channel warping report

8/10/2019 Channel Warping Report

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IMPERIAL COLLEGE LONDON

STUDY OF COMPOSITE CHANNEL WARPING 1

A STUDY OF WARPING OF CURED COMPOSITE PLATE AND U-

CHANNEL MADE WITH CARBON/EPOXY UNI-DIRECTIONAL PRE-

PREG

ARJUN RADHAKRISHNAN

CID: 00704997

SUBMITTED ON: 16/03/2012

"#$%&"'%(

A study of factors influencing distortions of cured laminated Carbon/Epoxy composite plates

and u-channels. Asymmetric, anti-symmetric and symmetric laminate sequences are used to

manufacture the specimens. Comparisons of actual and predicted distortions are reported.

Channels exhibit springing phenomenon and major factors are stacking sequence and

thermoelastic springing. In plates asymmetry and orthotropic thermal expansion coefficients

are the major influences. Understanding the phenomenon is essential to obtain dimensionally

stable composite parts. Applying the predictive capability to obtain naturally curved parts is a

novel application of the phenomenon observed here.


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1. OBJECTIVES

Manufacture channels with varying layup, simultaneously plates of similar layups as the

channels. Quantify the deformation in the cured products using measurement of change in

corner angles and arm curvatures. Estimate the curvature of the plates using LAMINATEANALYSIS PROGRAM (LAP). Further predict the channels corner angle and arm

curvatures, using thermal models and classical laminate theory (CLT). Compare the data and

comment, justify the qualitatively and quantitatively the channel warping.

2. INTRODUCTION

Composites are now extensively used to replace other materials as both structural and

non-structural members. As their production increases, the dimensional precision becomes

increasingly important. Composites exhibit orthotropic properties and hence like any other

properties their coefficient of thermal expansion (CTE) varies with directions. The force-

strain and moment-curvature equations of a composite laminate is expressed as 1:

!

! ! ! !

! ! !

!

From these equations it can be logically derived that there is coupling of shear,

extension, bending and twisting for a general case of asymmetric laminate. When

manufacturing laminates they would contract due to thermal effects from cure to the room

temperature. When this thermal contraction occurs a general laminate due to coupling effects

would come out deformed.

Figure 1 The spring back of angle is shown by the arrow 2

So the main factors influencing the distortion of the finished product would be:

1. Stacking sequence

2. Geometry of the product


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Table 1 List of material properties 6, 8

Fibre Carbon Matrix Epoxy

E 11 , (GPa) 129 ! 11 (strain/ oC) 1.50 e-06

E22

(GPa) 8 !22

(strain/ oC) 3.50 e-05

"12 0.31 Resin by Weight (%) 35

G 12 (GPa) 3 # $

0.024%11 0.32 e-04

%22 3.3 e-03 # r

0.299%through thickness 6.7 e-03

&V -0.0069

Mold CTE 23.1 e-06

Table 2 Laminate sequences

Sl

No:Layup Comments

1 ./01010102 Unsymmetrical

2 .010101/02 Unsymmetrical

3 ./01/010102 Anti-symmetric

4 ./0101/0102 Anti-symmetric

5 ./010101/02 Symmetrical

6 .0101/01/02 Anti-symmetric

7 .01/01/0102 Symmetrical

8 ./01010102 Unsymmetrical

!"3" ,('45 &67 %&648&9'4)*6:The mold as shown in fig 4 is made of aluminum with a PTFE coating on its surface.

Pre-preg is obtained in rolls and the required dimensions are cut from it. Care must be taken

to make sure the shorter side is considered as the 0 o direction. This should concur with the

definition of 0 o as the direction longitudinal to the channel section. The layups in table 1 are

defined outside to inside on the mould. While laying up 90 o fibres the lamina would have the

tendency to be flat due to high stiffness of the fibres. Tacky tape could be used to hold the

layer until cure.


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Figure 7 Oven at 40o

C to keep the specimens after manufacturing

A vacuum table can be used periodically to consolidate. Similar procedure is adopted

to manufacture the plates also. The laminates are subjected to the cure cycle as shown in fig 5

with the cure temperature of 180 oC. The setup used for autoclaving plate is shown in fig 6

and a similar setup to suit the channel is also used. Once manufacture all the channels and

plates except plate and channel 8 are kept in oven at 40 oC as shown in fig 7.

4. EXPERIMENTAL OBSERVATION

;"#" ,4)8&9( &55(&)&69( &67 '(


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table 3. All the channels, as shown in fig 8 and 9, except channel 7 exhibited either a spring

forward or backward and the observations are tabulated in table 3.

