Results in Physics 9 (2018) 1050–1056
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Results in Physics
journal homepage: www.elsevier.com/locate/rinp
Investigation on the hot melting temperature field simulation of HDPE water
supply pipeline in gymnasium pool
T
⁎
Zhiqiang Caia, Hongbin Daia, , Xibin Fub
a
b
Harbin University of Science and Technology, Harbin 150040, China
Xiamen Special Equipment Inspection Institute, Xiamen 361004, China
A R T I C LE I N FO
A B S T R A C T
Keywords:
High density polyethylene pipe
Thermal melt welding
Temperature field
In view of the special needs of the water supply and drainage system of swimming pool in gymnasium, the
correlation of high density polyethylene (HDPE) pipe and the temperature field distribution during welding was
investigated. It showed that the temperature field distribution has significant influence on the quality of welding.
Moreover, the mechanical properties of the welded joint were analyzed by the bending test of the weld joint, and
the micro-structure of the welded joint was evaluated by scanning electron microscope (SEM). The one-dimensional unsteady heat transfer model of polyethylene pipe welding joints was established by MARC. The
temperature field distribution during welding process was simulated, and the temperature field changes during
welding were also detected and compared by the thermo-couple temperature automatic acquisition system.
Results indicated that the temperature of the end surface of the pipe does not reach the maximum value, when it
is at the end of welding heating. Instead, it reaches the maximum value at 300 sand latent heat occurs during the
welding process. It concludes that the weld quality is the highest when the welding pressure is 0.2 MPa, and the
heating temperature of HDPE heat fusion welding is in the range of 210 °C–230 °C.
Introduction
In recent years, the concepts of sustainable development and environmental protection have gradually become deeply rooted in the
hearts of the people, with the rapidly develop of the science and
technology. Some kinds of new materials with low-carbon environment-friendly characteristics are widely used in industrial manufacturing and some other areas of life and production. Polyethylene is
one of these new materials which has a wide application prospect
[1].The water supply and drainage systems of the stadiums and
swimming pools usually require the drainage pipes to be wear resistant,
corrosion-resistant, and stress cracking resistant. Most traditional metal
pipelines are currently used. But the corrosion resistance and processing
performance of metal pipes are poor, which are unable to meet the
requirements of actual use. Polyethylene pipes, which can be connected
by hot melt, have rather excellent chemical resistance and wear resistance, and their flow resistance is small. Therefore, this polyethylene
pipes are gradually used to replace the traditional metal pipes.
The large-scale use of polyethylene pipes began in the 70s of last
century. With the progress of society and technology, polyethylene
pipes have been widely applied in the fields of water supply and gas
transportation, and have become an essential pipeline system to
⁎
maintain the normal life of urban residents. The way of connection of
polyethylene pipes is most widely used in hot melt welding. In the
process of laying water and gas pipelines, the performance of hot-melt
welded joints is particularly important for pipeline system safety [2,3].
It has proved that the most easily damaged and leaked part of
polyethylene pipe is the interface part of pipeline, so the key to success
of pipeline system is the quality of pipe connection. The most easily
damaged parts of polyethylene pipes in service are the welded joints, so
the welded joints properties are particularly important to the safety of
the pipelines [4–7].
During the hot-melt welding process of polyethylene pipes, the
technological defects caused by improper selection of welding parameters are easy to be ignored, leaving a major hidden danger for
polyethylene pipeline system. As we all known, the welding joints are
weak links of polyethylene pipeline system due to its’ complex process
parameters and operations. So the welding quality is greatly influenced
by human factors.
Hot-melt welding has become one of the most important technologies in the field of polyethylene pipe connection. The research on it has
been started in 80s of last century. The United States International
Pipeline Research Committee believes that welded joints are the most
vulnerable parts of pipeline system, because the material of welded
Corresponding author at: The School of Material Science & Engineering, Harbin University of Science and Technology, China.
