A novel method and tool which enhance the fatigue life of structural components with fastener holes

The fatigue failure around fastener holes in metal structures is particularly dangerous, since it leads to inevitable accidents. The presence of residual compressive hoop stresses around these holes, imparted by means of mandrel coldworking methods (including split sleeve cold expansion and split mandrel coldworking) closes the existing cracks and impedes the formation of new ones and thereby extends the fatigue life of corresponding component. Although this type of pre-stressing of fastener holes has been of great advantage over recent years it has some disadvantages: significant and nonsymmetrical with respect to the plate middle plane axial gradient of the residual hoop stresses due to axial force flow passing through the plate, considerable surface upset, etc. This article presents a novel patented method and a tool ensuring “pure” radial cold hole expansion – introducing nearly uniform residual hoop stresses around the hole along its axis having a minimized and symmetric gradient with respect to the plate middle plane. Because of this advantage, the method has been called by the authors “symmetric cold expansion”. Another advantage of the method is that it ensures one-sided process like the split sleeve and split mandrel. The carried out finite element (FE) simulations proved that the residual stresses in the novel method are much more uniformly distributed in comparison with the mandrel coldworking methods. The fatigue life improvement of the novel method in comparison with mandrel coldworking methods has been proved experimentally through fatigue cyclic test.