Ultrasonic Monitoring of Fastener Holes Using Load Modulated Energy Algorithms for Early Detection of Fatigue Cracks

Ultrasonic techniques have the potential to provide early detection of fatigue induced cracks originating from fastener holes with the long term goal of structural health monitoring. This paper considers energy-based algorithms for analyzing ultrasonic waveforms obtained using fixed angle beam transducers in a through-transmission configuration. The transducers were located on either side of the fastener hole with the sound beam oriented along the loading direction, and waveforms were obtained as a function of applied load. Previous work has shown that the ratio of the received ultrasonic energy at under tensile loading to the energy with the sample under no load is a robust method for determining the presence of cracks. The reason for the effectiveness of this metric is that the energy reduction from the crack opening is accentuated by the applied load. For structural health monitoring applications, the earliest possible detection of cracking is desired, and the total energy ratio method does not utilize information contained in different time and frequency windows of the received signals. Considered here are energy ratio parameters derived from multiple time and frequency windows in order to optimize sensitivity of detection to the widest range of crack sizes and geometries. An algorithm was developed to automate crack detection and was applied to the various energy ratio parameters. Results indicate that cracking can be detected significantly earlier in the fatiguing process compared to the total energy ratio method.