A time reversed guided wave method for bolt loosening detection of aerospace thermal protection system panels

Thermal protection systems (TPS) plays a critical role to ensure the aerospace vehicle safety and structural integrity. In order to detect loosening failure in bolted joints of TPS at it initial stage, this paper developed a novel modified time reversed guided wave method. For the modified time reversal (MTR) method, the response signal due to a tone burst input receives at the healthy structure is time reversed and saves as referenced reemitting signal (RRS). The peak amplitude of the focused wave packet in the reconstructed signal is calculated and utilized as tightness index. By bonding PZT sensors at different locations of the structure, a tightness index vector TI can be defined by gathering tightness indices at each sensor in sequence. Each component of TI represents the correlation coefficient of the corresponding local structure to its healthy condition. Experiments at a simulated TPS panel are conducted to verify the feasibility of the MTR method for bolt loosening identification. Different bolt loosening cases are considered and repeating experiments have been done. Experiment results illustrates that the proposed MTR method can be used to identify and localize the bolt loosening in a simulated TPS panel. By analyzing the damage index vector TI, the loosening bracket can be localized.