Volume 4, Issue 2 (February 2007)
Fatigue Crack Growth Analyses of Aerospace Threaded Fasteners—Part II: Material/Stress State and Bolt Strength
Because fatigue failure in a threaded fastener can cause the loss of an aircraft, fatigue strength analyses are required. Understanding the material and stress state within the threads of the bolt was one objective of this research because this information would provide insight into the cyclic strength of the bolt. The second objective was to determine the fatigue strength in the threads of a nut-loaded, stainless steel, aerospace, roll-threaded bolt under tensile fatigue conditions. The resulting S-N curves can then be used to improve the accuracy of fatigue life estimations. X-ray diffraction was used to determine the residual stress in the thread root of the test bolts while light and scanning electron microscopy were used to image the thread root grain structure. 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 between 0.0<R<1.0. The cycles to failure (Nf) and corresponding stress (S) were recorded for 143 specimens.
The thread root, residual compressive stress reached 65 % of the bolt UTS. The residual stresses are the result of the cold-rolled thread forming and grain distortion/alignment. The S-Nf plots showed a 10 % reduction in allowable fatigue stress due to a flaw. The unflawed bolt S-Nf curves showed typical variation with loading ratio (R) and reductions in fatigue strength below that of the fatigue strength of the bolt material (unnotched).