19 Effect of temperature on tensile properties of HDPE pipe material.pdf

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 2 10 to 70 u C 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 2 10 to 70 u C. 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

Introduction

Polyethylene comes in three different general grades: low

density polyethylene (LDPE), medium density polyethy-

lene (MDPE) and high density polyethylene (HDPE).

The increase in density results in the variation of

material properties. In general, the yield strength s ys ,

the modulus of elasticity E and the melting temperature

T m increase with density while the elongation %El and

toughness decrease. Medium density polyethylene and

more and more higher density polyethylene are being

extensively used for gas, water, sewage and wastewater

distribution systems.

The mechanical properties of high density polyethy-

lene like all polymers are very sensitive to service

temperature. In general, all polymers at temperatures

signiﬁcantly below their glass transition temperatures T g

undergo brittle fracture. In the region above the brittle

fracture regime, but below T g , polymers usually yield

and undergo plastic deformation as the modulus of

elasticity decreases.

Hitt and Gilbert 1 have studied the tensile properties of

PVC at temperatures ranging from 23 to 180uC. They

found that stress at break decreased steadily with

increasing temperature, whereas elongation at break

revealed a maximum between 80 and 90uCanda

minimum between 130 and 170uC. Ye et al. 2 have

reported effects of strain rate and temperature on

fracture behaviour of poly(4-methyl-1-pentene) (TPX)

polymer. The results showed that the fracture behaviour

of TPX polymer was highly dependent on cross-head

rate and temperature. Merah et al. 3 have investigated

the effect of temperatures ranging from 210 to 70uCon

the mechanical properties of CPVC. They found that the

yield strength and elastic modulus decreased linearly

with temperature. Brittle fracture occurred at tempera-

tures below room temperature while ductile fracture

occurred at room temperature and temperatures above.

Bronnikov et al. 4 investigated the thermal and

mechanical properties of drawn polymers over a wide

temperature range. They used polyethylene tetraphthalte

(PET), nylon 6 and nylon 610 for analysis. They have

shown that the mechanical properties of drawn polymers

are directly related to the thermal expansion and have

used this approach to show the temperature dependence

of Young’s modulus and yield stress over a wide

temperature range. They reported that Young’s modulus

and yield stress decrease with increasing temperature.

According to Bond, 5 the tensile strength of HDPE

pressure pipe material is shown to decrease from

21 MPa at 23uC to 10 MPa at 60uC. Other researchers 6,7

have also found that the increase in temperature leads to

a drastic decrease in polymer strength and stiffness.

Therefore, owing to the dependence of mechanical

properties on a large variety of parameters and mainly

Mechanical Engineering Department, King Fahd University of Petroleum

and Minerals, Dhahran, 31261, Saudi Arabia

* Corresponding author, email nesar@kfupm.edu.sa

2006 Institute of Materials, Minerals and Mining

Published by Maney on behalf of the Institute

Received 19 November 2005; accepted 30 June 2006

DOI 10.1179/174328906X103178

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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.

Variations of the mechanical properties such as yield

stress, modulus of elasticity and yield strain with

temperature are studied. The effect of temperature on

the mechanical properties of HDPE is also compared

with that of CPVC and PVC materials used in piping

systems for similar applications.

1 Load–elongation curves for HDPE at different tempera-

tures

load–elongation curves are illustrated in Fig. 1. It can

be seen that the stiffness of the material as well as the

load bearing capacity decreased with increasing test

temperature.

The results in terms of yield stress and modulus of

elasticity, along with their average values, in different

test conditions are provided in Table 1. The yield

strength is deﬁned here as the true stress at the

maximum load and the modulus of elasticity is obtained

from the initial linear portion of the stress–strain

(elongation) curve. It can be seen that except for tests

at 70uC, the scatter in the values of yield strength and

modulus of elasticity is minimal. The average value of

the yield strength obtained at 23uC is very close to the

typical value reported by the pipe manufacturer. This is

an indication that the process of specimen preparation

described above did not result in altering the mechanical

properties of the pipe material.

The typical engineering stress–elongation curves

developed from the load–elongation results at each

Experimental procedure

The specimens for tensile testing were prepared from

commercially available 4 inch (100 mm) Class V PE-100

pressure pipes manufactured by extrusion by a local

company in Saudi Arabia (typical compound density

960 kg m 23 , typical tensile stress at yield 23 MPa).

