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2007 Michell Medal Oration

This oration by Chief Scientist Francis Rose at DSTO discusses the significant contributions of the Michell Brothers to structural integrity engineering, particularly in the context of the F-111 aircraft. Key topics include the sole operator program, innovations in bonded repair substantiation, and optimization of hole shapes for stress reduction. The presentation highlights the historical legacy of the Michell family in engineering and the importance of rigorous testing and design for maintaining aging aircraft safety.

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2007 Michell Medal Oration

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  1. 2007 Michell Medal Oration F-111 Structural Integrity Support Francis Rose Chief Scientist, Platforms Sciences Lab, DSTO

  2. OUTLINE • Michell Brothers • F-111 Sole Operator Program • Hole Shape Optimisation • Bonded Repair Substantiation • Loose Ends & Acknowledgements

  3. Anthony George Maldon Michell 1870-1959 • Michell Structures • Tilting-pad Thrust-bearing

  4. John Henry Michell 1863-1940 • The Wave Resistance of a Ship • Stress Compatibility Equations

  5. 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”

  6. F-111 SOLE OPERATOR PROGRAM • Background • Hole Shape Optimisation • Bonded Repair Substantiation

  7. 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

  8. INNOVATIONS, ACCIDENTS & WATERSHEDS • de Havilland Comet (1953-54) • General Dynamics F-111 (1969) • Aloha Airlines Boeing 737 (1988)

  9. F-111 SWING WING MECHANISM

  10. CRACKING IN THE WING PIVOT FITTING Fuel Flow Vent Holes (FFVHs) Stiffener Runouts (SROs) Typical crack Typical crack Inside WPF upper plate

  11. in-service fatigue cracking FUEL VENT HOLES: WEIGHT REDUCTION PROGRAM FFVH 14 FFVH 13 FFVH 11 FFVH 12

  12. HOLE SHAPE OPTIMISATION • Optimal hole characterised by (piecewise) constant hoop stress • Iterative boundary deformation to achieve constant hoop stress

  13. 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

  14. 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)

  15. 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.

  16. BENEFIT FOR INSPECTION INTERVAL new position current position

  17. TOOLING FOR RE-WORK • optimal reworks manufactured into a test wing by electro discharge machining

  18. NEXT MOST CRITICAL LOCATIONS FFVH 14 FFVH 13 FFVH 11 FFVH 12

  19. WING DAMAGE ENHANCEMENT • Static tests are used to validate FE model • Cyclic test results are interpreted for Durability and Damage Tolerance

  20. BUCKLING ANALYSIS OF WING PIVOT FITTING CPLT Load: Blueprint configuration

  21. REPAIR SUBSTANTIATION

  22. 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

  23. LOAD FLOW & LOAD TRANSFER

  24. CRACK LOCATION

  25. LOCAL GEOMETRY

  26. FRACTOGRAPHY OF CRACKING

  27. PANEL SPECIMEN

  28. BOX SPECIMEN

  29. BOX SPECIMEN TESTING

  30. FATIGUE CRACK GROWTH COMPARISON

  31. RESIDUAL STRENGTH RESULTS

  32. 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

  33. 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

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