Abstract:

In fatigue, the boundary between the crack initiation and propagation phases needs to be resolved. The Manson-Coffin equation and the Paris equation are the two fundamental equations, which are used in crack-free and cracked structures, respectively. By transforming these two equations, a relatively reasonable quantitative boundary has been proposed.

When the same idea was used in a cracked structure, the crack growth rate has been calculated by the Manson-Coffin equation containing two parameters——the magnitude of stress intensity factor and the stress ratio. Through the analysis of stress ratio and residual stress, a plausible meaning of the threshold region is explained. Pre-overload experiments show that overload is the cause of the threshold region.