Kurdistan Journal of Applied Research (KJAR) | Print-ISSN: 2411-7684 – Electronic-ISSN: 2411-7706 | kjar.spu.edu.iq

Volume 2 | Issue 3 | August 2017 | DOI: 10.24017/science.2017.3.26

Flexural Behavior of CFRP Laminate at Elevated

Temperature

Rzgar M. Abdalrahman
Production Engineering and Metallurgy

Sulaimani Polytechnic University
Sulaimani, Iraq

rzgar.abdalrahman@spu.edu.iq

Abstract: A carbon fibre reinforced polymer (CFRP)

laminate forms the surface part of an integrally heated

tool. It was made up of carbon non-crimp triaxial fibre

and SR8100 epoxy in accordance to the stacking

sequence of [(0, ±45)/ (90, ±45)] S, using the resin

infusion (RI) method. The laminate is heated up to

90ºC when the tool is operated; therefore under-

standing the effect of temperature on the flexural

properties is quite significant. This experimental study

is carried out to investigate the flexural behaviour of

the CFRP laminate and finding its flexural properties

under the effect of elevated temperatures. For this

purpose, various CFRP specim-ens were prepared and

tested, using three point bending test method, at

different temperature levels from room temperature to

90ºC. The results show that each of the flexural peak

load, modulus and strength of the laminate decreases

consistently with the increase of temperature. Also the

laminate becomes slightly more flexible and significant

loss occurs in its flexural modulus when the

temperature elevates from 75ºC to 90ºC. The reduction

in the flexural behaviour of CFRP is imputed to

thermal softening of the epoxy polymer matrix

whenever becomes closer to (HDT).

KEYWORDS : CFRP laminate, flexural modulus,

high temperature and flexural test.

INTRODUCTION

Nowadays, application of carbon fibre reinforced

polymer (CFRP) composites have become quite popular

in aerospace, automotive, defence, marine, sporting

goods and infrastructure industries, because CFRP

compared to aerospace aluminium and steel alloys can

provide lower density and expansivity (low CTE) as well

higher specific modulus (modulus/density) and higher

specific strength (strength/density) and chemical

inertness [1]. The CFRP structure is polymer matrix

composites (PMC) reinforced by carbon fibre. The

premixed advanced compound of CFRP, known as

prepreg, is be produced by retaining the polymer matrix

in CFRP combination in a partially cured condition over

prolonged period of time [2]. Both the fibre and

reinforcement components are insoluble in each other

and they are differing in their physical form and

chemical composition.

Properties of the fibre content are generally dominant

in composites and improve their mechanical properties

such as strength and stiffness. However the matrix

stiffness and its mechanical strength are lower than those

of reinforcement, it acts as binder, holds the fibre

together, transfers mechanical loads through the fibres to

the rest of the structure and protects fibres from

mechanical (impact, abrasion and corrosion) and

environmental actions [3, 4]. One of the important

characteristic specific to solid materials is flexural

modulus, therefore many researchers have determined

this characteristic, using the three point bending test, to

understand the effect different factors on the adaptable

mechanical properties of composites [5]. CFRP

composite can provide well-bonded structures with

excellent strength and hardness after curing, but the

viscoelastic properties of polymers causes the main

problem of polymeric materials that results in creep and

a high sensitivity to temperature [6].

The test material, in this study, is a CFRP laminate

used in building an integrally heated tool. The laminate

structure is an epoxy system of SR8100 reinforced by

non-crimp fabric (NCF) triaxial carbon and it is exposed

to heat up to 90°C when the tool is been used. Therefore,

the current study aims to investigate the flexural

behaviour of the CFRP laminate at elevated temperature

and base on this a number of CFRP specimens are tested

to failure, using three points bending, at different levels

of temperature between room temperature and 90ºC.

Results presented that the increase of temperature

reduces the flexural properties consistently and drastic

decrease is observed in the flexural modulus after

increasing the test temperature from 75ºC to 90ºC that a

reaches the heat distortion temperature (HDT).

