Stress Analysis of an Interference Fit Life Extension Option for a Cold Expanded Elongated Fuel Flow Vent Hole on the F-111C Aircraft.
DEFENCE SCIENCE AND TECHNOLOGY ORGANISATION CANBERRA (AUSTRALIA)
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This investigation has been undertaken as part of a program of work having the aim of determining a suitable fatigue life enhancement option for the non-circular fuel flow vent hole number 13 in the wing pivot fitting of the F-111C aircraft. Two types of stress analysis have been undertaken for a finite width rectangular plate of D6ac steel containing an elongated hole. Firstly, plate stress distributions due to interference fitting obtained from elastic two dimensional finite element analyses were compared to those measured experimentally using strain gauges and thermoelasticity. Secondly, two-dimensional elasti-plastic finite element analyses were undertaken to quantify the effect on critical plate stresses due to enhancement by combined cold expansion with interference fitting, in the presence of subsequent representative cold proof test loading and a sample spectrum loading. The predicted stresses for the elastic analysis cases agreed well with the experimental results, which also demonstrated the suitability of a proposed tapered plugsleeve design to achieve effective interference fitting of an elongated hole. Overall, enhancement through combined cold expansion and interference fitting was considered to be significantly better than interference fitting alone. For example, the combined enhancement case, as compared to interference fitting only, led to a change in the critical hoop stress from 603 MPa, to -87 MPa. These favourable results indicate that such an enhancement procedure would potentially be suitable for extending the fatigue life of the fuel flow vent hole number 13 region of the F-111C aircraft, pending the results of appropriate static and fatigue tests.
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