This paper presents the Scanning Laser Vibrometry (SLV) imaging of fatigue cracks by taking advantage of the nonlinear ultrasonic guided wave scattering and mode conversion phenomena. The investigation starts with the numerical modeling using the Local Interaction Simulation Approach (LISA) to demonstrate the distinctive scattering and mode conversion features at rough fatigue cracks. During the wave crack interactions, nonlinear higher harmonics are generated from Contact Acoustic Nonlinearity (CAN). In addition, the microscale rough crack surface condition may introduce mode conversion between the symmetric and antisymmetric Lamb modes. After the theoretical analysis, SLV experiments are conducted on an aluminum plate, where fatigue cracks are nucleated from a rivet hole. The damage imaging scheme utilizes the post-processing techniques via Fast Fourier Transform (FFT), frequency domain filtering, and Inverse Fast Fourier Transform (IFFT) to eliminate the linear wave field, leaving only the scattered higher harmonics in the images. In this way, the fatigue cracks can be distinguished from structural features such as rivet holes and stiffeners. This paper finishes with summary, concluding remarks, and suggestions for future work.
In this paper, a bandgap meta-surface is carefully designed for enhancing the identifiability of nonlinear ultrasonic superharmonics for fatigue crack detection. In the unit cell design stage, modal analysis with Bloch-Floquet boundary condition is performed to obtain the dispersion features of guided waves in the meta-surface. Then, a finite element model (FEM) for a chain of unit cells is simulated to verify the bandgap effect. In practice, due to the inherent nonlinearity from the electronic instrument and bonding adhesive, the corresponding weak superharmonic components will adversely affect the identifiability of the nonlinear characteristics raised by wave crack interactions. In the current approach, the guided waves generated by the transmitter propagate into the structure, carrying the inherent nonlinearity with them. Immediately afterwards, they pass through the meta-surface with optimized transmission of the fundamental excitation frequency and complete mechanical filtration of the second harmonic component. In this way, the appearance and amplitude of the second harmonic in the sensing signal become evidently indicative of the presence and severity of the fatigue crack along the wave path between the meta-surface and the receiver. The proposed method possesses great potential in future SHM and NDE applications. Nonlinear ultrasonic experiments with the designed meta-surface are conducted to verify the theoretical and numerical investigations as well as to demonstrate the practical application of metamaterial in SHM and NDE. The paper finishes with summary, concluding remarks, and suggestions for future work.
This paper presents the investigation of nonlinear scattering features of guided waves from fatigue cracks. The fatigue cracks nucleated from a rivet hole are studied as the representative case. A small-size numerical model based on the Local Interaction Simulation Approach (LISA) is introduced, which enables the efficient analysis of the Contact Acoustic Nonlinearity (CAN) of guided waves. 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. Several distinctive aspects of the nonlinear scattering phenomenon are discussed: (1) the directivity and mode conversion features, which addresses the scattering direction dependence of fundamental and superharmonic wave mode components; (2) the amplitude effect, which stems from the rough crack surface condition with initial openings and closures; (3) the nonlinear resonance phenomenon, which maximizes the nonlinear response during the wave crack interactions at certain excitation frequency ranges. All these features may provide insights and guidelines for nonlinear guided wave based Structural Health Monitoring (SHM) system design. The numerical studies are compared with experimental data. The paper finishes with discussion, concluding remarks, and suggestions for future work.
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