Microstructural information on damage in a metastable austenitic steel developed under low-cycle fatigue conditions is extracted from the acoustic monitoring of Poisson's ratio supplemented by the eddy current, and, under certain conditions, specific weight data. These data imply that damage has the form of мicropores of strongly oblate, crack-like shapes (aspect ratios of 0.01–0.04). This means that crack density – and not porosity – is the proper parameter of their concentration, in whose terms the effective properties should be expressed. It is also found that the microcrack density at the fracture point is proportional to relative volume of the strain-induced martensite. We also discuss relations between porosity and crack density.