Effect of temperature on tensile properties of
HDPE pipe material
N. Merah*, F. Saghir, Z. Khan and A. Bazoune
The properties that make plastic of direct interest to designers and engineers are its good
strength to weight ratio, low manufacturing and installation costs and high durability. The strength
of polymers is known to be sensitive to temperature and this generally limits their use under
service temperatures lower than the glass transition temperature. The present work addresses the
effect of temperatures ranging from 210 to 70uC on the tensile properties of high density
polyethylene PE-100 pipe material. Tensile tests are performed on dog bone type ASTM standard
specimens. Yield stress and modulus of elasticity are found to decrease linearly with temperature.
The average yield strength decreased linearly from 32 to 9 MPa when the temperature is
increased from 210 to 70uC. The modulus of elasticity varied in the same fashion as the yield
strength. The yield strain, however, showed a slight increase in this temperature range. Ductile
fracture is observed to be the controlling failure mechanism at all the temperatures of interest. The
deformation at room and high temperatures is accompanied by considerable necking. The
temperature effect on the tensile properties of PE-100 pipe material is compared with that of
CPVC and PVC pipe materials, used in comparable applications. In general, a similar effect was
observed on yield stress, modulus of elasticity and yield strain in all these materials.
Keywords: Polyethylene, HDPE, CPVC, Tensile properties, Temperature effect, Yield stress, Modulus of elasticity, Yield strain
revealed a maximum between 80 and 90uC and a
Introduction
minimum between 130 and 170uC. Ye et al.2 have
Polyethylene comes in three different general grades: low
reported effects of strain rate and temperature on
density polyethylene (LDPE), medium density polyethy-
fracture behaviour of poly(4-methyl-1-pentene) (TPX)
lene (MDPE) and high density polyethylene (HDPE).
polymer. The results showed that the fracture behaviour
The increase in density results in the variation of
of TPX polymer was highly dependent on cross-head
material properties. In general, the yield strength sys,
rate and temperature. Merah et al.3 have investigated
the modulus of elasticity E and the melting temperature
the effect of temperatures ranging from 210 to 70uCon
Tm increase with density while the elongation %El and
the mechanical properties of CPVC. They found that the
toughness decrease. Medium density polyethylene and
yield strength and elastic modulus decreased linearly
more and more higher density polyethylene are being
with temperature. Brittle fracture occurred at tempera-
extensively used for gas, water, sewage and wastewater
tures below room temperature while ductile fracture
distribution systems.
occurred at room temperature and temperatures above.
The mechanical properties of high density polyethy-
Bronnikov et al.4 investigated the thermal and
lene like all polymers are very sensitive to service
mechanical properties of drawn polymers over a wide
temperature. In general, all polymers at temperatures
temperature range. They used polyethylene tetraphthalte
significantly below their glass transition temperatures Tg
(PET), nylon 6 and nylon 610 for analysis. They have
undergo brittle fracture. In the region above the brittle
shown that the mechanical properties of drawn polymers
fracture regime, but below Tg, polymers usually yield
are directly related to the thermal expansion and have
and undergo plastic deformation as the modulus of used this approach to show the temperature dependence
elasticity decreases. of Young s modulus and yield stress over a wide
Hitt and Gilbert1 have studied the tensile properties of temperature range. They reported that Young s modulus
PVC at temperatures ranging from 23 to 180uC. They and yield stress decrease with increasing temperature.
found that stress at break decreased steadily with According to Bond,5 the tensile strength of HDPE
pressure pipe material is shown to decrease from
increasing temperature, whereas elongation at break
21 MPa at 23uC to 10 MPa at 60uC. Other researchers6,7
have also found that the increase in temperature leads to
Mechanical Engineering Department, King Fahd University of Petroleum a drastic decrease in polymer strength and stiffness.
and Minerals, Dhahran, 31261, Saudi Arabia
Therefore, owing to the dependence of mechanical
*Corresponding author, email [e-mail] properties on a large variety of parameters and mainly
ß 2006 Institute of Materials, Minerals and Mining
Published by Maney on behalf of the Institute
Received 19 November 2005; accepted 30 June 2006
226 DOI 10.1179/174328906X103178 Plastics, Rubber and Composites 2006 VOL 35 NO 5
Merah et al. Effects of temperature on tensile properties of HDPE pipe materials
the temperature, it is difficult for the designer to select a
certain material without knowing all these parameters.
