Characterization of Reinforced Carbon Composites with Full Field Measurements: Long Gauge Length Compressive Apparatus

doi:10.4236/ojcm.2013.31002

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Open Journal of Composite Materials, 2013, 3, 7-15

http://dx.doi.org/10.4236/ojcm.2013.31002 Published Online January 2013 (http://www.scirp.org/journal/ojcm)

Characterization of Reinforced Carbon Composites with

Full Field Measurements: Long Gauge Length

Compressive Apparatus

Mathieu Colin de Verdiere1,2*, Alexandros A. Skordos1, Andrew Walton3

1Composites Centre, Cranfield University, Cranfield, UK; 2Fluid Structure Interaction, School of Engineering, Southampton Univer-

sity, Hampshire, UK; 3Cranfield Impact Center, Cranfield, UK.

Email: *[email protected]

Received September 2nd, 2012; revised October 15th, 2012; accepted October 27th, 2012

ABSTRACT

A new compressive testing apparatus is developed and used in this research. It has long gauge length to allow digital

image correlation monitoring and anti buckling guides to prevent buckling. It allows the optical recording of strains and

displacements. The novel setup is used to study the compressive response of tufted and untufted Carbon non crimp fab-

ric composites with full field measurements. Experimental results show that the specimens are not bending in the appa-

ratus under compression. Results also show reduced strain concentrations and a large strain field that provides a good

environment for material compressive stiffness characterization. The test proves particularly successful for bias direc-

tion layup of [+45/−45] for which large damage mechanism occurs. However for [0/90] specimens a scatter in com-

pressive ultimate strength was noticed which is due to the difficulty to prepare specimens with best minute accurate

geometry. The compressive apparatus has shown to be a good alternative to existing setups and to provide significantly

more information as well as having the possibility to be used in dynamics with a drop tower.

Keywords: Carbon Composite; Compression Testing; Full Field Measurements

1. Introduction

Composite structures undergo complex deformations

depending on the laminate design, part geometry and the

loading itself. Such structures are subjected to in-plane

tension, shear, compression and out of plane stresses. In

order to simulate the material response for structural de-

sign purposes, tensile, shear, compression, and out of

plane data are required and gathered from tests. Com-

pression testing remains difficult compared to other sets

of tests as specimen misalignments, material defects, and

poor specimen geometry lead to premature catastrophic

failure at the specimen level not representative of the

material itself. Compressive loading of polymeric com-

posites is often characterized by damage occurrence such

as micro buckling (axial) and matrix cracking (bias di-

rection) mechanism. Many researchers investigated these

responses through better compressive apparatus designs

allowing better specimen characterization. Meanwhile in

the last ten years digital image correlation (DIC) has gain

accuracy and a more trouble-free usage due to better al-

gorithms and optics. It can provide full field information

exceeding the strain gauges elongation. The method is

often used in tension to characterize polymeric materials

through failure strain and stiffness measurements. As

stiffness and damage in compression is much different

than in tension the need to investigate this difference

with a full field experimental approach is of interest.

Current compressive apparatus do not allow good use of

full field measurements in compression as the specimens

are often too small or direct view is diverted by the com-

pressive apparatus. In this research a new compressive

testing setup was designed that allowed the use of full

field digital image correlation on polymeric composites

and provided added information on compressive failure,

compressive strain, compressive stiffness and compres-

sive damage behavior. The capacity of the apparatus has

been introduced briefly in [1] but is presented in this pa-

per in greater detail. The paper reviews existing com-

pressive apparatus first and then describes the new com-

pressive design. Subsequently, results using a modified

standard apparatus and the new compressive apparatus

on standard and tufted non crimp fabric composites are

reported. Finally, the results are discussed and com-

pared to the published literature. Conclusions on the va-

lidity of the tests and the material response are presented.

*Corresponding author.

Characterization of Reinforced Carbon Composites with Full Field Measurements:

Long Gauge Length Compressive Apparatus

1.2. Existing Methods for Compressive Loading

Test of Composites

Compressive data are difficult to acquire because fibrous

composite material undergo micro-buckling, splitting,

inter-laminar shear, and the specimen itself is likely to

buckle. Furthermore, load and specimen misalignments

are possible throughout a test which would affect the

strain gage readings and failure strength.

