Nondestructive monitoring of fatigue damage evolution in austenitic stainless steel by positron-lifetime measurements

Abstract

Positron-lifetime measurements have been performed on austenitic stainless steel during (i) stress- and (ii) strain-controlled fatigue experiments for different applied stress and strain amplitudes, respectively. For this purpose a generator-detector assembly with a ${}^{72}\mathrm{Se}{/}^{72}\mathrm{As}$ positron generator [maximum activity $25\ensuremath{\mu}\mathrm{Ci}$ (0.9 MBq)] has been mounted on mechanical testing machines in order to measure the positron lifetime without removing the specimens from the load train. The average positron lifetime has been determined by a ${\ensuremath{\beta}}^{+}\ensuremath{-}\ensuremath{\gamma}$ coincidence. The feasibility to use the average positron lifetime for monitoring the evolution of fatigue damage and to predict early failure has been examined. In strain- and stress-controlled experiments the average positron lifetime shows a pronounced increase within the first 10% and 40% of the fatigue life, respectively. In stress-controlled experiments the average positron lifetime at failure depends significantly on the applied stress amplitude. In strain-controlled experiments significantly different positron lifetimes for different applied plastic strain amplitudes are obtained within the first 1.000 fatigue cycles, whereas differences get wiped out during further cycling until failure.