Holographic assessment of edge delamination on foam-adhesive bonded joints

Failure of laminated structures is frequently triggered by delamination at the adherend/adhesive interface and along the edge of bonding where high peeling stresses are present, hence the need to monitor the propagation of edge delamination during service. In this paper, the technique of whole-field, non-contacting, double-exposure holography is applied to the detection and assessment of artificially created edge delamination in a cantilever bar whose end is bonded to a rigid support with foam adhesive. Unlike the normal holographic interferograms which show distinct fringe perturbation at the defective regions, the lack of conspicuous fringe perturbation at the delaminated edge, attributed to the highly resilient behaviour of the foam, causes difficulty in detecting the defect. Through the use of two dimensionless parameters determined from the fringe pattern, a simple iterative method, based on the theory of beams on an elastic foundation, is proposed which readily identifies first the presence and then estimates the length of edge delamination. Although this investigation deals only with the case of a cantilever bar, the technique presented should be extensible to the study of interfacial edge delaminations in laminated composites.