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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: gigliotti@emse.fr

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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