Plasticity-induced martensitic transformation around fatigue cracks in type SUS304 austenitic stainless steel

The present study investigates plasticity-induced martensitic transformation around fatigue cracks in Type SUS304 austenitic stainless steel. Volume fraction of alpha' martensite transformed in uniformly stretched SUS304 plates was measured and expressed as a function of the applied strain level. The distributions of a' phase fraction in the plastic wake regions produced around fatigue cracks were then measured by ferrite scope in fatigued SUS 304 plate specimens. The results were compared with the distributions of vertical magnetic flux density B-z above the fatigue cracks in the specimens magnetized by a strong magnetic field higher than 0.4 T. It was revealed that the B-z distributions reflected the alpha' phase fraction distributions in the wake regions: i.e., the distance between two outermost peaks of the B-z distributions had good linear correlations with real fatigue crack length, and the maximum and the minimum values of B-z also showed good linear relationships with the applied stress intensity factor range DeltaK. These results imply that not only crack length but also the applied DeltaK level or the applied stress range Deltasigma level can be detected effectively in an electromagnetic non-destructive way.