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

Model Benchmarking and Reference Signals for Angled-beam Shear Wave Ultrasonic NDE Inspections

NDE modeling and simulation are important tools to support the development and validation of enhanced localization and characterization techniques. Previously, important achievements were made by the USAF to address crack detection in aircraft structures using angled-beam shear wave inspection techniques. However, new work on model benchmarking is needed to move beyond detection and achieve reliable crack characterization. To achieve this goal, simulated studies are needed to verify that models can accurately represent all of the key variables with the inspection of multilayer structures with fastener sites and varying crack conditions. Often with model benchmark studies, the accuracy of the model is evaluated based on the change in response relative to a selected reference signal. During recent simulated and experimental studies, some challenges were discovered concerning the creation and/or selection of a reference signal in a plate with a vertical hole and crack. The focus of this paper is on key findings concerning model benchmarking using CIVA-UT for angled-beam shear wave inspections. The use of a side drilled hole (SDH) in a plate was found to be somewhat problematic as a reference signal for angled beam shear wave inspection. Previously, only a limited number of studies have looked at model benchmarking for angled beam shear wave inspections. Systematic studies were performed with varying SDH depth and size, and varying the ultrasonic probe frequency, focal depth, and probe height. Care must be taken in understanding the precise beam properties with these experiments. One issue is that there is some increased error with the simulation of angled shear wave beams, especially in the near-field. Even more significant, asymmetry in real probes and the inherent sensitivity of signals in the near-field to subtle test conditions were found to provide a greater challenge with achieving model agreement. Through these studies, conditions of good and poor agreement were observed. For some inspection conditions, the skip signal off of the far wall from the side drilled hole can provide a better reference than the direct reflected signal. All in all, these seemingly mundane studies were found to be important with providing guidance on reference signal selection for model benchmarking work on the inspection of fastener sites with cracks

Model Benchmarking and Reference Signals for Angled-beam Shear Wave Ultrasonic NDE Inspections

NDE modeling and simulation are important tools to support the development and validation of enhanced localization and characterization techniques. Previously, important achievements were made by the USAF to address crack detection in aircraft structures using angled-beam shear wave inspection techniques. However, new work on model benchmarking is needed to move beyond detection and achieve reliable crack characterization. To achieve this goal, simulated studies are needed to verify that models can accurately represent all of the key variables with the inspection of multilayer structures with fastener sites and varying crack conditions. Often with model benchmark studies, the accuracy of the model is evaluated based on the change in response relative to a selected reference signal. During recent simulated and experimental studies, some challenges were discovered concerning the creation and/or selection of a reference signal in a plate with a vertical hole and crack. The focus of this paper is on key findings concerning model benchmarking using CIVA-UT for angled-beam shear wave inspections. The use of a side drilled hole (SDH) in a plate was found to be somewhat problematic as a reference signal for angled beam shear wave inspection. Previously, only a limited number of studies have looked at model benchmarking for angled beam shear wave inspections. Systematic studies were performed with varying SDH depth and size, and varying the ultrasonic probe frequency, focal depth, and probe height. Care must be taken in understanding the precise beam properties with these experiments. One issue is that there is some increased error with the simulation of angled shear wave beams, especially in the near-field. Even more significant, asymmetry in real probes and the inherent sensitivity of signals in the near-field to subtle test conditions were found to provide a greater challenge with achieving model agreement. Through these studies, conditions of good and poor agreement were observed. For some inspection conditions, the skip signal off of the far wall from the side drilled hole can provide a better reference than the direct reflected signal. All in all, these seemingly mundane studies were found to be important with providing guidance on reference signal selection for model benchmarking work on the inspection of fastener sites with cracks.</p

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