polymers

Article
Tapered Waist Tensile Specimens for Evaluating Butt Fusion
Joints of Polyethylene Pipes—Part 1: Development
Sunwoo Kim 1,2 , Taemin Eom 1 , Wonjae Lee 1 and Sunwoong Choi 1, *

1 Department of Polymer Science and Engineering, Hannam University, Daejeon 34054, Korea;
swkpado@krict.re.kr (S.K.); xoals2342@gmail.com (T.E.); dnjswo3983@naver.com (W.L.)
2 Chemical Materials Solutions Center, Korea Research Institute of Chemical Technology, Daejeon 34114, Korea
* Correspondence: swchoi957@gmail.com; Tel.: +82-10-3426-7938

Abstract: The structural integrity of butt fusion (BF) joints in thermoplastic pressure piping systems
is critical to their long-term safe use. The tapered waist tensile (TWT) specimen was developed
to alleviate issues associated with the currently used ISO 13953 waisted tensile (WT) specimen for
evaluating BF joints. Experimental and finite element analyses were performed to obtain optimum
TWT specimen designs for the BF joint destructive test. For TWT specimens, depending on the pipe
size, the displacement at onset necking was reduced by 30~100%, and the tested BF area increased by
60~80% compared to the WT specimen. In addition, the transverse specimen deflection was lower
thus providing better experimental stability. Furthermore, it showed the same BF displacement at the
maximum force local to the BF bead, indicating that the tapered waist geometry provides equivalent
deformation constraint and BF failure mode designed for the BF joint in the WT specimens. Therefore,
TWT specimens offer simplicity, adaptability, stability, and accuracy in specimen preparation, testing,
and analysis compared to WT specimens.



Keywords: polyethylene (PE) pipes; tapered waist tensile (TWT); waisted tensile (WT); butt fusion


(BF) joints; fracture energy
Citation: Kim, S.; Eom, T.; Lee, W.;
Choi, S. Tapered Waist Tensile
Specimens for Evaluating Butt Fusion
Joints of Polyethylene Pipes—Part 1:
Development. Polymers 2022, 14, 1187.
1. Introduction
https://doi.org/10.3390/ In creating a thermoplastic piping system for the conveyance of fluid, pipes are joined
polym14061187 together, and the structural integrity of the joints produced is essential for ensuring their
safety and reliability during their designed lifetimes. With the structural requirement
Academic Editor: Byoung-Ho Choi
of the joint met, polyethylene (PE) pipes are being used in a wide range of demanding
Received: 2 February 2022 applications across the water, gas, oil, and energy industries including nuclear [1]. Among
Accepted: 10 March 2022 several available jointing methods for thermoplastic pipes, thermal butt fusion (BF) [2]
Published: 16 March 2022 and electrofusion (EF) [3] jointing are popularly used, and their performance has proven
Publisher’s Note: MDPI stays neutral
dependable over decades of piping system operations. BF is one of the most frequently used
with regard to jurisdictional claims in in the PE piping system, and more stringent joint assessment requirements are considered
published maps and institutional affil- as their applications widen into higher pressures and larger diameters pipes. Various
iations. specifications and standards for fusion procedure, long-term life design, performance, and
installation procedures are utilized to meet such requirements, and new ones are made. In
addition, several international, regional, and ad hoc standards are available that evaluate
the BF joints by the destructive and non-destructive methods.
Copyright: © 2022 by the authors. Destructive methods range from static tensile, bending, tensile creep, fatigue, and
Licensee MDPI, Basel, Switzerland. high-speed tensile tests. The static tensile method is most popularly used, and they include
This article is an open access article waisted tensile (WT) [4,5], dogbone tensile [6], and low-temperature tensile (LTT) [7]
distributed under the terms and specimens. All have different geometries and contain BF joints at the center perpendicular
conditions of the Creative Commons
to the specimen axis. A WT specimen maintains the BF bead, whereas dogbone can be
Attribution (CC BY) license (https://
tested either with a bead or without the bead. On the other hand, the low-temperature
creativecommons.org/licenses/by/
tensile (LTT) test is performed with the bead removed. WT specimen is designed to provide
4.0/).

