Detection of Skin Disbond in Honeycombs and Coating Detachment by a Laser Acoustic Technique

Many engineering structures are composite and include for example a protective coating or a bonded layer. A novel technique, close to laser‐ultrasonics but significantly different, has been developed for the detection of disbonds between the coating or the bonded layer and the substrate. It is also applicable to the detection of core unbonds in honeycomb structures. The technique is based on the thermoelastic excitation by a pulsed laser of the top layer or top skin which is driven into vibration if it is detached from the substrate underneath. This vibration is then detected by a second laser coupled to a photorefractive interferometer. The technique can be made very flexible by using optical fiber coupling. One foresees its application to the in‐service inspection of aerospace structures for the detection of core unbonds in honeycombs or near surface delaminations. Examples of application to honeycombs and to various coatings are presented.