T. FÍLA et al.: CREEP BEHAVIOUR OF A SHORT-FIBRE C/PPS COMPOSITE
413–417
CREEP BEHAVIOUR OF A SHORT-FIBRE C/PPS COMPOSITE
VEDENJE KRATKIH VLAKEN C/PPS KOMPOZITOV PRI LEZENJU
Tomá{ Fíla1, Petr Koudelka2, Daniel Kytýø2, Jiøí Hos1, Jan [leichrt1
1Czech Technical University in Prague, Faculty of Transportation Sciences, Konviktská 20, 110 00 Prague 1, Czech Republic
2Institute of Theoretical and Applied Mechanics, Academy of Sciences of the Czech Republic, Prosecká 76, 190 00 Prague, Czech Republic
xfila@fd.cvut.cz
Prejem rokopisa – received: 2014-08-22; sprejem za objavo – accepted for publication: 2015-04-24
doi:10.17222/mit.2014.208
Composite materials with a polymeric matrix reinforced by carbon fibres are nowadays widely used as high-tech structural
materials with excellent mechanical properties (particularly their stiffness and strength). The application of this type of
composite to structural parts exposed to thermal loading has recently been proposed. Such an application requires an
investigation and analysis of the mechanical behaviour under long-term exposure to simultaneous thermal and mechanical
loading. In this paper the measurements and results of the creep behaviour of a composite with a polyphenylene sulphide matrix
reinforced with chopped poly-acrylonitrile carbon fibres (C/PPS) are presented. The measured compound is proposed for use as
a structural material for a jet-engine frame in the aerospace industry and the internal parts of aircraft with possible thermal
loading. A custom experimental device designed for the creep measurements of composite materials was used for measurements
of the developing strain at a constant tensile stress and temperature. Short-term creep tests with continuous strain monitoring
were performed at a constant stress level at several elevated temperatures below and above the glass-transition temperature of
the matrix. The strain was measured using the digital image correlation (DIC) method. The measured data were processed to
find the strain-to-time dependency and the creep-compliance-to-time dependency. The creep-compliance-to-time data were also
fitted using Findley’s creep law for polymers to evaluate the model parameters and to analyse the applicability of the model for
a PPS polymer reinforced with chopped carbon fibres.
Keywords: creep, short fibre composite, C/PPS, Findley’s model, DIC
Kompozitni materiali s polimerno osnovo, okrepljeno z ogljikovimi vlakni, se dandanes pogosto uporabljajo kot
visokotehnolo{ki konstrukcijski materiali z izjemnimi mehanskimi lastnostmi, {e posebej to velja za njihovo togost in trdnost.
Pred kratkim je bila predlagana uporaba kompozita te vrste za konstrukcijske dele, ki so izpostavljeni toplotni obremenitvi.
Tak{na uporaba zahteva raziskavo in analizo mehanskega vedenja pri dolgotrajni izpostavljenosti isto~asni toplotni in mehanski
obremenitvi. Prispevek predstavlja meritve in rezultate deformacijskega vedenja kompozitov z matrico iz polifenilen sulfida,
okrepljenega z razcepljenimi poliakrilonitrilnimi ogljikovimi vlakni (C/PPS), ki naj bi se uporabljali kot konstrukcijski materiali
za ogrodje reaktivnih motorjev v letalski in vesoljski industriji ter za notranje dele letal, ki so pod potencialno toplotno
obremenitvijo. Za merjenje napredovane deformacije pri konstantni natezni obremenitvi in temperaturni obremenitvi, je bila
uporabljena posebna preizkusna naprava, izdelana za merjenje deformacije kompozitov. C/PPS vzorci so bili postavljeni v
toplotno komoro in segreti. Vzorci so bili nato s pomo~jo stiskalnice izpostavljeni konstantni natezni sili. Kratkotrajni preizkusi
lezenja, z nadzorovanjem neprekinjene natezne sile, so bili opravljeni pri konstantnih stopnjah obremenitve pri razli~nih
temperaturah, ki so bile vi{je ali ni`je od temperature prehoda v steklasto stanje osnove. Deformacija vzorca je bila izmerjena z
uporabo metode korelacije digitalne slike (DIC). Namen izmerjenih podatkov je bil poiskati odvisnosti deformacije od ~asovne
komponente in lezenja. Podatki o ~asu in sili lezenja so bili usklajeni s Findleyevim zakonom lezenja za polimere za oceno
parametrov modela in analizo veljavnosti modela za PPS polimere, ki so okrepljeni razceplenimi ogljikovimi vlakni.
