Kirk W. Olsen

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

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.

Fatigue Crack Growth Analyses of Aerospace Threaded Fasteners—Part I: State-of-Practice Bolt Crack Growth Analyses Methods

To perform accurate crack growth analyses on roll-threaded fasteners, aerospace designers are required to estimate fatigue crack growth in the threads of a nut-loaded, roll-threaded bolt under tensile fatigue conditions. Threaded fasteners are difficult to analyze due to their asymmetrical geometry and fastener fatigue crack growth data are scant, especially for nondimensionalized crack depths of (a/d)

Fatigue Crack Growth Analyses of Aerospace Threaded Fasteners—Part IV: Numeric Analyses and Synthesis of All Results

The objective of this research was to determine the stress intensity multiplication factor (Y), as a function of the nondimensionalized crack depth (a/d), in the threads of a nut-loaded, aerospace, roll-threaded bolt under tensile fatigue conditions as a/d approaches zero. The proposed Y(a/d) solution can be used to improve fatigue crack growth life estimations. The research objectives were achieved through bolt material/stress state characterization, cyclic testing, and numeric modeling. The fracture analysis code FRANC3D was used because it could predict crack front shape and stress intensity factor (K). The numeric models predicted a changing crack front and stress intensity factors similar to the test data. The numeric studies accounted for the residual stresses measured by X-ray diffraction within the thread root of the test bolts. The FRANC3D model predicted Y(a/d) at shallow crack depths in the range of 0.01