The Effect of Cold Expansion at Chamfered Fastener Holes

Abstract

Cold expansion method has been widely used to enhance fatigue properties of the aerospace structures with fastener holes. Basically, cold expansion method is that an oversized tapered mandrel goes through the hole and produces compressive residual stress as well as plastic deformation around the hole. One of the problems with the cold hole expansion process is lower compressive residual stress values of the entry hole position than those at other positions of the hole. Often, fatigue cracks are initiated at the entry position of the hole. In this study, applying chamfers on a hole is proposed to increase the residual stresses at the entry position. Finite element analysis (FEA) was conducted to predict residual stress distributions of cold hole expansion processed chamfered holes. FEA results show that the chamfered holes have equal distribution of deep compressive residual stresses around the hole while the holes without chamfers have lower compressive residual stresses at the entry hole position. Effects of chamfer geometries on residual stress distributions are also discussed.