Identification of the Location and Level of Loosening in a Multi-bolt Structure using Nonlinear Ultrasound

This work evaluated a nonlinear method for in-situ structural health monitoring (SHM) of the loosened state of a multi-bolted structure. The aim was to assess a bolted structure with an array of piezoelectric transducers and determine which bolt had loosened and the extent of loosening by evaluating nonlinear ultrasound responses. Linear methods generally focus on traditional methods that evaluated the change of the damping curve and signal energy as the torque on the bolts of a structure changes. Whereas the nonlinear methods used evaluated the second harmonic and modulated responses using a Nonlinear Acoustic Moments (NAM) method coupled with a frequency selective process based on the cross correlation of the nonlinear responses at various loosened bolt states. The ultrasound excitation method relied on modulation between a continuous single frequency signal and pump sweep signal (multi-frequency), which generated a broad band nonlinear response. By exciting over a broad range of frequencies, probability of bolt specific frequency excitation and thus determination was improved. The nonlinear techniques showed good ability to determine the bolt specific frequency for each bolt and the extent of loosening.