Whole-field NDT of thermal protection system using digital holography

Abstract

In the development of re-entry space vehicles, thermal protection systems (TPS), which protect the vehicle from the high thermal environments during re-entry, constitute one of the most critical systems. Both metallic and non-metallic materials are candidates for TPS. Generally, the TPS materials are bonded to the structure in vacuum under load. For the successful performance of the vehicle, the TPS should be intact to the vehicle without any debond and hence the detection of debonds in TPS is very important. Holographic interferometry is established as a reliable NDT tool for detecting debonds. Digital holography is a technique that uses digital recording and numerical reconstruction of holograms, which does not require photographic films, wet processing and laser reconstruction. In this paper the application of the digital holographic interferometry technique in detecting debonds in different types of thermal protection systems is presented. The results of the tests on a few samples made of ablative and non-ablative TPS materials with debond are presented. The paper also describes the experimental details and stressing techniques used for the detection of debonds in TPS. The practical application of this technique for the NDT of TPS is explored.