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This article describes a new damage visualization method to investigate and analyze propagating guided Lamb waves using analyses of wavefield spatial gradients. A laser ultrasonic interrogation system was used to create full-field ultrasonic data measurements for ultrasonic wavefield imaging. The laser scanning process was performed based on both a raster scan and a circle scan. From the high-resolution wavefield data, a spatial gradient–based image processing technique was developed using gradient vectors to extract features sensitive to defects. Local impedance changes at the damaged area would result in a local distortion of the waveform which was captured and quantified by the variation of the gradient vectors in the scanning area as time evolves. Such variation was accumulated over time with a statistical threshold filter to generate a gradient-orientation map for damage visualization. The proposed algorithm was capable of producing distinctive damage patterns when tested experimentally on a 3-mm aluminum plate with multiple simultaneous simulated defects. Compared to conventional techniques like local wavenumber estimation, the generation of the accumulated orientation map involves no filtering process in the frequency or wavenumber domain, at the expense of more accurate shaping of the defect. A spatial covariance analysis was adopted to locate damage from the results as well as to evaluate the correlation between different kinds of defects. Combining the proposed approach with conventional laser ultrasonic imaging techniques enables a fast and robust damage identification and characterization process which requires lower computational burden and practical operation.

Ultrasonic wavefield imaging (UWI) provides insightful spatial information about ultrasonic wave propagation in planar (2-D) space for nondestructive evaluation and structural health monitoring (NDE-SHM) applications. In all materials, the wavefronts of the incident and reflected waves propagate with unique patterns that may be represented by parametrized polar curves in 2-D geometric space. In this paper, a spatial ultrasonic wavefront characterization method based on a parametric curve laser scan is proposed to characterize the spatial ultrasonic wavefront for both isotropic and anisotropic materials. Three parametric curves (circular, hyperbolic, and cyclic-harmonic curves) were considered. Two wavefront characterization process were carried out, namely (i) deciding the parametric equation of the closed-form geometric plane curve via UWI, and (ii) measuring and updating the ultrasound via laser ultrasonic interrogation system (LUIS) and quantifying the values(s) of the predicted parametric curve equation using a temporal cross-correlation technique. The proposed method was tested on pristine aluminum and cross-ply CFRP plates to characterize the spatial incident and reflected wavefronts of the plates. The non-fiber direction region (105°⩽ϕS⩽165°) and the fiber direction region (165°⩽ϕS⩽195°) of the cross-ply CFRP plate were considered in the test. The laser circle scan and the laser cyclic-harmonic curve scan showed the ability to characterize the incident wavefronts of the S0 and A0 modes in the aluminum plate and the CFRP plate, respectively, followed by the laser hyperbolic curve scan. With the promising results obtained in the proposed method, the integration of the parametric curve scanning method into LUIS may provide a new approach to damage detection and useful information for ultrasonic algorithm design in NDE-SHM applications.

Numerous Lamb wave dispersion curve estimation methods have been developed to support damage detection and localization strategies in non-destructive evaluation/structural health monitoring (NDE/SHM) applications. In this paper, the covariance matrix is used to extract features from an ultrasonic wavefield imaging (UWI) scan in order to estimate the phase and group velocities of S0 and A0 modes. A laser ultrasonic interrogation method based on a Q-switched laser scanning system was used to interrogate full-field ultrasonic signals in a 2-mm aluminum plate at five different frequencies. These full-field ultrasonic signals were processed in three-dimensional space-time domain. Then, the time-dependent covariance matrices of the UWI were obtained based on the vector variables in Cartesian and polar coordinate spaces for all time samples. A spatial covariance map was constructed to show spatial correlations within the full wavefield. It was observed that the variances may be used as a feature for S0 and A0 mode properties. The phase velocity and the group velocity were found using a variance map and an enveloped variance map, respectively, at five different frequencies. This facilitated the estimation of Lamb wave dispersion curves. The estimated dispersion curves of the S0 and A0 modes showed good agreement with the theoretical dispersion curves.