Polymers 2022, 14, 1187. https://doi.org/10.3390/polym14061187 https://www.mdpi.com/journal/polymers

Polymers 2022, 14, 1187 2 of 23

stress concentration at the waisted BF joint, while the reduced section in the LTT specimen
is shaped by a large radius. In addition, WT and dogbone specimens are tested at room
temperature, while −80 ◦ C is used for the LTT test. Pull speeds of 5, 50, and 200 mm/min
are applied, for WT, dogbone, and LTT specimens, respectively.
Using dogbone specimens, earlier workers determined the conditions for achieving
good BF joints of polyethylene materials of different melt flow indexes [8] and of different
grade polyethylene, polybutene-1, and polypropylene [9]. It was also determined that with
the bead removed, it provided a clearer picture of the joint performance [9].
Tensile creep test [10] demonstrated that BF joints of large diameter and thick wall PE
pipes, produced by the single low pressure (SLP) and duel low pressure (DLP) [11], had
slow crack growth (SCG), starting at the stress concentration developed between the bead
and the parent pipe and the crack growth occurring into the parent pipe [12]. It was also
reported that BF made by the SLP procedure performed better than DLP.
The lifetime ranking of PE pipe BF joints by the SCG using the Pennsylvania notched
test (PENT) [13,14] and the full notch creep test (FNCT) [15,16] have been reported. Using
FNCT on thick wall BF joints produced by SLP, DLP, and SHP (single high pressure)
procedures all demonstrated to satisfy the designed lifetime requirements. However, it was
noted that the lifetime was less with SHP but not significantly. More recently, SCG times in
BF joint were determined for different PENT-rated PE4710 pipes used for nuclear power
applications [17]. The method showed the SLP procedure for BF jointing gave longer SCG
failure times than the SHP method [18].
The long-term biaxial loading of BF joint using the plane strain grooved tensile (PSGT)
specimens [19,20], removed from the 630 SDR 11 and 914 SDR 17.6 pipe BF joints, showed
a comparable stress–rupture result to that of the sustained internal pressure pipe test [21].
Hence, this has provided an alternate means to stress–rupture test BF joint of large diameter
pipes.
Stress controlled fatigue test on BF joints was reported. It showed that the BF bead
reduced the low-cycle tensile fatigue lifetime and had a negligible effect on the medium and
high-cycle lifetimes [22]; however, low and medium cycle regions were affected when tested
in bending mode [23]. In addition, a deflection controlled fully reversed cantilever bending
fatigue test was applied to PE4710 4” SDR 11 BF joints and showed better performance
with unimodal than the bimodal resin [24]. A high-speed tensile impact test on BF joints
using a tensile impact speed of 152 mm/min is often used to correlate to the indications
from the non-destructive examination for determining between good and bad joint, based
on the failure mode [25,26].
Evaluation of a BF joint using the bending mode is available through the technological
bend test of DVS 2203-5 [27] and guided strip bend test of ASTM F3186 [28]. Bend angle
(or ram displacement) and resulting deformation and fracture of the BF joints are used to
assess the condition of the joints. More recently, Wermelinger proposed a modified strip
bend test that evaluates BF joints with ease, quickness, and good accuracy [26].
All of the BF joint destructive tests described above was based on the tensile coupon
test in which the coupons are removed from different locations in the BF joint. Studies of
BF joint integrity using the whole pipe tensile test have also been reported. More recently, a
whole pipe axial tensile test using hydrostatic pressure was developed and demonstrated
its effectiveness by comparing it to the mechanical axial tension test result of the whole
pipe BF joint [29]. Tensile creep tests on whole pipe BF joints using hydraulic jack [30],
and hydrostatic pressure (HAT) [31,32] have also been reported. Both tests were made to
test the BF joint with factors like residual stresses and fusion strength variation around
the joint cross-section, otherwise not considered in the coupon creep tests. HAT has been
shown to produce comparable results to the whole pipe mechanical creep test [21,31] and
has the advantage of testing whole pipe BF joint of different sizes using already available
hydrostatic pressure testing equipment.
Comparison of waisted tensile (WT) specimen to other specimens was discussed [30,33],
and one of the advantages is that it allows tensile tests to be performed on BF joints from
Polymers 2022, 14, 1187 3 of 23

thick-wall pipes. In addition, a recent international round-robin on the performance of several

