Health Monitoring of a Thermal Protection System using Lamb Waves

The applicability of guided waves as a structural health monitoring (SHM) tool to predict the point of impact and detect delamination in a thermal protection system (TPS) is studied. A model TPS was designed by bonding ceramic porous tiles to a 2.2 mm thick 2124-T351 aluminum alloy plate. The delamination defect may be caused by the impact phenomenon or due to other reasons such as manufacturing defect, thermal, or mechanical fatigue, etc. Impact phenomenon is simulated by dropping a ping pong ball on the tiled structure. The delamination at the interface between the ceramic tile and the aluminum plate is simulated by removing the adhesive bond at the selected interface regions during the specimen fabrication process. The conventional triangulation technique cannot predict the point of impact in a tiled plate structure because the triangulation technique assumes that the wave speed is independent of the direction of propagation which is not the case for ceramic tile mounted plates. An alternative method based on the optimization scheme was proposed by Kundu et al. [1] to locate the point of impact in plates by analyzing the time of arrival of the ultrasonic signals received by passive sensors attached to the plate. This objective function based method is used to locate the impact point on the tiled plate. Experiments are carried out by dropping a ping pong ball on the tiled structure and recording acoustic signals by passive transducers adhesively bonded to the plate at three different locations. The impact point is predicted and compared with the actual location of impact. Delamination detection studies were conducted in the pitch-catch mode, by changing the angle of strike and the frequency of the transducer excitation to generate the appropriate guided wave mode. The delamination defect could be detected and the impact point could be identified underlining the importance of the use of guided waves as an SHM tool for TPS.