No abstract

No abstract

A B S T R A C T A recognized way of improving the fatigue resistance of a fastener hole is to introduce compressive tangential residual stress around it. This can be achieved by using a cold expansion method in which an oversized pin or ball is forced through the hole to produce a local plastic region surrounded by an elastic one. Once the pin or ball is removed allowing the elastic region to spring back it results in compressive tangential residual stress around the hole. In practise, however, it is found that such a cold expansion method creates a nonuniform residual stress distribution through the plate thickness and even tensile residual stress can be created at the entrance and exit faces. In this paper a new method of cold expansion is proposed. It uses a tapered pin with a mating tapered split sleeve and creates an almost uniform compressive residual stress around the hole as shown by FE method. Also, fatigue tests were carried out to verify that the method does significantly improve fatigue life. Finally the tangential residual stress distribution and fatigue life improvement of this new method were compared with those of a well-established cold expansion method and it was shown that the new method is more efficient in improving fatigue life.

No abstract

A B S T R A C T Cold expansion of fastener holes creates compressive residual stresses around the hole.This well-known technique improves fatigue life by reducing tensile stress around the holes. However, cyclic loading causes these compressive residual stresses to relax, thus reducing their beneficial effect. Estimation of the fatigue life without considering the residual stress relaxation might lead to inaccurate results. In this research, numerical studies were carried out using 2D finite element (FE) models to determine the initial tangential and radial residual stress distributions generated by cold expansion and their relaxation under cyclic loading. To predict the stress relaxation, four nonlinear kinematic hardening models were applied in simulation of stress/strain path. The results obtained from the FE analysis were compared with available experimental results. A good agreement between the numerical and experimental results was observed.Keywords cold expansion; fastener hole; finite element simulation; residual stress relaxation. N O M E N C L A T U R Eā = current centre of the yield surface in the deviatoric stress spacē a 4 = threshold term C i = nonlinear kinematic hardening model parameter E = elastic modulus H = plastic modulus m = nonlinear kinematic hardening model parameter M = number of segments required for multilinear representation of the uniaxial stress-strain R = stress ratio s¯= deviatoric stress tensor S u = tensile strength ᾱ = current centre of the yield surface in total stress space δ = multiaxial parameter in the kinematic hardening rule ε p = plastic strain tensor γ i = nonlinear kinematic hardening model parameters ν = Poisson's ratio σ = stress tensor σ y = yield stress σ 0 = size of the yield surface