CREATION OF MODAL-BASED DAMAGE DETECTION METRICS FOR A THERMAL PROTECTION SYSTEM

Damage detection in aircraft is critical for improving the safety and reliability of a vehicle while reducing downtime and maintenance costs. Automating the damage detection process with structural health monitoring techniques can allow damage detection to be performed accurately and efficiently. Various structural health monitoring techniques, such as statistical pattern recognition and the electro-mechanical impedance method, have been used to detect various types of damage, but they require either extensive experimental testing to train a statistical algorithm and/or high-frequency sensors to detect structural damage. This research utilizes a methodology that does not require extensive experimentation or high-frequency sensors to quantify the failure of fasteners connecting composite thermal protection system plates to the backing structure of a hypersonic flight vehicle. The methodology for detecting and localizing fastener failure damage begins with the formation of a finite element model to simulate the physics of a structure. Experimental results are then collected for the low-frequency dynamics of the structure in its healthy state to validate the finite element model. The validated finite element model is then used to simulate many damage states in significantly less time than would be required experimentally. The healthy and damaged mode shapes obtained with the validated finite element model are used to calculate existing and created modal metrics. The existing modal metrics presented in this paper are the modal assurance criterion (MAC) and the coordinate modal assurance criterion (COMAC). The created metrics for damage localization are the normalized coordinate modal assurance criterion (NOCOMAC) and the normalized coordinate modal assurance criterion summation (NOCOMACSUM). While the existing modal metrics showed some damage detection and localization capabilities, the created damage metrics demonstrated significant improvement in localizing the damage to the correct portion of the structure compared to the existing modal metrics.