Volume 3, Issue 5 (May 2006)
Modeling the Formation and Growth of Cracks from Cold-Worked Holes
First generation crack formation and crack growth predictive approaches for cold-worked holes—based on the growth of a single noninteracting crack in a stationary residual stress field—fail to account fully for the physical mechanisms by which cracks form and grow from cold-worked holes. This failure leads in many cases to large differences between predicted and actual lives. Factors not accounted for in first generation approaches include the following: 1. 3-D nature of residual stress field due to mandrel pull-through. 2. Multistage crack growth involving a progressively spreading system of cracks. 3. Multiple potential initial crack sites, and the effect of site upon the multistage crack growth path. 4. Relaxation of the residual stress field due to overloads/underloads, or due to cyclic reyielding from crack growth. 5. Interaction of the hole with adjacent structural elements for multilayer joints. A physics-based second generation methodology for accounting for these factors is described. This methodology separates into individual building blocks each of the various mechanisms controlling the formation and relaxation of residual stresses, and the nucleation and progression of a system of cracks. Because it explicitly models each of these mechanisms, it is capable of eliminating or reducing the uncertainty over the life improvement potential of the cold-working process, allowing the full potential of cold-working to alleviate the aging aircraft problem to be untapped.