BF joint test specimens has demonstrated that the WT test is one of the methods that can better
discriminate between good and bad PE pipe BF joints [26].
The BF joint strength or fracture energy value is measured in the WT test. The joint
strength is then compared to the pipe strength, and the “weld factor” (ratio between
joint strength and pipe strength) is obtained [34,35]. The “weld factor” value determines
between good versus the flawed joint. However, a shortcoming of this method is that
joint fractures can occur in some flawed BF joints after achieving the same joint strength as
no-flaw BF joints. Thus, a personal judgment is made by observing the fracture mode on
the fracture surface to determine the state of the BF joint integrity. This makes the method
subjective and can lead to erroneous conclusions of the BF joint quality. In the context of
published studies of BF joint integrity, fracture energy is more frequently reported than
strength [36], as fracture energy also takes into account the energy spent on fracturing
the joint after reaching the maximum load. The quantitative fracture energy is estimated
from the total area under the load–displacement curve. However, traction holes in the
unreduced parts of the WT specimen for accommodating loading rods deform upon loading
and become a significant added source of unnecessary specimen displacement included in
the BF joint fracture energy calculation. In addition, in some pipe sizes, for a given standard
dimensional ratio (SDR–ratio between pipe diameter and thickness), the number of WT
specimens produced from the BF joint decreases with increasing pipe diameter. Hence, for
225 SDR 11 to 914 SDR 11 WT specimens, the % total area of the BF joint tested is between
19.5% and 23.1%. Therefore, the reliability of the test results in representing the entire BF
joint can be questioned. In addition, the size, shape, and traction holes in the WT specimen
can make the specimen machining to dimensions more demanding. Recently, work to
optimize the WT specimen dimensions has been carried out [37]. However, although the
modified WT specimen dimensions were changed to create ductile failures in the no-flaw
BF joint and the parent pipe, it still retained the use of loading pins.
In this work, a tapered waist tensile (TWT) test specimen for testing the BF joint is
developed that eliminated the use of loading pins. In addition, by redesigning the shape of
the “waist” in the form of a tapered waist fusion zone notch, a single specimen width is
made to cover all pipe sizes and pipe thicknesses. As a result, the specimens are shorter
and simpler in shape with no traction holes, thus making the specimen machining simpler.
In addition, a simple push-in tensile grip design is described, making the tensile test more
stable and simpler. Experimental and numerical analyses are performed on TWT and WT
specimens of high-density PE BF joints to compare their merits. Therefore, TWT specimens
offer simplicity, adaptability, stability, and accuracy in specimen preparation, testing, and
analysis compared to WT specimens.

2. Materials and Methods

2.1. Material
Materials were received as PE100 (HE3490-LS, Borouge, Abu Dhabi, UAE) black
compound extruded pipes of 110 SDR 9*, 225 SDR 11, 315 SDR 11, and 450 SDR 9 size
from a local pipe extruder (Cosmo I & D, Sejong, Korea). The material manufacturer
declared tensile yield strength, fracture elongation, and modulus of elasticity were 25 MPa,
>600%, and 1.1 GPa, as measured by ISO 527-2. The density and melt flow index of the PE
resin were 960 kg/m3 (ISO 1183-12, Method 1) and 0.25 g/10 min (ISO 1133), respectively
(https://borouge.com/IndustrySolution/PDF%20Files/DataSheetBB2581.pdf, assessed
on 4 February 2022).
For example, a 110 SDR 9 pipe size refers to a pipe with 110 mm nominal outside
diameter (dn ) and a standard dimensional ratio (SDR–ratio between pipe diameter and
thickness) of 9. Therefore, the nominal pipe wall thickness (t) was 12.2 mm.
Polymers 2021, 13, x FOR PEER REVIEW 4 of 24
Polymers 2022, 14, 1187 4 of 23

