Shearographic and Holographic Assessment of Defective Laminates With Bond-Lines of Different Elasticities

While defects in laminated plates are readily revealed using double-exposure shearography and holography, when an incremental vacuum pressure is applied between the exposures, the apparent defect size, measured from the boundary of the perturbed fringes, may over-estimate the actual defect size depending on the elasticity of the bond-line. In this paper, the perturbed fringes were described by a mathematical model derived from the theory of thin plates supported by a Winkler type elastic foundation which takes into consideration the elasticity of the bond-line. An iterative technique is subsequently presented for estimating the size and depth of the defect from the fringe pattern without the need to pre-determine the current elasticity of the bond-line. Laminates with a thick and highly elastic bond-line, found in foam-adhesive bonded metal laminates, and those with a thin and brittle bond-line, found in multi-layered glassflbre reinforced plastic plates, were tested. Results have shown that the defects in these laminates were accurately assessed.