A NEW TECHNIQUE USING DIGITAL SPECKLE CORRELATION FOR NONDESTRUCTIVE TESTING OF CORROSION

When the speckle pattern (laser or white light) of a corroded specimen surface is compared to that of a virgin surface, the difference cart be quickly and clearly revealed by the cross-correlation coefficient o f the two speckle patterns. This technique can reveal pitting corrosio n areas as small as 5 x 5 mm (0.2 x 0.2 in.) wherein exists a cluster of pits of 0.3 min (0.012 in.) in diameter and 0.1 mm (0.004 in.) in d epth. An artificial stress corrosion crack teas used to demonstrate th e validation of this method for detection of stress corrosion cracking . For detection of crevice corrosion, laser illumination must be appli ed. Digital speckle correlation takes the advantage of laser speckle p roperties. The speckle decorrelation caused by out-of-plane displaceme nt in the region with a crevice corrosion under the specimen's surface is bigger than that without defects. This different is calculated by a digital correlation algorithm, which reveals the crevice corrosion.