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The Effects of Loading History and Manufacturing
Methods on the Mechanical Behavior of High-Density
Polyethylene
Article in Journal of Elastomers and Plastics · September 2011
DOI: 10.1177/0095244311404181
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The Effects of Loading History and
Manufacturing Methods on the
Mechanical Behavior of High-Density
Polyethylene
NECMI DUSUNCELI*
Department of Mechanical Engineering, Aksaray University, Aksaray
68100, Turkey
BULENT AYDEMIR
The Scientific and Technological Research Council of Turkey
(TUBITAK), National Metrology Institute, Kocaeli 41470, Turkey
ABSTRACT: This article describes a series of experiments conducted to
determine the effects of loading history and manufacturing techniques on
mechanical behavior of high- density polyethylene (HDPE). The main reason for
undertaking the research was to investigate multiple creep, multiple relaxation,
and cyclic loading on uniaxial tension. The samples used for tensile tests were
obtained from extruded pipe and compression-molded sheets. The stress–strain
responses of both samples under uniaxial tensile were found to be independent
of the loading history. It was observed that the compression-molded specimens
exhibit greater deformation ratio than the extruded specimen. Understanding
the deformation behavior under different loading can offer the designer of highdensity polyethylene products reliable data relevant to practical applications.
KEY WORDS: high-density polyethylene, loading history, mechanical properties, creep, relaxation, manufacturing techniques.
*Author to whom correspondence should be addressed.
E-mail: dusunceli@gmail.com
JOURNAL OF ELASTOMERS AND PLASTICS Vol. 43–September 2011
0095-2443/11/05 0451–18 $10.00/0
DOI: 10.1177/0095244311404181
ß The Author(s), 2011. Reprints and permissions:
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451
452
N. DUSUNCELI AND B. AYDEMIR
INTRODUCTION
been replacing metallic materials in
many engineering applications such as load-bearing components and
applications involving the use of high-explosive materials and expected
to perform as reliably as the components that they replace. Polymers
present a very complex nonlinear behavior depending on external
factors and structural parameters and are classified by taking into
account three basic characteristics that greatly influence the processing
and end-use properties. These characteristics are density, molecular
weight, and molecular weight distribution. High-performance thermoplastics such as high- density polyethylene (HDPE) are semi-crystalline
polymeric materials having a microstructure that has both crystalline
and amorphous regions. HDPE has a ratio of up to 90% crystallinity;
however, low-density polyethylene has a ratio of up to 40% crystallinity.
Thus, it can be seen that the greater the crystalline regions, the higher
the density of polymeric materials and this affects a number of physical
and mechanical properties of HDPE. Generally, increasing degree of
crystallinity results in greater yield strength and stiffness.
The mechanical behavior of HDPE is viscoelastic, and because of its
semi-crystalline composition, it is very complex, being directly related to
time and temperature. The crystalline region that accounts for the
elastic response to forces is stiff; the amorphous region accounting for
the viscous fluid-like response is soft.
If polymeric materials are subjected to a constant load, initially they
undergo a rapid deformation and then the deformation continues at a
slower rate until eventually the material ruptures. This deformation is
named viscoelastic creep and this may occur even at room temperature
and under modest stress level below the yield point of polymeric
materials.
Relaxation can be defined as a change in the stress level with time
when the strain is held constant. Relaxation is one of the most basic
techniques to determine viscoelastic behavior. Initially, stress relaxation
occurs rapidly, then gradually decreases, and after some time, the stress
level reaches equilibrium [1–4].
Structural components to be subjected to severe loading conditions
must be reliable. Hence, performance analysis needs to be carried out
prior to production. The first step in this process is inelastic analysis,
which provides information about stresses and strains during manufacturing and lifetime. For the inelastic analysis and lifetime predictions
of engineering components, both experimental results and constitutive
models are needed to estimate the deformation behavior of these
P
OLYMERIC MATERIALS HAVE
Effects on the Mechanical Behavior of HDPE
453
materials as accurately as possible. In order to develop an experimentalbased constitutive model, the mechanical response of polymer needs to
be investigated under different loading conditions, such as uniaxial and
multiaxial, monotonic and cyclic, and single and multiple loadings.
