ESPI and Digital Speckle Correlation Applied to Inspection of Crevice Corrosion on Aging Aircraft

A recent advance in the nondestructive inspection of crevice corrosion is reported. Crevice corrosion is one of the most hazardous defects that threaten the integrity of aging aircraft. Two optical techniques, electronic speckle pattern interferometry (ESPI) and digital speckle correlation (DSC), are employed to reveal the existence of crevice corrosion, which is usually undetectable from outside by visual inspection. The ESPI is configured to measure out-of-plane displacement. Both thermal loading and vacuum loading are used. The anomaly of fringe patterns reveals the existence of crevice corrosion hidden between two metallic plates. Not only the location of defects, but also the approximate size can be estimated. The digital speckle correlation technique takes advantage of laser speckle properties. The speckle decorrelation due to out-of-plane displacement in the region with crevice corrosion is larger than the region without defects. This difference is calculated by a digital correlation algorithm, which reveals the crevice corrosion. These two techniques can be used alternatively for field inspection of aircraft. While digital speckle correlation offers a simple, rapid approach to qualitatively detect defects, ESPI can be used as a supplementary tool if a more sensitive and quantitative evaluation is required.