Abstract
In the past, we have conducted the tests on the macroscopic through-crack propagation of ductile steels in creep, and found out the nonlinear fracture mechanical behavior. The investigation of fracture mechanics of microcrack growth is important to clarify the characteristics of creep fracture as well as to indicate a guide for the evaluation of cracks or flaws in high temperature structures.
In this study, a continuous observation of intergranular microcrack growth at the surface of coarse grained smooth specimens of 316 stainless steel was made during creep tests at 650 and 800°C in vacuum. The initiation of surface cracks was detected at the early stage of steady state creep. Surface cracks increased in number, and some of them grew intermittently to the length of a few grain boundaries, while the crack opening displacements increased rather momentarily. The aspect ratio scattered from 0.6 to 1.5 for shallow cracks of one or two grain boundary length. It was found that the growth rate of every cracks at the surface had a roughly linear relation to the creep J-integral estimated on the basis of the steady creep rate and modified by crack shape correction factors, and that the relation corresponded with that of macroscopic through-crack propagation rate of the CCP specimens. A better correlation was obtained between the surface crack length and the cumulative creep J-integral estimated by the crack opening displacement.
