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.

Synopsis :Laser ultrasonics have been studying for many years as a promising technique for evaluating industrial materials. The non-contact technique, however, still have some problems in practical use for large structures such as pipes, tanks, bridges, etc. Authors have adopted the scanning laser source technique for imaging defects in a plate-like structure to solve one of the problems that elastic wave cannot be measured stably with laser interferometory due to the unstable detection of scattering light at rough and inclined surfaces of existing structures. In this study, a remote experimental system of the defect imaging technique with the scanning laser source, which does not require cables between receiving transducers and experimental equipments, was developed. In the experimental system, laser emission signal detected by a photo-detector was used as trigger signal that requires quick response for accurate measurements. The other data that does not require such quick responses were transffered with local area network (LAN) communications. Using the remote defect imaging system, we confirmed that defect images can be obtained clearly as the conventional cablewired experimental system was used. Moreover, we obtained defect images at the distances of 2.6 m and 7.6 m between the plate specimen and laser equipment.

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