LITERATURE REVIEW

Many studies have been conducted to define effects

of different factors on the bending stiffness of different

anisotropic composite plates, using a three-point support

bending test. They deduced that the composite properties

are tailorable and they are not only functions of the

constituents but they are also affected by many other

factors, such as; the nature of matrix and reinforcement,

fibre volume fraction, the compatibility between the

components, materials processing technology and

conditions, the dispersion or distribution of the filler in

the matrix, working environment (temperatures,

humidity. etc.), the interfacial structure and morphology,

fibre orientation, laminate stacking, surface waviness

and moulding temperature [5, 7-10]. For instance;

Azzam A. & Li W. [5] deduced that quasi-isotropic

types of stacking sequences of composite laminate

structure exhibits a brittle behaviour, while the

unbalance type exhibits a progressive failure mode

consisting of fibre failure.

Considerable amount of research were conducted

recently to investigate the effect of different environment

mailto:rzgar.abdalrahman@spu.edu.iq

factors, such as; humidity, vibration and especially

temperature on various CFRP structures [11]. Most of

the researches have been carried out on fibre reinforced

polymers (FRPs) that are used in construction,

strengthening and repairing of reinforced concrete

structures [12-14], for example; Gamage et al. [15]

concluded that that epoxy loses its strength rapidly at the

temperature around 73ºC. Also, Reis, J. M. L. [16]

studied the effect of a range of temperatures varying
from room temperature to 90ºC on the performance of

epoxy and polyester polymer mortars (mixture of

foundry sand with the thermoset resin binder) and they

concluded that both the flexural and compressive

strength are changing inversely with the increase of

temperature and the epoxy polymer mortars are more

sensitive to temperature variation.

During testing several concrete specimens externally

bonded with CFRP, Di Tommaso et al. [17] obtained

that the failure load at 40ºC lower than that at room

temperature, while Klamer et al. [18], found

contradictory results because the failure load increased at

40ºC and 50ºC and suddenly decreased above a

threshold temperature of 65ºC. That behaviour was due

to the fact that the epoxies will undergo a transition from

a hard rigid state to a more pliable (rubbery state) when

it reaches the glass transition temperature (Tg),

(midpoint of a transition temperature range) that depends

on the resin or the polymer type [19]. Aforementioned

review illustrates that the mechanical properties of

polymers change considerably at high temperature,

especially when it reaches its HDT.

EXPERIMENTAL WORK

Material:

The material is a CFRP laminate of 3.1 mm thick

and 0.53 fibre volume fraction (Vf), which forms the
surface part of an integrally heated tool. The laminate is

prepared according to the stacking sequence of [(0, ±45)/

(90, ±45)] s, using resin infusion under flexible tooling

(RIFT) processing method. Four layers of non-crimp

fabric (NCF) triaxial carbon of 660 g/m2 areal-weight

from Sigmatex Industries [20], are applied as the

reinforcement, each layer consists three lamina of

parallel fibres laid in three different orientations of (0, ±

45) Figurer 1.

Table 1: properties of the SR 8100 Epoxy (at 20ºC) [21]

Properties

Value

Viscosity (m.Pas) 930±100

Density (g/cm3) 1.158

Tg (°C) 74-81

HDT (°C) 85

Tensile strength (MPa) 60

Tension modulus of elasticity (MPa) 2700

Flexion modulus of elasticity (MPa) 2850

Elongation at break (%) 12

The matrix is the two components, injection and

infusion, epoxy system SR8100 with SD8824 standard

hardener from Sicomin Composites [21]. The resin

properties at room temperature provided by the

manufacturer illustrated in Table 1. The combination

was allowed to cure for 3 days at room temperature and

then the laminate is post-cured in an oven for 16 hours at

80ºC with a ramp rate of 3ºC/min. The laminate is

considered as the most appropriate material due to its

good structural performance at room temperature, but

there is lack of information about its flexural behaviour,
when the tool works and temperature rises to 90ºC.

Figure 1: The CF layers before processing according to

stacking sequence of [(0, ±45)/ (90, ±45)] s.

Test procedure:

Different uniform rectangular specimens, Figure 2,

are cut from the produced CFRP laminate with the

dimensions illustrated in Table 2 and at a span to depth

ratio of 32, in accordance with ISO 14125 [22].

Table 2 : The main dimensions of the test specimen

according to ISO 14125 [22]

Figure 2 : CFRP specimens prepared for flexural test.