E-mail address: hongbindai95@126.com (H. Dai).
https://doi.org/10.1016/j.rinp.2018.04.019
Received 22 March 2018; Received in revised form 9 April 2018; Accepted 9 April 2018
Available online 13 April 2018
2211-3797/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 9 (2018) 1050–1056
Z. Cai et al.
joints and pipes is difficult to be exactly the same. The welding joints
and welding defects are considered as “stable failure factors” [8]. The
VEG Gas Association of Holland has summed up the two major categories of short-term and long-term mechanical performance test
methods for the performance testing methods of hot-melt welded joints
in polyethylene pipes.
Because the welding joint is a relatively weak part of the pipeline
system, many scholars have gradually begun to study the failure mode
of the hot melt welded joint of polyethylene pipe. In early 80s, B.
Cherry et al. [9] had founded two kinds of failure modes of welded
joints by studied the tensile test of the joint specimen: one failure mode
Table 1
Range of parameters in DVS 2207-1.
Heating
Temperature (T/
°C)
Welding
Pressure
(P1/MPa)
Heating
Times (t2/
s)
Switching
Times (t3/s)
Cooling Time
of Pressure
Welding (t5/
min)
190–240
0.2
70–120
6–8
10–16
Fig. 1. Flow chart of welding process. a) Milling Process; b) Effect after milling; c) Heating process of pipe; d) Cooling process of pressure welding; e) Basic
technology of fusion welding.
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the hot-melt welded joint, and the last one is the crack far away from
the joint wall.
In twenty-first Century, L. Daigle et al. [14] from Canada Building
Research Association had tested the tensile property of the polyethylene
pipe butt welded joints with different inclusions, they analysis the
tensile stress–strain curve, obtained the stress–strain relationship of
different failure mode, and divided it into several quality levels. M.
Troughton and A. Scandurra [15] had perform the high temperature
hydrostatic test of hot melt inclusion with different amount of welding
by using high temperature tube stretching equipment, the failure mode
was then obtained. Through tensile test, dynamic mechanical analysis
(DMA) and slow crack growth (SCG) test of high density polyethylene
pipe hot melt connection, D. Shim [16] had found that the pipe material
and fusion zone is basically the same, but the fusion zone more easily
than material pipe failure, shorter life expectancy.
For the connection of polyethylene pipes, welding technology is an
important part that affects the safety application of pressure piping, and
it has a very important influence on the welding quality of pipe fittings
and the change of temperature field during welding process. In this
study, one-dimensional unsteady heat transfer model of polyethylene
pipe welded joints was established by MARC software. The temperature
and stress fields distribution were simulated. The temperature field of
welding was detected by temperature inspecting instrument, and the
simulation results were compared with experiments. The mechanical
properties of welded joints were analyzed by bending test. The fracture
morphology of welded joints was compared by scanning electron microscope (SEM), and the relationship between welding temperature and
weld performance was obtained.
Fig. 2. Schematic diagram of test specimen.
is the fracture failure from outer edge root to inner edge root, the other
one is the fracture failure of fusion surface. They also found that the
rigidity of crack edge rolling and the crack angel between rolling edge
and pipe wall have important influence on the performance of stiffness.
Since then, R. Parmar and J. Bowman [10] had tested the fatigue
property of hot melt welding joints and found that the initial crack edge
from the end of the germination, extended to the outer edge of the
middle, and the failure of joints all brittle fracture. In 1997, R.S cavrzzo
et al. [11] studied the fatigue and fracture behavior of hot melt welded
joints of polyethylene pipes by rapid tensile test and bending fatigue life
test. The testing results showed that the bending fatigue resistance of
pipe is greater than that of the pipe welding joint. During the bending
fatigue tests, the failure modes of hot melt welding joint are as follow:
the first step is the crackinitiation from the groove at the outer edge and
the pipe, then the heat affected zone extends to the edge groove after
fracture. Results from the rapid tensile test of the welding joints indicated that the fracture occurred in the heat of fusion zone, and the
fracture cross section showed the local fatigue fracture extension
character.
J. Bowman and R. Parmar [12,13] of Brunel University in 1989
adopted the isothermal fatigue and life tests with constant temperature
and pressure to exam the life of the joints with polyethylene joints in
the hot melt welding of polyethylene pipes. Through the analysis of the
test results, they found that when the life of the fusion welded joint is
guaranteed, the misalignment rate can't exceed 0.09, and the current
standard requires that the wrong side number should not exceed 10%.