The mechanical properties such as yield strength and
elastic modulus are usually given as a range for plastics
and usually only at room temperature. Therefore, the
accurate determination of mechanical properties of
polymers with respect to environment and material
variables is very important.
The present paper addresses the temperature effects
on the mechanical properties of PE-100 pipe material.
The effect of temperature is investigated by performing
tensile tests at 210, 0, 23, 40, 50 and 70uC. This range
encompasses the temperatures at which this type of
pipes may be used in the different areas of the world. 1 Load elongation curves for HDPE at different tempera-
Variations of the mechanical properties such as yield tures
stress, modulus of elasticity and yield strain with
temperature are studied. The effect of temperature on
load elongation curves are illustrated in Fig. 1. It can
the mechanical properties of HDPE is also compared
be seen that the stiffness of the material as well as the
with that of CPVC and PVC materials used in piping
load bearing capacity decreased with increasing test
systems for similar applications.
temperature.
The results in terms of yield stress and modulus of
Experimental procedure
elasticity, along with their average values, in different
test conditions are provided in Table 1. The yield
The specimens for tensile testing were prepared from
strength is defined here as the true stress at the
commercially available 4 inch (100 mm) Class V PE-100
maximum load and the modulus of elasticity is obtained
pressure pipes manufactured by extrusion by a local
from the initial linear portion of the stress strain
company in Saudi Arabia (typical compound density
(elongation) curve. It can be seen that except for tests
960 kg m23, typical tensile stress at yield 23 MPa).
at 70uC, the scatter in the values of yield strength and
Additives such as carbon (.2%) were also added to
modulus of elasticity is minimal. The average value of
improve the physical and mechanical properties of the
the yield strength obtained at 23uC is very close to the
pipes. Rings were cut from the pipe section and slit into
typical value reported by the pipe manufacturer. This is
two halves. After heating for y60 min at 130uC in an
an indication that the process of specimen preparation
electric oven, the rings were straightened in a specially
described above did not result in altering the mechanical
designed mould, following the procedure described by
properties of the pipe material.
Irfan.8 Temperature setting and exposure time were
The typical engineering stress elongation curves
carefully monitored to obtain the same heating history
developed from the load elongation results at each
for all the specimens. During flattening, the pressure was
carefully applied to the material to avoid compressing
the plate after flattening and to conserve the original Table 1 Results of monotonic tests performed on HDPE
specimens
thickness. The specimens for tensile tests were machined
from the straightened plates according to the ASTM
Yield strength, Modulus of
D638 Standard method of test for tensile properties of
Serial no. MPa elasticity, MPa
plastics.9 Tensile loading was performed in the direction
perpendicular to the extrusion direction to obtain the 210uC
132.61 1032
material resistance to hoop stress created by internal
231.79 1038.5
pressure.
Average 32.20 1035.25
An Instron 8501 material testing frame was used for
0uC
testing (load capacity Ä„100 kN). The machine is
329.49 923.25
equipped with a hydraulically actuated self-aligning
430.00 925.35
gripping system. To ensure the vertical alignment of
530.45
Average 30.00 924.30
the specimen, specially machined inserts were used
23uC
during the tests. The deformation was measured by an
623.85 670.30
Instron clip-on extensometer with a gauge length of
723.27 665.10
200 mm. Environmental chambers with an accuracy of
Average 23.56 667.70
Ä„1uC were used for tests in non-ambient conditions.
40uC
Two to three tests were performed at each of the
816.40 407.50
temperatures 210, 0, 23, 40, 50 and 70uC and at a strain 915.69 392.45
Average 16.05 399.00
rate of 661024 s21. The results obtained from these
50uC
tests are presented and discussed in the following
814.21 291.95
sections.
