Static and Fatigue Testing of Mechanical Life Extension Options for Plates Containing Elongated Holes

F-111 representative static and fatigue testing has been undertaken to assess the suitability of AMRL designed stress-bridge and interference-fit plug options for the fatigue life enhancement of plates containing non-circular elongated holes of 2:1 aspect ratio. For the stress-bridge design, an improvement in strain concentration factor of 38% was demonstrated for lower magnitude remote loads as compared to plates with unenhanced holes. Conversely, during typical high loads, the alternating strain range was negligibly improved due to slipping of the feet. Hence during the fatigue tests there was extensive specimen cracking. However, excellent results were achieved with the interference-fit plug design, where the peak elastic strain concentration factor was reduced by 79% as compared to the unenhanced case. This design also prevented the introduction of detrimental residual tensile stresses during high loads. Consequently, for these enhanced specimens, crack growth during fatigue testing was completely prevented, and very large extensions in fatigue lives were demonstrated. Thus the interference-fit plug is considered a very suitable option for the fatigue life extension of elongated holes such as the critical fuel flow vent holes in the F-111 wing pivot fitting.