Table 3 Experimental observations of channels

Channel

No:Springing

Surface appearance Bowing(Length

wise)

Bowing

(Arms)Twisting

Top Bottom

1 Back Rough with

wrinkled

texture

Smooth

surface

No All arms

bowing up

Yes

2 Forward Rough Smooth with

wrinkledtexture

No All arms

bowingdown

No

3 Back Rough with

wrinkled

texture

Smooth No Slight

bowing up

on all arms

Yes

4 Back Rough with

wrinkled

texture

Smooth No Slight

bowing up

on all arms

Yes

5 Back Rough Smooth with

wrinkled

texture

No Nil No

6 Forward Rough Smooth with

wrinkled

texture

Yes Slight

bowing

down in all

arms

No

7 Back Rough Smooth No Nil Yes

8 Back Rough with

wrinkled

texture

Smooth with

wrinkled

texture

No All arms

bowing up

No

;"!" (7:(, >8 'A( 9A&66(+,

The edges of the channels are zones prone to the maximum errors during

manufacturing. A paper edge effect is noticed on all the channels as shown in fig 11. There


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are also errors due to misplacing of the laminates as shown again in fig 11. There is excess

resin at all the free edges they are brittle and susceptible to chip off as shown in fig 12.

;";" 5+&'( 94)B&'4)(,

The plates, as shown in fig 10, exhibited varying curvatures except for plates 5 and 7. Two

stable curvatures are observed for plates 3, 4 and 6. Top surface is rough and bottom surface

is smooth for all the plates. Observations of the 8 plates are summarized in table 4.

Table 4 Experimental observations for plate

Plate

No:Curvature

Surface appearance

Top Bottom

1 Positive in y direction Rough Smooth

2 Positive in x direction Rough Smooth

3 Double curvature with negative in y

direction

Rough Smooth

4 Double curvature with positive in y

direction

Rough Smooth

5 Nil Rough Smooth

6 Double curvature with positive in y

direction

Rough Smooth

7 Nil Rough Smooth

8 Negative in y direction Rough Smooth

Figure 8 (Right to left) Cross-sectional view of the channels 1-8

Figure 9 (Right to left) Top view of the channels 1-8


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Figure 11 Edge and texture of composite channel

Figure 12 Brittle edges of the channel

Figure 13 Bottom surface of the specimen


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5. THEORY AND INFERENCES

C"#" %&648&9'4)*6: (88(9',

The smoothness of the underside of all the specimens could be attributed to the PTFE

coating on the mold for both the plates and channels. On the other hand the roughness on the

top surface could be because of the bleeder cloth kept on top while curing. The glass bleeder

cloth could slightly influence and cause a non-uniform removal of resin while under vacuum.

As observed there were no variation on texture of the surface between the different plates. But

for the channels there was a large variation in the textures. A trend can be identified in this

difference in texture correlating it with the layup orientation used. For preparing a channel

pre-pregs are placed over a channel, a 90 o lamina would be perpendicular to the length of the

channel, as explained in the manufacturing section. The lamina would be stiffer and would

have the tendency to spring back to a stable configuration. Forcing the lamina would result in

wrinkling of the fibres. This correlates well with the observations as the channel with 90 o on

the surface creates a wrinkled texture for that surface, be it upper or lower surface.

C"3" ,'&9D*6: ,(E4(69( &67 9>(88*9*(6' >8 'A()%&+ (6

The plate curvature could be attributed to the asymmetry of the layup. The asymmetry

of the plate layup used in current experiment would generate a coupling between extension

and other distortions, like bending and twisting. Shear-extension would not exist since there

are only 0 o and 90 o plies. From the observation of all the plates it could be suggested that

there is extension-bending coupling, but there is no visual evidence to suggest an extension-

twisting coupling. Once the laminate has been cured at 180 oC, it is cooled down to room

temperature. Due to thermal shrinkage there would be strain in both x and y axes. When there

is a coupling it results in other forms of distortions, resulting in plates like in the current

experiments. This would suggest that the plates to be warped rather than have a dominant

curvature in any axis, as observed in these plates. CTE is higher transverse to the fibre than

along the fiber direction for the material used in current experiment. Hence, larger the number

of 90 o laminates greater is the chance for a curvature in y-axis. This theory seems to be

consistent with the observed curvatures in all the plates except 1 and 2. In plate 1 the

curvature is upwards in y directions indicating a possible reversal in layup. The curvature is in

x-axis for plate 2 indicating an error in cutting the length of the laminate.


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composite parts. Aluminum has a CTE that is greater than that of the laminate as can be seen

by comparing property from table 1 and 6. Hence during the curing stage the mold would be

subjecting the laminate to undue stresses, resulting in residual stresses within the material.

