Vibration energy analysis of a plate for defect imaging with a scanning laser source technique

Vibration energies generated by laser irradiation to a plate with a crack were calculated by the semi-analytical finite element method to elucidate the principle of defect imaging using a scanning laser source. For normal incidence in the ablation regime, the incident energy increases when the incident source is located in the vicinity of the crack, owing to the effect of the non-propagating A1 modes. For dipole loading in the thermoelastic regime, the vibration energies are completely different, depending on the position of the crack opening. If the crack opening is located opposite the incident source, the vibration energy increases abruptly in the vicinity of the crack, which is affected by the higher-order non-propagating modes as well as the A1 modes. When the crack opening and the incident source are located on the same side, the generated energy approaches zero as the source moves closer to the crack. The energy reduction around the crack is caused by the superposition of the incident wave from dipole loading and the phase-inverted reflected wave. The results of experiments conducted to verify the energy variations in the vicinity of a crack were in good agreement with the numerical results for dipole loading.