Nucleation mechanism of stress corrosion cracking from notches

✍ Scribed by Y. Hirose; T. Mura

• Publisher: Elsevier Science
• Year: 1984
• Tongue: English
• Weight: 850 KB
• Volume: 19
• Category: Article
• ISSN: 0013-7944
• DOI: 10.1016/0013-7944(84)90026-2

✦ Synopsis

Crack nucleation mechanism of hydrogen assisted cracking at notched cracks in aqueous solutions is investigated, using the compact type specimens with various notch radius in low-tempered 4340 steel. A detached crack initiates at some distance ahead of the notch root. The crack nucleation at the notched root is determined by the electrical potential method. When the crack initiates, the voltage difference starts to increase. The crack nucleation site is examined by SEM. The time for crack nucleation increases with the notch root radius, p, and decreases with the apparent stress intensity factor K, A linear relationship between the crack nucleation time, t,, and the parameter {2K&rp)"*-(2Kd(rp)"*),h} is seen in semi-log diagram, where (~K&TP)~~'),,, is almost equal to the yield shear strength. In order to explain these experimental results, a new model of micromechanics is proposed on the basis of stress induced diffusion of hydrogen in the high stress region ahead of the notch root. This model suggests that the detached crack initiates at the elasto-plastic boundary where the hydrogen concentration is from 2 to 5 times higher than that of the notch root surface. The theory agrees with experiments with respect to {2Kd(~rp)"~ -(2K,,/(np)"*),h} vs tn and t,, vs p. The empirical equation holds under constant t,, K, = Kll(dPefl)'" where Ke is the stress intensity factor with p = 0 under the present environment, pef is the effective notch radius and m is constant. The value of m is 0.25 for the crack nucleation time (tn)rh corresponding to the threshold stress intensity factor (K,,)rh, 0.5 for 1, < &,),I, and 0 for p 5 pef. The above equation agrees with the theoretical equation proposed by Tanaka and Mura for any t,, and pee.