2.2. Butt Fusion (BF) Jointing

2.2. Butt Fusion (BF) Jointing
BF joints containing no flaws were produced on 110 SDR 9, 225 SDR 11, 315 SDR 11,
and 450BF SDR
joints11containing no flaws
pipes. BF joints fromwere produced
the former threeon 110sizes
pipe SDRwere
9, 225 SDRusing
made 11, 315 SDR
a T-315
BF jointing machine (Georg Fischer, Schaffhausen, Switzerland), and 450 SDR 11 was pro-a
11, and 450 SDR 11 pipes. BF joints from the former three pipe sizes were made using
T-315 BF
duced jointing
using machine
a BT216 (Georgmachine
butt fusion Fischer, Schaffhausen,
(WMS, Seoul, Switzerland), and 450
Korea), according SDRBF
to the 11pro-
was
produced using a BT216 butt fusion machine (WMS, Seoul, Korea), according
cedure of ISO 21307. For all pipe sizes, a single low-pressure (SLP) procedure using 1.5 to the BF
procedure of ISO 21307. For all pipe sizes, a single
bar was followed with the heating plate set at 230 °C. low-pressure (SLP) procedure using 1.5
bar was followed with the heating plate set at 230 ◦ C.
All butt-fused pipes were left at ambient temperature for 24 h before machining into
All butt-fused pipes were left at ambient temperature for 24 h before machining into
TWT and WT specimens.
TWT and WT specimens.
2.3.
2.3.TWT
TWTand
andWTWTSpecimen
SpecimenPreparation
Preparation
Tapered
Tapered waist tensile (TWT)and
waist tensile (TWT) andwaisted
waistedtensile
tensile(WT)
(WT)specimens
specimenswerewerecut
cutfrom
fromthethe
butt-fused
butt-fused pipe along the pipe axial, as shown in Figure 1. The BF bead is centeredon
pipe along the pipe axial, as shown in Figure 1. The BF bead is centered oneach
each
specimen
specimenand andmachined
machinedto todimensions
dimensionson onaaSimplex-2
Simplex-2CNC CNCmilling
millingmachine
machine(Hawcheon,
(Hawcheon,
Gwangju,
Gwangju,Korea).
Korea).The
Thefinal
finalshapes
shapesandanddimensions
dimensionsof ofTWT
TWTand andWTWTspecimens
specimensmademadefrom
from
110 SDR 9, 225 SDR 11, 315 SDR 11, and 450 SDR 9 pipe BF joints and the
110 SDR 9, 225 SDR 11, 315 SDR 11, and 450 SDR 9 pipe BF joints and the maximum numbermaximum num-
ber of TWT
of TWT and and
WTWT specimens
specimens possible
possible fromeach
from eachpipe
pipesize
sizeare
aregiven
given in
in Section
Section 3.1.1.
3.1.1. All
All
specimens
specimens were conditioned at 23 C, 50% RH for at least 24 h prior to the tensiletest.
were conditioned at 23 °C,
◦ 50% RH for at least 24 h prior to the tensile test.

(a) (b)
Figure
Figure1.1.Diagram
Diagramof
ofrelative
relativepositions
positionsof
ofType
Type A
A specimens
specimens (unit:
(unit: mm):
mm): (a)
(a)TWT;
TWT; (b)
(b) WT.
WT.

2.4.TWT
2.4. TWTand
andWT
WTTest
Test
TWTand
TWT andWT WTspecimens
specimenswere weretensile
tensiletested
testedusing
usingaauniversal
universaltesting
testingmachine
machine(AGS-
(AGS-
X, Shimadzu,
X, Shimadzu, Kyoto,Kyoto, Japan).
Japan). AA50 50kNkNload
loadcellcell and
and aa dual-camera
dual-camera video video extensometer
extensometer
(TRViewX 800D,
(TRViewX 800D, Shimadzu,
Shimadzu, Kyoto,
Kyoto,Japan)
Japan)with
withupupto to800
800mm mmfield
fieldofofview
viewmeasurement
measurement
capability were employed. For the TWT specimen, a specially
capability were employed. For the TWT specimen, a specially designed universal TWT designed universal TWT
push-ingrip
push-in grip(see
(seeFigure
Figure2a)
2a) was
was used.
used. Traction
Tractionholes
holeswere
wereusedusedwith
withWT WT specimens
specimensto to
accommodate the
accommodate the loading
loading pins
pins for
for tensile
tensile testing
testing (Figure
(Figure 2b). TheThe 55 mm/min
mm/mindisplacement
displacement
ratewas
rate wasused
usedfor forTWT
TWTand andWT WTspecimens.
specimens.Specimen
Specimendisplacement
displacementwas wasmeasured
measuredin intwo
two
ways. One used the grip to grip (machine crosshead) separation distance.
ways. One used the grip to grip (machine crosshead) separation distance. And the other by And the other by
the video extensometer with a gage length set at ± 10 mm from the center
the video extensometer with a gage length set at ±10 mm from the center of the bead width of the bead width
forall
for all pipe
pipe sizes
sizes (Figure
(Figure 2).
2). All
Allspecimens
specimenswere weretested
testedtotofailure, andand
failure, thethe
load–displacement
load–displace-
curves were obtained for further analysis to determine the BF joint
ment curves were obtained for further analysis to determine the BF joint fracture fracture energy.energy.
Tensile
tests were
Tensile testscarried out in the
were carried outsame
in theenvironment as conditioning.
same environment as conditioning.
Polymers 2022, 14, 1187 5 of 23
Polymers 2021, 13, x FOR PEER REVIEW 5 of 24

