Laser Ultrasonic Imaging of Wavefield Spatial Gradients for Damage Detection

Laser ultrasonic techniques (LUTs) are increasingly used in non-destructive evaluation (NDE) applications because they can provide full-field ultrasonic wavefield data with high resolution in a three-dimensional (3D) space–time domain. This paper investigates the properties of the wavefield spatial gradient as a detector and localizer for defects that distort the local wavefield. A laser ultrasonic interrogation method based on a 527-nm Q-switched laser scanning system was used to interrogate 3D ultrasonic signals in a 3-mm aluminum plate. The plate was tested with and without multiple artificial scatterers to test the proposed method. During the scanning process, the ultrasonic waves were generated by the pulsed laser at an n-by-m grid of sampling points and recorded by a single fixed PZT transducer. The whole scan generates a 3D matrix which allows the accessibility of showing the wave propagation over time in movie form with a high spatial resolution. After the scanning process, the full-field ultrasonic data of the plate without and with artificial damage were obtained, respectively. Then, spatial gradient vectors were computed from each frame of the full-field ultrasonic data along the time axis. For a space-domain frame \(\left\{ {U_{i} } \right\}^{n \times m}\)at a time \(i \in t\), the gradient vectors were determined by two components, \(\left\{ {G_{i,x} } \right\}^{n \times m}\)and \(\left\{ {G_{i,y} } \right\}^{n \times m}\), which reflected the spatial derivatives of every pixel with respect to the position coordinates. Once the gradient vectors of the full-field ultrasonic data (intact and damage cases) were determined, the spatial gradient ultrasonic wavefield imaging was generated with the gradient orientation profile \(\left\{ {\theta_{i} } \right\}^{n \times m}\)and the gradient magnitude profile \(\left\{ {A_{i} } \right\}^{n \times m}\)as the features used for damage detection. The wavefield spatial gradients of the defect-free case were used as a baseline to compare with the wavefield spatial gradients of the damaged case. The gradient profiles of the wavefield spatial gradients could detect and localize the damage by the inconsistency of the wave pattern in the 2D space domain. The wavefield spatial gradients also showed the ability to remove the complexity of identifying the damage among multi-mode waveform. Future work will be focusing on applying the algorithm to complex structures such as CFRP composites.