Evaluation of fatigue damage using nonlinear guided waves

Abstract

This research develops an experimental procedure for characterizing fatigue damage inmetallic plates using nonlinear guided waves. The work first considers the propagation ofnonlinear waves in a dispersive medium and determines the theoretical and practicalconsiderations for the generation of higher order harmonics in guided waves. By usingresults from the nonlinear optics literature, it is possible to demonstrate that both phaseand group velocity matching are essential for the practical generation of nonlinear guidedelastic waves. Next, the normalized acoustic nonlinearity of low cycle fatigue damagedaluminum specimens is measured with Lamb waves. A pair of wedge transducers is used togenerate and detect the fundamental and second harmonic Lamb waves. The results showthat the normalized acoustic nonlinearity measured with Lamb waves is directly related tofatigue damage in a fashion that is similar to the behavior of longitudinal and Rayleighwaves. This normalized acoustic nonlinearity is then compared with the measuredcumulative plastic strain to confirm that these two parameters are related, and toreinforce the notion that Lamb waves can be used to quantitatively assess plasticitydriven fatigue damage using established higher harmonic generation techniques.