Application of the holographic non- destructive testing method to the evaluation of disbondings in sandwich plates

Abstract Disbondings between core and facesheets of sandwich structures can cause severe reductions in their load-bearing capacity and, for reliability and safety, this necessitates the introduction of adequate methods of control. By using holographic interferometry, one of the non-destructive testing methods which could be used in these cases, even the smallest subsurface anomalies in sandwich panels can be made visible under stress, showing them as local irregularities in a fundamental fringe pattern. Good results for fault detection were obtained for specimens which has been stressed or deformed by the application of a partial vacuum to the outside of the specimen, even when the vacuum had been applied prior to — and not during — the holographic measuring procedure. By using this holographic method for testing industrially manufactured sandwich panels — consisting of rigid synthetic foam cores and skins of either glass fibre-reinforced plastics or steel sheets — its high accuracy for detecting even small disbonds, found in the laboratory stage of evaluation, could be verified. The results obtained offer some interesting aspects for future application in quality control and in-service testing of sandwich materials.