On the feasibility of nonlinear assessment of fatigue damage in hardened IN718 specimens based on non-collinear shear wave mixing

Abstract

Recent studies have shown that various ultrasonic wave mixing techniques can be exploited for assessing the nonlinearity of both intact and damaged materials. It has been reported that one particular type of ultrasonic wave mixing, non-collinear shear wave mixing, is sensitive to the excess nonlinearity caused by plastic deformation and low-cycle fatigue in Al2014-T4 aluminum alloy. In this study we investigated the feasibility of using the same non-collinear shear wave mixing technique to detect plastic deformation and fatigue damage in fully hardened IN718 engine alloy specimens. We implemented numerous technical improvements over the earlier developed non-collinear shear wave mixing system to improve its detection sensitivity and image resolution. In spite of these enhancements, we found that in fully hardened IN718 this technique is sensitive only to plastic deformation and, to a much lesser degree, to highly accelerated low-cycle fatigue, but it is not sensitive to either typical low-cycle fatigue or high-cycle fatigue. These observations highlight a potential deficiency in our current understanding of nonlinear material-wave interaction and suggest that further research is needed to explain why certain classes of dislocation networks remain hidden from this type of inspection.