Calculation method for maximum low-cycle fatigue loads using FRASTA reconstruction data

A new method is proposed to calculate the load on a specimen during a fatigue failure using a post-mortem analysis of the fracture surfaces. This method uses the fracture-surface topography analysis to infer the plastic strains that have developed during the failure. That is, based on the previously proposed simple bar hypothesis, the fracture surfaces can be assumed to be composed of independent rectangular bars. After dividing the plastic deformation into single bars, the original lengths of these bars are calculated and then the global strains of these bars during the course of failure are calculated. According to the relationship between true stress and true strain for the material, the normal stress on the cross section of each bar is determined. Adding all loads on all bars together provides the total applied load of the specimen. As illustrations, the method is applied to fracture surfaces obtained from double-edge notched specimens made of two kinds of metallic alloy, broken under low-cycle fatigue. Results show that the calculated maximum fatigue load is almost equal to that recorded during testing.