FEM simulation of nonlinear lamb waves for detecting a micro-crack in a metallic plate

Nonlinear ultrasonic technique has been employed to detect microcracks since conventional linear elastic ultrasonic technology is just sensitive to gross defects. However, most of nonlinear ultrasonic researches to date have been experimental generally using bulk waves or Rayleigh waves, few numerical studies exist, especially for lamb wave ultrasonic. In this chapter, finite element method (FEM) is applied to simulate nonlinear lamb waves interacting with a micro-crack in a thin metallic structure. A pitch and catch approach is introduced containing two symmetric piezoelectric transition (PZT) wafers as actuators to generate single S0 mode signal and one PZT wafer as receiver. Generated S0 mode lamb waves propagate along the structure, interact with the micro-crack, obtain nonlinear features, and are picked up by the receiver. An undamaged plate and seven plates with different crack length are simulated. The received simulation signal from a micro-cracked plate contains a S0 mode wave-packet and a new wave-packet. A nonlinearity index (NI) is proposed to show the degree of nonlinear effect. The simulation results show that employing NI, a received signal of S0 mode can provide information on the damage severity of a micro-crack and the new wave packet signal can be used as an early indicator for the existence of a micro-crack.