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) Inside WPF upper plate Typical crack

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

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

13. 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 ITERATIVE BOUNDARY DEFORMATION (constraint: only material removal allowed, multi-peak stress minimisation)

14. Uniform stress regions are very flat, indicating true optimality. (20% & 6% reduction in maximum +ve peaks, 22% reduction in –ve peaks) INITIAL AND FINAL STRESS (constraint: only material removal allowed, multi-peak stress minimisation)

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. traditional reshape optimal reshape nominal shape For F-111 case optimal reshapes provide: 25 – 35% stress reduction compared to traditional reshape 40 – 50% stress reduction compared to nominal shapes 10% stress reduction = double fatigue life OPTIMAL RE-WORK SHAPE

17. BENEFIT FOR INSPECTION INTERVAL current position new position

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

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

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

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

23. REPAIR SUBSTANTIATION

24. 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

25. LOAD FLOW & LOAD TRANSFER

26. CRACK LOCATION

27. LOCAL GEOMETRY

28. FRACTOGRAPHY OF CRACKING

29. PANEL SPECIMEN

30. BOX SPECIMEN

31. BOX SPECIMEN TESTING

32. FATIGUE CRACK GROWTH COMPARISON

33. RESIDUAL STRENGTH RESULTS

34.  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 FEATURES OF MECHANICAL REPAIRS New Crack Repair Doubler Skin Filler Stringer

35.  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 Original Crack Repair Stringer Skin Doubler FEATURES OF BONDED REPAIRS