Growth modes of cracks in creeping type 304 stainless steel

Abstract

The modes of initial growth of cracks in creeping Type 304 stainless steel were studied in the 600–775°C range in plane strain and three different test geometries consisting of center cracked panels, double edge notched bars, and compact tension specimens in both annealed material and in some material with 5% initial cold work. In all cases it was found that in samples with pre-fatigue sharpened cracks the initial growth was branched and on planes at 50° with the median plane of the crack. In other cases of samples with blunt cracks having machined radii of curvature, initial growth was found to remain in the median plane of the crack. In further growth, however, it was noted that while the direction of the branched cracks became later parallel to the median crack plane, the initially planar cracks showed later evidence of branching. This meandering behavior of cracks is a result of the different nature of crack tip strain concentration for initially sharp and initially blunt cracks. While data were limited, a dependence of the average crack speed on a power function of the C∗ parameter was found for all cases in which crack growth was intergranular.