Improving the Fatigue Life of the Mirage IIIO Wing Main Spar.
AERONAUTICAL RESEARCH LABS MELBOURNE (AUSTRALIA)
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An increased life-of-type requirement for Mirage IIIO aircraft operated by the RAAF was associated with a specific need to develop a refurbishment technique for extending the lives of fatigue-cracked wing main spars. This led to a comprehensive series of flight-by-flight fatigue tests on specimens of a geometry and thickness 32 mm closely representing the inboard lower rear flange section of the spar. Of the various options investigated, the most promising was the use of interference-fit steel bushes in bolt holes-either alone or in combination with a modified anchor-nut sub-assembly which eliminated the need for some rivets in the spar flange. Specimens with oversize reamed holes using oversized sleeved bolts performed poorly. Cold-expansion of the critical bolt hole using the Boeing split-sleeve process was unsatisfactory because of small hole-to-spar edge distances and the proximity of small rivet holes. This investigation has shown that acceptable extensions in fatigue life for wing main spars of the Mirage IIIO are possible by use of combinations of standard bolts and 0.3 interference-fit steel bushes of 1.5 mm wall thickness in the critical bolt holes, with the provisos that pre-existing fatigue cracks are completely removed and that after reworking bolt holerivet hole separation distances are adequate. In some circumstances acceptable increases in life may be obtained by reducing the outside diameter of the bush and using stepped undersize bolts.
- Attack and Fighter Aircraft
- Couplers, Fasteners and Joints