Klju~ne besede: lezenje, kompozit s kratkimi vlakni, C/PPS, Findleyev model, DIC
1 INTRODUCTION
Composite materials with various matrices reinforced
by short or continuous fibres are nowadays widely used
as general structural materials with good mechanical
properties (particularly their stiffness and strength). One
of the very popular composite compounds that are
widely used in the aviation industry consists of a poly-
phenylene sulphide (PPS) matrix and carbon-fibre rein-
forcement. This material, in continuous fibre form, is
typically used in the production of external aircraft com-
ponents because of its excellent mechanical properties,
chemical resistance to aerospace fluids and low density.1
Composite compounds with carbon fibres and PPS
matrices (C/PPS) also have a good resistance to heat and
flames. Therefore, they are able to meet the smoke and
toxicity requirements of aviation legislation1 and recently
they have been used for the construction of aircraft’s
interior parts, particularly seats.1,2
Attempts to simplify the fabrication process for air-
craft parts and to reach a higher cost efficiency in terms
of production have led to the introduction of C/PPS in
the form of a material reinforced by chopped or short
carbon fibres.3 The parts used in the aviation industry,
e.g., seats, manufactured using a composite with a short
reinforcement can be easily fabricated using injection
moulding and their shape can be very complex and
thick.3 However, such composites consist of unit cells
with discrete boundaries, various fibre arrangements and
distributions. Thus, these materials exhibit significantly
lower strengths and are in comparison with continuous-
fibre reinforcements susceptible to the occurrence of
creep.4 For this reason the creep behaviour of short-fibre
composites has to be properly described and taken into
Materiali in tehnologije / Materials and technology 50 (2016) 3, 413–417 413
UDK 620.1:539.376:669.018.25 ISSN 1580-2949
Professional article/Strokovni ~lanek MTAEC9, 50(3)413(2016)
account as one of the most important parameters for an
estimation of a part’s lifetime.
In this study a series of isothermal creep measure-
ments was performed with the composite compound
consisting of a PPS matrix reinforced with short carbon
fibres fabricated using an untested method involving
heating cables (details of the manufacturing method are
classified, and thus only the main features of the method
will be described in the text). The tested material is
planned to be used for the construction of monocoque
shell parts of the fuselage and for structural parts with
large volumes (such as engine suspension, etc.) and thin
reinforcements with possible thermal loading.
2 EXPERIMENTAL PART
Material samples with a dog-bone shape were tested
in a custom-designed device for uniaxial creep tests at a
constant stress level. The creep was measured on a set of
samples at various temperatures above and below the
glass-transition temperature of the matrix. The strain was
observed using a digital single-lens-reflex (DSLR) came-
ra and evaluated using the digital image correlation tech-
nique (DIC). As a result, the strain-time and the creep-
compliance-time dependencies were evaluated and the
creep behaviour of the tested compound was described.
2.1 Material
The material samples were manufactured as a new
type of cost-efficient material. The composite compound
(Carbon AS4/PPS, TenCate Advanced Composites) con-
sists of a polymeric polyphenylene sulphide (PPS)
matrix and chopped carbon fibres with a length of appro-
ximately 10 mm. The material was fabricated as a
2.5-mm-thick sheet using the technique of mixing
chopped fibres with PPS particles and baking in a mould
with heating cables at 310–340 °C under 4 MPa
pressure. The outer parts of the sheet were manufactured
with an 8 mm thickness for the clamping of the samples
into sample holders. Part of the sheet is depicted in
Figure 1.
2.2 Specimen preparation
The dog-bone-shaped specimens were cut from a
sheet of material using a water-jet cutting machine. The
middle part of the specimen was prepared with a width
of 10 mm. The overall length of the specimen was 160
mm, the maximum width was 25 mm and the thickness
of the middle part was 2.5 mm. The maximum distortion
caused by the water-jet cutting was experimentally deter-
mined to be better than 0.15 mm (in all dimensions on
both surfaces of the specimen). Such a manufacturing
tolerance was considered to be sufficient for the experi-
ment and the surfaces were not additionally modified.
All the specimens were measured before the test and
particular dimensions were used for the evaluation of the
experiment. The dimensions were selected to be similar
to the dimensions of small longitudinal reinforcements
used in, e.g., aircraft seats. The specimen surface was
sprayed using an airbrush with a granite effect for better
pixel identification in the DIC technique. A specimen
with a granite coating is displayed in Figure 2.