The existing compressive testing set ups are classified

in three different categories depending on the manner the

load is introduced in the specimen.

 The first category is called the end loading method

and introduces the load directly at the end of the

specimen; it is described in standard D695-91 [2].

The standard recommends a long specimen gauge

length and use of anti buckling guides. This set up is

cost effective and versatile but has the disadvantages

of not being able to obtain a complete stress strain

surve and of high sensitivity to specimen geometry

[3]. Westberg [4] recommends the addition of an end

cap at the specimen top to prevent brooming.

 The second category of compressive set ups is the

shear loading method which introduces the load in

shear by clamping the short tabbed specimen in grips.

This type of setup is described in standard D34109/D/

D34010M-03 [5]. The apparatus is called the Cela-

nese or the IITRI rig (Illinois Institute of Technology

of Research Institute).

 More recently a final category of setups emerged as it

was found that combinations of end and shear loading

methods reduced stress concentrations at the tab tip

and at the loaded end [6]. The RAE rig used this

method in the early eighties [7] but in recent times

new advanced compressive apparatus such as Wyo-

ming CLC (combined loading compression) [8] and

ICSTM (Imperial College London) [9,10] using com-

bined loading were developed and led to standard

D6641/D6641M [11].

Comparison of compressive testing methods via round

robins or more limited research occurred more than 15

years ago [3,4,7,12-15] and showed some contradicting

trends. Seng [14] established that the Celanese apparatus

provided lower compressive strength than other testing

methods, which opposed the findings of Aoki [16].

Woolstencroft [17] showed that the RAE apparatus con-

ferred a compressive strength near the true value of the

material. The end loading method was reported to be

superior to others and yielded a higher compressive

strength [6] for thick specimens. Westeberg [4] and Ab-

dallah [12] found that the differences between shear

loading and end loading methods were small in their op-

timum configurations, as long as the end did not crush

and that in either case the tabs did not debond [12]. Fi-

nally specimen preparation was found to be critical to

achieve optimum compressive testing conditions [4,12,

16,17].

1.3. Notes on Experimental Compressive Testing

Loading introduction can lead to stress concentrations

that affect strain readings on small specimens, leading to

improper material characterization. Adams [12] found

that shear loaded specimens had higher stress concentra-

tions at the tab tip while end loaded specimens showed

more stress at the end loaded tip.

As the specimen becomes thicker, stress concentra-

tions occurred at the end tabs making the estimation of

stiffness difficult and lowering the overall compressive

strength [14]. Bogetti [18] found that end effects on the

IITRI apparatus could modify the Young’s Modulus to

30% of its true value. Therefore Harper [3] recommends

the use of longer specimens. Adams [12] acknowledged

that specimen gauge length had little influence on com-

pressive strength, but then the failure may occur away

from the strain gage location.

Nisitani [19] found that strength was constant from 3.2

to 12.7 mm specimen gauge length using a Celanese rig.

Small gauge length specimens provided a value close to

the true strength but bared more difficulties with strain

measurements. Salvi [20] tested off axis composite speci-

mens in compression and recommended long specimen

gauge length with anti-buckling guides for proper char-

acterization. It is understood that longer gauge length

provides a uni-axial strain field in the middle of the spe-

cimen (Figure 1).

According to Harper [3] misalignments create a bend-

ing moment in the specimen that makes the strain re-

cording on both sides of the composite different giving

improper stress strain curve and modulus. Similarly

Woolstencroft [7] with the RAE rig found that in-plane

misalignments reduced the compressive strength by 50%.

After reviewing the literature, it is understood that

Clear strain

field

Stress

concentrations

Stress

tti

(a) Small specimen

unloaded

(b) Small specimen

loaded

Figure 1. Stress concentrations in compression.

Characterization of Reinforced Carbon Composites with Full Field Measurements:

Long Gauge Length Compressive Apparatus

most compressive apparatus have been designed specifi-

cally to reduce the scatter in ultimate strength of the ma-

terial, and only in secondary for compressive modulus or

strain measurements. For practical reasons many resear-

chers use an extensometer to characterize the specimen

compressive displacement from which they compute an

overall strain to avoid gluing a small strain gauge on

such reduced specimen size.

