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.
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
8
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
(c) Longer specimen
loaded
Figure 1. Stress concentrations in compression.

Characterization of Reinforced Carbon Composites with Full Field Measurements:
Long Gauge Length Compressive Apparatus
9
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
(c) Schematics of specimen
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
10
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
0
-1.00% -0.75%-0.50% -0.25%0.00%
s
t
ra
i
n
s
t
ress
(MP
a
)
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
11
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
0
-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
-0.80%-0.55% -0.30% -0.05%
s
t
ra
i
n
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
12
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
0
-10.00% -8.00%-6.00%-4.00%-2.00%0.00%
shear strain ε
12
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
-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
-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
13
(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
14
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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