2007 michell medal oration f-111 structural integrity support francis rose chief scientist,...
TRANSCRIPT
2007 Michell Medal Oration
F-111 Structural Integrity Support
Francis RoseChief 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)
F-111 SWING WING MECHANISM
CRACKING IN THE WING PIVOT FITTING
Fuel Flow Vent Holes (FFVHs) Stiffener Runouts (SROs)
Inside WPF upper plate
Typical crack Typical crack
FUEL VENT HOLES: WEIGHT REDUCTION PROGRAM
in-service fatigue cracking
FFVH 11FFVH 11
FFVH 13FFVH 13
FFVH 14FFVH 14
FFVH 12FFVH 12
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
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)
-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)
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
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
TOOLING FOR RE-WORK
– optimal reworks manufactured into a test wing by electro discharge machining
Electrode plate
Finishing electrode
Roughing electrode
Locating probe
NEXT MOST CRITICAL LOCATIONS
FFVH 11FFVH 11
FFVH 13FFVH 13
FFVH 14FFVH 14
FFVH 12FFVH 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
Blueprint configurationBlueprint configuration
CPLT Load:CPLT Load:
REPAIR SUBSTANTIATION
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
LOAD FLOW & LOAD TRANSFER
CRACK LOCATION
LOCAL GEOMETRY
FRACTOGRAPHY OF CRACKING
PANEL SPECIMEN
BOX SPECIMEN
BOX SPECIMEN TESTING
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
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
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
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