Abstract: The feasibility of using a nonlinear ultrasonic Lamb wave approach for accurately evaluating fatigue damage in solid plates was theoretically analyzed.With the established experimental system for ultrasonic Lamb waves,the fundamental-and second-harmonic amplitude-frequency curves of Lamb waves,propagating in an aeronautic aluminum sheet without being subjected to any cyclic loading,were measured.Reasonable physical explanations were given for the data measured.The tension-tension fatigue loading was respectively applied to the two given aeronautic aluminum sheets.The fundamental-and second-harmonic amplitude-frequency curves of ultrasonic Lamb waves,propagating in the two specimens subjected to tension-tension fatigue loading for different numbers of loading cycles,were measured under the condition that ultrasonic Lamb waves had a strong nonlinearity.It was found,in the early stages of fatigue damage in the specimens,that the change in the Stress Wave Factor (SWF) of the fundamental waves of ultrasonic Lamb waves with the number of loading cycles (N) was not obvious,and that there was not a clear monotonic relationship between the SWF of the fundamental Lamb waves and N.In the initial stage of cyclic loading,however,the change in the second-harmonic SWF of ultrasonic Lamb waves was very sensitive to N,and there was a clear monotonic relationship between second-harmonic SWF of ultrasonic Lamb waves and N.It could be concluded that the nonlinear effects of ultrasonic Lamb waves and the corresponding second-harmonic SWF might offer a potential for accurately evaluating the early stages of fatigue damage in solid plates.