914.55 287.35
Average 14.38 289.65
70uC
Results and discussion
10 9.09 223.10
Temperature effects on stress strain curves
11 7.45 201.06
12 10.44 237.15
Load elongation curves were obtained using a PC
Average 8.99 220.65
interfaced with the testing frame. Representative
Plastics, Rubber and Composites 2006 VOL 35 NO 5 227
Merah et al. Effects of temperature on tensile properties of HDPE pipe materials
4 Effects of temperature on modulus of elasticity
2 Representative stress %elongation curves for HDPE at
different temperatures
Yield stress and modulus of elasticity
The average values of yield strength sys are plotted
temperature are illustrated in Fig. 2. Several features of
against absolute temperature T in Fig. 3. It can be seen
these curves are worth noting: increasing the tempera-
that in the range 263 to 343 K (210 to 70uC), the yield
ture produces a decrease in elastic modulus, a reduction
strength varies linearly with temperature. The slope of
in tensile strength and an enhancement of ductility. For
the regression line in this temperature range has a value
this set of results, the yield strength drops from y32 to
of 20.305, with a linear correlation coefficient of 0.99.
7.5 MPa as the temperature is increased from 210 to
The linear dependence of yield stress on temperature for
70uC. It is evident in Fig. 2 that ductile fracture occurs
HDPE can be expressed as
with a definite yield point characterised by a maximum
in the stress strain curve. A considerable amount of
sys~112:85 0:305T 263 KÅ‚
TÅ‚
343 K (1)
plastic deformation, which is usually associated with the
crazing phenomenon, can also be evidently observed.
This behaviour falls in line with Eyring s theory of
The pipe material undergoes plastic deformation,
viscosity expressed in its simplest form as
illustrated in the hump, at all the temperatures. The
deformation after the yield point at 210 and 0uC is
DH R e
mainly by shear yielding while at 23uC and above, sys~ z ln T (2)
V V Ae
deformation is by shear yielding and cold drawing. In
the cases of 0 to 70uC, after a sufficient amount of strain, where R is the universal gas constant, V is the activation
the slope of the stress strain curve begins to increase volume also known as the Eyring flow volume, e is the
after reaching a minimum stress value. This is produced strain rate, DH is the change in enthalpy and Ae is a
by the alignment of HDPE chains in the strain direction material constant. For tests conducted at constant strain
resulting in material strain hardening. The slope of the rates, the above model predicts a linear relationship
curve continues on increasing until the plastically between yield strength and temperature. Equation (1)
can be used to develop temperature de-rating factors for
deformed sample eventually breaks. It should be noted
here that at 23uC and higher temperatures, the speci- the present HDPE material at any service temperature
within the specified range.
mens did not break and the test was interrupted after a
The values of sys reported in Ref. 5 for HDPE, Ref. 7
considerable amount of elongation was produced,
for PVC and Ref. 3 for CPVC pipefitting material are
usually .200%. The effect of temperature on the main
also shown in Fig. 3 for comparison purposes. As
tensile properties such as yield strength, yield strain and
expected, both PVC and CPVC have higher strength
elastic modulus is discussed in detail in the following
than HDPE at all temperatures. The temperature
sections.
sensitivity of the strengths of PVC and CPVC materials
is more than that of HDPE; the yield strength sys for
PVC and CPVC decreases at a faster rate than that for
HDPE. The slope of the regression line for CPVC is
y1.5 times that of HDPE. The variation of temperature
is shown to have an even higher effect on the yield
strength of PVC where the slope of the regression line
has a value of more than twice that for HDPE.
