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CLASSIFICATION
UNCLASSIFIED
– Approved For Public Release
Life Extension of F/A-18 LAU-7 Missile
Launcher Housings Using Rework
Shape Optimisation
Manfred Heller, Jaime Calero, Ron Wescott, Simon
Barter, Jireh Choi and Gregory Surtees
Presented at 11th International Fatigue Congress, Melbourne, 2-7 March 2014
Outline
1. Project definition and aims
2. Background for rework shape optimisation
3. Design and manufacture of optimised rework for launcher housing
4. Fatigue testing and NDI developments
5. TFSPO fleet trial
6. Current activities and conclusions
1. Project definition and aims
F/A-18A/B fleet
issue
FWD hanger
Fatigue cracking has been reported for the housing guide rails
Cracking located near the forward hanger
Managed via periodic NDI
Fleet cracking
Outboard
Fwd
Forward
hanger
Forward hanger detail
Fleet cracking
FWD
Guide rail
ASRAAM
FWD hanger
Cracking from a small radius
Housing replaced when cracking is detected
Corresponds to crack depth of about
0.3 to 0.5 mm
LAU-7
Cracking
Tensile fracture
Fatigue crack
Fleet cracking examples
1 mm
Multiple initiation sites & shallow depth compared with length
Steady crack propagation, no evidence of tearing
Aims and proposed
approach
Request from TFSPO
DSTO to investigate & develop a repair option for cracked launchers
- This is expected to save costs by reducing component replacement
and/or inspections & improve availability
DSTO approach
Use DSTO capabilities in rework shape optimisation, failure analysis, fatigue
testing, fractography and NDT
2. Background for rework
shape optimisation
Context – Standard blend repairs
Common approach for removal of damage
Applied to flat or curved surfaces
May extend fatigue life depending on stress level
local radius, R
repair radius, r
depth, d
Concept of optimised blending (rework)
Improved blend shapes remove the damaged material and minimise stresses
Crack
Initial shape
Traditional blend
(limited benefit)
Improved blend
(lowest stress)
Numerical Method
Single peak stress minimisation
σth
∆σ = |σi | − σth
 σ − σ th 
 sc
d i =  i
σ
th


