Reflection phenomena of a set of coupled longitudinal waves/shear wave striking obliquely against the mechanically stress-free boundary surface of a uniform nonlocal elastic solid half-space with double porosity structure have been investigated. Employing the appropriate nonlocal stress-free boundary conditions at the surface, the formulae for reflection coefficients and their corresponding energy ratios have been derived for the dissipative and non-dissipative models. Their variations have been computed against the angle of incidence for a specific model. It is found that all the reflection coefficients depend upon the presence of the nonlocality, porosities of both types, frequency and angle of incidence. Their impact on various coefficients is also studied numerically and depicted graphically.

Fibre-reinforced plastic (FRP) is an anisotropic material and is used in aircraft, civil and architectural structures. The anisotropic property sometimes makes it difficult for inspectors to detect flaws in FRP because the phase and group velocities of the ultrasonic waves in anisotropic materials depend on the propagation direction. The velocities of these ultrasonic waves in a material can be determined using the dynamic elastic constants of the material. Therefore, determining the dynamic elastic constants of anisotropic materials such as FRP is an important task. In this research, a new dynamic elastic constant estimation method using laser ultrasonic visualisation testing is developed for FRP. The laser ultrasonic visualisation technique is incorporated to measure the ultrasonic wave velocities, which are required for elastic constant estimation. In this paper, the procedure of the elastic constant estimation method developed is described with the anisotropic elastodynamic theory. An image processing technique for the visualised data obtained using laser ultrasonic visualisation testing is utilised to determine the phase and group velocities of the ultrasonic waves. The elastic constants of an FRP specimen are estimated using this proposed method. In addition, the simulation of ultrasonic wave propagation in the FRP with the estimated elastic constants is implemented using the finite difference time domain (FDTD) method and the results are compared with those obtained using laser ultrasonic visualisation testing to validate the proposed method.

In recent years, nondestructive testing for civil engineering structures has become increasingly important. Ultrasonic testing is one of nondestructive inspection methods for civil structures. However, the inspection of civil engineering structures takes much time because of the extensive scope of the inspection. Moreover, in the field of nondestructive testing, there are also concerns about a future shortage of inspectors, so that an innovative effective nondestructive method needs to be developed. This study proposes an automatic defect detection approach using pre-trained convolutional neural network for laser ultrasonic visualization testing. The effectiveness of the proposed method is confirmed by applying it to a concrete structure with a surface defect. Grad-CAM demonstrates that the created CNN model in this study accurately predicts the position of a surface defect of concrete specimens.

This study develops a method to visualize the state of steel-concrete interface with ultrasonic testing. Scattered waves are obtained by the UT pitch-catch mode from the surface of the concrete. Discrete wavelet transform is applied in order to extract echoes scattered from the steel-concrete interface. Then Linearized Inverse Scattering Methods are used for imaging the interface. The results show that LISM with Born and Kirchhoff approximation provide clear images for the target.

In recent years, there has been increasing attention on the development of non-destructive evaluation (NDE) methods using guided waves for long-length materials such as thin plates and pipes. The guided waves are capable of long-distance propagation in thin plates and pipes, and they exhibit properties such as multimodality and dispersion. These characteristics of the guided waves make inspection using guided waves challenging. In this study, we apply a 2-D elastodynamic time-reversal method to detect surface breaking cracks of a thin plate where guided waves are present. The finite element method (FEM) is used to calculate the scattered waves from surface breaking cracks and their corresponding time-reversal waves. We also employ topological sensitivity as an assessment index for detecting surface breaking cracks using the time-reversal method. As numerical examples, we demonstrate guided wave propagation, scattering, and the time-reversal wave propagation obtained by using the FEM. Finally, we present the results of surface breaking crack detection in a thin plate and discuss the validity and effectiveness of the proposed method.