These stresses could result in an additional springing angle, possibly a spring back.

6. MEASURED CHANNEL DISTORTIONS

To quantify the distortions the channel was split into five sections as shown in fig 14.

The sections 2 and 4 are quantified by the change in included angle while curvature is used to

quantify the other sections.

Figure 14 Example of splitting channel into sections

Figure 15 Measurements for the channel

The cross-section of the channels is drawn onto a grid sheet paper with 0.5 HB pencil.

The transition points from flat to curved part is marked by comparing it with mold and then

tracing it into the drawing directly. The gridded sheet was scanned and imported into

SEASHORE, simple co-ordinate system based image software. The grid is measured

using the software and compared with the actual grid size. The ratio of digital to actual size

gives the scaling ratio and is calculated to be 2.


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For sections 1, 3 and 5 as shown in the fig the ' and l are measured. The radius of

curvature and curvature are obtained using eqn 2 and 3. This is applied both for channel and

plates.

Equation 1

! ! !!

!

!

! ! ! ! ! ! !

Equation 2

! !

!!

!

! ! !

! !

Equation 3

! !!

!

The included angles for section are measured using similar methods to other sections.

As shown in fig 15 and using eqn 2 the radius of curvature is measured. Now using eqn 4 the

included angle is measured.

In an ideal channel the sections 1, 3 and 5 would have zero curvature and the included

angle for sections 2 and 4 would be 90 o based on the geometry. Therefore the distortions can

be quantified using eqn 3 and 4. The measured distortions for the channels are tabulated in

table 5.

Equation 4

! ! ! ! !"# ! ! !! ! !

! !

Table 5 Measured channel distortions

Channel

No:

&$SMALL &$LARGE &' yy

Section 1 Section 2 Section 3

1 /"; 33"3C -13.4 -21.23 -18.60

2 F/"! F#/"// 15.44 25.79 54.74

3 #H"3H C"!! -9.30 -15.01 -2.55

4 3"3I #"!/I 0 0 0

5 #"3H !"!I 0 0 0

6 F0"!/ F3"!I 6.41 13.24 16.48

7 #"#! 3"I/ 0 0 0

8 ##"I 3C"C/ -13.32 -21.23 -18.60


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7. ESTIMATED CHANNEL DISTORTIONS

The section 1, 3 and 5 can be quantified using classical laminate theory to estimate the

curvature. There are several methods in literature to estimate the thermoelastic and non-

thermoelastic springing. Prediction model proposed by Kedwards et al is used to estimate thethermoelastic springing 2, as shown in eqn 5. A sum of CLT prediction in combination with

thermoelastic model is used to predict the springing, as shown in eqn 7. The estimated cure

shrinkage influence is calculated by using eqn 8.

Equation 5

!" ! ! !"#$% ! ! ! ! ! ! ! ! ! ! !

Figure 16 Material properties into LAP


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Figure 17 Loading and Layup into LAP


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Laminate Analysis Program (LAP) by anaglyph is used to predict the effects of asymmetry.

The material property, laminate orientation, cure temperature and room temperature are fed to

the system as inputs, as shown in fig 16. The program provides the complete laminate

properties as shown. The thermal effect of the stacking sequence is obtained by inputting the

temperatures as shown in fig 17. To obtain the curvature in y-axis, the x-axis curvature is

constraint to zero as shown in fig 17. The curvature obtained from LAP is used in eqn 6 to

obtain the change in included angle due to asymmetry. The total change in induced angle is

calculated using eqn 7. The estimated curvatures and included angles are tabulated in table 6.

Hence there are two problems encountered in LAP:

1. Obtaining the effect of cool down from cure on laminate

2. Restricting the double curvature

Equation 6

!" !"#$% ! !! ! ! ! !"# ! !" !! ! !

Equation 7

!" !"!#$ ! !" !"#$% ! !" ! ! !"#$%

Equation 8

!" ! ! !"#$% ! ! ! ! ! ! ! ! ! ! !