HDPE is a widely used raw material in the manufacture of pipes. Even
though there are large numbers of experimental and analytical
investigations on HDPE, few have examined the effects of manufacturing techniques on the small and finite deformation behaviors of HDPE.
Since HDPE is a semi-crystalline polymeric material, the degree of
crystallinity, molecular morphologies, and molecular structure extensively affect its mechanical behavior. Different manufacturing methods
result in different molecular morphologies and molecular structure in
the final product.
Various experimental studies have investigated singly or combined
aspects of deformation such as loading–unloading, relaxation, creep,
multiple creep behavior of HDPE. However, this study has only been
carried on one manufactured sample such as compression-molded or
injection-molded sample. Understanding the deformation behavior
under different loadings can offer the designer of HDPE products
reliable data relevant to practical applications. The study of the loading
history is important in engineering design because the mechanical
properties and deformation mechanism may be heavily dependent on the
applied loading types.
Khan and Zhang [5] performed a series of tests on polytetrafluorethylene (PTFE) samples that included monotonic loading and multiple
creep, multiple relaxation tests. This investigation revealed that the
mechanical behavior of PTFE is independent of loading history. Zhang
and Moore [6] investigated the multiple creep, relaxation, and cyclic
compression behavior of HDPE including the testing of specimens after
loading and partial unloading. Khan [7] performed a series of loading–
unloading tests and multiple relaxation and creep tests on thermoplastics samples. He aimed to compare the effect of loading history on
relaxation and creep tests at different loading and unloading stress–
strain levels. Avanzini [8] carried out a fully reversed symmetric
tensile and compression loading test on ultra-high molecular weight
polyethylene (UHMWPE) samples and reported that strain softening
and stress–strain hysteresis were observed after cyclic loading. Meyer
and Pruitt [9] performed cyclic tensile loading tests on UHMWPE
samples at large deformation levels and indicated that the mechanical
behavior of UHMWPE was related to microstructural variables.
Therefore, an increasing amount of strain and number of cycles
increases the residual plastic strain, which reduces density.
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N. DUSUNCELI AND B. AYDEMIR
Zrida et al. [10] investigated the mechanical behavior of three grades of
polypropylene (PP) at various loading conditions that include monotonic
loading, cyclic loading–unloading, and multiple relaxations. This study
showed that mechanical behavior of PP is highly nonlinear and
hysteretic. This behavior depends on strain rate level. Zhang and
Chen [11] performed multiaxial ratcheting test on solid cylindrical
PTFE samples under constant axial stress with cyclically controlled
shear strain at room temperature. They concluded that the ratcheting
behavior of PTFE strongly depends on loading histories and increasing
number of cycles decreases the ratcheting strain rate. Drozdov and
Christiansen [12] conducted uniaxial tensile tests, relaxation tests, creep
tests, and cyclic tests on two grades of HDPE. Their work focused on the
mechanical behavior of HDPE associated with molecular weight.
Increasing molecular weight affects crystalline morphology by making
it more regular. This regularity causes an increasing elastic modulus
and a slowing down of the viscoelastic processes.
In the literature, much investigation has been conducted on the effect
of strain rate, loading types, and temperature on the mechanical
behavior of polymeric materials. A few studies have investigated the
effect of loading history, cyclic deformation behavior, and manufacturing methods. The aim of experimental work reported in this article was
to explore the mechanical behavior of HDPE associated with loading
history, manufacturing methods, and cyclic loading–unloading. Multiple
creep and relaxation tests were performed by loading two and three
stress–strain levels. Cyclic loading–unloading tests were conducted for
various strain and strain rate levels, and for each of the manufacturing
methods.
EXPERIMENTAL WORK
The experiments are carried out with a Zwick 250 tensile
machine at the National Metrology Institute in The Scientific
and Technological Research Council of Turkey (TUBITAK). The
load-cell capacity of the tensile machine can be adjusted for loads of
10, 20, 50, and 100 kN. A load-cell with capacity of 10 kN is used in
tensile tests.
