Baseline-free estimation of the residual fatigue life using a modulated nonlinear ultrasound method

This work develops a modulated nonlinear elastic wave spectroscopy (NEWS) method for the evaluation of a metallic components residual fatigue life. An aluminium specimen (AA6082-T6) was tested at predetermined points throughout its fatigue life using a dual-frequency ultrasound method. A modulated nonlinear parameter was derived, which described the relationship between the generation of modulated (sideband) responses of a dual frequency signal and the linear response. The sideband generation from the dual frequency was shown to increase as the residual fatigue life decreased, and as a standalone measurement method it can be used to show an increase in a materials damage. Although analytical evaluation of the method was required to develop a baseline-free method, work done by Meo and Amerini combined the Paris law and the Nazarov-Sutin crack equation developing the theoretical model. The results showed good correlation between the derived theoretical model and the modulated nonlinear parameter, allowing for material residual fatigue life estimation.