Polymers 2021, 13, x FOR PEER REVIEW 5 of 24

(a) (b)
(a)Figure 2. Grip-specimen assembly showing the line sticker gage (b)location for optical measurement
Figure 2. Grip-specimen assembly showing the line sticker gage location for optical measurement
(unit: mm): (a) TWT test; (b) WT test.
Figure
(unit: mm): 2. Grip-specimen assembly
(a) TWT test; (b) showing the line sticker gage location for optical measurement
WT test.
(unit: mm): (a) TWT test; (b) WT test.
3. Results and Discussion
3. Results and Discussion
3.1.3.3.1.
TWT TWT Specimen
Results Development
and Discussion
Specimen Development
3.1.1.
3.1.
3.1.1. Specimen
TWT Geometry
Specimen
Specimen andDimensions
Development
Geometry and Dimensions
3.1.1. The geometry and dimensions of the proposed TWT specimens with a BF bead at the
TheSpecimen
geometry Geometry and Dimensions
and dimensions of the proposed TWT specimens with a BF bead at the
center are given in Figure 3a,b, and Table 1. Similarly, Figure 3c–e, and Table 2 show the shape
center areThegiven
geometry and dimensions
in Figure 3a,b, andofTablethe proposed TWT specimens
1. Similarly, Figure 3c–e, withanda BFTable
bead 2 atshow
the the
and dimensions of the WT specimens per ISO 13953. The characteristic design of the TWT
center
shape and aredimensions
given in Figure of 3a,b,
the and specimens
WT Table 1. Similarly,
per Figure
ISO 3c–e,The
13953. andcharacteristic
Table 2 show the shape of the
design
specimen has a tapered waist cut in angles to provide tapered support for the tensile load.
TWT andspecimen
dimensions hasofathe WT specimens
tapered per ISO 13953.to The characteristic design of the TWT
Two TWT specimen types A waist
(Figurecut 3a)in angles
and B (Figure provide
3b), cover tapered
the BFsupport forfor
tensile test theall tensile
specimen
load. Two has
TWT a tapered
specimen waist
types cutAin angles
(Figure to
3a)provide
and B tapered
(Figure support
3b), for
cover the
the tensile
BF load.test for
tensile
pipe sizes. In comparison, WT specimens require three types (two Type A (Figure 3c,d)
Two TWT specimen types A (Figure 3a) and B (Figure 3b), cover the BF tensile test for all
all pipe
and asizes.
Type B In(Figure
comparison,
3e)). TheWT TWT specimens
specimen is require
smallerthree types (two
and shorter Type
than the WTAspecimen
(Figure 3c,d)
pipe sizes. In comparison, WT specimens require three types (two Type A (Figure 3c,d)
andduea Type
to theB (Figure
absence 3e)). The TWT
of traction holes specimen is smaller
on the specimen, and shorter
as shown in Figurethan the WT specimen
3. However, the
and a Type B (Figure 3e)). The TWT specimen is smaller and shorter than the WT specimen
width
duedueto to ofabsence
thethe the BF joint
of tested isholes
traction the same on in both
the specimens
specimen, as (D = 25 mm
shown in in Figure
Figure 3. 3, Tables the
However,
absence of traction holes on the specimen, as shown in Figure 3. However, the
width1 and
of of2). In
the BFBFaddition,
joint while the width of both
the WT specimen(D (dimension BininFigure
Table 2) in-
width the jointtested
testedisisthe
the same
same in in specimens
both specimens (D = 25= 25mmmm in Figure 3, Tables
3, Tables 1
creases
and1 2). In with pipe
addition, dimensions,
while the the
width width
of the is fixed
WT at 60
specimen mm for TWT
(dimension specimens
B in for
Table all
2) pipe
increases
and 2). In addition, while the width of the WT specimen (dimension B in Table 2) in-
with sizes.
pipeHence, with thethe
dimensions, TWT specimen,
width is fixedthe atBF60joint
mm areafortested
TWT isspecimens
larger by 60%~80% than
creases with pipe dimensions, the width is fixed at 60 mm for TWT specimensfor forall
all pipe
pipe sizes.
the WT specimen for pipes having greater than 160 mm nominal diameter, as shown in
Hence,
sizes.with
Hence,thewith
TWT thespecimen,
TWT specimen,the BFthe joint
BF area
joint tested is larger
area tested by 60%~80%
is larger by 60%~80% thanthan the WT
Table 3. The absence of traction holes in TWT specimens also provides simplicity of spec-
the WT for
specimen specimen
pipes for pipes greater
having having greater
than 160 thanmm 160nominal
mm nominal diameter,
diameter, as shown
as shown in inTable 3.
imen machining, ease of testing, and experimental stability. At the same time minimizes
TheTable
the
3. Theof
absence
specimen
absence
traction
tensile
of traction
holes in
displacement
holes
TWT in TWT specimens
specimens
contribution fromalso
also
places
providessimplicity
provides
other than
simplicity of
the BF area. ofspec-
specimen
imen machining, ease of testing, and experimental stability.
machining, ease of testing, and experimental stability. At the same time minimizes the At the same time minimizes
the specimen
specimen tensiletensile displacement
displacement contribution
contribution fromfromplaces
placesother
other than
than the
theBF BFarea.
area.