Based on the results above, they also found three kinds of cracks that
cause the failure of the joint. The first one is the MDPE specific axial
crack under fatigue test, the second one is the circumferential crack on
Experiment
In order to study the effect of temperature on the performance of the
weld, tests are performed according to the DVS2207-1 standard of
polyethylene pipe hot melt. The equipment used in this research is the
Italy Ritmo company's CNC ASIA 250 FA automatic butt fusion welding
machine, the welding temperature range is set from 190 °C to 240 °C,
and the welding temperature interval is 10 °C. The welding temperature
parameters are listed in Table 1. The welding parameters include
heating temperature (T/), welding pressure (P1/Mpa), heating time
(t2/s), conversion time (t3/s) and pressure welding cooling time (t5/
min). The common welding method of polyethylene pipes is thermal
fusion welding. Fig. 1 shows basic technology of fusion welding. The
material we used in this research is PE100 water pipe with its’ diameter
is 200 mm, SDR is 17, and the wall thickness is 11.76 mm.
The bending specimen is prepared according to the German standard DVS 2203-5 (bending test), as shown in Fig. 2. After the sample is
prepared, it should be placed in the environment of 23 °C ± 2 °C,
6 h ± 30 min for state adjustment to eliminate the influence of residual
stress and temperature difference on tensile test.
Results and discussion
The three point bending test is carried out at room temperature
Fig. 3. Cross sections of samples after bending tests.
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Fig. 4. The microstructure of cross section of sample bending at 190 °C. a) black area; b) area between black and white; c) white area.
Fig. 5. The cross section SEM of samples after bending at 240 °C.
Table 3
Thermo physical parameters of polyethylene solid phase and melt phase.
Density ρ/kg·m
Specific heat at constant pressure Cps/kg °C
Thermal conductivity Ki/W/(m °C)
Thermal diffusivity αi/m2/S
3
Table 2
Physical performance parameters of HDPE.
Thermal
conductivity/
W·(m °C)−1
Isobaric heat capacity
Cp/kJ·(kg °C)−1
Enthalpy/
kJ kg−1
0
50
100
125
132
150
200
250
0.445
0.405
0.34
0.300
–
0.265
0.264
0.263
1.83
2.05
2.87
5.56
15.51
2.65
2.67
3.03
0
95
200
318
485
535
700
900
Melting phase
955
2.31
0.49
2.23 × 10−7
766
2.512
0.24
1.23 × 10−7
(25 °C) with 50 mm/min extrusion speed. Under different welding
procedure specifications. Fig. 3 shows the photographs of samples after
bending tests. It can be clearly seen that the bending specimens failed to
produce cracks and surface cracks at 200 °C, 210 °C, 220 °C, and 230 °C
in 120°bending angle. Moreover, the fracture occurred in the center of
welding joints at 190 °C and fracture along the junction of the parent
material and the weld seam at 240 °C, respectively.
From Fig. 3, it can be seen that the fracture at 190 °C is mainly
divided into two regions in black area and white area, and only in white
area at 240 °C. The occurrence of the white area can be attributed to the
“white stress” phenomenon of HDPE, which generated at the bending
state. This “white stress” phenomenon can be explained as follow: when
the polymer is under the action of external force, the internal holes and
defects are constantly forming, growing and connecting, then cause the
formation of silver grains in perpendicular to the direction of stress.
This silver grains can change the refraction rate of the polymer and
almost totally reflected the incident light at the serious deformational
region, resulting the white morphology of the sections.
The section after failure was cut off and placed in the ultrasonic
cleaning instrument. After ultrasonic cleaned for 15–20 min and natural
dried, the section was spray by gold. Then SEM was used to observe the
section. Figs. 4 and 5 are microstructures of cross sections after bending
at 190 °C and 240 °C, respectively.
As can be seen from Fig. 4a), the black area presents lamellar
structure, which is due to the non-sufficient entanglement of the PE
Fig. 6. Finite element model of HDPE pipe.
