Fatigue damage assessment using nonlinear critically refracted longitudinal (LCR) wave in pure iron: Experiments and FEM

Abstract

To achieve early-stage microstructure evolution assessment and damage monitoring of fatigue, a new thickness-independent and nondestructive method based on nonlinear critically refracted longitudinal (LCR) wave is proposed. The nonlinear ultrasonic (NLU) parameter is sensitive to the accumulated fatigue damage, showing two peaks around the 5th and 1000th cycles (0.03% and 6% of fatigue life), respectively. This phenomenon is comprehensively discussed in terms of dislocation structure evolution based on the dislocation string model. A correction considering the weakening of nonlinearity induced by the rough surface is applied to the experiment measurements. The corresponding results are closer to the finite element modeling (FEM) quantitation based on the dislocation-dependent nonlinear constitutive relation. Compared to linear LCR wave, nonlinear LCR wave has weaker sensitivity to roughness effect and higher sensitivity to damage such as dislocation, which offers a new method for monitoring fatigue in engineering components.