A number of flexural tests are carried out for the

specimens at 22ºC, 35ºC, 50ºC, 65ºC, 75ºC and 90ºC on

an Instron 3367 machine (Figure 3) at a loading speed of

0.03 mm/sec using the three-point bending method in

accordance with ASTM D790-07 [23]. Each test is

repeated three times to verify results. Prior to each

bending test, the specimen is exposed to heat inside a

temperature controlled oven to reach the desired test

temperature. Then the heated specimen is wrapped by

insulator during transferring to the test jig to prevent heat

loss.

Dimensions Value (mm)

Depth (h) 3.13
Width (d) 14.8

Span length (Ls) 120

Total length (L) 150

Figure 3 : Three-point bending test on the Instron 3367

machine.

RESULTS AND DISCUSSION

This research work was achieved with the aim of

understanding the flexural behavior of CFRP laminate

and finding its flexural properties under the effect of

different levels of temperature from room temperature to
90ºC. Different rectangular CFRP specimens, according

to standards, were prepared for this purpose, using

cutting saw and grinder equipment.

The bending test results of the CFRP specimens, at

each particular temperature level, are illustrated as load-

deflection curves in Figure 4. The figure shows that with

the increase of temperature the CFRP material loses 60%

of peak load (flexural load resistance) at a rate of about 4
N/ºC from 500 N to 198 N. Also it shows that the

flexural behaviours of elastic deformation and sudden

drop of the load for all the CFRP specimens tested at the

temperatures range from room temperature to 75 ºC. The

initial part of each load-deflection curve that has a linear

appearance exhibits the bending stiffness to peak load

and elastic deformation of the specimen because no

delamination and oscillations were observed before the

peak load.

Figure 4 : Load-displacement curves of the CFRP

specimens at different test temperatures

The sudden drop of the load, after reaching the peak

value, represents brittle behaviour of the laminate due

fibre cracking as it was noticed during the tests and

presented in Figure 5a. After the load drop, it was

observed that the specimens continued to afford flexural

load but never exceeded previous peak load because

only the reinforcement resisted. Compared with the load-

displacement curves for the specimens tested at

temperatures below 90ºC, obvious difference can be

noticed in the curve shape of that tested at 90ºC, also the

tested specimen, as shown in Figure 5b, was the only

that formed around the test machine indenter without

fracture. This difference in behaviour indicates a sudden

change in the property of the specimen material that

becomes pliable and less brittle because its temperature

has reached the HDT of the epoxy component, Table 1,

which relates closely to the resin glass transition

temperature and corresponds to the initial softening

point.

Figure 5: Failure of the CFRP specimens in flexural test (a)

at 35ºC (b) at 90ºC

The flexural modulus (𝐸𝑓𝑙) and flexural strength (𝜎𝑓𝑙)

of each test specimen at each specified temperature level

are calculated from the specimen dimensions (Table 2),

the slop (𝑚) and the peak load (𝐹𝑃) values of the
correlated load-displacement curve, as shown in Figure

4, according to the following equations [22].

𝜎𝑓𝑙 =
3𝐹𝑝𝐿

2𝑏ℎ2
(1)

𝐸𝑓𝑙 =
𝑚𝐿3

2𝑏ℎ2
(2)

The obtained values are plotted in Figure 6 versus the

test temperatures range from room temperature to 90ºC.

Accordingly a consistent decrease of 60.4%, over the

entire increasing of test temperatures, can be observed in

the flexural strength of the CFRP laminate at a rate of
10-2 GPa/ºC from 0.61 GPa to 0.24 GPa. Furthermore,

between the room temperature and 75 ºC, the laminate

loses 29% of its flexural modulus gradually from 45

GPa to 31.8 GPa at a rate of 0.2 GPa/ºC, but significant

decrease of 40% occurs from 32 GPa to14 GPa at a rate

of 1.2 GPa/ºC when the temperature elevates from 75 ºC

to 90 ºC.

