Application of a Local Stress Based Approach in Aluminum Aerospace Joint Design

Fatigue performance characterizations using current structural analysis techniques are shown to be the corner stone in improving stress-life predictions and optimizing inspection requirements. Example calculations are presented to illustrate how an enhanced test-analysis interface using a more physics based local criterion can improve fatigue calculation accuracy. Additionally, current computer based analysis methods are used for improved extrapolation of test results and expanded test data applicability. Analysis verification test are presented on a frame splice representing a fairly common rotorcraft airframe configuration. The test articles were built-up from 7075-T73 aluminum machined components and 7075- T6 aluminum sheet metal details. Cracking was naturally generated under the constant amplitude loading. Many current fatigue methods require the determination of a stress concentration factor (Kt) and ultimately a fatigue reduction factor (Kf) with fatigue predictions subsequently determined using global stress-fields. A typical airframe configuration is reviewed and redesigned using a local criteria for both weight and life optimization. A nonstandard geometric configuration is also reviewed as an example redesign to demonstrate the ability of a local based criterion to extrapolate test data. The redesign illustrates how locally based calculations can create a more robust and improved understanding of fatigue behavior through better analysis correlation with test data.