ADVANCED IMAGE-PROCESSING TECHNIQUES FOR AUTOMATIC NONDESTRUCTIVE EVALUATION OF ADHESIVELY-BONDED STRUCTURES USING SPECKLE INTERFEROMETRY

In conventional optical nondestructive evaluation (NDE) of structures using shearography or electronic speckle pattern interferometry (ESPI) , results are typically provided in the form of fringe patterns or def ormation contour plots. However, in order to fully automate the proces s of defect detection, it is desirable to obtain simpler results which are easier to interpret. We present here one such optical system base d on additive-subtractive shearography/ESPI. This system processes add itive-subtractive fringe patterns and provides the sizes and locations of defects such as disbonds in adhesively-bonded composite structures . This is achieved by exciting the structure under inspection using an acoustic stressing mechanism which sweeps a range of vibration freque ncies of the structure. Since the defective areas of the structure hav e different mechanical properties from their neighboring regions, vary ing and complex fringe patterns are obtained at different stressing fr equencies. We propose an algorithm which enables the automatic identif ication and selection of relevant additive-subtractive fringe patterns that pertain only to localized deformations associated with defects, and which excludes images that pertain to any overall modes of the ent ire structure. The algorithm also includes a pixel-by-pixel adjustable thresholding scheme which compensates for intensity variations due to nonuniform reflectivity from unpainted and dirty test objects. Morpho logical processing is then performed to extract the shapes of the defe ct from the processed fringe clusters. Various structures, from simple aluminum specimens with simulated defects to a complex honeycomb-base d aviation repair patch specimen, have been successfully evaluated usi ng this system.