1.4. Compressive Failure and Damage

Composite materials have lower properties in compres-

sion than in tension because of catastrophic instability

via microbuckling, inter laminar shear failure [10], and

stress concentrations near the tabs [19]. Failure is initi-

ated by local shear, followed by delamination and buck-

ling of layer [3,21]. Berbinau [15] found that kink bands

were initiated at the laminate free edge, at a void, or at

points of high fibre curvature. As microbuckling occurs

due to the buckling of fibres whilst kink bands are

formed at peak stress when the matrix is deformed plas-

tically [22]. The best model to predict microbukling was

the Xu-Reifsnider model [23] and to predict kink bands

the Budiansky model [22].

2. Experimental Procedures and Apparatus

2.1. New Compressive Testing Apparatus

Light reflection on the specimen can make the calibration

of the optical system difficult and time consuming.

Therefore the new compressive apparatus is designed to

provide maximum and regular enlightenment of the sam-

ple (Figure 2). The specimen is loaded via end and shear

loading at the bottom end, while it is simply end loaded

at the top end. In order to improve the end load introduc-

tion in the composite and reduce stress concentration, 1.5

mm aluminum tabs are added at each end of the speci-

men. To prevent any delamination or splitting a cap is

added on top of the composite. Anti-buckling guides are

composed of steel struts 3 × 5× 50 mm (Figure 3).

Possible specimen thicknesses that can be tested vary

(a) Drawing view (c) Apparatus with specimen

Figure 2. Long gauge length compressive rig.

from 1 to 4.5 mm. The specimen length is 80 mm, the

width 25 mm and the gauge length can vary from 70 to

60 mm.

2.2. Specimens

Specimens are manufactured from carbon fibres and ep-

oxy resin. TENAX HTA 6 K carbon fibre arranged in a

NCF (Non CrimpFAbric) of 540 g/m2 areas density with

a +/−45 degrees bi-diagonal architecture are used. Some

of the specimens are reinforced through the thickness by

tufting with a 1K carbon thread and 5 × 5 mm density.

The matrix is RTM6 resin. The specimens are made via

resin infusion at high temperature. Specimens are sprayed

in white and black paint to form a speckle pattern (Fi-

gure 3). Paint dot positions are recorded and interpreted

throughout the tests with 2 cameras. The layups and size

of specimens are reviewed in Table 1 for the different

test configurations.

2.3. Strain and Displacement Recording

Strain gauges measurements are limited by the gauge

size and the maximum deformation they can hold before

failing and the necessity of gluing and wiring them. An

alternative solution such as DICM is more time consum-

ing in data post processing but it provides further infor-

mation as strain and displacement are monitored on the

whole surface. Optical strain field measurement requires

the use of one camera (in 2D) or two cameras (in 3D);

the latest provides the opportunity to check for out of

plane deformation with positions and angles of the cam-

eras computed via pictures recorded with a calibration

target (Figure 4(a)).

The camera position and light intensity are important

features of the design of the new compressive set up

(Figure 4). Optical recording provides vertical displace-

Aluminium

Composite

Aluminium

(a) Steel anti

buckling guides

(b) End

cap

with tabs and speckle pattern

Figure 3. Anti buckling guide cap and specimen.

Table 1. Apparatus and specimen used.

Lay up Specimen size Specimen

gauge lenght

Specimen

numbers

[0/90]6s and [+45]6s 80 - 72 mm±55 mm 35

Characterization of Reinforced Carbon Composites with Full Field Measurements:

Long Gauge Length Compressive Apparatus

ment on the specimen area. The large strain field also

permits to analyze the specimen failure in compression

throughout the loading using several strain extraction

methods (Table 2). This permits to gain useful informa-

tion such as possible bending or strain concentration on

the specimen that could invalidate the tests. Displace-

ment data can be extracted in specified locations such as

top and bottom of the specimen from which an overall

strain can be calculated. If bending or misalignments

occur then the two extractions technique yields signifi-

cant difference. Comparing the two measurements can be

used to check that the specimen is not bending and that

the test is valid.