First, the extruded samples for the tensile tests were obtained from
PE100 (HDPE) pipe with a diameter 160 and wall thickness 5 mm. The
extruded samples were created according to the ISO 6259-1 and ISO
Effects on the Mechanical Behavior of HDPE
455
6259-3 standards for the determination of the properties of tensile
deformation in polymeric pipes [13,14].
The compression-molded samples were punched out from compression-molded 2-mm thick HDPE sheets. The density of this material is
0.954 g m 3 and the trade name TotalXS10B. The samples were crated
according to the ISO 527 Standard [15,16]. Figure 1 shows the
dimensions of the extruded and compression- molded samples.
To investigate the behavior of HDPE samples, the following
experiments were performed.
1. Uniaxial loading and unloading at different strain rates, 1.E 4 s 1.
2. Cyclic loading–unloading at different strain rates: 1.E 3 and
1.E 4 s 1 and various loading–unloading strain levels.
3. Multiple creep at different stress levels, 6–16, 6–13, 13–16, and
6–13–16 MPa for 1200 s at the strain rate of 1.E 4 s 1.
4. Multiple relaxation at strain levels of 2–15%, 5–15%, 10–15%, and
5–10–15% at the strain rate of 1.E 4 s 1 for 1200 s.
All tests were performed at 23 18C (room temperature).
RESULTS AND DISCUSSIONS
Loading–Unloading Behavior
The uniaxial tension loading and unloading tests were performed on
extruded and compression- molded HDPE at room temperature. The
samples were loaded with a constant strain rate up to 15% strain and
unloaded at the same strain rate. The uniaxial stress–strain behavior of
HDPE at room temperature at 1.E 4 s 1 on both extruded and
compression-molded samples are shown in Figure 2. The stress–strain
curves have a similar overall behavior but the stress levels are different.
The stress–strain curves of both the extruded and compression-molded
samples demonstrated elastic–viscoelastic–viscoplastic mechanical behavior. The results of the uniaxial loading–unloading experiments
performed at 15% strain levels showed that the maximum stress and
yield stress of the extruded specimen were much higher than the
compression-molded samples. The maximum stress levels of the
extruded samples were 5 MPa higher than the compression-molded
samples. The reason for this diversity is the degree of crystallinity and
difference in the manufacturing technique that involves the reorientation of the polymer chains.
456
N. DUSUNCELI AND B. AYDEMIR
FIGURE 1. The dimensions of the samples (all dimensions are in mm).
FIGURE 2. Comparison of loading–unloading behaviors of extruded and
compression-molded HDPE at 1.E 4 s 1.
Loading–Unloading Strain Rate Change Dependency
Two tests consisting of three loading–unloading steps were conducted
to investigate the strain rate history and unloading behavior. The first
test was conducted at the same strain rate, 1.E 4 s 1, and it consisted of
three loading–unloading steps at 5%, 10%, and 15% strain levels.
Effects on the Mechanical Behavior of HDPE
457
FIGURE 3. Comparison of cyclic and single loading–unloading behaviors of extruded
HDPE at 1.E 4 s 1.
FIGURE 4. Comparison of cyclic loading–unloading behaviors of compression-molded
HDPE at 1.E 4 and 1.E 3 s 1.
These test results are given in Figures 3 and 4 for the extruded and
compression-molded samples, respectively. The second test was performed three loading–unloading steps at 5% strain level at 1.E 4 s 1,
followed by 10% at 1.E 3 s 1, and 15% strain level at 1.E 4 s 1
458
N. DUSUNCELI AND B. AYDEMIR
FIGURE 5. Comparison of cyclic and single loading–unloading behaviors of extruded
HDPE at 1.E 4 and 1.E 3 s 1.
strain rate; see Figures 5 and 6 for studying the extruded and
compression-molded samples, respectively. The strain rate of the last
step for each test was the same; so, the stress level at the same strain
rate could be directly compared for different histories.
