Detection of open and partially closed surface defects in plates using ultrasonic enhancement

Enhancement of the frequency content of multimodal Lamb waves incident on surface breaking defects has been investigated for use in defect detection and characterization, for different thickness plates with machined defects of varying depth. A fully optical method, using laser ultrasonics, was used to study the enhancement as a function of defect depth when passing either the detection (SLD) or generation (SLS) laser over the defect. Time frequency representations (TFRs) were used to identify the wave modes present at each detection point, and the frequency magnitude of various wave modes was tracked, with an increase in magnitude observed for both scanning laser detection and scanning laser source at the defect. The scanning laser detection enhancement is explained by a superposition of incident waves with those reflected and mode converted from the defect. Scanning laser source enhancement can be explained in terms of several mechanisms; including truncation of the source, the change in geometry at the defect and the change in generation boundary conditions at the defect. The TFR analysis was also applied to samples containing partially closed defects caused by stress corrosion cracking (SCC). Scanning laser detection analysis was shown to be incapable of detecting the defects; however, scanning laser source analysis showed large enhancements over the defect site, illustrating the effectiveness of this method for detection of partially closed cracks.