Study of the Residual Strain in Lap Joints

Abstract

Both experiments and finite element analysis were carried out to study the residual strain (stress) in the riveted lap joint for a better understanding of the fatigue life of fuselage lap joints. A force-controlled riveting process was used to apply a constant load ramp to install the rivets. Strain variations on the joint surface were measured using microstrain gauges during the riveting process. Because neutrons are known to penetrate through many centimeters of aluminum alloys, neutron diffraction was used to provide a nondestructive technique to determine strains at certain depths in the joint. Parallel to the experimental testing, a two-dimensional axisymmetric finite element model was developed to simulate the riveting process. Both material and geometric nonlinearities, as well as nonlinear contact boundary conditions, were used in this numerical model. Comparisons between the numerical simulations and experimental results focused on the rivet driven head deformations and strain variations, and the results showed that the current two-dimensional axisymmetric finite element model using the proper boundary conditions can reliably be used to determine the residual strains (stresses) present in joints that were induced during the riveting process. Nomenclature C = material parameter D = rivet shank diameter Dmax = maximum rivet shank diameter after riveting d = spacing between lattice plane d0 = stress-free lattice spacing E = Young’s modulus m = material parameter r = distance to fastener hole centre e = normal strain etrue = true strain θ = diffracted neutrons’ angle λ = wavelength of the incident neutron beam ν = Poisson’s ratio σ = normal stress σtrue = true stress beyond the initial yield stress σy = initial yield stress