Temperature/°C
Solid phase
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Fig. 7. Contours of heating process.
Fig. 8. Isothermal distribution of heating process.
Fig. 9. The cooling curve of heating temperature at 210 °C.
will break out at once. In this process, the stress is continuously applied,
the stress concentration is more serious, and the new microfiber is more
easily fractured. Finally, the white area appears in the macro fracture
area.
Fig. 5 shows a cross section SEM image of samples after fractured at
240 °C. It can be seen that although the wrinkle is obvious, the particles’
size of the filler is smaller and the compatibility is better. However, the
orientation of crazing microfiber tends to be in the radial direction of
the pipe, and the number of crazing microfiber with axial orientation is
very small. Due to the very high temperature and pressure during
welding, a large number of high temperature molten material is extruded to form a welding edge, resulting the reduction of the thickness
of weld area at melting zone. Moreover, the rather low temperature at
residual molten zone will cause the weaken of the thermal movement of
molecules at the low welding temperature. At this case, the fusion area
form a loose spiral lamellae, resulted the decreased of fusion surface
combination. Fig. 4 b) shows that the connecting piece of crystal is not
close in bending the relative sliding force. With the increasing of stress,
the lamellae is splitted, then forms a plurality of fine grain blocks, while
the molecular chain is elongated. These elongated molecular chains
tangle each other, some voids or defects are formed. From Fig. 4c),
there is a clear fold between the fracture surfaces and the morphology
of the large diameter filling. At this time, with the increase of bending
stress, the micro void gradually increases, and the micro voids begin to
gather on the plane perpendicular to the maximum principal stress,
thus forming microfibers around the micro holes gradually. Continuous
stress is applied, and the microfiber is gradually elongated under the
action of force. When the microfiber is elongated to a certain extent, it
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Fig. 10. Comparison of the axial temperature curves of different positions at the same time.
heat transfer coefficient of the outer wall of the pipeline is 7.22 W/
(m2·°C). The convection heat transfer coefficient of the inner wall of the
pipeline is 0.89 W/(m2·°C).
The heat conduction in the welding process can be regarded as a
dynamic local heating process. In order to accurately respond the relationship between material mechanical properties and temperature,
the selection of thermophysical properties of materials is particularly
important, which directly affects the accuracy of simulation results.
HDPE is a polymer material with high crystallinity, its’modulus is related to temperature and time. According to the relationship between
temperature and modulus of polymer, it can be concluded that the state
of the material at different temperatures is charactered as the glass
state, high elastic state and viscous flow. Moreover, it has two regions
includes the conversion of the glass transition region and viscous state
transformation zone. As we all known, the long chain structure of
polymer materials depends on the motion formin the material inside the
transition between different states of non isothermal process. The specific heat of polymers significantly affects by temperature. In the process of curing, polymers can release the curing latent heat. So the
equivalent specific heat method can be choosed to determine properties
of amorphous polymers at different temperatures. The melting point of
polyethylene is about 130 °C, which is of great significance for the study
of polyethylene crystallization. The density of HDPE is 0.95 kg/m3, and
other thermal physical and mechanical properties with the change of
temperature are shown in Tables 2 and 3.
Fig. 7 shows the distributed cloud chart of the HDPE pipeline during
welding at 180 s and 300 s. The isotherm diagrams at 180 s and 300 s
were made according to the cloud chart. As shown in Fig. 8, it can be
seen that the isothermal surface is pushed forward uniformly in the
axial direction, the temperature is gradually reduced and the end face
temperature is the highest. The thicknesses of the melting layer at 180 s
are about 1.5 mm, and the thicknesses of the thawing layer are about
3.5 mm at 300 s.
For the reliability of numerical simulation verification of polyethylene pipe, several holes are drilled, which located at 2 mm, 4 mm,
6 mm, 8 mm, 10 mm, 12 mm, 14 mm, 16 mm, 18 mm from outer surface
to the end of the pipe. The size of each hole is about 5 mm in diameter
and 0.5 mm in depth. K type thermocouple wires were buried in these
nine holes and the changes of temperature are recorded by the temperature inspection instrument during the welding process. Fig. 9 is the
curve of temperature changing with time at different axial test points at
210 °C under 0.2 MPa.