Thus it could be deduced that the effect of

temperature is quite significant, despite the flexural

properties being dominated by the fibres at the outer

surfaces, because the laminate loses properties

constantly whenever the specimen temperature becomes

closer to the HDT of the epoxy component. This will be

important to select proper epoxy with suitable HDT for

designing the CFRP laminates that are working at

elevated temperatures and providing high thermal

degradation resistance.

On the other hand, however, temperature elevation

100

200

300

400

500

600

0 4 8 12 16 20 24 28 32

L
o

a
d

[
N

]

Midspan deflection [mm]

22ºC

35ºC

50ºC
65ºC

75ºC
90ºC

Figure 6 : Flexural modulus and ultimate stress of the

CFRP specimens as function of temperature.

decreases the laminate properties of flexural strength and

modulus; it softens the laminate and makes it slightly

more pliable that can improve some features like impact

and vibration resistance as well as slightly higher peel

strength. This flexible feature is also useful for the CFRP

laminates that are bonded to dissimilar materials with

different coefficients of thermal expansion (CTE) as

occurred in the integrally heated tool and increases the

delamination resistance of the tool.

CONCLUSION

In this study, the flexural test results of the CFRP

laminate specimens at different levels of temperature up

to 90ºC show that:

1. The temperature dependency of the CFRP laminate

properties related strongly to the resin HDT. This

will be important in selecting proper epoxy with

suitable HDT during designing the CFRP laminates

that are working at elevated temperatures to

achieve high thermal degradation resistance.

2. The CFRP laminate retains its behaviours of elastic

deformation, sudden load drop and brittle fracture

At the temperature levels from room temperature to

75ºC.

3. After load drop, the laminate continues in resisting

the flexural load till final failure but never exceeds

the previous peak load because only the fibre

resists.

4. The CFRP laminate loses consistently 60% of peak

load (from 500 N to 198 N) and flexural strength

(from 0.61 GPa to 0.24 GPa) at rates of about 4

N/ºC and 10-2 N/ºC, respectively, over the entire

test temperatures from room temperature to 90ºC.

5. The CFRP laminate loses 29% of its flexural

modulus from 45 GPa to 32 GPa, at a rate of 0.2

GPa/ ºC, gradually due to the temperature change

from the room temperature to 75ºC but the losing

increases drastically to 40% (from 32 GPa to 14

GPa) and the rate to 1.2 GPa when the temperature

elevated from 75ºC to 90ºC.

6. The CFRP laminate becomes slightly more flexible

with the increase of temperature and more when it

reaches HDT.

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0.0

0.2

0.4

0.6

0.8

1.0

20 40 60 80 100

F
le

x
u

r
e
a

l
st

r
e
n

g
th

[
G

P
a

]

F
le

x
u

r
a

l
m

o
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u
lu

s
[G

P
a

]

Temperature [ºC]

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ACKNOWLEDGEMENTS

The author would like to acknowledge the support that

both the Polytechnic University of Slemani and the

University of Plymouth have given to complete this

work. Also the author would like to thank Barzan Akram

Mohammed (Sulaimani Polytechnic University) who

assisted with some aspects of the experimental work.

BIOGRAPHY

Dr. Rzgar Mhammed Abdalrahman is a member of

teaching staff at the production engineering and

metallurgy department of technical college of

engineering at Sulaimani Polytechnic University. He

started as an assistant lecturer in 1990 then promoted to

lecturer in 2010.

He awarded B.Sc. degree in production engineering
in 1981-1985 and M.Sc. degree with a thesis entitled

(Behavior of Cutting Tool in Threading) in 1988 at

production engineering and metallurgy department of

University of Technology / Baghdad-Iraq. Also awarded

Diploma of Membership of Plymouth University in

2015-2016 and PhD in recognition of a programme of

work entitled (Design and Analysis of Integrally-Heated

Tooling for Polymer Composites) in 2015-2016 at

faculty of science and engineering / School of marine

science and engineering of Plymouth University / UK.

He published six articles in the journals of; Zankoy

Sulaimani in 2010, Pure and Applied Sciences /

Salahaddin University in 2010, Materials and design in

2014, Reinforced plastics and composites in 2016 as

well as in 2017 and an article at the proceeding of
ECCM16 conference in 2014.

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http://www.matrix-composites.co.uk/prod-data-sheet/old/sr-8500-860x-uk.pdf
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