Fatigue of metastable austenitic steel: Micromechanics aspects

Abstract

Microstructural changes accompanying fatigue of a metastable austenitic steel comprise two main factors: (a) microcracking and (b) nucleation and growth of the martensitic phase. Both factors produce changes in the effective elastic properties that can be monitored by high-precision acoustic techniques. In order to focus on monitoring microcracking, its effect on the elastic properties should be separated from the effect of the martensitic phase, since the latter has elastic constants that are different from the ones of the austenitic phase. To this end, the eddy current technique is used to monitor the volume fraction of martensite. The volume fraction parameter is generally insufficient for the prediction of the elastic properties since they depend not only on the volume fraction but, also, on the shapes of the martensitic particles – that are complex and not fully known. However, if the elastic contrast between the austenitic and martensitic phases is weak-to-moderate, the effect of shapes can be neglected, so that the volume fraction information is sufficient. This allows non-destructive monitoring of microcracking by a combination of the acoustic and the eddy current data.