Airframe Life Extension by Optimised Shape Reworking Overview of - - PowerPoint PPT Presentation

Airframe Life Extension by Optimised Shape Reworking

Overview of DSTO Developments

• M. Heller1, M. Burchill1, R. Wescott2, W. Waldman1, R. Kaye1, R. Evans1,
• M. McDonald1

1 Air Vehicles Division, DSTO, 2 QinetiQ Aerostructures

Presented at the 25th ICAF Symposium – Rotterdam, 27-29 May 2009

Context – Airframe life extension

• Airframes contain many stress raising features
• Most shapes consist of straight lines & circular arcs
• Traditional shapes have localized peak stresses
• Cracking at only a few locations can define the

economic fatigue life for an aircraft structure

• Hence reducing stresses at a few locations can

provide significant benefits to the RAAF

• safety, aircraft availability, cost saving

Context – Standard blend repairs

• Very common approach for removal of damage
• Applied to flat or curved surfaces
• May extend fatigue life
• Stresses higher than original shape

depth, d repair radius, r local radius, R

Concept of optimal reworking

• Optimal shape removes the damaged material and minimises stresses
• For many practical problems there are no analytical solutions

Initial shape Traditional re-shape Optimal re-shape (free-form) (limited benefit) (lowest stress)

Blueprint geometry Traditional rework Optimised rework

Crack

Outline

Context / concept Numerical approach F-111 WPF application & lessons learned Fatigue life trends Other design studies / applications Transitioning issues

Numerical approach

Single peak stress minimisation

i th th th i i

sc d σ σ σ σ σ max = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − =

σth

∆σ = |σi | − σth

1 n Node position on boundary Hoop stress i

• Aim is constant local stress
• Iterative FE method based on biological growth
• Net material removal only
• Remeshing algorithms used – DSTO code

Numerical approach

2D multi-peak stress minimisation

S2 S2

2:1 elliptical hole 2:1 optimal rework

• 21% stress reduction compared to elliptical hole
• 43% stress reduction compared to circular hole

Numerical approach

3D multi-peak stress minimisation

j=v

:

j=2 j=1 i-1 i+1

i

2:1 aspect ratio Remote stresses, S2 = -S1/4

• 14 % stress reduction compared to elliptical hole

F-111 wing pivot fitting application

runouts holes

Requirement:

• Improve shapes for fatigue

prone holes and runouts

• Achieving PWD & extend

inspection intervals

F-111 WPF application

Stiffener runouts

Typical SRO after machining

Rework shape

• 30-40% reduction in peak stress
• 4 unique optimal shapes for 4 different locations
• buckling strength considered

F-111 WPF application

Fuel flow vent holes

• 25 - 50 % stress reductions
• 4 unique optimal shapes for 4

different locations

F-111 WPF application

Manufacturing rework shapes

• Electrical discharge machining
• Worked - but complex with difficult access

Electrode Plate Finishing Electrode Roughing Electrode Locating Probe

F-111 WPF application

Experimental validation

• 20000
• 16000
• 12000
• 8000
• 4000

4000 8000 30 60 90 120 150 180 210 240 270 300 330 360 Angle about Hole (Degrees) Strain (µε) FE (optimal) Test (optimal) FE (blueprint) Prior test (baseline)

Static tests Fatigue tests

• durability
• damage tolerance

F-111 WPF application

Lessons

Reshaped holes

Requirements: 1. Account for variations in fleet nominal geometry 2. Increased understanding re interaction of:

• Hole size & aspect ratio
• Manufacturing constraints
• NDI constraints
• Fatigue lifing philosophy

3. Need simpler in-situ manufacturing methods

Reshaped SRO

Fatigue life trends

Safe life approach

Shape optimisation increases life by reducing stress concentration

Kt

Flight time, t (hours)

Kt1 Kt2 t1 t2

Non-optimal Optimal

2 4 6 8 10 12 14 0.6 0.7 0.8 0.9 1.0 Kt2 / Kt1 tt2 / tt1 A= -0.2 A= -0.227, F18 APOL A= -0.314, AP-3C A= -0.4

A

K K t t

/ 1 t1 t2 t1 t2

⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ =

+ ) ( = ) Β t Α K

t t

ln ln(σ

Fatigue life trends

Crack growth / SBI

Baseline is initial circular hole, r = 20mm

y σr

σy

a/ σr

1.0 1.5 2.0 2.5 3.0 3.5 5 10 15 20 a (mm) Beta factor

3.0 2.5 2.0 1.5

Kt

) / ( 12 . 1 where

remote y σ

σ β =

Fatigue life trends

Crack growth / SBI

5000 10000 15000

0.01 0.1 1 10 100

ai (mm)

Residual life (hrs)

