The lead crack fatigue lifing framework

Abstract

A fatigue lifing framework using a lead crack concept has been developed by the DSTO for metallic primary airframe components. The framework is based on years of detailed inspection and analysis of fatigue cracks in many specimens and airframe components, and is an important additional tool for determining aircraft component fatigue lives in the Royal Australian Air Force (RAAF) fleet. Like the original Damage Tolerance (DT) concept developed by the United States Air Force (USAF), this framework assumes that fatigue cracking begins as soon as an aircraft enters service. However, there are major and fundamental differences. Instead of assuming initial crack sizes and deriving early crack growth behaviour from back-extrapolation of growth data for long cracks, the DSTO framework uses data for real cracks growing from small discontinuities inherent to the material and the production of the component. Furthermore, these data, particularly for lead cracks, are characterized by exponential crack growth behaviour. Because of this common characteristic, the DSTO framework can use lead crack growth data to provide reasonable (i.e. not overly conservative) lower-bound estimates of typical crack growth lives of components, starting from small natural discontinuities and continuing up to crack sizes (thus encompassing short-to-long crack growth) that just meet the residual strength requirements. Scatter factors based on engineering judgement are then applied to these estimates to determine the maximum allowable service life (safe life limit). The aim of the paper is to present the framework of assumptions and observations used in conjunction with a unique measure of the initiating discontinuity and a simple crack growth law to predict a lower bound fatigue life estimate.