempered at higher tempering temperatures. In this case, the only exception was the material tempered at 300°C. This could be due to tempering embrittlement of the material.
(3) da/dt-KI curves obtained by two types of loading were in excellent agreement over their applicable KI limits.
