Detection of the point of crack initiation using multi-stage random sampling (MRS) and spatial point pattern (SPP)

Abstract

Porosity is a major defect in cast aluminum alloys affecting in particular, the fatigue strength. The pores serve as points of stress concentration and points of crack initiation for eventual failure. In this work, fractal analysis was used to numerically characterize the pores in uni-directionally solidified Al 4.5 wt % Cu alloy micrographs, transverse section at a distance of 14mm from the metal/chill mold interface. The Spatial Point Pattern (SPP) and the Multi-stage random sampling (MRS) methods were used to determine the distribution of the pores and the point of crack initiation leading to eventual failure. The MRS method reveals that all the pores considered are of irregular shapes, i.e shrinkage pores, with sphericity β < 0.3. The "worst" of the shapes is the pore in the upper left region with β= 5.3078e-010 and D = 1.8949. The SPP method confirms the result of the MRS method because crack initiation will commence in a region with clustered pores.