The feasibility of using the nonlinear effect of primary Lamb wave propagation for assessing accumulated fatigue damage in solid plates is theoretically analyzed. After the aluminum sheets are subjected to tension-tension fatigue loading for different numbers of loading cycles, they are subjected to ultrasonic tests near the driving frequency where Lamb waves have a strong nonlinearity. This is followed by the measurement of the amplitude-frequency curves for second harmonics of the considered Lamb waves. The experimental results show that the effect of second-harmonic generation by Lamb wave propagation is very sensitive to the accumulation of fatigue damage of solid plates.

In this work, we analyze theoretically and observe experimentally the frequency mixing response induced by the collinear interaction of two primary Lamb waves with different frequencies in an isotropic and homogenous plate. The internal resonance conditions for generating the combined harmonics are analyzed by using a perturbation approach and a normal-mode-expansion technique for waveguide excitation. The second- and third-order combined harmonics generated by the collinear wave mixing of two primary Lamb waves propagating in a specimen are analyzed, and the existence of the combined harmonics at specific frequencies is predicted. An experimental procedure is proposed to measure the combined harmonics induced by the collinear cross-interaction of the two specific primary Lamb waves at given frequencies. The theoretical prediction of the appearance of the combined harmonics at the specific mixing frequencies is consistent with experiment. The theoretical analysis and experimental observation provide a clear physical insight into the frequency-mixing response induced by the collinear cross-interaction of two primary ultrasonic Lamb waves.

Static component (SC) generation of guided waves (GWs), which combines the high sensitivity of acoustic nonlinearity to micro-damage and low attenuative effect, has great potential for damage assessment in large composite structures. The present work explores the use of SC generation of GWs for assessing damages in carbon fiber reinforced polymer (CFRP) composite laminates. The features including mode, waveform, and cumulative effect of the generated SC in composites are numerically investigated by three-dimensional finite element modeling and simulation. A dynamic displacement measurement method based on piezoelectric transducers is accordingly proposed and experimentally verified. The cumulative SC pulse generated from primary GW tone-burst with a finite duration, is observed and verified numerically and experimentally. It is found that the magnitude of the generated SC pulse is linearly proportional to the quadratic material nonlinearity. Experimental results demonstrate that the generated SC pulse of GW under group velocity matching condition, is an effective means to assess the hygrothermal damage and low-velocity impact damage in CFRP composite plates. The performed experimental examination validates the feasibility of the proposed approach for damage assessment in CFRP composites.