Crack Propagation Assisted with Hydrogen under Cyclic Stress in Ni–Cr–Mo Steel

Abstract

Hydrogen cracking tests under cyclic stress were carried out on hydrogen pre-charged specimens, and the effect of stress ratio on the relation between frequency f and the decreasing rate of crack propagation velocity 1−β was investigated. 1−β equals {(da⁄dt)S−(da⁄dt)R}⁄(da⁄dt)S, and (da⁄dt)S and (da⁄dt)R are the crack propagation velocities under static and cyclic stress, respectively. The results were analyzed by using an asymmetrical internal friction model proposed.There appear two peaks on a 1−β vs. f curve, as they do in a delayed failure test under cyclic stress in water, i.e. this phenomenon is controlled by the diffusion and concentration process of hydrogen atoms to the triaxially tensile stressed position near a crack tip. With an increase in stress ratio R=Kmin⁄Kmax, the two peaks approach each other, the height of peaks decreases, and the frequencies at which two peaks appear shift to the higher frequency range.The existence of two peaks on a 1−β vs. f curve, and the characteristic change in this curve with a change in the stress ratio can be explained by assuming the interaction between hydrogen atoms and the cyclic moving of triaxially stressed position. Furthermore, 1−β vs. f relations calculated by using the asymmetrical internal friction model were in good agreement with the experimental tendencies.