Metamaterial improved nonlinear ultrasonics for fatigue damage detection

This article presents an improved nonlinear ultrasonic technique for fatigue damage detection utilizing a kind of carefully designed aluminum-lead composite metamaterial. It focuses on developing a bandgap metamaterial to improve the accuracy and identifiability of the superharmonic features from fatigue cracks by eliminating the inherent nonlinear components in the nonlinear ultrasonic technique. The study starts with the unit cell design through modal analysis by applying the Bloch-Floquet boundary condition to obtain the band structure. Based on the local resonance mechanism, by adjusting the height ratio between the aluminum and lead cylinders, the bandgaps covering the required frequency ranges can be opened up. Then, a chain of regularly arranged unit cells is modeled to analyze the spectral response and verify the bandgap effect through harmonic analysis. The targeted ultrasonic frequency component of guided waves within the bandgap can be mechanically filtered out. Subsequently, a finite element model of the pitch-catch active sensing procedure for fatigue crack detection is constructed via the coupled-field transient dynamic analysis. Nonlinear ultrasonic experiments with the designed metamaterial are carried out to verify the theoretical and numerical investigations. This paper demonstrates that the metamaterial, with its outstanding wave manipulation capability, shows great potential on structural health monitoring and nondestructive evaluation applications. The paper finishes with summary, concluding remarks, and suggestions for future work.