A Review of Recent Developments in the Holographic Observation of Disbonds

Holographic techniques have been employed to observe nondestructively the presence of disbonds or bond degradation in adhesives of thin and thick laminates. It is shown that when panels of honeycomb aircraft skin are exposed to prolonged humidity, the time history of bond weakening can be holographically observed as the humidity attack proceeds inward from the edges. While heat stressing is suitable to the holographic observation of disbonds in “thin” laminates, it generally fails in thick structures. A second technique of exciting resonance patterns of disbonds and observing them with time-averaged holography is suitable for use with thicker materials. Various mode patterns of disbonds have been observed in “thick” laminates of the ceramic-fiberglass epoxy composites considered for use in applications such as missile radomes.