The variation of modulus of elasticity E as a function
of absolute temperature for HDPE is shown in Fig. 4. It
can be observed that in the present temperature range, E
also decreases linearly with increasing temperature,
much similar to what was observed with sys. The linear
dependence obtained in the present study is similar to
that reported by Povolo et al.7 for PVC and Merah
et al.3 for CPVC. The variation of E with absolute
3 Effect of temperature on yield stress temperature T for HDPE pipe material can be expressed
228 Plastics, Rubber and Composites 2006 VOL 35 NO 5
Merah et al. Effects of temperature on tensile properties of HDPE pipe materials
5 Relationship between t and G for HDPE
as
6 Variation of yield strain with temperature for HDPE
E(T)~3912:8{11T (3)
for semicrystalline polymers. Merah et al.3 have also
A comparison of the variation of E for HDPE with that
used Kitagawa s model in their study of CPVC and
for CPVC (Ref. 3) and PVC (Ref. 7) with absolute
reported the value of exponent n equal to 1.69. Figure 5
temperature is also shown in Fig. 4. An analysis of these
is a log log plot according to equation (4) for the values
curves reveals that the regression lines for HDPE and
of t and G obtained for HDPE tested over the
CPVC seem to be parallel to each other, which leads to
temperature range of 210 to 70uC. The Poisson s ratio
the conclusion that the variation of temperature has a
was assumed constant, equal to 0.46 (Ref. 12), over the
similar effect on the stiffness of these two materials. The temperature range of interest. The line drawn on the
stiffness of PVC, however, decreases at a faster rate than graph has a slope of 0.659 and all the points fall very
that of HDPE and CPVC. close to this line with a coefficient of regression of 0.975.
The value of slope is less than that reported for
Relationship between elastic modulus and yield
semicrystalline polymers because of the presence of
strength additives in the HDPE pipe material chosen for testing.
Elastic modulus and yield strength are linearly related to
Yield strain
each other. Hence, any variable that affects elastic
Figure 6 shows the variation of yield strain with
modulus will also affect the yield strength. Argon and
temperature. The yield strain (denoted by ey) is defined
Bessonov10 derived the analytical relationships between
as the ratio of yield stress to modulus of elasticity, i.e.
elastic modulus and yield strength over a wide tempera-
ey5sys/E. The yield strain remains fairly constant for the
ture range. These theories show excellent agreement with
temperature range studied; the regression line shown in
Argon s experimental results but their analytical form
the graph has a slope of 261024. Similar results were
is too complex to be applied to practical situations.
obtained by Povolo et al.7 and Merah et al.3 for PVC
Kitagawa11 has expanded and generalised Argon s
and CPVC respectively.
theory to arrive at a relationship between shear stress t
and shear modulus G which can be represented by a
power law relationship of the form
Conclusions
n
Tot ToG
The effect of temperature on the mechanical properties
~ (4)
of high density polyethylene PE-100 pipe material was
Tto TGo
studied by performing a number of tensile tests at six
where To is the reference temperature, the values to and
different temperatures (210, 0, 23, 40, 50 and 70uC).
Go are of shear yield stress and shear modulus at some
The following conclusions are obtained from the
To (conveniently taken as the ambient temperature), and
analysis of tensile test results.
n is a temperature independent exponent.
1. The yield stress and elastic modulus decrease
The tensile modulus and yield strength are converted
linearly with temperature.
into the corresponding shear modulus and shear yield
2. Ductile fracture occurred at all the temperatures.
strength for using Kitagawa s relationship. This can be
3. The temperature dependence of HDPE strength is
performed using the following equations of solid
lower than that of PVC and CPVC; the yield strength
mechanics
for HDPE decreases at a slower rate than that for PVC
and CPVC.
E(T)
G(T)~ (5)
4. The variation of temperature has a similar effect on
2ð1znÞ
the stiffness of HDPE and CPVC.
5. Shear stress and shear modulus are related by a
sys(T)
t(T)~ (6)
Kitagawa power law with an exponent of 0.66.
31=2
6. Variation of temperature has a limited effect on the
where n is the Poisson s ratio.
yield strain of HDPE.
Kitagawa in agreement with Argon showed that a
relationship of the form of equation (4) held over a wide
Acknowledgement
range of temperatures for most polymers. He also found
that the exponent n had a unique value of 1.63 for all The authors acknowledge the support of the King Fahd
amorphous polymers and a value between 0.80 and 0.90 University of Petroleum and Minerals.
Plastics, Rubber and Composites 2006 VOL 35 NO 5 229
Merah et al. Effects of temperature on tensile properties of HDPE pipe materials
7. F. Povolo, G. Schwartz and E. B. Hermida: J. Polym. Sci., 1996,
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