2.3 Experimental setup
The experimental setup consists of a custom-de-
signed experimental device for the creep measurements
and a DSLR camera with accessories.
The creep experimental device was designed for uni-
axial creep testing at a constant stress level. The device
consists of a heating chamber SFL 3119 (Instron, USA)
with a temperature range of –70 °C to 350 °C, a rigid
structural steel frame, two independent aluminium alloy
lever arms with ratio of 1:10 and set of dead weights.
The device is equipped with two load cells (VTS Zlín,
Czech Republic) with a loading capacity up to 10 kN.
The strain can be measured in two possible ways: exten-
someters or DIC. Two custom-designed extensometers
(VTS Zlín, Czech Republic) with a maximum measur-
414 Materiali in tehnologije / Materials and technology 50 (2016) 3, 413–417
T. FÍLA et al.: CREEP BEHAVIOUR OF A SHORT-FIBRE C/PPS COMPOSITE
Figure 2: Image of a specimen with sprayed granite coating on its
surface
Slika 2: Prikaz vzorca s prevleko iz granita na povr{ini
Figure 1: Image of a C/PPS sheet used to prepare a specimen
Slika 1: Posnetek C/PPS plo~evine, ki je bila uporabljena za pripravo
vzorca
able extension of 10 mm and a heat resistance up to
150 °C allow the accommodation of two samples in the
device’s heating chamber and their independent loading.
In contrast the DIC allows the measurement of only a
single specimen, but it is not limited by the maximum
temperature because all of the measuring equipment is
situated outside the heating chamber. The experimental
device is shown in Figure 3.
The DIC was used as a tool for an evaluation of the
strain held in the experiments. The specimen in the
heating chamber was observed with a DSLR camera
EOS 550D (Canon, Japan) with a macro-objective
EF100mm/1:2.8L Macro (Canon, Japan) situated on a
tripod in front of the heating chamber. The sample
surface was illuminated with a laboratory LED light
source KL 2500 (Shott, Germany). The overall view of
the experimental setup is shown in Figure 4.
2.4 Experiment
Prior to the experiment the specimen was put in the
heating chamber and the positions of the illuminators
and the camera stand were adjusted. The reference image
of the sample surface in the heating chamber was taken
to verify the focalization, the field of view and to detect
and eliminate possible reflections of illuminating light
on the chamber window. The unloaded specimen was
then heated to a given temperature and for a 30-min
period was held to reach a uniform temperature distri-
bution throughout all the heated parts. After that the
image-capturing sequence was started and the specimen
was loaded with a constant tensile force of 1.5 kN (one
measurement with a loading force of 2.5 kN was also
performed). The loading force was selected according to
data obtained during quasi-static measurements in
tension of specimens with an identical shape. A value of
1.5 kN was situated in a linear elastic region of the
material and represented approximately 70 % of the
material’s yield strength. The creep test was continued
until specimen rupture or for at least 20 h. The glass-
transition temperature of the matrix material was
according to the manufacturer’s datasheet approximately
85 °C and therefore the creep tests were performed at
temperatures of 60 °C (certainly below the glass-transi-
tion temperature), 90 °C (slightly above the glass-transi-
tion temperature), 110 °C, 130 °C and 140 °C (signifi-
cantly above the glass-transition temperature). Images
were taken at equidistant time intervals and were
labelled with a unique timestamp for precise synchroni-
zation of the DIC and the experiment time. The strain
was evaluated from the image sequence using custom
DIC software5 based on the Lucas-Kanade algorithm.6
3 RESULTS AND DISCUSSION
The evaluated strain-time dependencies are shown in
Figure 5. The creep-compliance-time dependencies were
calculated from the data using the following Equation
(1):7,8
J t
t
c
c
c
( )
( )
=



(1)
where Jc(t) represents the creep compliance in time t,

c(t) is the actual creep strain in time t (excluding the
initial strain) and c is the applied constant tensile
stress. A graph of the creep-compliance-time depen-
dency is shown in Figure 6.
