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Residual Curing Stresses in ThinResidual Curing Stresses in Thin[0/90] Unsymmetric Composite Plates[0/90] Unsymmetric Composite Plates
Marco Gigliotti°, Michael R. Wisnom, Kevin PotterDepartment of Aerospace Engineering, University of Bristol, UK
°current address: Département MEM, Ecole des Mines de Saint-Etienne, France contact: [email protected]
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COMPAVS Research ProgramUni of Bristol, Airbus UK, QuinetiQ, AugustaWestland, Bombardier Shorts
Aim :Understanding, predicting and controlling
residual stresses, distortions and variability coming from the cure of high temperature composite parts
Understanding of basic phenomena is needed
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1.1. IntroductionIntroduction2.2. Experimental activity3.3. Simulations4.4. Conclusions
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Main sources of residual curing stresses are:
- Cooling (Tcure-Troom)
- Resin Chemical Shrinkage
- Tool interaction
- Thermal, degree of cure and Vf gradients- ….
AUTOCLAVE
Tool
Laminate
Curing cycle
t
T
P
Generalities on Residual Curing StressesAUTOCLAVE MOULDING TECHNIQUE
Introduction (1/2)
Tg
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Introduction (2/2)
Use of flat thin [0/90] unsymmetric samples
Aim : Elimination or minimisation of many parameters, such as:- thermal, degree of cure and Vf gradients through the thickness- cure shrinkage- tool interaction
Investigation on the residual deformation of partially or totally cured samples
Method :
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1.1. Introduction2.2. Experimental activityExperimental activity3.3. Simulations4.4. Conclusions
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Experimental activity (1/13)Physical principles :
What we measure :- the curvature k after partial or total cure- the stress free temperature Tsf, at which samples are flat- intermediate curvatures between Troom and Tsf
0°
0°/90°T
90°
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Experimental activity (2/13)Physical principles :
Tsf Tg
T
Strain/Curvature
Tinitial > Tg
Cooldown + Reheating
Tinitial
t
T
Tg
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Experimental activity (3/13)Physical principles :
A: only thermoelastic strains are in the structureB: non-thermoelastic strains are in the structure
Tinitial T
Tinitial Tg
Strain/Curvature
A
B
Cooldown
t
T
Tg
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Experimental activity (4/13)Physical principles :
A: only thermoelastic strains are in the structureB: non-thermoelastic strains are in the structure
Tinitial Tsf
Tinitial Tg
T
Strain/Curvature
Tsf > Tinitial
A
B
Reheating
t
T
Tg
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Experimental activity (5/13)Measurement apparatus :
oven
CCD video-cameraPC
L’
ht
then k 22' 4
8
hL
h
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Experimental activity (6/13)Interrupted Cure Cycles (ICC) :
20
40
60
80
100
120
140
160
180
200
0 50 100 150 200 250 300 350
Time (min)
Tem
per
atu
re (
C)
Temperature
ICC
A
B
C D E F G H
material: AS4/8552, oven curing samples: 300mm x 30mm x 1mm
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20
40
60
80
100
120
140
160
180
200
0 50 100 150 200 250 300 350
Time (min)
Tem
per
atu
re (
C)
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
Cu
rvat
ure
(1/
m)
Temperature
Curvature
A
B
C
D E F G H
Experimental activity (7/13)Results : Residual curvatures
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20
40
60
80
100
120
140
160
180
200
0 50 100 150 200 250 300 350
Time (min)
Tem
per
atu
re (
C)
Temperature
Stress Free Temperature
A
B
C
D E H
Experimental activity (8/13)Results : Stress free temperatures
The reaction rate slows down at the vitrification pointTsf > Tcure for samples cured beyond vitrification
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Experimental activity (9/13)Results : Reheating sample B
0
0,2
0,4
0,6
0,8
1
1,2
20 30 40 50 60 70 80 90
Temperature (C)
Cu
rvat
ure
(1/
m) CYCLE B:Heat-up
CYCLE B:Cooldown
Tsf
The increase of Tsf indicates post-cure
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Experimental activity (10/13)Results : Post curing effects
20
40
60
80
100
120
140
160
180
200
0 50 100 150 200 250 300 350
Time (min)
Tem
per
atu
re (
C)
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
Temperature
Curvature
