Algorithm to 3-dimensional Analysis of Cleavage Facet and Its Application for Brittle Fracture Surface of Steels

Computer image processing technology was applied to cleavage fracture surface analysis and an algorithm for the estimation of 3-dimensional cleavage facet boundaries and 3-dimensional facet areas were developed. An angle αi, j between relative normal vectors of a mesh plane Mi, j and the adjacent mesh plane Mi+1, j along X axis was calculated and αi, j was compared to α threshold angle αth. If αi, j is larger than αth and αi+1, j is smaller than αth, the mesh plane Mi+1, j was decided to be the facet boundary. The same calculation was continued along both X- and Y-axes to obtain facet numbers contained in the analyzed area. After the estimation of facet boundaries, 3-dimensional facet areas were calculated from the 2-dimensional facet areas divided by cosβi, j, which is the angle between average normal vector of α facet and vertical vector. The algorithm developed in this study was applied to analyze the cleavage fracture surfaces of mild steel and HT80 steel and the effect of the threshold angle on the estimated facet numbers, distribution of the values of α and the 3-dimensional facet areas were discussed. The developed method is very useful for analyzing the cleavage facet size and for discussing a relationship between cleavage fracture and microstructures of steels, such as ferrite and the prior austenite grain sizes.