A fractographic study of fatigue crack growth from a cold expanded fastener hole at an engineering "crack initiation" scale

The effects of cold expansion on fatigue crack growth from fastener holes in aircraft structure are a well-studied problem, with many contributions focusing on both residual stresses and "long crack" experimental data. In this study, the authors have used quantitative fractography techniques to measure fatigue crack growth from naturally occurring discontinuities on an engineering "crack initiation" scale. Fatigue crack growth rates were measured through the full range of crack depths, allowing the effects of beneficial residual stresses to be directly observed. The almost identical early crack growth rates in cold expanded and non-cold expanded specimens were demonstrated up to crack depths of approximately 0.1-0.2 mm. Beyond this depth, fatigue crack growth in cold expanded specimens was markedly slowed to the point at which failure ultimately arose from cracks away from, or growing into, the fastener hole. The available life improvement factor for cold expansion is therefore maximized through a crack growth-based lifing approach. Sub-millimeter fatigue crack growth rates from cold-expanded fastener holes were measured.Service lives for cold-expanded holes under crack initiation and durability crack growth fatigue criteria were compared.Fatigue crack growth rate data from natural cracks suitable for correlation of analytical models is presented.