Materiali in tehnologije / Materials and technology 50 (2016) 3, 413–417 415
T. FÍLA et al.: CREEP BEHAVIOUR OF A SHORT-FIBRE C/PPS COMPOSITE
Figure 4: Image of the experimental setup and a specimen mounted in
the thermal chamber
Slika 4: Prikaz preizkusnega sklopa in vzorca, ki je bil name{~en v
toplotni komori
Figure 3: Visualization of the custom-designed device for the creep
measurements
Slika 3: Prikaz posebne naprave za merjenje deformacije
The creep-compliance data were also fitted using
Findley’s creep law for polymers, assuming steady-state
creep behaviour according to the Equation (2):8
J t b tc
b( ) = 0 1 (2)
where J tc ( ) represents the predicted creep compliance in
time t (in seconds) and b0, b1 are material parameters
evaluated by a regression of the experimental data. The
evaluated regression curves are shown in Figure 7 and
the calculated material parameters are summarized in
Table 1.
Table 1: Evaluated parameters of Findley’s creep model
Tabela 1: Ocenjeni parametri Findleyevega modela lezenja
Temperature Loading
b0 b1(°C) (kN)
60 1.5 4.432×10–10 0.9386
90 1.5 2.286×10–9 0.6818
110 1.5 2.419×10–7 0.5711
130 1.5 1.271×10–7 0.6712
140 1.5 1.078×10–6 0.3475
140 2.5 1.832×10–7 0.7044
Based on the experimental data it can be stated that
the creep behaviour of the tested material was brittle
above the glass-transition temperature. The test of each
specimen above this temperature ended with a sudden
specimen rupture after a short time period (max. approx.
6 h).
Creep under (test at 60 °C) and slightly above (test at
90 °C) the glass-transition temperature can be considered
immeasurable because the measured maximum displace-
ments of approximately 10 μm correspond only to
double the pixel-size of the DIC image (approx. 5 μm)
and thus subject to the noise and reliability of the
method.
The majority of the measured dependencies agreed
with the standard polymer creep theory and their creep
strain-rate increased with increasing temperature or
loading. A regression analysis showed a good correlation
of the experimental data and Findley’s creep model for
polymers (Figure 7). The fitted model’s parameters have
different values than usual for clear polymers8–10 because
of the brittle behaviour of the material and the short
times to specimen failure. Still, Findley’s creep model
can be considered as suitable for a description of the
creep behaviour for the used C/PPS compound.
However, disagreement with the basic theory can be
seen in the test with a loading of 1.5 kN at a temperature
of 140 °C that exhibited a lower strain rate than ex-
pected. The strain rate at the beginning of the test was
appropriate for the temperature, but at a strain of appro-
ximately 1×10–3 a sudden decrease was observed and the
strain remained low until specimen rupture (Figure 5).
Moreover, several specimens ruptured during the initial
loading at forces even 3 times lower than the testing
force of 1.5 kN and are not presented in the results. This
discrepancy was caused by significant imperfections
detected in the material’s microstructure (missing matrix
binder, etc.) caused by an imperfect fabrication process
and size effect due to the combination of a random
material structure and the small width of the specimen
(10 mm). Hence, the delivered material was considered
to be unsuitable for parts with small dimensions in terms
of structural and thermal loading.
416 Materiali in tehnologije / Materials and technology 50 (2016) 3, 413–417
T. FÍLA et al.: CREEP BEHAVIOUR OF A SHORT-FIBRE C/PPS COMPOSITE
Figure 7: Graph of fits of Findley’s creep model on the experimental
data
Slika 7: Prikaz prilagoditev Findleyevega modela lezenja na podatke
pri preizkusu
Figure 6: Graph of measured creep-compliance-time dependencies
Slika 6: Diagram odvisnosti izmerjenega lezenja od ~asa
Figure 5: Graph of measured strain-time dependencies
Slika 5: Diagram odvisnosti izmerjenih obremenitev in ~asa
4 CONCLUSIONS
A set of isothermal creep tests with constant loading
of the C/PPS composite with chopped fibres was per-
formed. The C/PPS material was fabricated with an
innovative technique of baking in a mould using heating
cables. The experiments were performed at several diffe-
rent temperatures below and above the glass-transition
temperature and the strain was evaluated using the DIC
technique. The experimental results showed a good
correlation with the standard creep theory for polymers
and with Findley’s creep law. However, the used
fabrication technology brought several imperfections to
the material’s structure, which together with the size
effect caused by small dimensions of the specimen, led
to a discrepancy in the results. Thus, the tested material
was considered unsuitable for small structural parts
exposed to structural and thermal loading and so further
improvements to the fabrication process are required.
Acknowledgements
The research was supported by The Technology
Agency of Czech Republic (grant No. TA03010209), by
the Grant Agency of the Czech Technical University in
Prague (grant No. SGS15/225/OHK2/3T/16) and by
RVO: 68378297. All the support is gratefully acknow-
ledged.
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