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![Page 1: 2007 Michell Medal Oration F-111 Structural Integrity Support Francis Rose Chief Scientist, Platforms Sciences Lab, DSTO.](https://reader030.fdocuments.net/reader030/viewer/2022032803/56649e3c5503460f94b2ef8f/html5/thumbnails/1.jpg)
2007 Michell Medal Oration
F-111 Structural Integrity Support
Francis RoseChief Scientist, Platforms Sciences Lab, DSTO
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OUTLINE
• Michell Brothers
• F-111 Sole Operator Program
• Hole Shape Optimisation• Bonded Repair Substantiation
• Loose Ends & Acknowledgements
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Anthony George Maldon Michell 1870-1959
Michell Structures
Tilting-pad Thrust-bearing
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John Henry Michell 1863-1940
The Wave Resistance of a
Ship
Stress Compatibility
Equations
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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”
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F-111 SOLE OPERATOR PROGRAM
• Background
• Hole Shape Optimisation
• Bonded Repair Substantiation
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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
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INNOVATIONS, ACCIDENTS & WATERSHEDS
• de Havilland Comet (1953-54)
• General Dynamics F-111 (1969)
• Aloha Airlines Boeing 737 (1988)
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F-111 SWING WING MECHANISM
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CRACKING IN THE WING PIVOT FITTING
Fuel Flow Vent Holes (FFVHs) Stiffener Runouts (SROs)
Inside WPF upper plate
Typical crack Typical crack
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FUEL VENT HOLES: WEIGHT REDUCTION PROGRAM
in-service fatigue cracking
FFVH 11FFVH 11
FFVH 13FFVH 13
FFVH 14FFVH 14
FFVH 12FFVH 12
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Cd jth
jth
jij
i
σ
σσ
ji
jth σmaxσ
HOLE SHAPE OPTIMISATION
• Optimal hole characterised by (piecewise) constant hoop stress
• Iterative boundary deformation to achieve constant hoop stress
-2
-1
0
1
2
3
4
0 20 40 60 80 100
% arc length around boundary
Str
ess
Initial hole Optimal hole
s1 s1s3
s2s4
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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)
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-2
-1
0
1
2
3
4
0 20 40 60 80 100
% arc length around boundary
Str
ess
Initial 2:1 ellipse Optimal hole
s1 s1s3
s2s4
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)
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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.
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BENEFIT FOR INSPECTION INTERVAL
0
2000
4000
6000
8000
1000 2000 3000 4000 5000
Peak von Mises stress (MPa)
Insp
ecti
on
inte
rval
(h
ou
rs)
estimated inspectioninterval trend
current position
new position
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TOOLING FOR RE-WORK
– optimal reworks manufactured into a test wing by electro discharge machining
Electrode plate
Finishing electrode
Roughing electrode
Locating probe
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NEXT MOST CRITICAL LOCATIONS
FFVH 11FFVH 11
FFVH 13FFVH 13
FFVH 14FFVH 14
FFVH 12FFVH 12
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WING DAMAGE ENHANCEMENT
– Static tests are used to validate FE model
– Cyclic test results are interpreted for Durability and Damage Tolerance
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BUCKLING ANALYSIS OF WING PIVOT FITTING
Blueprint configurationBlueprint configuration
CPLT Load:CPLT Load:
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REPAIR SUBSTANTIATION
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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
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LOAD FLOW & LOAD TRANSFER
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CRACK LOCATION
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LOCAL GEOMETRY
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FRACTOGRAPHY OF CRACKING
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PANEL SPECIMEN
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BOX SPECIMEN
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BOX SPECIMEN TESTING
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FATIGUE CRACK GROWTH COMPARISON
Panel specimen crack growthunder cycle-by-cycle spectrum loads
40
45
50
55
60
65
70
75
80
0 10,000 20,000 30,000 40,000 50,000
Equivalent flight hours
Crack length tip-to-tip (mm)
PATCHED PANELS
UNPATCHED PANELS
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RESIDUAL STRENGTH RESULTS
0 50 100 150 200 250 300 350 400 450
Patched, Cracked After30,000 Flight Hours
Patched, Cracked, -40C
Patched, Cracked, +110C
Patched, Cracked, RT
Unpatched, Cracked, RT
Unpatched, Uncracked
FAILURE STRESS (MPa)
2a = 40 mm
2a = 40 mm
2a = 40 mm
2a = 63 mm
2a = 40 mm
D.U.LD.L.L
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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 REPAIRSFEATURES OF MECHANICAL REPAIRS
New Crack
Repair
Doubler
Skin
Filler
Stringer
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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