Using a Beta (β) Correction to Improve the Life Predictions of Cold-Expanded Holes in 2024-T3 & 7075-T6 Aluminum Alloys

Abstract

Fastener holes represent the most common fatigue details found in airframe structures. In order to minimize the impact fastener holes have on the fatigue behavior of critical aircraft components, many are processed by cold expansion (Cx). Cx imposes a residual compressive stress field around a hole that retards fatigue crack growth (FCG) and increases the fatigue life of the component. In order to take analytical advantage of the Cx process, a “one size fits all” method of reducing the initial flaw size to a minimum of a 0.005 in. (0.127 mm) symmetric corner crack is often employed. This approach leads to predictions that do not represent the physics of crack growth behavior within materials that have been processed using Cx. A physics-based approach was investigated to take analytical advantage of the Cx process. The authors developed an empirical-based factor (β) to characterize the effect of Cx on the FCG behavior within 2024-T351 and 7075-T651 aluminum alloys. This method takes into account the interaction of the fatigue crack and the residual stress field. This β can be used in FCG analysis to provide an accurate model of the crack growth behavior at Cx holes. In order to develop accurate da/dN versus ΔK plots for the Cx configuration, finite element modeling was used to calculate stress intensities along the crack front. By using the fracture-mechanics-based similitude principle, the nonCx and Cx da/dN versus ΔK data were compared and β corrections representing the residual stress field in the body were calculated.