A VIBRATION TECHNIQUE FOR SPACE-SHUTTLE TILES BOND ASSESSMENT .1. MODELING

This paper presents a novel technique that combines laser-based vibrat ion measurement and finite element modeling to evaluate the bond condi tion of the thermal protection system tiles of the Space Shuttle orbit er. This technique is based on characterizing the response of tiles wh en excited by audible acoustic energy. Finite element models for tile assemblies attached to the orbiter aluminum skin are first developed. The tiles' eigenvalues and eigenvectors are determined. Then frequency responses of tiles excited by simulated sound pressure are computed. The computed frequency response at selected points of a partially bond ed tile indicates a decrease in its natural frequencies. This is used to quickly identify the disbonded tiles. However, the exact size and l ocation of the disbond are determined from the computed rigid-body mod es of the tile. A companion paper presents the experimental results of test panels that are obtained by a laser rapid scan system and shows excellent agreement between the finite element and experimental result s. These results demonstrate that experimental modal analysis, when co mbined with finite element analysis, can successfully be used as a rel iable nondestructive, non-contact technique for tile bond assessment.