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This paper describes an investigation to define statistical fatigue in terms of the initial quality of fastener holes. The objective is to develop and verify equivalent initial flaw size (EIFS) concepts for fatigue durability analysis of typical production quality structures.
A special test procedure was used to obtain a statistically large sample of data under spectrum loading by testing a specimen containing 24 holes. It is possible to obtain a maximum of 96 fatigue data sets in the test of two specimens up to 100 000 flights in a reasonable time. The method involved removing the early cracked holes with a circular saw cutter and continuing the testing of the remaining holes. Marker cycles, which were designed to be an integral part of the spectrum, have served the intended purpose of identifying crack origins and microcrack growth rates using scanning electron microscope (SEM) fractography. The surface mechanical and material defects responsible for the origin of microcracks were examined quite fully.
Through quantitative analysis of a large amount of microcrack growth data and surface macrocrack growth data, many parameters affecting apparent fatigue scatter were resolved. A method was developed to define statistical fatigue quality of fastener holes in terms of EIFS distribution. The EIFS should be a function of initial surface quality and independent of other secondary variables that affect crack growth rate. The procedure involves the use of a master crack growth curve developed from the experimental data of microcrack growth ranging from initial physical flaws as small as 0.005 mm (0.0002 in.). The essential physical defects in 2024-T3 as origins of microfatigue cracking are the tool marks and inclusions.
statistical fatigue, fractography, flaws, equivalent initial flaw size, spectrum testing, crack growth, 2024-T3, fastener holes
Senior Engineer, Structural Mechanics, Douglas Aircraft Company, McDonnell Douglas Corporation, Long Beach, Calif.