Holographic Inspection of Plates Containing Areas of Localized Thickness Variation

Circular plates, under unknown clamping conditions and containing simulated defects in the form of circular localized thinning or thickening, are inspected by double-exposure holography. With an incremental uniform pressure applied between exposures, eccentric defects are readily revealed from the distinct irregular fringe patterns. In the case of central circular defects, however, the absence of distinct irregular fringe patterns does not enable easy visual detection of the defects. The simple method of analysis described in this paper, based on the fact that the displacement in a defective plate differs from that in a defect-free plate, allows easy deduction of central and eccentric defects from the fringe patterns. Furthermore, this method enables identification of the type of defect (localized thinning or thickening), the extent of thickness variation, as well as an accurate estimation of the location and size of the defect.