Investigation of Bolt Loosening in Super-alloy Thermal Protection System Panel using High-speed Digital Image Correlation

The Thermal protection system (TPS) panel was designed and fabricated to protect the hypersonic vehicle interiors from the extreme conditions, such as high heat flux, vibrations, and acoustics that can affect the vehicle structure and its payloads. One problem with the TPS is the maintenance cost and inspection time required to ensure that it is in good condition before the next flight. This study examines the method to investigate the damage state of a super-alloy TPS panel through the modal analysis. The experiment is implemented in three conditions to account for good condition, insulation effect, and bolted-joint loosening. In the experiment, the TPS panel was suspended vertically by two fine-nylon cord. The TPS panel was excited by applying an impulse force with an impact hammer in the dynamic testing. Two high-speed cameras based on digital image correlation (DIC) technique were used to record the response of the TPS panel. The response in the time domain was transferred to the frequency domain to get the frequency response function. After filtering and averaging data, the natural frequencies were determined by the peaks of the frequency response function. The first mode of the healthy TPS panel was at 192 Hz while it was 172 Hz for the damaged state at one corner of the TPS panel. Based on changes in natural frequencies and mode shapes due to the TPS failure, the bolt loosening could be detected. As a result, the damage state such as a loose fastener reduces the stiffness of the TPS panel which results in the natural frequencies degradation. The method demonstrated the advantages of the DIC method in measuring the full-field and non-contact deformation of the aerospace structure.