Abstract
Stress corrosion crack growth rates are measured at sveral stress intensity levels for low-tempered 4340 steel in 0.1N H 2SO 4 solution. The characteristics of the growth rates are divided into three regions of stress intensity factors: Region I near K 1 SCC ; Region III near unstable fracture toughness, K 1 SC ; and Region II, which lies between the two. K 1 SCC is the value of K at which no crack growth can be detected after 240 hr. In order to explain these experimental results, the crack initiation analysis reported in a previous paper is extended to the growth rates. A detached crack initiates and grows at the tip of an already existing crack. When the detached crack reaches the tip of the main crack, the process repeats as a new existing crack. A relationship between crack growth rate, v, and stress intensity factor, K, is obtained as a function of b/ a and a = b + d, where b is the distance from the tip of the main crack to the detached crack, and d is the ydrogen atom saturated domain. The experimental data are in good agreement with the theoretical values in Region II when a = 0.02 mm, b/ a = 0.8, c 1/ c 0 = 2.8 for 200°C tempered specimens and a = 0.015 mm, b/ a = 0.7, c 1/ c 0 = 3.0, ρ b = 0.055 mm for 400°C tempered specimens, where ρ b is a fictitious notch radius. The plateau part in Region II for 400°C tempered specimens is also successfully explained by the present theory. For Region III, the value of b/ a will be almost equal to 1 because v → ∞ for b/ a → 1. On the other hand, for Region I, b/ a will be zero, since the value of v becomes negligibly small and no crack growth is observable.
Published Version
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