Additives such as carbon (.2%) were also added to

improve the physical and mechanical properties of the

pipes. Rings were cut from the pipe section and slit into

two halves. After heating for y60 min at 130uCinan

electric oven, the rings were straightened in a specially

designed mould, following the procedure described by

Irfan. 8 Temperature setting and exposure time were

carefully monitored to obtain the same heating history

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

thickness. The specimens for tensile tests were machined

from the straightened plates according to the ASTM

D638 Standard method of test for tensile properties of

plastics. 9 Tensile loading was performed in the direction

perpendicular to the extrusion direction to obtain the

material resistance to hoop stress created by internal

pressure.

An Instron 8501 material testing frame was used for

testing (load capacity ¡100 kN). The machine is

equipped with a hydraulically actuated self-aligning

gripping system. To ensure the vertical alignment of

the specimen, specially machined inserts were used

during the tests. The deformation was measured by an

Instron clip-on extensometer with a gauge length of

200 mm. Environmental chambers with an accuracy of

¡1uC were used for tests in non-ambient conditions.

Two to three tests were performed at each of the

temperatures 210, 0, 23, 40, 50 and 70uC and at a strain

rate of 6610 24 s 21 . The results obtained from these

tests are presented and discussed in the following

sections.

Table 1 Results of monotonic tests performed on HDPE

specimens

Yield strength,

MPa

Modulus of

elasticity, MPa

Serial no.

2 10 u C

32 . 61

1032

31 . 79

1038 . 5

32 . 20

1035 . 25

Average

0 u C

29 . 49

923 . 25

30 . 00

925 . 35

30 . 45

30 . 00

924 . 30

Average

23 u C

23 . 85

670 . 30

23 . 27

665 . 10

23 . 56

667 . 70

Average

40 u C

16 . 40

407 . 50

15 . 69

392 . 45

16 . 05

399 . 00

Average

50 u C

14 . 21

291 . 95

14 . 55

287 . 35

14 . 38

289 . 65

Average

Results and discussion

Temperature effects on stress–strain curves

Load–elongation curves were obtained using a PC

interfaced with the

70 u C

9 . 09

223 . 10

7 . 45

201 . 06

10 . 44

237 . 15

8 . 99

220 . 65

Average

testing

frame. Representative

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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 s ys are plotted

against absolute temperature T in Fig. 3. It can be seen

that in the range 263 to 343 K (210 to 70uC), the yield

strength varies linearly with temperature. The slope of

the regression line in this temperature range has a value

of 20 . 305, with a linear correlation coefﬁcient of 0 . 99.

The linear dependence of yield stress on temperature for

HDPE can be expressed as

s ys ~112 : 85 0 : 305T

temperature are illustrated in Fig. 2. Several features of

these curves are worth noting: increasing the tempera-

ture produces a decrease in elastic modulus, a reduction

in tensile strength and an enhancement of ductility. For

this set of results, the yield strength drops from y32 to

7 . 5 MPa as the temperature is increased from 210 to

70uC. It is evident in Fig. 2 that ductile fracture occurs

with a deﬁnite yield point characterised by a maximum

in the stress–strain curve. A considerable amount of

plastic deformation, which is usually associated with the

crazing phenomenon, can also be evidently observed.

The pipe material undergoes plastic deformation,

illustrated in the hump, at all the temperatures. The

deformation after the yield point at 210 and 0uCis

mainly by shear yielding while at 23uC and above,

deformation is by shear yielding and cold drawing. In

the cases of 0 to 70uC, after a sufﬁcient amount of strain,

the slope of the stress–strain curve begins to increase

after reaching a minimum stress value. This is produced

by the alignment of HDPE chains in the strain direction

resulting in material strain hardening. The slope of the

curve continues on increasing until the plastically

deformed sample eventually breaks. It should be noted

here that at 23uC and higher temperatures, the speci-

mens did not break and the test was interrupted after a

considerable amount of elongation was produced,

usually .200%. The effect of temperature on the main

tensile properties such as yield strength, yield strain and

elastic modulus is discussed in detail in the following

sections.

263 KƒTƒ343 K

(1)

This behaviour falls in line with Eyring’s theory of

viscosity expressed in its simplest form as

s ys ~ DH

V z R

A e

V ln

(2)

where R is the universal gas constant, V is the activation

volume also known as the Eyring ﬂow volume, e is the

strain rate, DH is the change in enthalpy and A e is a

material constant. For tests conducted at constant strain

rates, the above model predicts a linear relationship

between yield strength and temperature. Equation (1)

can be used to develop temperature de-rating factors for

the present HDPE material at any service temperature

within the speciﬁed range.

