Results are presented from laboratory experiments and simulations designed to determine the ability to localize and characterize fatigue cracks around fastener holes using spherically focused ultrasonic (UT) probes for shear-wave inspections. In designing and evaluating inspection protocols, the number of cases that can be studied through laboratory experiments is severely limited by cost and time constraints. Simulations therefore stand to play a significant role in the design and optimization of inspection strategies for those conditions that can be accurately modeled. Moving from benchmark studies for relatively simple geometries toward more realistic conditions creates significant challenges. For shear-wave inspections of fastener holes these challenges include the complex energy field in the thin plates, reflections off the borehole, the complexity of making measurements in the near-field, material anisotropy, cracks as small as 1mm square, and a sealant layer between aluminum sheets. To achieve comparable modeling and simulation data requires a very accurate experimental setup that allows the probe angle, probe height and scan path to be precisely set. For the modeling, care must be taken to match the applied gain and gates used during acquisition of the experimental data. Initial results presented include sensitivity studies to determine how probe variables (frequency, focal depth, diameter), crack variables (size, shape, location, angle with respect to the probe), and the experimental setup affect results. Simulated and experimental C-scan images for 5 and 10 MHz probes are shown in Figure 1 for a fatigue crack that intersects the back wall. Comparison of CIVA UT simulations and experimental measurements for the fatigue crack shown in the photograph in the top row. The picture is followed by three views of the borehole and crack in the CIVA model. The C-scan simulation results (green background) and corresponding experimental results (blue background) are shown in the bottom row for 5 MHz and 10 MHz shear-wave scans (45°) for probes with a focal depth of 50mm and a diameter of 12.7mm. The red boxes indicate reflections off the borehole, which was filled with water for the experiments.
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