Cold expansion technology is an extended method used in aeronautics to increase fatigue
life of holes and hence extending inspection intervals. During the cold expansion process, a mechanical
mandrel is forced to pass along the hole generating compressive residual hoop stresses. The most
widely accepted geometry for this mandrel is the tapered one and simpler options like balls have
generally been rejected based on the non-conforming residual hoop stresses derived from their use.
In this investigation a novelty process using multiple balls with incremental interference, instead
of a single one, was simulated. Experimental tests were performed to validate the finite element
method (FEM) models and residual hoop stresses from multiple balls simulation were compared with
one ball and tapered mandrel simulations. Results showed that the use of three incremental balls
significantly reduced the magnitude of non-conforming residual hoop stresses and the extension of
these detrimental zone.
