Validation Testing and Analysis of Cracked-Hole Continuing Damage Solutions

Abstract

Abstract In performing damage tolerance analysis (DTA) for aircraft structures, there are instances in which it is necessary to determine not only the life of a crack from a hole to the edge of a part, but the growth of a secondary crack on the opposite side of the hole to failure. These types of fracture mechanics models have recently been added to the commercially available crack growth software programs used for DTA. Southwest Research Institute and the United States Air Force performed testing designed to simulate continuing damage scenarios in which a crack from a hole has failed the primary ligament and a secondary crack is growing from the opposite side of the hole. This testing included constant amplitude testing of two geometries for fracture mechanics model validation and spectrum testing at four combinations of material, geometry, and stress level for validation of retardation parameters. Test results were then compared analytically to stress intensity values calculated using AFGROW, NASGRO, DSTO-developed equations, and the p-version finite element software StressCheck. Because of the nature of the cracking, there is significant in-plane bending, and the different methods available to model the boundary conditions played a large role in the accuracy of the results. When boundary conditions were applied in a manner that closely modeled the testing, stress intensities matched well except near the transition from corner crack to through crack. Deviations from test results in this area and in spectrum test results seem to be caused by differences in crack aspect ratio compared to what is expected analytically. Further investigation is recommended—and planned—to determine the reasons for differences in crack aspect ratio across geometries and loading.