The shapes of the unloading curves are nonlinear concave curvature
for each of the samples at each of the loading–unloading steps. For the
unloading, only the initial stress level affected the evolution of the
unloading curve. Increasing the stress level shifted the loading curve
inward and increased the amount of recovery on unloading. The
magnitudes of the recovered strain magnitudes are given in Table 1.
The unloading behavior of both samples at all strain rates is nonlinear
and the viscoelastic recovery during the unloading (until zero stress
level) increases with an increase in the strain level. The extruded
samples are more recovered than compression-molded samples.
Figures 5 and 6 show that despite the second step loading–unloading
strain rate on the second test not being the same as the other two steps;
there is no difference in the last loading curve. Finally, in Figures 3
and 4, the loading–unloading stress–strain curves at 15% at 1.E 4 s 1
are compared with the cyclic loading–unloading stress–strain curve and
it can be seen that cyclic loading–unloading history has no effect on the
mechanical behavior of HDPE.
459
Effects on the Mechanical Behavior of HDPE
FIGURE 6. Comparison of cyclic loading–unloading behaviors of compression-molded
HDPE at 1.E 4 and 1.E 3 s 1.
Table 1. The percentage of viscoelastic recovered strain during unloading.
Number of
step
Single
1
2
3
1
2
3
Strain
rate (s 1)
1.E
1.E
1.E
1.E
1.E
1.E
1.E
4
4
4
4
4
3
4
Strain
level (%)
Extruded
sample (%)
Compression-molded
sample (%)
15
5
10
15
5
10
15
10.34
3.65
6.96
10.34
3.65
6.2
10.34
8
2.57
5.34
8
2.57
5.1
8
Creep
Creep is a strain increasing with time when the stress level is kept
constant. For the prediction of the durability and reliability over the
lifetime of components made from polymeric materials, the creep
responses of the polymers are fairly important. Creep deformation of
polymeric materials can occur at relatively small stress level at room
temperature. Two- and three-step multiple creep tests were conducted
to investigate the effects of stress and loading history. Multiple
creep tests were performed for both sample types at different stress
460
N. DUSUNCELI AND B. AYDEMIR
FIGURE 7. Comparison of creep stress–strain behaviors of extruded HDPE at various
stress levels.
levels: 6–16, 6–13, 13–16, and 6–13–16 MPa for 1200 s at the strain rate
of 1.E 4 s 1 in Figures 7 and 8. The percentages of creep strain were
calculated for both the samples and are presented in Table 2. The
amount of creep strain increases with the rising stress level and is
independent of the effect of the loading history. These results indicated
that the compression-molded samples had greater creep strain magnitude especially at high-stress level than the extruded samples.
The strain versus time curves are given in Figures 9–11. All the
viscous effects are related to the delayed response of the polymer chains.
It is observed that at higher stress levels, compression-molded samples
yielded greater change in strain than the extruded samples.
Relaxation
Relaxation is a drop in stress over time when the strain level is
maintained. For polymeric materials, even at small strains, relaxation is
observed. According to Krempl and Khan [17], relaxation is constant in
the inelastic region of the stress–strain curve. To investigate both this
case and the effect of the loading history, multiple relaxations at the
strain levels of 2–15%, 5–15%, 10–15%, and 5–10–15% at the strain rate
1.E 4 s 1 were performed for 1200 s. Increasing strain level increases
the stress drop. Finally, the relaxation was found to be at the same
461
Effects on the Mechanical Behavior of HDPE
FIGURE 8. Comparison of creep stress–strain behaviors of compression-molded HDPE
at various stress levels.
Table 2. The percentage of strain increasing during creep test at different
stress levels.
Stress level (MPa)
6
13
16
Extruded sample (%)
Compression-molded sample (%)
0.434
1.43
1.58
0.75
3.25
6.23
magnitude in the inelastic region. The stress versus strain at the strain
levels of 2–15%, 5–15%, 10–15%, and 5–10–15% are shown in Figures 12
and 13 for the extruded and compression-molded samples, respectively.
