Abstract
The symmetric zero-frequency mode induced by weak material nonlinearity during Lamb wave propagation is explored for the first time. We theoretically confirm that, unlike the second harmonic, phase-velocity matching is not required to generate the zero-frequency mode and its signal is stronger than those of the nonlinear harmonics conventionally used, for example, the second harmonic. Experimental and numerical verifications of this theoretical analysis are conducted for the primary S0 mode wave propagating in an aluminum plate. The existence of a symmetric zero-frequency mode is of great significance, probably triggering a revolutionary progress in the field of non-destructive evaluation and structural health monitoring of the early-stage material nonlinearity based on the ultrasonic Lamb waves.
Highlights
Material non-destructive evaluation and structural health monitoring during the early stage of material degradation are crucial for structural integrity and safety [1]
In order to obtain the solution of zero-frequency mode, one single mode is considered for simplicity
Wesymmetric present theoretical analysis andisexperimental simulation results to demonstrate that the zero-frequency mode effective for and evaluating the early-stage demonstrate that the symmetric zero-frequency mode is effective for evaluating the early-stage material nonlinearity
Summary
Material non-destructive evaluation and structural health monitoring during the early stage of material degradation are crucial for structural integrity and safety [1]. Narasimha et al [34,35], Jacob et al [36] and Nagy et al [37] have shown that the zero-frequency mode varies linearly with the propagation distance Their studies are mainly confined to longitudinal acoustic waves. To overcome these limitations, Sun et al (theory) [38] and Wan et al (simulation) [39] have shown that the signal of the zero-frequency mode is stronger than that of the traditional nonlinear harmonics for Lamb waves. Solutions for the second harmonic, sum- and difference-frequency components are obtained via modal decomposition [28] This procedure originally assumes that the nonlinearly generated secondary wave fields (perturbation solution) can be expressed as a superposition of the Lamb wave modes, since an orthogonality exists between different Lamb wave modes (completeness of Lamb wave modes is assumed).
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