The experimental data and the simulation results under the optimum welding process parameters, namely 210 MPa and 0.2 s, were
polyethylene, then prevent the penetration of polyethylene chains into
the lattice, results that the entanglement between molecular chains is
not sufficient, while reduce the contents of tie molecular chains. All the
changes in molecular chains will cause that the radial orientation of the
weld area of chains increase, while the axial orientation of tie molecules
chain reduce. There are many differences between the lace molecules in
the weld and the lace molecules in the base material. The performance
weakening area is produced at the joint between the weld and the base
metal, which leads to the short weld line and the fracture along the
weld joint and the base metal under the bending stress.
Simulation of temperature field in HDPE tube hot-melt welding
As the main means of welding of polyethylene pipe, the quality of
welding joint directly affects the safety of the whole pipeline operation
system. Through the process test, it is found that the temperature of
welding affects the degree of fusion of the material during the welding.
For further investigate the welding temperature and the welding
quality, the process of heat-stress coupling method is choosed by authors to simulate the temperature field in the welding process by MARC
software. The changes on temperature field during heating, switching
and compression stage are obtained. The welding temperature field is
analysized by finite element method and results are compared with the
experiment.
For the high density polyethylene pipe PE100, the diameter of the
pipe is 200 and SDR value is 17. Before the analysis, there are some
assumes should be noted during the welding process, which are listed as
below: 1. the tubes are evenly distributed in each radial direction; 2. the
thermal physical properties of material can only be a function of the
temperature. The a rectangular model is established with 100 mm as
length and 11.8 mm as thickness. A plane 55 model with two-dimensional axisymmetric 4 node solid element plane55 is choosed and the
model is divided into non uniform grid, the grid subdivision in the
heating near the tip of the model contains 25,020 nodes and 22,284
elements. The finite element model is shown in Fig. 6. The temperature
is set at 210 °C, the heating pressure is 0.2 MPa, the ambient temperature is 28 °C, and the temperature distribution along the axial direction
is the result of the heat conduction inside the tube. In the calculation
model, the boundary condition between the air and the polyethylene is
the convective heat transfer boundary condition. During welding, the
convection heat transfer between the inner and outer walls of the tube
is very complicated. According to the Prandtl criterion, the Guerra
Shchev criterion, the Nusselt criterion and the Reynolds criterion, the
convective heat transfer coefficient is determined, and the convection
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of the swimming pool stadium.
compared. Results when welding at 180 s and 300 s were compared
with the simulated and experimental temperature curves at different
locations at the same time, as shown in Fig. 10.
From Fig. 10, it can be seen that the actual measurement of the
temperature field distribution of polyethylene pipes is basically the
same as that obtained by simulation, which proves that the finite element model is reasonable. But when the experiment reaches 300 s, the
temperature at the center of the weld is 163 °C, which is higher than
that obtained by the finite element simulation. Moreover, the axial
temperature of the model decreases steadily. The axial temperature in
the experiment starts to decrease rapidly and then becomes stable. The
reason for the slight difference between the simulation results and the
experimental results is that the physical parameters of HDPE are more
sensitive to the temperature change, while the simulation process is
regarded as an ideal process. In practice, the convection heat transfer
coefficient of polyethylene is not the same as that of the pipeline.
However, in order to simplify the model in the same way, there is a
certain error between the simulation results and the experimental results.
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Conclusions
1. The best weld quality is obtained when the welding pressure is
0.2 MPa and the heating temperature of HDPE heat fusion welding is
within the range of 210 °C to 230 °C.
2. The welding temperature field in the welding process was measured
by MARC software, which was in agreement with the results measured by the experiment.
3. The melting amount of HDPE at weld joint will be affected by
temperature, which resulting the reduce of bending strength at
welding joints.
According to the analysis above, it is concluded that the hot-melt
welding HDPE pipe is suitable for the water supply and drainage system
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