(a) (b) (c) (d) (e)

(a) (b) (c) (d) (e)

Figure 3. Shape and relative dimensions: (a) Type A TWT specimen (t < 25 mm); (b) Type B TWT
(t ≥ 25 mm); (c) Type A WT specimen (t < 25 mm), dn ≤ 160 mm; (d) Type A WT (t < 25 mm),
dn ≤ 160 mm; (e) Type B WT (t ≥ 25 mm).
Polymers 2022, 14, 1187 6 of 23

Table 1. Dimensions of TWT specimens.

Symbol
Description (mm) Type A (t < 25 mm) Type B (t ≥ 25 mm)
(See Figure 3a,b)
A Overall length 100 120
B Width at ends 60
Length of narrow
C NA 20
parallel sided portion
Width of narrow
D 25
portion
Tapered waist fusion
R 10
zone notch radius
Tapered (Grip)
Φ 30◦
support angle

Table 2. Dimensions of WT specimens per ISO 13953.

Symbol Type A (t < 25 mm) Type B

Description (mm)
(See Figure 3c–e) dn ≤ 160 dn > 160 (t ≥ 25 mm)
A Overall length 180 250
B Width at ends 60 80 100
Length of narrow
C NA 25
parallel sided portion
D Width of narrow portion 25
R Radius 5 10 25
Initial distance between
G 90 165
grips
Diameter of the traction
I 20 ± 5 30 ± 5
holes

Table 3. Percent BF area per pipe covered by TWT and WT specimens.

Number of Samples from a Whole Pipe BF Area Evaluated (%)

Pipe Size
TWT WT TWT WT
110 SDR 9 4 4 32.6 32.6
225 SDR 11 8 5 31.1 19.5
315 SDR 11 12 7 33.4 19.5
450 SDR 9 18 10 35.8 19.9
914 SDR 11 39 23 37.4 22.0

3.1.2. Tapered Support Angle Grip Design

In the TWT specimen, the tensile displacement from the specimen gripping is mini-
mized by using a tapered support angle (Φ in Figure 3) in the newly designed push-in grip
that supports reaction to the tensile load shown in Figure 4a.
 of 30° was chosen to provide an optimum design for resistance against specimen slippage
and gripping instability.
Figure 4c illustrates the experimental load–displacement curves of TWT specimens
with grip support angles 30° and 35°. The load–displacement behavior is similar, but the
Polymers 2022, 14, 1187 35° grip support angle exhibits a larger displacement at maximum load by about 55%7than
of 23
at 30°.

(a) (b) (c)

Figure 4.
Figure 4. (a)
(a) Tapered
Tapered support
support angle
angle of
of push-in
push-in grip
grip for
for TWT
TWT specimens;
specimens; (b)
(b) finite
finite element
element analysis
analysis
result; (c) experimental load–displacement curves with 30° and 35° grip support angle.
result; (c) experimental load–displacement curves with 30◦ and 35◦ grip support angle.

