The Stress State Identification of Critical Steel Bridge Components Using Nonlinear Acoustics

Increasing demand in transportation and variations in structures due to renovations may increase the contribution of dead load stress, and as a consequence, unexpected failure. While bridges are regularly inspected or continuously monitored using strain gauges and accelerometers, there is no nondestructive evaluation method available to measure the actual stress-state of structurally critical bridge components. The proposed concept is to use nonlinear acoustoelastic theory in order to detect the actual load and stress state on gusset plates. Acoustoelasticity is the dependence of ultrasonic wave speed and polarization on stress. In this research a small ultrasonic perturbation is introduced to the structure through an ultrasonic transducer. Numerical studies were performed in the time domain and frequency domain to identify critical device design parameters. In the time domain study, a minimum wave frequency was determined based on the common gusset plate thicknesses. With a set perturbation frequency, a stressed study was performed in the frequency domain to confirm the expected frequency shift. A paired wedge device was constructed to easily attach to the surface of an exposed Gusset Plate. Upon initial testing, ultrasonic coupling and surface repeatability issues were identified. Methods to mitigate the large effects of varying surface coupling conditions were developed and implemented. The relationships between applied stress and peak perturbation frequency shift were explored. Three critical areas of testing were performed: the uniaxial case, the biaxial parallel loading case and the biaxial angled loading case. Satisfactory linear or bilinear relationships were found for all cases, except the biaxial angled loading case. A field test was conducted on a fracture critical Pratt Truss bridge gusset plate in Chicago. The results of the field test when compared with a numerical model confirm the frequency shift with increased loading stress. The accuracy of the field test is affected from suspected shear influence factors in the biaxial angled loading case.