Crack nucleation and propagation of corrosion fatigue in high-strength steel

✍ Scribed by Y Hirose; T Mura

• Publisher: Elsevier Science
• Year: 1985
• Tongue: English
• Weight: 851 KB
• Volume: 22
• Category: Article
• ISSN: 0013-7944
• DOI: 10.1016/0013-7944(85)90114-6

✦ Synopsis

Crack nucleation and growth behavior of corrosion fatigue are investigated experimentally and theoretically for 200°C tempered AISI 4340 steel in 35% NaCl solution. The crack nucleation at notch roots is determined by the electrical potential method. The cycle for crack nucleation, N,, increases with the notch radius p and decreases with the apparent stress intensity factor AK,. A linear relation between N, and parameter {?AK,l(np)"' -(?AK,/(np)"'),,,} is observed m a semilog diagram, where 2(2AK,/(npl""),t, is almost equal to the yield shear strength. The influence of stress ratio. R, and cycle frequency. f. on the crack growth rate, 15, is clearly observed. The 18 increases with decreasing f, and the threshold stress intensity factor, AIYFsfC, 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 notchtip, 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. A detached crack initiates and grows to the tip of the already existing crack. When the detached crack extends to the tip of the main crack. the process of crack initiation repeats at the tip of a new existing crack. The theory agrees with all the experimental observations.