The Lamb wave time reversal method has widely been investigated as a baseline-free damage detection technique for structural health monitoring. Due to the mode tuning effects from the transducer-wave interactions, even for a pristine wave path, the reconstructed signal waveform may differ much from the original excitation waveform. Consequently, it becomes difficult to distinguish the differences between undamaged and damaged wave paths. This article presents an enhanced Lamb wave virtual time reversal (VTR) algorithm with transducer transfer function compensation to eliminate the transducer influence for dispersive, multimodal Lamb waves. This VTR procedure builds upon a complete 2D analytical model for Lamb wave generation, propagation, and reception. The analytical solution shows that, with the transducer transfer function compensation, a perfect reconstruction of the original excitation waveform can be achieved for both symmetric and antisymmetric Lamb modes. In addition, finite element modeling and experimental validations are further performed to verify the enhanced time reversal procedure. Finally, a time reversal tomography experiment is conducted with a piezoelectric transducer array for structural damage imaging. The enhanced VTR method can achieve more accurate and robust damage imaging results. The paper finishes with discussion, concluding remarks, and suggestions for future work.

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.

This article presents the investigation of nonlinear scattering features of guided waves from rivet hole nucleated fatigue cracks considering the rough contact surface condition. A small-size numerical model based on the local interaction simulation approach is developed, enabling the efficient analysis of the contact acoustic nonlinearity during the wave crack interactions. The study starts with an idealized breathing crack model possessing smooth, perfectly kissing contact surfaces. Then, the nature of rough crack surfaces is considered with randomly distributed initial openings and closures. Several distinctive aspects of the nonlinear scattering phenomenon are discussed: (1) the amplitude effect, which renders significantly different nonlinear response under various levels of excitation wave amplitudes; (2) the directivity and mode conversion features, which addresses the scattering direction dependence of the fundamental and superharmonic wave mode components; (3) the nonlinear resonance phenomenon, which maximizes the nonlinear response during the wave crack interactions at certain excitation frequency ranges. This study demonstrates that these nonlinear features are substantially influenced by the crack surface condition and differ much between an idealized breathing crack and a rough crack in most practical cases. Fatigue tests on a thin aluminum plate with a rivet hole is conducted to induce cracks in the specimen. An active sensor array surrounding the crack zone is implemented to generate and receive ultrasonic guided waves in various directions. Current work emphasizes on using a highly efficient numerical model to explain the nonlinear features of scattered waves from fatigue cracks considering the rough crack surface condition. These special features may provide insights and guidelines for nonlinear guided wave based nondestructive evaluation and structural health monitoring system design. The paper finishes with discussion, concluding remarks, and suggestions for future work.

In this article, ultrasonic guided wave propagation and interaction with the rivet hole cracks has been formulated using closed-form analytical solution while the local damage interaction, scattering, and mode conversion have been obtained from finite element analysis. The rivet hole cracks (damage) in the plate structure gives rise to the non-axisymmetric scattering of Lamb wave, as well as shear horizontal (SH) wave, although the incident Lamb wave source (primary source) is axisymmetric. The damage in the plate acts as a non-axisymmetric secondary source of Lamb wave and SH wave. The scattering of Lamb and SH waves are captured using wave damage interaction coefficient (WDIC). The scatter cubes of complex-valued WDIC are formed that can describe the 3D interaction (frequency, incident direction, and azimuth direction) of Lamb waves with the damage. The scatter cubes are fed into the exact analytical framework to produce the time domain signal. This analysis enables us to obtain the optimum design parameters for better detection of the cracks in a multiple-rivet-hole problem. The optimum parameters provide the guideline of the design of the sensor installation to obtain the most noticeable signals that represent the presence of cracks in the rivet hole.