'Post-cured' curvature
A
B
C
D E F GH
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Experimental activity (11/13)Results : Reheating curves
-0,5
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
20 40 60 80 100 120 140 160 180 200 220
Temperature (C)
Cu
rvat
ure
(1/
m)
CYCLE B (Half of second ramp)
CYCLE C (End of second ramp)
CYCLE D (20 mins after second dwell)
CYCLE E (40 mins after second dwell)
CYCLE H (End of the cycle)
Linear behaviour, curves have the same slope
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Experimental activity (12/13)Results : Tool effect
Tooling system Surface against the
tool
Average curvature (1/m)
Stnd Deviation (1/m)
90 3.56 0.014 Aluminium plate (with release film)
0 2.89 0.041
90 3.65 0.064 Carbon plate (with release agent)
0 3.01 0.112
90 3.43 0.045 Aluminium plate (without release film)
0 2.74 0.08
No significant differences (level of confidence 5%)
Autoclave cured samples
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Experimental activity (13/13)Results : Tool effect
Sample Average Curvature (1/m) Standard Deviation (1/m)
Cured with release film 4.11 0.16
Cured without release film 3.98 0.08
No significant differences (level of confidence 5%)
Oven cured samples
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1.1. Introduction2.2. Experimental activity3.3. SimulationsSimulations4.4. Conclusions
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Simulations (1/6)
Model generalities
A FE Abaqus code is used for modelling the thermoelastic behaviour of 0/90 thin plates
during the cooldown from the stress free temperature
- shell 4 node elements with reduced integration (S4R)- temperature differentials applied uniformly in one static step- option nlgeom (small strain, moderate rotations)
Material properties AS4/8552
EL (MPa) ET (MPa) LT GLT (MPa) T(1/°C) 135000 9500 0.3 5000 3*10-5
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Simulations (2/6)
Remarks on the thermoelastic behaviour of 0/90 thin plates:
- according to the Classical Lamination Theory (small strain, small displacement) deformed shapes are saddles
- due to large displacements, plates in some range of in-plane dimensions (or thickness) exhibit cylindrical deformed shapes and/or strong non-linear behaviour with temperature
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Simulations (3/6)
Remarks on the thermoelastic behaviour of 0/90 thin plates:
For narrow plates (AR>10) the deformed shape is almost a saddle with curvatures which vary almost linearly with temperature.
AR>10
principal curvature lateral bow
For such samples, predictions from CLT are good
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Simulations (4/6)
0
20
40
60
80
100
120
140
160
180
200
0 50 100 150 200 250 300 350
time (mins)
T (
°C)
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
Cu
rv (
1/m
)
Cure cycle
Experimental curvatures
Predicted curvatures
Stress free temperatures
Results :
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Simulations (5/6)Results :
-0,5
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
20 40 60 80 100 120 140 160 180 200 220
Temperature (C)
Cu
rvat
ure
(1/
m)
CYCLE B (Half of second ramp)
CYCLE C (End of second ramp)
CYCLE D (20 mins after second dwell)
CYCLE E (40 mins after second dwell)
CYCLE H (End of the cycle)
CYCLE B (Model)
CYCLE C (Model)
CYCLE D,E,H (Model)
3*10-5 1/°C
The offset indicates the non-thermoelastic portion of residual curvature (< 5%)
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Simulations (6/6)Results :
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
20 40 60 80 100 120 140
T (°C)
Cu
rva
ture
(1
/m)
Tcure=Tsf
ABAQUS
Experimental
HTA/913C Composite system (cured at 120°C)
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1.1. Introduction2.2. Experimental activity3.3. Simulations4.4. ConclusionsConclusions
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Conclusions (1/1)
residual curvature and stress free temperature monitoring givesexhaustive information about the cure process of composites
the stress free temperature of AS4/8552 samples cured beyond thevitrification point is found to be higher than Tcure
the stress free temperature of HTA/913C samples is found to be equal to Tcure
simulations allow us to find values of T (below Tg) and to estimate non-thermoelastic sources of residual stress for the AS4/8552
non-thermoelastic sources of stress may be ascribed to resin chemical shrinkage