2002 Volume 42 Issue 12 Pages 1412-1418
A computer program of automated image processing was developed for fractal analysis of creep fracture surface profiles in this study. Change in the creep fracture surface patterns during crack growth was examined by the computer-aided image processing and analysis. Fractal analysis was then made using software on the processed images. Creep crack growth experiments were carried out on the surface notched specimens of the austenitic 21Cr-4Ni-9Mn steel at 973 K. Creep crack growth rate was lower in the specimens with serrated grain boundaries than in those with straight grain boundaries. The fractal dimension of the grain-boundary fracture surface profiles was larger in the former specimens than in the latter ones. The fractal dimension was larger in the specimens tested under the lower stress, and decreased with increasing distance from notch root. Effects of grain-boundary microstructures and stress on the fracture patterns were correlated to the microstructure and stress dependence of the density of grain-boundary microcracks linked to the fracture surface. Quantitative evaluation of fracture surface patterns may give an important information about the fracture origin or crack growth direction.
