Theoretical and experimental measurement of intrinsic and fatigue induced material nonlinearities using Lamb wave based nonlinearity parameters

The paper presents development and application of amplitude and physics based parameters for measuring intrinsic and fatigue induced material nonlinearities of plate materials considering Lamb wave motion into them. The amplitude based parameter is derived using Lamb wave equations and expressed in terms of amplitudes of the harmonics. This parameter can be evaluated directly from the results of experiments. The physics based parameter is derived considering the fundamental dislocation theory and it depends on percent fatigue life. It is used to construct a theoretical nonlinearity curve (TNC). Both the parameters are evaluated for Al 7075-T651 at different percents of fatigue life and compared. The close agreement of results show that with reference of TNC, the amplitude based parameter obtained from a field test can be used for estimating the remnant useful life of a plate structure as formation of dislocation substructures during fatigue loading is a prestage of microcrack formation.