Stress Analysis of Coldworked Fastener Holes.

Abstract : The stress and strain distributions around an initially coldworked hole in a plate and the subsequent redistribution of the stresses and strains when the plate is subjected to a uniform tensile loading are evaluated both analytically and experimentally. An elastic-plastic finite element idealization is developed for a one-quarter inch diameter hole in a 7075-T6 aluminum alloy plate one-quarter inch thick. Stress and strain distributions are provided for a 6 mil radial expansion of the hole and at tensile load levels of 10,000 lb., 25,000 lb., 37,500 lb., and 42,500 lb. Moire interferometry was used to verify that the elastic-plastic finite element analysis is representative of the strain distributions in an actual fastener coldworking procedure which involves inserting a cylindrical sleeve into a nominal one-quarter inch diameter hole and then drawing an oversized mandrel through the hole to produce a symmetrical distribution of residual stresses around the hole. Previous experimental strain evaluations of holes in plates involved hole diameters of at least one inch. The moire mismatch technique and photographic enlargements of the moire patterns provided the accuracy required. The conditions in the vicinity of the hole associated with the sleeve insertion procedure did not reflect the idealized conditions used in the finite element analysis which prohibited a valid comparison between the experimental and computed results. (Author)