A photograph
The finite elementof theresult
newlyofdesigned push-in
the gripping tensilein
is shown grip and TWT
Figure 4b forspecimen
the supportassembly
angle
is shown in ◦Figure 5a.
◦ Since the specimen width is the same
◦ independent
between 25 and 35 [38]. The grip support angle of 35 provides significant specimen of the pipe size,
one push-in
slipping while grip
thewas made angle
support to fit all specimens
deflection with
is the wall thickness
smallest under the≤ 315 SDRload.
tensile 11 (Figure 5b).
And vice
Multiple
versa withpush-in grips That
a 25◦ angle. can beis, assembled together
the larger the for add-on
grip support thickness
angle, the moreforis thicker speci-
the specimen
mens,but
slips as smaller
illustratedthe in Figure
grip 5c. under the tensile load. Therefore, a grip support angle
deflection
◦
of 30 was chosen to provide an optimum design for resistance against specimen slippage
and gripping instability.
Figure 4c illustrates the experimental load–displacement curves of TWT specimens
with grip support angles 30◦ and 35◦ . The load–displacement behavior is similar, but the
35◦ grip support angle exhibits a larger displacement at maximum load by about 55% than
at 30◦ .
A photograph of the newly designed push-in tensile grip and TWT specimen assembly
is shown in Figure 5a. Since the specimen width is the same independent of the pipe
size, one push-in grip was made to fit all specimens with wall thickness ≤ 315 SDR 11
Polymers 2021, 13, x FOR PEER REVIEW
(Figure 5b). Multiple push-in grips can be assembled together for add-on thickness 8 of for
24
thicker specimens, as illustrated in Figure 5c.

(a) (b) (c)

Figure
Figure5.5.(a)
(a)Assembly
Assemblyofofpush-in
push-ingrip
gripand
andTWT
TWTspecimen;
specimen;(b)
(b)example
exampleofof315
315SDR
SDR11
11(single-grip
(single-grip
assembly for t ≤ 30 mm specimens); (c) example of 450 SDR 9 (multi-layering of grip assembly for
assembly for t ≤ 30 mm specimens); (c) example of 450 SDR 9 (multi-layering of grip assembly for
testing thicker specimens of t > 30 mm) (unit: mm).
testing thicker specimens of t > 30 mm) (unit: mm).
3.1.3.
3.1.3.Experimental
ExperimentalDevelopment
DevelopmentofofTWTTWTSpecimen
Specimen
The
Thefusion
fusionzone notch
zone radius
notch of the
radius of Type A TWT
the Type specimen
A TWT was decided
specimen by tensile
was decided bytesting
tensile
5testing
mm and5 10 mm radius notches. The load–displacement curves obtained at
mm and 10 mm radius notches. The load–displacement curves obtained 5 mm/min cross-
at 5
head speed are given in Figures 6a and 7a for 110 SDR 9 and 225 SDR 11 TWT specimens,
mm/min crosshead speed are given in Figures 6a and 7a for 110 SDR 9 and 225 SDR 11 TWT
respectively. The corresponding failed specimens are presented in Figures 6b,c and 7b,c. For
the 110 SDR 9 TWT specimen, 5 and 10 mm notch radius show practically the same result
(Figure 6a and Table 4). In addition, the maximum load is similar to the WT specimen (5
mm notch radius); however, the corresponding displacement is reduced by 52% with TWT
Polymers 2022, 14, 1187 8 of 23

specimens, respectively. The corresponding failed specimens are presented in Figure 6b,c
and Figure 7b,c. For the 110 SDR 9 TWT specimen, 5 and 10 mm notch radius show
practically the same result (Figure 6a and Table 4). In addition, the maximum load is similar
to the WT specimen (5 mm notch radius); however, the corresponding displacement is
reduced by 52% with TWT specimens, as expected (Table 4). The failure mode is completely
Polymers 2021, 13, x FOR PEER REVIEW 9 of 24
ductile and occurs after full necking displacements in all specimens (Figure 6b–d). These
results indicate that a 5 or 10 mm fusion zone notch radius is suitable for use with the Type
A 1104.SDR
Table 9 TWTload
Maximum specimen.
and corresponding displacement of Type A, 110 SDR 9 TWT, and WT spec-
imens.