3. Experimental Results

The long compressive apparatus provides a long area of

interest that allowed the characterization of [0/90] and

[+45/−45] tufted and untufted specimens. The strain field

allows the identification of maximum strains regions

which are in agreement with the locations of specimen

failure. Tufted and untufted specimens are tested in axial

compression on [0/90] specimens and bias direction com-

pression on [±45] specimens.

3.1. Axial Behavior [0/90]

a) Untufted material

Figure 5 describes the behavior of the untufted [0/90]

specimen in compression. It is observed that the strain at

failure is higher than the other extracted strains, which

Camera

(a) Calibration target (b) Top view of full field strains

recording on specimen

Figure 4. Optical strain recording set up.

Table 2. Different extraction methods.

Strains extraction

methods Corresponding method

Failure The strains were extracted on the failure area

Average centre The strains were average on the

whole specimen.

“Average displacement”

“global extensometer”

The strains were calculated from the

displacement at the top and bottom of the

sample.

“Local central point”

or “middle”

The strains were extracted in the middle of the

sample and were assumed to be similar to a

strain gauge.

means that the failure is relatively localized. The average

displacement and the average centre strain are very close.

Thus it is deduced that the specimen was not bending and

that the test was therefore valid. The specimen cross sec-

tion was observed with an optical microscope.

The failure modes identified are delamination, kink

bands, matrix cracking and fibre breaking (Figure 6).

The “0” plies can be observed in white and the “90” plies

in dark grey. It can be observed that the non crimp plies

have some waviness throughout the thickness of the ma-

terials even so they are not weaved. It can be also noticed

that little damage occurs away from the failure area. Fi-

nally the 0 degrees fibers are rarely at a strict 0 degrees

from the specimen axis.

-500

-400

-300

-200

-100

-1.00% -0.75%-0.50% -0.25%0.00%

ress

(MP

)

at fa ilure po int

average c e ntr e

aver a ge d isplaceme nt

local centr al poin t

Figure 5. Compressive stress strain response of [0/90] un-

tufted specimens.

(a)

(b)

Figure 6. Optical microscope images of untufted [0/90] spe-

cimens; (a) specimen failure, (b) fibre waviness.

Characterization of Reinforced Carbon Composites with Full Field Measurements:

Long Gauge Length Compressive Apparatus

b) Tufted material

Results of tufted specimens in compression are also

monitored with the optical strain field apparatus. The

specimen behavior at compressive failure is shown in

Figure 7. On that sample the failure occurred near the

top tabs on the monitored side. In most case the failure

would occur in the middle of the specimen, also it was

not always the case. The image show that failure was

much localised (blue) and that after failure the rest of the

specimen unloaded itself proof of some overal minor

damage except at failure location.

Results also show that the specimen is not bending as

the average calculated displacement strain and the aver-

age center strain are close (Figure 8).

The specimen was observed under a stereoscope. Fi-

gures 9 (a) and (b) shows the resin rich area caused by

the through thickness tufts.

The global strain is used to compare and analyze the

results. The global strain is an average of the strain in the

gauge area.

This strain is preferred to the use of failure strain be-

cause the tufted specimen failure showed that failure

could in some cases be localized on one side of the speci-

men due to the tufts holding the material together. The

responses of the tufted and untufted composites in com-

pression are illustrated in Figure 10.

-0.01125 eyy -0.00

Figure 7. Strain field on the tufted specimen.

-500

-400

-300

-200

-100

-1.00% -0.75% -0.50% -0.25%0.00%

strain

stress (MPa)

at f ail ure point

average centre

average displacement

l ocal central poin t

Figure 8. Compressive stress strain response of [0/90] tufted

specimens.

Untufted materials show a stiffer and stronger respon-

se than the tufted NCF composites.

This is partly explained by the resin rich area at the

tuft site which lowered the fibres volume fraction locally

and the induced fibre misalignment. The data of the ma-

terials in compression are summarized in Table 3.

