A Rectangular-Region, Speckle Correlation Approach for the Measurement of Interference Levels of Installed Aircraft Rivets

Experiments were conducted to determine the effectiveness of digital imaging, speckle correlation techniques for the measurement of rivet upsets. Rivets were installed in test coupons with various interference stress levels typical of current aircraft manufacturing practices. The coupons were digitally photographed before and after the installation of the rivets. These digital images were then compared using a rectangular-region correlation algorithm to determine the surface displacements caused by the rivet installation. The algorithm divides the two images into rectangular segments and then determines how far the segment of the after, or deformed, image has been displaced from the before, or undeformed, image. From this displacement data it is possible to determine the state of strain around the rivet installation, and thus the quality of the rivet installation. Experiments have shown that the algorithm is capable of determining the whole field displacement data quickly and with acceptable accuracy.