Beneficial residual stress from the cold expansion of large holes in thick light alloy plate

Abstract

Beneficial residual compressive stress is induced around fastener holes by the process of cold expansion in civil aircraft. Fatigue performance is enhanced by increasing the allowable fatigue stress levels which are accounted for in the design. Much work exists that explains both the cold-expansion process and its effects when applied to smaller diameter holes and smaller plate thicknesses, i.e. 12mm or less. Since the introduction of larger diameter holes and larger plate thicknesses of 25mm and greater to current large civil aircraft, investigations have revealed that the cold-expansion process has several unique features that are attributable to the larger size. While superficially the two scales of component may appear to have both static and dynamic similitude, in fact the scaling is not total when the complete manufacturing system is considered. All the dimensions are scaled up with the important exception of the flat portion of the mandrel; it is this easily missed feature that gives rise to the significant three-dimensional effect under investigation. This paper is written from an industrial perspective and reviews the result of experimental stress analyses that demonstrate the difference in the process between small and large holes. Strains were measured in the time domain and the strain progression against time indicates definite three-dimensional effects in the process which are compared with a closed-form solution and a three-dimensional finite element analysis.