Micrographs of failure are illustrated in Figures 11 (a)

and (b). Both specimen types show kink bands, large

delamination, fibre failure and kinking. But the tufted

specimen shows that the damage was smaller on the spe-

cimen surface but propagated through the specimen

thickness. The untufted specimen shows more micro de-

lamination and a more concentrated damage area.

3.2. Compression in the bias direction [+45/−45]

In plane bias direction tests are commonly performed in

tension following standard procedure with a laminate at

[+45/−45]. The new long gage length allow for compres-

(a) (b)

Figure 9. Through the thickness resin rich area due to the

tuft (a) resin rich area and (b) resin rich area bigger magni-

fication.

-400

-300

-200

-100

-0.80%-0.55% -0.30% -0.05%

stress (MPa)

Tufted global

Untuf ted global

Figure 10. Comparison of [0/90]s tufted and untufted strain

stress response.

Table 3. Mechanical properties.

Untufted Tufted

Compressive modulus (GPa)63 56

Compressive ultimate strain (%)0.70 0.71

Compressive strength (MPa)418 374

Characterization of Reinforced Carbon Composites with Full Field Measurements:

Long Gauge Length Compressive Apparatus

sive bias direction testing since the specimen gauge

length is twice as long as its width. Effectively the [±45]

tows needs to slide freely relatively to one another to

allow the shearing behavior to occur fully. On small

specimens the width is often wider that the gauge length

which means that then the tabs prevents the full bias di-

rection shearing to occur fully. On the other hand the

long specimen gauge length and conventional width of

the new specimens permit such mechanism to occur and

to be monitored fully. Again different information is ex-

tracted from the strain field to follow the specimen com-

pressive shear response.

a) Untufted material

The untufted specimen response is illustrated in Fi-

gure 12(a). The strains recorded in the specimen middle

and at failure are similar because failure occurs in the

middle of the specimen. Lower strain was measured be-

fore ultimate failure as some of the specimen tends to

relax when part of it is failing. This difference is ex-

plained by the fact that failure is localized to a specific

area of the specimen.

b) Tufted material

The strain recording on the tufted specimen is similar

to the untufted specimen and can be seen in Figure 13.

Difference between middle and failure strain indicates

that the shear failure does not take place in the middle.

Large strain at failure compared to the overall strain

(a) (b)

Figure 11. (a) Untufted specimen; (b) Tufted specimen.

-16 0

-14 0

-12 0

-10 0

-80

-60

-40

-20

-10.00% -8.00%-6.00%-4.00%-2.00%0.00%

shear strain ε

stress (MPa)

e12 middle

e12 global as ext ensiometer

e12 failure

Figure 12. Stress strain response in the bias direction; un-

tufted.

highlights the localization of the damage area.

c) Comparison between tufted and untufted on ±45

Comparison of the tufted and untufted material is il-

lustrated in Figure 14. The tufted material shows a

stronger response meaning that the specimen will absorb

considerably more energy during failure. The strain at

failure was similar for the two materials.

The untufted specimen shows large shear delamination

of the layers through the thickness with some shear fail-

ure and delamination of the tows on the surface (Figure

15). Tufted specimens show less severe delamination on

the surface and through the thickness. Nevertheless de-

lamination is well spread through the thickness.

4. Discussion

The work of Bogetti [18] illustrates that the compressive

Young’s Modulus could be up to 30% off its true value

with the IITRI apparatus due to stresses and strains con-

centrations. Part of the literature recommends the use

long gauge length to avoid stress concentrations; such a

configuration is particularly promising combined with

the use of DICM.

-16 0

-14 0

-12 0

-10 0

-80

-60

-40

-20

-0.09 -0.07-0.05 -0.03 -0.01

strain e12

stress (MPa)

e1 2 middle

e12 global as extensiometer

e12 fail ure

Figure 13. Stress strain response in the bias direction;

tufted.

-160

-140

-120

-100

-80

-60

-40

-20

-0.1-0.08 -0.06 -0.04 -0.020

strain e12

stress (MPa)

U nt ufted t cnc542middle!e12 failure

T uft ed e12 f ail ur e

Figure 14. Comparison stress strain response in the bias

direction for tufted and untufted composites.

