Further Investigations to Improve the Fatigue Life of the Mirage IIID Wing Main Spar

摘要

Wing main spars of Mirage IIID aircraft have undergone a refurbishment program to extend their fatigue lives by the installation of interference fit steel bushes. A supplementary investigation has been carried out to assess two potential techniques for reducing the influence of empty rivet holes (namely the installation of interference-fit steel pins and adhesively-bonded aluminum rivets), and to assess the improvements in fatigue life which might be introduced by a redesign of the spar at the previously critical location. It was found that both treatments of the rivet holes resulted in a further small improvement in the life of specimens refurbished with interference-fit steel bushes at the bolt holes. and that these lives were not significantly different from that of specimens of the new design of spar in which the cross-sectional area was increased and the river holes omitted. However, considerable improvements in life were obtained when the bolt holes in the 'redesigned spar' specimens were either cold-expanded ( a factor of 4.1) or had interference-fit steel bushes installed in them ( a factor of 3.4).