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

Anthony George Maldon Michell Michell Structures Tilting-pad Thrust-bearing

John Henry Michell 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 ( ) & RAAF ( ) 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 ( ) 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

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

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

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)

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.

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

BENEFIT FOR INSPECTION INTERVAL current position new position

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

NEXT MOST CRITICAL LOCATIONS FFVH 11 FFVH 13 FFVH 14 FFVH 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 configuration 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

RESIDUAL STRENGTH RESULTS

 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

 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