Characterization of Reinforced Carbon Composites with Full Field Measurements:

Long Gauge Length Compressive Apparatus

(a)

(b)

(c)

Figure 15. Image of tufted and unutfted specimen in bias

direction failure (a) surface untufted damaged area; (b)

large through the thickness delamination for untufted

specimen, tufted specimen limited delamination of the tows,

localised fibers failure on the surface.

The compressive apparatus provides information that

is not obtainable in compression with standard setups

with strain gauges or extensometer recording equipment.

This is due to the damaging nature of compressive re-

sponse that is rather complex and particularly well moni-

tored by the long gauge length and full field measure-

ments. Full field measurements permit to distinguish

between different areas of the material and provide sig-

nificantly more information that allows checking that the

specimen does not bend under compressive loading. The

DICM method is an adequate tool to measure the mate-

rial response with the new compressive apparatus. In

addition to the test validation, the tool provides full stress

strain curves for both axial and bias direction compres-

sive loading.

The apparatus is understood to provide complete com-

pressive bias direction characterization that was not pos-

sible in previous setups as the gauge length relative to the

specimen width was too small.

The new setup proves to be reliable on cross ply lami-

nates as on [0/90] laminate the ultimate strength can have

some scatter. The cap end and added tab improve load

introduction in the specimen preventing any splitting or

brooming. Nevertheless the standard deviation of

strength is relatively high on [0/90] specimens; such re-

sponse is also observed by Aoki [16] and is inherent to

axial compression response. This is explained by several

phenomena:

- The possible slight misalignments of the 0 degrees

plies from one layer or specimen to the other.

- The specimen end not being perfectly flat and paral-

lel.

- A very small amount of specimen misalignment is

inevitable.

Micrographs taken showed that 0 degrees plies could

have a slight out of the plane waviness that would vary

from specimen to specimen; this would reduce the axial

compressive strength significantly and also increase the

scatter.

To finish off the advantages and disadvantages of the

new long gauge apparatus are listed below, with the ad-

vantages being:

- Possibility to check for misalignment in 3 dimen-

sions.

- Full strain field on the specimen until compressive

failure.

- Possibility to use the visual strain recordings as ex-

tensometer to check for misalignments.

- Full stress strain curves recordings in the axial direc-

tion.

- Full stress strain curves in compression shear.

- Images of the displacements and strains.

- The possibility to perform photo mechanics on the

images to generate new fields.

- Very reproducible results on cross plies laminates.

- Permit the study of open hole specimen in compres-

sion.

- Possibility to use the rig in dynamic conditions.

Also the disadvantages of the new apparatus are:

- The rig requires near perfect specimen preparation on

[0/90] or [0] layups, (flat end, accurate layup, low

laminate waviness (good manufacture)).

- Only the use of optical recording can be truly trusted

with this set up as failure might occur away from the

strain gauge location and the failure strain might ex-

ceed its capacity.

- Tabbing is required for improved load introduction.

- Low loading speed is required in the axial direction

(0.3 mm/min) as 1or 2 mm/min can be used in the

bias direction.

Characterization of Reinforced Carbon Composites with Full Field Measurements:

Long Gauge Length Compressive Apparatus

5. Conclusions

The new long gauge length apparatus provide a uniform

strain field prior to damage of the material itself, that

allows characterization of the material in compression

with repeatable results in terms of stiffness and strength

on the [±45] laminate and on stiffness for [0/90]s but for

this layup less repeatable results are observed on ultimate

strength.

From a material point of view tufts reduce stiffness

and strength by about 12% but prevent extensive speci-

men delamination in the axial direction. In the bias direc-

tion tufts oppose the shearing of the plies

The new compressive apparatus has shown to be a

good alternative to previous setups. As the DIC usage

evolves and improves, further understanding could be

captured in compression. Further work has looked at dy-

namic compression testing with such apparatus and the

generation of kinematic fields [24].

6. Acknowledgements

EU financial support through ITOOL/(ITOOL-FP6/

516146), DICM equipment Limess Messtechnik and

Software GmbH as well as Professor A. K. Pickett.

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