3.0 2.5 2.0 1.5

Kt

• Assume through crack
• Use “effective block” approach
• F/A-18 spectrum

da K C

i

a m

∫

=

f

a ref

] ) ( /[ 1 Life

a Kref π σ σ σ

ref remote y

) / ( 12 . 1 where =

Reduced rate of crack growth – typically gives longer inspection intervals

Simplified in-situ manufacturing method

Non-circular hole in steel stiffener – F-111 FFVH test case

• Precise templates in conjunction with air-powered tooling
• Two main steps: 1. Coarse sanding drum,
• 2. Fine abrasive drum, followed by polishing

Simplified in-situ manufacturing method

Non-circular hole in steel stiffener coupon wing pivot fitting

Non-circular hole in closely spaced al. alloy stiffeners

Simplified in-situ manufacturing method

For difficult to access locations Two main steps:

• 1. Carbide burr cutting tool,
• 2. Diamond coated abrasive tool, followed by polishing

Simplified in-situ manufacturing method

Oversized circular hole in closely spaced al. alloy stiffeners

• Oversized circular hole
• Height above skin of 0.01″

Simplified in-situ manufacturing method

Non-circular runout in steel stiffener

Improved NC machining: F-WELD example

• Code for smoothing of raw FEA co-ordinates
• Shape has many circular arcs
• radius of curvature shown

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 15 30 45 60 75 90 Angle about hole, ฀ (degrees) Normalised radius of curvature 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 15 30 45 60 75 90 Angle about hole, π (degrees) Normalised radius of curvature

before after

1 2 3

• 10
• 8
• 6
• 4
• 2

2 4 6 8 10

Load inclination angle, θ

Peak Stress

Robust optimal Standard optimal

σnom σmis σnom σmis θ

Robustness for optimised shapes

• Robust optimal has constant and minimised peak stress over expected 10

degree load misalignment range

• For variation of load orientations or multiple load cases

Other design studies & applications

1. Generic optimal solutions for loaded plates with:

• Single holes
• Interacting holes
• Edge notch coupons
• Surface damage removal (3D)
• Shoulder fillets
• Crack stop holes

Other design studies & applications

• 2. Applications / demonstrators with optimal reworks:
• 1. F-111:

four fuel flow vent holes in WPF FEA, full scale tests, fleet

• 2. F-111:

four stiffener runouts in WPF FEA, full scale tests, fleet

• 3. F-111:

fuel pilot valve hole in upper skin FEA, full scale tests

• 4. F-111:

gravity refuel hole in upper skin FEA, full scale tests

• 5. F-111

wing pivot fitting bush FEA, full scale tests

• 6. F/A-18: aileron hinge

FEA, static tests

• 7. F-111

revised FFVH, SRO FEA, manuf. demo

• 8. AP-3C:

fuel flow hole in stiffener FEA, manuf. demo

• 9. B707:

surface damage removal FEA, manuf. demo

• 10. F/A-18: vertical tail stub attachment

FEA, manuf. demo

• 11. PC9/A: lower wing skin at up-lock

FEA

• 12. F/A-18: FS 470 bulkhead

FEA

• 13. F-35:

bulkhead access holes test case FEA

• 14. Other

low kt coupon design Fatigue tests, pending

• no interaction effects after optimisation
• optimal shapes approach half-circle as

e/h approaches zero

1.5 2.0 2.5 3.0 3.5 4.0 0.0 0.5 1.0 1.5 2.0 2.5 Minimum edge separation distance, e/h Stress concentration factor, K t

Circular holes Optimal holes

• 4
• 3
• 2
• 1

1 2 3 4

• 3
• 2
• 1

1 2 3 x/h y/h e/h = 4.92

Analytical solution from Cherepanov (1974) with corrections by Vigdergauz (1976)

S1 S1 S2 S2 Design study

Generic interacting optimal holes (loading S1=S2)

e r θ h w

Practical application example

F-111 WPF bush fillet redesign

nominal redesign

• used on F-WELD fatigue test
• 30% stress reduction
• Test life 12000 hrs, versus fleet replacement at

least every 1025.

Design study

Robust hole in a shear panel

31% reduction in peak stress

(a) (b) 99 Ksi

Options: 1. A family of morphed shapes,

• negligible increase in peak

stress 2. A local circular arc blend

• small increase in in peak

stress

• partial or full thickness

depth, d repair radius, r local optimal radius, R

• 140
• 100
• 60
• 20

20 60

• 120

120

Inner Optimal Middle Optimal Outer optimal 50 % Morphed shape

Transitioning issues

Repair if re-crack occurs at optimal

Acknowledgements

RAAF ASI – DGTA (Sponsorship) Staff working on F-111 Sole operator program, including F-WELD Other DSTO staff RAAF Amberley

• Industry
• Boeing, TAE, Amiga Eng., QinetiQ AeroStructures:

Conclusions

• Rework shape optimisation a useful approach for life extension
• Shapes either:
• Generic (symmetric)
• One off optimal shapes
• Key technical impediments overcome
• DSTO keen to transition approach more widely
• Approach applicable to initial design