The Use of Holographic Phase Shifting in Assessing Unbonds in GRP Plates

During holographic inspection of glass fiber reinforcesd plastic (GRP) laminated plates, perturbation in the fringe patterns not only reveals the presence of an unbond but also enables estimation of its depth. The accuracy of estimation depends, among other factors, on the accuracy in determining the fringe orders within the boundary of perturbation. This study shows that phase shifting using a single reference beam enables determination of fractional fringe orders at any point on the test surface. When applied to the assessment of the detected unbond, this simple technique gives a more accurate estimation of the unbond depth than conventional holography.