σ th = max σ i
Hoop
stress
1
i
Node position on boundary
Aim is constant local stress
Iterative FE method based on biological growth
Net material removal only
Remeshing algorithms used – DSTO code
n
Numerical Method
2D multi-peak stress minimisation
S2
S2
2:1 elliptical hole
• 21% stress reduction compared to elliptical hole
2:1 optimal rework
F-111 wing pivot fitting application
Typical improved hole
blend
DSTO developed
improved shapes for fatigue
prone holes and runouts
Aim was to achieve PWD &
extend inspection intervals
30 - 40% stress reduction
Implemented on 6 aircraft
Typical improved runout
blend
F-111 WPF bush fillet redesign
nominal
used on F-WELD fatigue test
30% stress reduction
Test life 12000 hrs, versus fleet replacement
at least every 1025 hrs
redesign
Simplified in-situ manufacturing
method
Non-circular hole in closely spaced aluminium alloy stiffeners
For difficult to access locations
Two cutting steps
3. Design and manufacture of
optimised rework for launcher
housing
Finite element
modelling
P2
P1
Fully
constrained
on cut surface
Fully
constrained
on cut surface
P1 = 20kN, P2 variable
P2 varied to match crack location and achieve a robust shape
Geometric constraints for shape
optimisation
Trade off between thickness of cutting tool, stress
reduction and cut depth (nominally 1.1 mm)
Comparison of elastic stress
concentration
Nominal
Optimal rework
Optimal rework shape removes crack and reduces elastic Kt ~ 33%
Optimal rework shape is robust to variation in P2 within design range
Reworking jig
Fixture is portable
Cutting tool has optimised shape
Typical manufactured
profile
Nominal
1.1 mm depth optimised rework
Typical manufactured profiles
Optimised Profile - 1.1 cut. 40KN Peak load
Line A
Line B
1
Y, mm
X, mm
Nominal shape
FEM optimesed shape
1
Variation in position of manufactured profiles is acceptable
Typical deviation +/- 0.2 mm
4. Fatigue testing and NDI
developments
Coupon fatigue testing
Normalised spectrum derived from FT245
Spectrum developed from FT245 Wing Tip jack loads
Represents 55 flight hours & included marker band for Quantitative Fractography (QF)
Peak load (20kN on each flange) to replicate in service cracking
Fatigue testing – typical failure
surfaces
Nominal profile coupon
Optimised profile coupon
Fatigue testing – crack growth
results
10
10
8
8
Crack depth (mm)
Crack depth (mm)
Depth is through flange
6
4
5_1
11_1
13_1
14_1
15_1
2
6
4
2
0
20B1
21B1
22B1
23F1
24F1
0
0
5
10
15
20
No. of blocks
25
30
35
40
0
5
10
15
20
25
30
No. of blocks
Nominal
Optimal rework
35
40
Fatigue testing – average crack depth
Crack depth (mm)
3
2
1
Nominal Average (Primary)
Optimal Average (Primary)
0
0
5
10
15
Life (no. of blocks)
20
25
30
Fatigue testing – typical life
extension
Nominal
2.0
1.1 mm deep optimal
Crack depth (mm)
1.5
1.0
0.5
Launcher removed from service
0.0
0
5
10
15
20
Life (no. of blocks)
Assume housing repaired by removing crack of 0.5 mm depth
Average life increased from 4 to 15 blocks
25
NDI developments for coupon
testing
Eddy current method conducted at
the end of each spectrum block
Probe for Nominal shape
Cracks of 0.2 mm deep by 1.13
mm surface length or greater were
consistently detected in reworked
profile
Measurements checked
subsequently via QF
For fleet trial a special NDI
technical instruction and a HFEC
kit have been developed based on
DSTO experimental work
Pencil probe for
Optimised shape
Calibration block
5. Fleet trial
Fleet trial
TFSPO is conducting fleet trial involving10 reworked launchers
Performance criteria:
-
Time (AFHRS) required for a fatigue crack to propagate to a
detectable size by HFEC NDI
-
Whether non-detected fatigue cracks will grow to 0.5 mm
depth or greater before the next scheduled inspection
To date, more than 1,500 AFHRS flown and no fatigue cracks have
been detected yet
TFSPO to assess the repair concept for fleet wide implementation
once sufficient data have been collected and analysed
6. Current activities and
conclusions
Further life enhancement - Cold
rolling
Introduces beneficial residual stresses & improves surface finish
Rolling tool developed, it has similar profile to the rework cutting tool
Laboratory testing completed
Cold rolled test results
2
Fatigue crack
growth rate up to
2 mm is slower
1.5
Crack depth (mm)
Coupon fatigue
life to 0.5 mm
depth increased
from 13 to 21
blocks
1
0.5
Nominal
Optimised
Optimised and
Cold rolled
0
5
10
15
20
Life (blocks)
25
30
35
Shape optimisation for life extension of new
launcher housings
Shallow optimised rework
reduces elastic Kt by ~ 46 %
Manufactured shallow depth
rework ~ 0.1 mm
Conclusions
Analysis and testing have shown that rework shape optimisation
is an effective method for recovering cracked housing rails
For example, repair of a 0.5 mm deep crack, gives a life increases from
4 to 15 spectrum blocks (220 to 825 SAFHRS)
To date none of the reworked launchers housings involved in fleet trail
have shown evidence of cracking
Fleet wide implementation will depend on results of flight trials
Successful implementation will lead to cost saving and improvements in
platform availability
Additional cold rolling of the rework shape fatigue critical corner further
enhanced fatigue life
Spares
Numerical Method
2D multi-peak stress minimisation
Initial hole
4
PLATE
σ1
3
Boundary of hole
Optimal hole
σ1
σ3
Γ
Stress
2
Nodes
ni
Hoop stress σi
di
ρ
u
k-1 k 1 2
1
0
ti
i+1
i–1 i
Polygonal constraint
boundary (optional)
-1
σ2
σ4
-2
0
20
40
60
80
% arc length around boundary
• Aiming for multiple constant stress regions
• Each subregion defined by zero crossings
 σ iq − σ thq 
 sc
d iq = 
q
 σ th 


σ thq = max σiq
100

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