A defect-mode elastic metamaterial enhancing nonlinear ultrasonic signatures for fatigue crack detection

• Metamaterials and nonlinear ultrasonics NDE technique join hands. • A defect mode metamaterial amplifies the originally weak nonlinear wave signal. • Nonlinear signal amplitude can reach six times higher than the conventional NDE setup. This study presents enhanced nonlinear features at the sensing terminal for improved fatigue detection through leveraging the wave control capability of metamaterials, utilizing a defect-mode metamaterial (DMM) system. Initially, an analytical investigation based on mass-spring chain models is explored to illustrate the defect cavity concept aiming at improving the current nonlinear ultrasonic sensitivity. Subsequently, by examining the complete band structure of the DMM supercell through modal analysis, distinct defect bands can be identified within a specific frequency range. Notably, one defect mode effectively localizes the antisymmetric Lamb waves, boosting the sensing signal nonlinearity. To validate the enhancement capability, harmonic analyses of both the DMM and a smooth plate system are conducted, revealing the transmitted wave energy spectrum and identifying the optimal frequency for targeting at the second harmonic component. A coupled-field transient finite element simulation follows, capturing the transient dynamic responses of both systems, and demonstrates the effectiveness of the DMM in amplifying the originally weak second harmonic signature generated by wave-crack interactions. Experimental demonstrations further substantiate these theoretical findings and numerical predictions. This approach holds promise for advancing conventional Nondestructive Evaluation (NDE) and Structural Health Monitoring (SHM) system via metamaterial enhancers.