Crack Nucleation and Propagation of Corrosion Fatigue in High-Strength Steel

摘要

Crack nucleation and growth behavior of corrosion fatigue are investigated experimentally and theoretically for 200 C tempered AISI 4340 stell in 3.5% NaCl solution. The crack nucleation at notch roots is determined by the electrical potential method. The cycle for crack nucleation, N sub i, increases with the notch radius p and decreases with the apparent stress intensity factor Delta Kp. A linear relation between N sub i and parameter 2 Delta Kp (the square root Pi x P) - (2 Delta Kp/(the square root Pi X P) is observed in a semilog diagram, where 2(2 Delta Kp/(the square root Pi x P) almost equal to the yield shear strength. The influence of stress ratio, R, and cycle frequency f, on the crack growth rate, v, is clearly observed. The v increases with decreasing f, and the threshold stress intensity factor, Delta KFSCC, decreases with decreasing R. In order to explain these experimental results, a new dislocation dipole model is proposed. A model of dislocation dipoles is considered at a notch-tip, and diffusion of hydrogen atoms along the line of the pile-up of dislocation dipoles is solved. A certain accumulation of hydrogen atoms at the leading edge of pile-up of dislocation dipoles leads to the crack initiation.