Response of Crack Propagation to Load Variation in Delayed Failure of Ni-Cr-Mo Steels

In order to clarify the crack propagation behavior in delayed failure under repeating load, the stress intensity factor K was trapezoidally applied to the specimen, and the relation was investigated between K increasing or decreasing time, t1 or t3, and the incubation time, tu or td, which appeared while K was held at a maximum or minimum value.The incubation time td is always longer than tu, and td rapidly and tu gradually decreases with increase of t1 and t3. Both tu and td decrease with increase of testing temperature T. The Arrhenius relation holds between 1⁄T and 1⁄tu or 1⁄td, giving the activation energy of 20 or 40 kJ/mol, which is comparable with that for the diffusion of hydrogen atoms in steel.The appearance of incubation time after the abrupt change in K can be explained by assuming that hydrogen diffusion occurs later than moving of triaxially tensile stressed position at a crack tip.