Modal frequency versus shearography in detecting and locating voids/delaminations in sandwich composites

This paper reports on the attempt by the authors to establish a novel non-destructive testing protocol in detecting the presence and location of subsurface defects, such as voids or delaminations, in sandwich composites. The work performed was based on the premise that the presence of a defect in a composite structure would affect its modal frequencies when subjected to vibratory excitation. Experiments were carried out using various types of sandwich composites fashioned as built-in cantilever beams, which were perturbed by airborne vibrations. As predicted by a parametric finite element analysis, the modal frequencies of the cantilever were modestly affected by the presence of a defect; however, its position along the length of the beam could not easily be determined. For comparison purposes, the same cantilever specimens were subjected to thermal excitation and inspected using a portable digital shearography system. As expected, the presence and location of the defects were readily and vividly identified.