Integrating Residual Stress Analysis of Critical Fastener Holes into USAF Depot Maintenance
Abstract: The objective of this Rapid Innovation Fund (RIF) program is to integrate finite element simulations of residual stress fields, physically based life prediction methods, and inspections for cold worked (CX) fastener holes into depot maintenance practices. For decades, the aircraft industry has used cold working of fastener holes to enhance the service life (fatigue performance) of critical, highly stressed structure. Although the structure directly benefits and is much less prone to failure, current requirements for structural inspections are very stringent. This is because the current analysis methods used to set inspection intervals do not take into account the beneficial residual stress fields that result from the cold work process. The efforts under this program have shown that the United States Air Force (USAF) can increase inspection intervals by as much as five times if the physically based residual stress fields are incorporated into analysis procedures. This program has successfully integrated a series of tools to accomplish the objective for A-10 application and capture the Return-On-Investment afforded by the implementation.
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“Accurate and reliable stresses and Stress Intensity Factors are required for determination of static and residual strength and for crack growth analyses in analysis tools such as AFGROW. For some geometries, industry solutions are either insufficient or nonexistent. The geometry, applied forces, and crack shapes and dimensions must be modeled reasonably well to obtain useful engineering data. The p-version finite element software StressCheck® (ESRD, Inc., St. Louis, Missouri, USA) is used to demonstrate how accurate finite element solutions can lead to good quality engineering analysis.”