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