Use of Small Crack Data to Bring about and Quantify Improvements to Aircraft Structural Integrity.
Final rept. Jan 81-Oct 82,
AIR FORCE WRIGHT AERONAUTICAL LABS WRIGHT-PATTERSON AFB OH
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Crack growth information has been used in ways to quantitatively evaluate and predict damage tolerance and slow crack growth life limits of structures. Recent advances in the area of crack growth at small crack sizes less than one millimeter have enabled increasingly quantitative studies into the specific mechanisms that affect initiation and growth at structural details. As an example, through the use of small crack data the USAFGeneral Dynamics study on Fastener Hole Quality was able to identify a manufacturing-related problem causing short structural lives, propose a modification to shop equipment, and quickly and specifically evaluate the resultant flaw growth improvement. The purpose of this paper is to describe the general procedures used in the derivation of small crack data and to present growth data for different structural manufacturing methods. The data will be presented in terms of equivalent initial flaw size populations, crack growth rate, and initiation life to a specific length for fractographically measured cracks within the range of .01 to 1 millimeter in length. Procedures will be discussed to utilize the small crack data from developing and verifying changes in fastener systems and manufacturing methods for improving the fatigue performance of aircraft structures.