Table 6 Estimated channel distortions

Channel

No:

&' yy &$SMALL &$LARGE ! yy

Thermal CLT Total Thermal CLT Total

1 -13.48 0"3! /"J; #0"0I F0"3! #H"/J #I"3# 3"#H(F0C

2 13.48 0"3! F/"J; F/"H# F0"3! F#H"// F#H"IH 3"#H(F0C

3 -7.1673 0"0J C"3! C"!# F0"0J /"0! /"## /"H3(F0H

4-1.739

0"0#

#"3I

#"3J

F0"0#

3"30

3"3#

;"H0(F0H

5 0 0"0# 0"00 0"0# F0"0# 0"00 0"0# !"/I(F0H

6 7.163 0"0J FC"3! FC"#C F0"0J F/"0! FJ"/C /"H0(F0H

7 0 0"0# 0"00 0"0# F0"0# 0"00 0"0# !"/I(F0H

8 -13.48 0"3! /"J; #0"0I F0"3! #I"03 #I"3C 3"#H(F0C

8KL MNN OPMQQRNS TPR RSTUVMTR OWLR SPLUQXMYR

UQZNWRQOR KQ S[LUQY \MOX US M[[LK]UVMTRN^

0"3K

%KUSTWLR M\SKL[TUKQ RZZROT KQ OWL_MTWLR ` 0"000/ V


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8. DISCUSSION

J"#" 9>%5&)*,>6 5)(7*9'*>6 &67 %(&,4)(%(6' >8 +&):() )&7**

The corner with larger radius, i.e. section 2, exhibits a variation in estimated and

predicted values as shown in fig. 18 Except for channel 3, 4 and 6 the estimated value is

lesser than the predicted. This seems to be valid as the prediction model does not account for

effects like cure shrinkage and non-uniform curing in the channel. At the same time there

could be error induced by the measurement technique adopted. In channels 3, 4 and 6 the

prediction is more than the measured included angle. Besides, measurement errors it could

also possibly be an over-estimation in the prediction as these laminated where anti-symmetric

and possessed double curvatures. Curvature in x-axis was constrained to obtain a single

curvature possibly leading to an increased estimation of the included angle. In symmetric

laminates only thermoelastic predictions are included as there is no effect due to coupling.

The angle measured for both channels 5 and 7 with symmetric layup is close to 3 o. At the

corners due to excess resin bleeding on the surface there could be a volume fraction gradient,

which in turn would affect the material properties. The excess in measured angle hence would

include cure effects and volume fraction effect of upto 0.2 o. Hence the large variation can

only be attributed to lack of accuracy in measurement and improper layup.

Figure 18 Distortion at larger radii corner


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J"3" 9>%5&)*,>6 5)(7*9'*>6 &67 %(&,4)(%(6' >8 ,%&++() )&7**

The smaller radii seem to be exhibiting a range of springing less than that of the larger

one, as shown in fig 19. This is expected as the major effect in all the channels except

symmetric channels 5 and 7 is coupling. As can be seen from eqn 6 there is a direct relation

between the change in included angle and the radii of the corner. The measured value again

seems to exhibit inconsistency and could largely be contributed by the technique, as said

before.

Figure 19 Distortion at smaller radii corner

J"!" (88(9' >8 ,'&9D*6: ,(E4(69(

The layup has a major influence on the spring forward/back phenomenon, it can be

seen that channels with inner ply being 90 o exhibits spring back while the vice versa leads to

spring forward. Thermoelastic springing in the current experiment results in only spring

forward as inferred from eqn 5 and logic discussed in theory section. It can be noticed with an

increase in number of 0 o plies the thermoelastic effect increases as shown in fig 21, the

channel 1, 2 and 8 has similar spring forward and maximum among all channels, of 0.23 o.

The lowest is for symmetric and [90/0] 2 laminates with 0.01 o for channels 4, 5 and 7. This

could be because of a balance achieved due to equal numbers of 0 o and 90 o about the mid-

plane. These trends are clearly due to the influence of the layups on the laminate thermal

properties.


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J";" (88(9' >8 94)( ,A)*6D&:(

Cure shrinkage is another major factor that influences springing. Anders analytically

measured the effect of cure shrinkage to be close to 0.21 o for 46 % and 0.19 o for 50% fiber

volume fraction. In the current experiment due to lack of cure shrinkage in tangential

direction, it was assumed to be the same as estimated by Anders 2. A spring back of 0.2 o is

predicted. But it should be understood that with decrease in the % of matrix there would be

lesser influence of cure shrinkage. Although not directly related there is a possibility that with

increasing number of 0 o plies there would be reduced spring back effect 4. A direct

measurement of effect of cure shrinkage is not available to compare the prediction.

Figure 20 Distortion in the arms

J"C" 9>%5&)*,>6 >8 5)(7*9'*>6 &67 %(&,4)(%(6' >8 =>?*6: *6 &)%,

The bowing in the arms were quantified and the results are shown in comparison with

the predicted curvature in fig 20. All the predictions are below the measured value that is

expected. Channels 5 and 7 have no bowing owing to their symmetric layup. Channel 4 has a

slight predicted bowing but no bowing could be measured with the current technique. The

bowing is less because channel 4 has achieved a balance about its mid plane as discussed

earlier. The section 3 prediction of channel 2 could be a measurement error, as it seems to be

far off the predicted value. A general trend observed is section 2 has a slightly higher

curvatures, this could be attributed to the restrain at both sides of section and the effect of

springing of corners (section 2 and 5). There is also a slight consistency in measurementobserved as, channel 1 and 8 with similar laminate sequence had similar measured values.


