Evaluation of Surface Defects Using Surface Acoustic Waves Generated by Phase Velocity Scanning of Laser Interference Fringes

In order to maintain the reliability of structural materials such as ceramics, it is important to nondestructively detect surface defects that reduce the strength of materials. Therefore, we have developed the phase velocity scanning (PVS) method. The PVS method does not involve the use of coupler (noncontact) and has excellent directivity. We report that the sensitivity for detecting surface acoustic waves (SAW) was improved using a video signal. The video signal S/N ratio, which is defined as the ratio of generated SAW to background noise, is 46.5 dB. Back propagation waves were not detected within the limit of the S/N ratio. Utilizing this excellent directivity we succeeded in B-mode imaging to detect indentation cracks (Knoop indentation cracks). Scattered waves were detected on the indentations of above 100 gf but could not be detected for those of below 50 gf. This result implies that this method can be used to evaluate the critical load of initiating median cracks which is difficult using conventional methods.