Effect of loading history on stress corrosion cracking in high strength steel

Abstract

The effect of preloading on crack nucleation time was examined with compact tension specimens having various notch radius in 0.1N-H 2SO 4 aqueous solution for 200°C tempered AISI 4340 steel. Crack nucleation time t n increases by preloading for a given apparent stress intensity factor K p 2 . The curve K ϱ 2 vs. t n deviates upward from the curve for the non preloading case. A linear relationship between the crack nucleation time and parameter (2K ϱ 2 /(πϱ) 1 2 −(2K ϱ 2 /(πϱ) 1 2 ) th) is seen in semi-log diagram, where (2K ϱ 2 /(πϱ) 1 2 ) th is taken as the value at t n = α due to preloading. The apparent threshold stress intensity factor increases with K ϱ 2 which is the apparent stress intensity factor of preloading. A detached crack is nucleated at some distance from the notch root and extends in a form of circle. This distance increases with increasing K ϱ 2 . The effect of load reduction during crack growth was examined. When the K-value was reduced from K 1 to K 2, an incubation time was observed before the crack started growing under the K 2-value. The incubation time t m tends to increase with increasing ΔK = K 1- K 2. The threshold stress intensity factor was also found to increase for high load reduction. In order to explain these experimental results, a new dislocation model is proposed on the basis of stress induced diffusion of hydrogen in high stress region ahead of the notch root or a crack. This model suggests that the change in the crack nucleation time and the increase of the incubation time due to preloading or load reduction are caused by reducing the hydrostatic pressure and by spreading the hydrogen saturated region which requires more time for the hydrogen accumulation due to preloading or load reduction. The theory predicts the experimentally observed relations between (2K ϱ 2 /(πϱ) 1 2 − (2K ϱ 2 /(πϱ) 1 2 ) th) and t n and between log t in and ΔK.