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J"H (88(9' >8 %>*,'4)( &=,>)5'*>6

Channel 8 is exposed to moisture at 20 oC and at moisture content of 1.76 %. There

seems to be very negligible. There can be effect of through thickness expansion as it is higher

than other two directions as shown in table 1. If kept for a longer period the moisture content

absorbed by CFRP would be proportional to square root of time 7.

J"I" >'A() 8&9'>), 'A&' *68+4(69( 'A( ,5)*6:*6: 5A(6(%(6>6

Although, the current study does not comprise any conclusive data on effect of degree

of cure, resin flow, temperature gradient and interaction of the mould, it is worth discussing.

The temperature gradient induced when cooling along the thickness would be prominent as

the thickness increase. A 0.5 mm laminate would have very negligible effect due to this

phenomenon. The channels where subjected to one full curing cycle and the degree of cureshould be approximately close to 80%. This was not measured for the current work, but it

would have an influence on the springing phenomenon. The reduced curing would reduce the

number of cross-linking in the matrix in turn reducing the transverse stiffness. This would not

only affect the coupling influence but also affect the thermoelastic springing, possibly

reducing the spring back. The resin flow would have a more prominent effect on larger parts

since this would result in regions of excess and scarce resin. This in turn would result in

variation of stiffness pointing back to the same influence as degree of cure.A similar set of experiments where done by Oakeshott et al 5. There seem to be effect

of corner radius on the springing phenomenon with an increase in radius indicating an

increased springing. A similar conclusion is applicable in the current work also. As observed

by comparing the fig 22 and 8 it can be noticed the spring phenomenon is consistent. It is also

worth noticing that there is a slight spring back in [0] 4 channels observed by Oakeshott which

due to constraints where not studied in the current experiment.

9. FURTHER WORK AND RECOMMENDATIONS

The current work was undertaken to understand the effect of warping caused to

composite parts. The prediction model considered only of thermoelastic and coupling effect

on springing. Further understanding is required on including the other effects like cure

shrinkage, resin flow and material. An experiment to quantitatively measure the cure

shrinkage would have provided solid data to improve the predictive model adopted here. LAP

program had some inherent bugs that had to be understood to obtain the required results. An


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FEM analysis of the channel would have yielded slightly better predictions but its would

require extensive modeling to capture all the major effects simultaneously.

The accuracy of measurement could have been improved by using a Coordinate

measurement machine (CCM) to pinpoint the exact transition points. The handling of the

composites is of concern as repeatedly subjecting it to mishandling leads to damage. This is

important as the brittle edges could chip off leaving measurements that follow erroneous.

A wide variety of laminate sequences where adopted, but an understanding of

increasing the number of 90 o plies would have been achieved if a laminate sequence of

[90 3/0] or [0/90 3] was tested.

Another exercise to improve the understanding would be to predict the sequence of

laminate required to produce a prescribed curvature. This concept has been utilized in several

applications by imbibing asymmetry to produce the curvature 1.


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REFERENCES

(1) Jones R. Mechanics of composite materials.

(2) Holmberg JA. An approximate analysis of springback phenemenon. SICOMP. Report number:

95-006; 1995.

(3) Patterson JM, Springer GS. Experimental observations of the spring-back phenomenon. ; 1991.

(4) Fernlund G. Experimental and numerical study of the effect of cure cycle, tool surface,

geometry, and lay-up on the dimensional fidelity of autoclave-processed composite

parts. Composites.Part A, Applied science and manufacturing 2002;33(3): pp. 341.

(5) Oakeshott JL, Lemoine D. Experimental study of spring forward in cured laminated U-channels

made from unidirectionally reinforced carbon fiber-epoxy prepregs. PLAST RUB COMPOS

PRO 1998;27: pp. 190.

(6)

Arao, Y. Effect of moisture absorption on dimensional stability in carbon/epoxy composites,2007 .

(7) Springer GS editor. Environmental effects on composite materials. : Technomic; 1981.

(8) Advance composites datasheet, MTM 44-1 Prepreg.


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APPENDIX 1

ABD Matrix for all the laminate sequences ! !

! !

90/0/0/0

0/0/0/90

90/90/0/0

90/0/90/0


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90/0/0/90

0/0/90/90

0/90/90/0

90/0/0/0

channel warping report

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