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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“The p-type element has been used to great advantage in the finite element system ESRD StressCheck®, . This software provides the engineer with the means to conduct solution verification in an extremely straightforward manner by simply increasing the degree of the element, monitoring convergence and using Richardson extrapolation reliably to estimate the error. This can be conducted automatically by the software thereby enabling the engineer to concentrate on the engineering rather than the simulation. StressCheck® has also been used to develop ESRD’s Handbook and Toolbox applications. The first of these provides engineers with a repository of parameterised standard problems of the type found in texts like Roark’s “Formulas for Stress and Strain”, . The second, Toolbox, is a tool that can be used to parameterise a company’s range of components for rapid and reliable analysis by non-expert analysis. Toolbox then is an exemplary of the way in which the democratisation of simulation can be applied.”
Angus Ramsay, PhDEngineering Director, Ramsay Maunder Associates