The history of a holographic non-destructive test procedure

Holographic interferometry is being used to non-destructively test light-weight laminated panels for areas of de-lamination. In cross-section the panels consist of two outer aluminum plates and a thinner middle plate separated by corrugated aluminium material, the layers being brazed together at the interfaces. The purpose of the test is to establish the completeness of the braze. The panels are stressed by increasing the internal pressure; a weak area due to a mis-braze bulges slightly and is revealed as a closed pattern of fringes on the holographic interferogram. From measurements of the diameter and maximum deflection of a bulge as revealed by the hologram, it is possible to deduce at which interface the mis-braze has occurred. The paper describes the development of the test procedure, through the feasibility studies to the establishment of equipment for testing a production series of panels.