Sequential cold expansion and resulting beneficial residual stress prediction around adjacent fastener holes

Fatigue life of aircraft structure fastener holes are enhanced through cold expansion process, which creates an annular zone of compressive residual stresses around the hole. Durability and damage tolerance analysis of structures containing cold expanded adjacent holes require quantification of cold expansion-induced compressive residual stresses. Three-dimensional non-linear FE simulation of sequential cold expansion of two closely spaced adjacent holes in Al 7075-T651 plate is carried out to predict the beneficial residual stresses completely. Further, an experimental investigation on sequential cold expansion of adjacent holes is carried out using cold expansion tooling system developed in-house and the retained expansion levels are measured. Comparison of FE simulation results and results of experimental investigations indicate that developed simplified FE simulation approach is capable of adequately predicting cold expansion-induced beneficial residual stresses around the hole, including through thickness variations.