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2007 Michell Medal Oration. F-111 Structural Integrity Support Francis Rose Chief Scientist, Platforms Sciences Lab, DSTO. OUTLINE. Michell Brothers F-111 Sole Operator Program Hole Shape Optimisation Bonded Repair Substantiation Loose Ends & Acknowledgements.
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2007 Michell Medal Oration F-111 Structural Integrity Support Francis Rose Chief Scientist, Platforms Sciences Lab, DSTO
OUTLINE • Michell Brothers • F-111 Sole Operator Program • Hole Shape Optimisation • Bonded Repair Substantiation • Loose Ends & Acknowledgements
Anthony George Maldon Michell 1870-1959 • Michell Structures • Tilting-pad Thrust-bearing
John Henry Michell 1863-1940 • The Wave Resistance of a Ship • Stress Compatibility Equations
Michell Brothers Legacy • Contributions to both Fluid & Solid Mechanics • Application Driven • Uncompromising Intellectual Integrity • & Quality of Engineering Science • “Theory is the captain; practice the soldiers”
F-111 SOLE OPERATOR PROGRAM • Background • Hole Shape Optimisation • Bonded Repair Substantiation
F-111 SOP BACKGROUND • USAF (1967-1996) & RAAF (1973-2010) • USAF Early Retirement Announced Dec 1994 • RAAF Supportability Study 1995 – 96 • DSTO to address • Engineering Risk • Ageing Aircraft Risk
INNOVATIONS, ACCIDENTS & WATERSHEDS • de Havilland Comet (1953-54) • General Dynamics F-111 (1969) • Aloha Airlines Boeing 737 (1988)
CRACKING IN THE WING PIVOT FITTING Fuel Flow Vent Holes (FFVHs) Stiffener Runouts (SROs) Typical crack Typical crack Inside WPF upper plate
in-service fatigue cracking FUEL VENT HOLES: WEIGHT REDUCTION PROGRAM FFVH 14 FFVH 13 FFVH 11 FFVH 12
HOLE SHAPE OPTIMISATION • Optimal hole characterised by (piecewise) constant hoop stress • Iterative boundary deformation to achieve constant hoop stress
ITERATIVE BOUNDARY DEFORMATION (constraint: only material removal allowed, multi-peak stress minimisation) • Initial 2:1 elliptical hole 2:1 Optimal hole • 21% reduction in peak stress compared to an initial elliptical hole • 43% reduction in peak stress compared to a circular hole • Greater stress reduction with increasing hole aspect ratio
INITIAL AND FINAL STRESS (constraint: only material removal allowed, multi-peak stress minimisation) Uniform stress regions are very flat, indicating true optimality. (20% & 6% reduction in maximum +ve peaks, 22% reduction in –ve peaks)
FE Implementation • Only move nodes on one edge of a mesh generation block (B1, B2) • New element mesh created for each iteration (avoids mesh distortion) • It is also useful to maintain relative spacing of boundary nodes.
BENEFIT FOR INSPECTION INTERVAL new position current position
TOOLING FOR RE-WORK • optimal reworks manufactured into a test wing by electro discharge machining
NEXT MOST CRITICAL LOCATIONS FFVH 14 FFVH 13 FFVH 11 FFVH 12
WING DAMAGE ENHANCEMENT • Static tests are used to validate FE model • Cyclic test results are interpreted for Durability and Damage Tolerance
BUCKLING ANALYSIS OF WING PIVOT FITTING CPLT Load: Blueprint configuration
REPAIR SUBSTANTIATION REQUIREMENTS • Validation of design analysis by an independent method • Validation testing of a representative test article for • Static strength • Durability and Damage Tolerance Proper accounting for environmental effects
FEATURES OF MECHANICAL REPAIRS Repair • New stress concentrations at fastener holes • Difficult to detect cracks under patch • Low patching efficiency, cannot patch cracks • May damage hidden components • May cause corrosion problems • Simple to apply - no new technology Filler Stringer Skin New Crack Doubler
FEATURES OF BONDED REPAIRS Repair • No damage to structure or hidden components • High patching efficiency, can repair cracks • Can detect cracking under boron/epoxy patch • Minimises stress concentrations • No corrosion problems • Simple/effective surface treatment essential Stringer Original Crack Skin Doubler