Experimentally derived beta corrections to predict fatigue crack growth at cold expanded holes in 7075-T651 aluminum alloy

Fastener holes represent one of 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. Cold expansion imposes a residual compressive stress field around a hole that retards fatigue crack growth from the volume of cold working and increases the fatigue life of the component. According to the current United States Air Force's guiding documents state that a damage tolerance analysis can take advantage of cold expansion by reducing the initial flaw size to a minimum of a 0.005 inch corner crack. A more physically based approach was investigated to take analytical advantage of the cold expansion process. The author explored, through experimental fatigue testing, the development of an empirical correction factor j8 for the effects of cold expansion in 2024-T351 aluminum alloy. This method takes into account the interaction of the fatigue crack and the residual stress field found in the body. Many common current methods are unable to account for this critical interaction. The (3 correction factor can be used in fatigue crack growth and damage tolerance analysis and can be shown to provide a more accurate model of the crack growth behavior at cold expanded holes.