The Use of Interference-Fit Bolts or Bushes and Hole Cold Expansion for Increasing the Fatigue Life of Thick-Section Aluminium Alloy Bolted Joints

The detection of fatigue cracks at bolt holes in the main spar of the Mirage III wing during full-scale fatigue tests led to a requirement for refurbishment procedures to extend the fatigue lives at a number of critical locations. One of these, which is covered by this investigation, was the spar lower front flange. Flight-by-flight fatigue tests have been carried out to determine the relative fatigue performance of aluminum alloy bolted joint specimens of 28 mm thickness incorporating close-fit bolts, interference-fit bolts (0.4%), hold cold-expansion (3%), interference-fit steel bushes (0.3%) and a combination of cold-expansion and interference-fit bushes. Fractographic measurements of crack development from the bores of holes in specimens incorporating close-fit bolts in non cold-expanded (reamed) and cold-expanded holes clearly indicated much slower fatigue crack propagation rates from the cold-expanded holes until the crack length was close to the nominal region of transition from the residual compressive to tensile stress zone around the cold- expanded holes. Fatigue tests on cold-expanded hole specimens at different spectrum scaling factors indicated that, under the loading sequence used, each 10% increase in stress reduced the life to about half that at the lower stress level.