Damage Imaging in Metallic and Composite Structures using Scanning Air-coupled and Laser Ultrasound

A rapid, fully non-contact, hybrid system which encompasses an air-coupled transducer (ACT) and a laser Doppler vibrometer (LDV) is presented for profiling A0 Lamb wave dispersion curves as well as detecting different damages in metallic and composite structures. The influence of the ACT angle of incidence on Lamb wave excitation is investigated and Snell's law is used to directly compute Lamb wave dispersion curves including phase and group velocity dispersion curves in aluminum plates from incident angles found to generate optimal A0 Lamb wave mode while in composite plates the group velocity dispersion curves were obtained using Morlet wavelet transform (MWT) based on time-of-flight along different wave propagation directions. The phase and group velocity polar characteristic wave curves are also computed using the same methods.;In addition, a rapid imaging technique is proposed for imaging damage in metallic plates by using a zero-lag cross-correlation (ZLCC) imaging condition in the frequency domain. The technique was experimentally verified by employing the non-contact ACT/LDV system on three identical aluminum plates with same notch geometry (10 mm x 5 mm) but with various orientations at different locations. A damage image is constructed by crosscorrelating the forward and backward propagating wavefields in the aluminum plates which are separated by analyzing the actual LDV measured wavefield using a frequencywavenumber filtering post-processing technique.;The following technique was also applied for detecting barely visible impact damages (BVID) with different dimensions in a 16-ply [45/0/-45/90]2S carbon fiber reinforced plastic (CFRP) laminate. The experimental images highlight the damaged areas with higher ZLCC values compared to other parts of the inspected areas. However, it was found that depending on the damage size and if delaminations are happening all around the impacted spot, the delamination boundary on the right side of the impact is difficult to detect if the generated wavefield is traveling from left to right which interacts with the delaminations of the left side and vice versa. The experimental investigation has shown a good correlation between the ZLCC imaging condition and C-scan images, which demonstrate a strong capability of guided wave ZLCC imaging condition technique in approximating the location and size of relatively small BVIDs in thin composite structures and small defects in the isotopic material.;The final step in this research develops a two-dimensional (2-D) theoretical model to simulate the behavior of a fully non-contact air-coupled nondestructive inspection (NDI) system for a thin isotropic plate. The model is divided into three phases: (1) transmission phase, (2) elastic wave propagation phase and (3) reception phase. The validation of the complete model was carried out by modeling the same system by means finite element method (FEM) using COMSOL Multiphysics software. The results of the acoustic pressure excited by the transducer, the out-of-plane velocity amplitudes for the generated A0 Lamb wave mode, and the radiated pressure from the plate caused by the leaky Lamb wavefield were all compared between the two models and a reasonable degree of similarity was noticed between them.