The Effect of Cold Expansion on Fatigue Life of Closely Spaced Adjacent Fastener Holes

Aerospace strcutures contain numerous fastener holes which are closely spaced. The fatigue strength of these fastener holes are often enhanced through cold expansion process. In order to achieve maximum possible fatigue life enhancments from cold expansion process, it is required to optimize the process parameter such as level of hole expansion. The guidelines for selecting an optimum process parameter is standardized for typical hole sizes and metallic materials. These standrads are established by considering a single hole which is free from the infuence of adjacent holes. However, the actual strucures contain series of holes which are closely spaced and presence of adjacent holes influences the cold expansion process. Hence, the present work is aimed at predicting an optimum process paramater which gives maximum fatigue life enhancement benefit around fastneter hole under the infulence of adjacent hole.

In this work, a series of finite element simulations on cold expansion process for different levels of hole expansions are carried out by considering two closely spaced adjacent holes in aircraft grade Al 7075-T651 plate. For each level of expansion, the reduction of stress concentarion effects around hole regions are predcited. As a result, extent of fatigue life enhancements around adjacent hole regions are evaluated using J. Morrow critera. From the results, it is found that for2% expansion level, the fatigue life of adjacent holes is enhannced up to 6.6 times compared to non-cold expanded adjacent holes.