The values of s ys reported in Ref. 5 for HDPE, Ref. 7

for PVC and Ref. 3 for CPVC pipeﬁtting material are

also shown in Fig. 3 for comparison purposes. As

expected, both PVC and CPVC have higher strength

than HDPE at all temperatures. The temperature

sensitivity of the strengths of PVC and CPVC materials

is more than that of HDPE; the yield strength s ys 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 s ys . 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

temperature T for HDPE pipe material can be expressed

3 Effect of temperature on yield stress

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Merah et al. Effects of temperature on tensile properties of HDPE pipe materials

5 Relationship between t and G for HDPE

6 Variation of yield strain with temperature for HDPE

E(T)~3912 : 8{11T (3)

A comparison of the variation of E for HDPE with that

for CPVC (Ref. 3) and PVC (Ref. 7) with absolute

temperature is also shown in Fig. 4. An analysis of these

curves reveals that the regression lines for HDPE and

CPVC seem to be parallel to each other, which leads to

the conclusion that the variation of temperature has a

similar effect on the stiffness of these two materials. The

stiffness of PVC, however, decreases at a faster rate than

that of HDPE and CPVC.

for semicrystalline polymers. Merah et al. 3 have also

used Kitagawa’s model in their study of CPVC and

reported the value of exponent n equal to 1 . 69. Figure 5

is a log–log plot according to equation (4) for the values

of t and G obtained for HDPE tested over the

temperature range of 210 to 70uC. The Poisson’s ratio

was assumed constant, equal to 0 . 46 (Ref. 12), over the

temperature range of interest. The line drawn on the

graph has a slope of 0 . 659 and all the points fall very

close to this line with a coefﬁcient of regression of 0 . 975.

The value of slope is less than that reported for

semicrystalline polymers because of the presence of

additives in the HDPE pipe material chosen for testing.

Relationship between elastic modulus and yield

strength

Elastic modulus and yield strength are linearly related to

each other. Hence, any variable that affects elastic

modulus will also affect the yield strength. Argon and

Bessonov 10 derived the analytical relationships between

elastic modulus and yield strength over a wide tempera-

ture range. These theories show excellent agreement with

Argon’s experimental results but their analytical form

is too complex to be applied to practical situations.

Kitagawa 11 has expanded and generalised Argon’s

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

Yield strain

Figure 6 shows the variation of yield strain with

temperature. The yield strain (denoted by e y ) is deﬁned

as the ratio of yield stress to modulus of elasticity, i.e.

e y 5s ys /E. The yield strain remains fairly constant for the

temperature range studied; the regression line shown in

the graph has a slope of 2610 24 . Similar results were

obtained by Povolo et al. 7 and Merah et al. 3

for PVC

and CPVC respectively.

Conclusions

The effect of temperature on the mechanical properties

of high density polyethylene PE-100 pipe material was

studied by performing a number of tensile tests at six

different temperatures (210, 0, 23, 40, 50 and 70uC).

The following conclusions are obtained from the

analysis of tensile test results.

1. The yield stress and elastic modulus decrease

linearly with temperature.

2. Ductile fracture occurred at all the temperatures.

3. The temperature dependence of HDPE strength is

lower than that of PVC and CPVC; the yield strength

for HDPE decreases at a slower rate than that for PVC

and CPVC.

4. The variation of temperature has a similar effect on

the stiffness of HDPE and CPVC.

5. Shear stress and shear modulus are related by a

Kitagawa power law with an exponent of 0 . 66.

6. Variation of temperature has a limited effect on the

yield strain of HDPE.

T o t

Tt o ~ T o G

(4)

TG o

where T o is the reference temperature, the values t o and

G o are of shear yield stress and shear modulus at some

T o (conveniently taken as the ambient temperature), and

n is a temperature independent exponent.

The tensile modulus and yield strength are converted

into the corresponding shear modulus and shear yield

strength for using Kitagawa’s relationship. This can be

performed using the

following equations of

solid

mechanics

G(T)~ E(T)

21zn

(5)

t(T)~ s ys (T)

3 1 = 2

(6)

where n is the Poisson’s ratio.

Kitagawa in agreement with Argon showed that a

relationship of the form of equation (4) held over a wide

range of temperatures for most polymers. He also found

that the exponent n had a unique value of 1 . 63 for all

amorphous polymers and a value between 0 . 80 and 0 . 90

Acknowledgement

The authors acknowledge the support of the King Fahd

University of Petroleum and Minerals.

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