A Review of Australian and New Zealand Investigations on Aeronautical Fatigue During the Period April 1999 to March 2001
DEFENCE SCIENCE AND TECHNOLOGY ORGANISATION VICTORIA (AUSTRALIA) AERONAUTICAL AND MARITIME RESEARCH LAB
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Optimal rework shapes for the most fatigue critical stiffener runout SRO locations in the F-111 wing pivot fitting WPF upper plate region have been determined using a recently developed finite-element-based gradientless shape optimisation procedure 20. The WPF region is shown in Figure 4. The resulting precise free-form shapes which typically remove cracked material render the local notch stress distributions near uniform and typically provide a 30-40 reduction in peak elastic stresses as compared to current rework shapes that exist for aircraft in service with the Royal Australian Air Force. They also typically represent a 50 improvement in stresses, as compared to the nominal blueprint shapes. The unique optimal shapes of different sizes have been determined for four of the most fatigue prone SRO locations in the F-111 WPF. The final peak stress level achieved at each of the optimised runouts was relatively consistent therefore, a similar inspection interval can be expected for each location. A sample comparison of elastic stresses is given in Figure 5, where it should be noted that the key aim was to reduce the peak compressive stress to minimse residual tensile stresses after cold proof load testing. The recommended rework configuration provides a manageable compromise between minimising the runout stresses and maintaining the WPF buckling strength to within acceptable limits. A number of important issues have been addressed in the present practical problem, including use of higher-order elements for efficient robust stress prediction accounting for the effect of size constraints on the optimal shapes and assessing the robustness of the idealised optimal shapes to perturbations away from idealised conditions, such as those due to potential manufacturing errors. As part of an associated validation program, the precise shapes have been manufactured in two full-scale static test wings.