The use of carrier fringes in shearography for locating and sizing debonds in GRP plates

Abstract

Debonds in laminated plates when subjected to vacuum stressing are readily revealed using conventional shearography. In estimating the depth of these debonds from the perturbed fringes, difficulty often arises in assigning accurately the maximum fringe order which corre-sponds to the maximum deflection gradient of the surface of the debonded region. By superposing a set of linear reference carrier fringe pattern onto a conventional shearogram, it is shown in this paper that debonds embedded in a glassfibre-reinforced plastic (GRP) plate may be detected easily from the resulting perturbed fringe pattern. The size and depth of debonds may also be estimated with good accuracy, whether the perturbed carrier fringes are overdriven or non-overdriven. Analysis also indicates that it is generally desirable to use a dense set of reference carrier fringe patterns, which results in a non-overdriven fringe pattern, because it enables a more accurate estimation of the maximum fringe order; the accuracy in the determination of the debond depth is thereby improved. © 1991.