Following the relaxation experiments, the same sample was reloaded
at different strain levels. It was observed that stress levels were
almost the same after reloading, as can be seen Figures 12 and 13.
Such an observation reveals that material is not history dependent: to
a large extent, prior loadings do not affect the subsequent behavior.
The percentages of the stress drop during relaxation test at different
strain levels are given in Table 3. The results of three relaxation tests
together with stress–time curves are compared in Figures 14–17 for all
strain levels. It can be seen that evolving of trend of the curves is
462
N. DUSUNCELI AND B. AYDEMIR
FIGURE 9. Comparison of creep behaviors of extruded and compression-molded HDPE
at a stress level of 6 MPa.
FIGURE 10. Comparison of creep behaviors of extruded and compression-molded HDPE
at a stress level of 13 MPa.
Effects on the Mechanical Behavior of HDPE
463
FIGURE 11. Comparison of creep behaviors of extruded and compression-molded HDPE
at a stress level of 18 MPa.
FIGURE 12. Comparison of relaxation stress–strain behaviors of extruded HDPE at
various strain levels.
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N. DUSUNCELI AND B. AYDEMIR
FIGURE 13. Comparison of relaxation stress–strain behaviors of compression-molded
HDPE at various strain levels.
Table 3. The percentage of stress drop during relaxation test at different
strain levels.
Strain level (%)
2
5
10
15
Extruded sample (%)
Compression-molded sample (%)
27
24.9
22.09
21.6
28
23.2
24.3
23.6
approximately equidistant for all the strain levels. One of the interesting
aspects of the relaxation experiments is that stress drop is approximately the same in the both the extruded and compression- molded
samples, as seen in Table 3. These results indicate that manufacturing
methods have no influence on the relaxation behavior of HDPE.
CONCLUSIONS
Comprehensive research on the mechanical behavior of HDPE was
carried out to investigate the effects of loading history and
Effects on the Mechanical Behavior of HDPE
465
FIGURE 14. Comparison of relaxation behaviors of extruded and compression-molded
HDPE at a strain level of 2%.
FIGURE 15. Comparison of relaxation behaviors of extruded and compression-molded
HDPE at a strain level of 5%.
466
N. DUSUNCELI AND B. AYDEMIR
FIGURE 16. Comparison of relaxation behaviors of extruded and compression-molded
HDPE at a strain level of 10%.
FIGURE 17. Comparison of relaxation behaviors of extruded and compression-molded
HDPE at a strain level of 15%.
Effects on the Mechanical Behavior of HDPE
467
manufacturing methods on different manufactured samples. Uniaxial
cyclic loading–unloading, multiple creep, and multiple relaxation tests
were conducted at room temperature. The HDPE exhibited nonlinear
time- dependent and loading history-independent behavior in the
uniaxial tension test. The deformation behaviors of both samples have
the same trend except for the stress–strain level.
In the cyclic tests, the unloading behaviors of both samples at all
strain rates were nonlinear and the viscoelastic recovery during
unloading (until zero stress level) rises with an increase in the strain
level. The extruded samples recovered more readily than the compression-molded samples on unloading. The creep behavior of HDPE
was found to be independent of loading history and the amount of
creep strain increases with the rising stress level. The amount of
creep strain in compression-molded sample was greater than in the
extruded samples at especially high-stress levels. The relaxation
behavior of HDPE is independent of loading history and the stress
drop magnitude decreases with increasing strain level. In the flow
stress region, stress drop is found to be independent of the strain level.
The percentage of stress drop is very similar for both samples at all
strain levels.
The results of this study show that manufacturing techniques affect
the mechanical behavior of HDPE in terms of elastic modulus and
tensile strength. Decrease of such mechanical properties is related as the
function of the crystalline ratio or molecular orientation. However, the
extruded and compression-molding manufacturing techniques have
little impact on the overall mechanical behavior of HDPE and the
loading history effects. However, it is important that designers of HDPE
products are fully aware of these impacts and accessible accurate data
must be made available.
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