Localization and Characterization of Fatigue Cracks Around Fastener Holes Using Spherically Focused Ultrasonic Probes

Results are presented from laboratory experiments and simulations designed to determine the ability to localize and characterize fatigue cracks around fastener holes using spherically fo-cused 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 mod-eled. 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. This work is supported by the U.S. Air Force Research Laboratory (AFRL) through Research Initiatives for Materials State Sensing (RIMSS) contract with Universal Technologies Corp., Contract No: FA8650-10-D-5210

Localization and Characterization of Fatigue Cracks Around Fastener Holes Using Spherically Focused Ultrasonic Probes

Results are presented from laboratory experiments and simulations designed to determine the ability to localize and characterize fatigue cracks around fastener holes using spherically fo-cused 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 mod-eled. 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. This work is supported by the U.S. Air Force Research Laboratory (AFRL) through Research Initiatives for Materials State Sensing (RIMSS) contract with Universal Technologies Corp., Contract No: FA8650-10-D-5210.</p