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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

2007 Michell Medal Oration

F-111 Structural Integrity Support

Francis Rose

Chief Scientist, Platforms Sciences Lab, DSTO

outline
OUTLINE
  • Michell Brothers
  • F-111 Sole Operator Program
    • Hole Shape Optimisation
    • Bonded Repair Substantiation
  • Loose Ends & Acknowledgements
anthony george maldon michell 1870 1959
Anthony George Maldon Michell 1870-1959
  • Michell Structures
  • Tilting-pad Thrust-bearing
john henry michell 1863 1940
John Henry Michell 1863-1940
  • The Wave Resistance of a Ship
  • Stress Compatibility Equations
michell brothers legacy
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
F-111 SOLE OPERATOR PROGRAM
  • Background
  • Hole Shape Optimisation
  • Bonded Repair Substantiation
f 111 sop background
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
INNOVATIONS, ACCIDENTS & WATERSHEDS
  • de Havilland Comet (1953-54)
  • General Dynamics F-111 (1969)
  • Aloha Airlines Boeing 737 (1988)
cracking in the wing pivot fitting
CRACKING IN THE WING PIVOT FITTING

Fuel Flow Vent Holes (FFVHs)

Stiffener Runouts (SROs)

Typical crack

Typical crack

Inside WPF upper plate

hole shape optimisation
HOLE SHAPE OPTIMISATION
  • Optimal hole characterised by (piecewise) constant hoop stress
  • Iterative boundary deformation to achieve constant hoop stress
iterative boundary deformation
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
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
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
BENEFIT FOR INSPECTION INTERVAL

new position

current position

tooling for re work
TOOLING FOR RE-WORK
  • optimal reworks manufactured into a test wing by electro discharge machining
next most critical locations
NEXT MOST CRITICAL LOCATIONS

FFVH 14

FFVH 13

FFVH 11

FFVH 12

wing damage enhancement
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
BUCKLING ANALYSIS OF WING PIVOT FITTING

CPLT Load:

Blueprint configuration

repair substantiation requirements
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

slide33

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

slide34

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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