ISSN: 1546-962X
CODEN: JAIOAD
Published Online: 1 March 2007
Page Count: 12

Fatigue Crack Growth Analyses of Aerospace Threaded Fasteners—Part III: Experimental Crack Growth Behavior

Olsen, Kirk W.
Senior Engineering Specialist, LORD Corporation, Erie, PA

Rimnac, Clare M.
Professor and Director, Musculoskeletal Mechanics and Materials Laboratories, Case Western Reserve University, Cleveland, OH

(Received 3 December 2004; accepted 14 November 2006)

Abstract

Because fatigue crack growth in a threaded fastener can cause the loss of an aircraft, damage tolerant analyses are required. Because designers commonly perform crack growth analyses on stainless steel fasteners with scant data or simplifying assumptions, or both, the objective of this research was to determine the stress intensity multiplication factor (Y) in the threads of a nut loaded, stainless steel, aerospace, roll-threaded bolt under tensile fatigue conditions as the nondimensionalized crack depth (a/d) approaches zero. Y(a/d) can then be used to improve the accuracy of fatigue crack growth life estimations. Unflawed and flawed aerospace bolts were fatigue tested at a maximum stress (S) ranging from the ultimate tensile strength (UTS) to the surface endurance limit of the test bolt and at several loading ratios of 0.1⩽R⩽0.9. Data collection from the fracture surfaces yielded crack front shape characterization and striation spacing. The shape of the crack front in the unflawed and flawed test bolts was different and both changed as the crack grew. Fifty-one striation spacing measurements ranging from 5.6×10⁻⁵ to 7.9×10⁻⁴ mm were found at crack depths of a/d>0.035. These striation measurements were directly related to crack growth rates so that stress intensity factor ranges (ΔK) and in turn Y(a/d) could be estimated.

Keywords:
aerospace bolts, fatigue crack growth, stress intensity multiplication factor, rolled threads, crack front shape

Paper ID: JAI100489
DOI: 10.1520/JAI100489
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