Specimen Notch Radius Fmax Displacement @ Fmax

(mm) (kN) (mm)
TWT_5 8.97 ± 0.14 8.1 ± 0.2
TWT_10 9.08 ± 0.13 8.3 ± 0.1
WT_5 8.79 ± 0.31 17.4 ± 1.4

The Type A, 225 SDR 11 TWT specimen with a 5 and 10 mm radius fusion zone notch
exhibits different load–displacement behavior as given in Figure 7a. Slightly higher maximum
load and lower fracture displacement are shown for a 5 mm notch radius (Table 5). These may
be partly due to the rate effect, where a 5 mm radius notch provided approximately two
times higher nominal strain rate than a 10 mm notch radius at the BF zone. On the other
hand, the 10 mm notch radius gives a similar load–displacement curve to the WT speci-
men (10 mm notch radius) but smaller specimen displacement by about 30% at the maxi-
mum load (Table 5). The corresponding failure mode is a mixed-mode failure with lower
ductility for a 5 mm notch radius (Figure 7b). In contrast, similar complete ductile failure
is seen6.with
Figure a 10ofmm
Results 110notch
SDR 9radius
(Type ATWT (Figurepipe
specimens) 7c) BF
and WT(unit:
joint (Figure 7d)
mm): (a)specimens. Hence
load–displacement
for the Type A, 225 SDR 11 TWT specimen, a fusion zone notch radius
curves; (b) TWT_5 mm; (c) TWT_10 mm; (d) WT_5 mm fusion zone notch radius.of 10 mm is suitable.

(a) (b) (c) (d)

Figure
Figure 7.
7. Results
Results of
of 225
225 SDR
SDR 11
11 (Type
(Type A
A specimens)
specimens) pipe
pipe BF
BF joint
joint (unit:
(unit: mm):
mm): (a)
(a) load–displacement
load–displacement
curves; (b) TWT_5 mm; (c) TWT_10 mm; (d) WT_10 mm fusion zone notch radius.
curves; (b) TWT_5 mm; (c) TWT_10 mm; (d) WT_10 mm fusion zone notch radius.

Table 5. Maximum load and corresponding displacement of Type A, 225 SDR 11 TWT, and WT
specimens.

Specimen Notch Radius Fmax Displacement @ Fmax

(mm) (kN) (mm)
TWT_5 14.28 ± 0.39 9.0 ± 0.2
TWT_10 13.70 ± 0.12 8.9 ± 0.2
WT_10 13.77 ± 0.31 12.4 ± 0.6

Type B TWT and WT specimens are illustrated in Figure 3b,e. The TWT specimen
had a shorter reduced parallel side (dimension C in Tables 1 and 2). In addition, the grip
Polymers 2022, 14, 1187 9 of 23

Table 4. Maximum load and corresponding displacement of Type A, 110 SDR 9 TWT, and WT
specimens.

Specimen Notch Radius Fmax Displacement @ Fmax

(mm) (kN) (mm)
TWT_5 8.97 ± 0.14 8.1 ± 0.2
TWT_10 9.08 ± 0.13 8.3 ± 0.1
WT_5 8.79 ± 0.31 17.4 ± 1.4

The Type A, 225 SDR 11 TWT specimen with a 5 and 10 mm radius fusion zone
notch exhibits different load–displacement behavior as given in Figure 7a. Slightly higher
maximum load and lower fracture displacement are shown for a 5 mm notch radius
(Table 5). These may be partly due to the rate effect, where a 5 mm radius notch provided
approximately two times higher nominal strain rate than a 10 mm notch radius at the BF
zone. On the other hand, the 10 mm notch radius gives a similar load–displacement curve
to the WT specimen (10 mm notch radius) but smaller specimen displacement by about 30%
at the maximum load (Table 5). The corresponding failure mode is a mixed-mode failure
with lower ductility for a 5 mm notch radius (Figure 7b). In contrast, similar complete
ductile failure is seen with a 10 mm notch radius TWT (Figure 7c) and WT (Figure 7d)
specimens. Hence for the Type A, 225 SDR 11 TWT specimen, a fusion zone notch radius of
10 mm is suitable.

Table 5. Maximum load and corresponding displacement of Type A, 225 SDR 11 TWT, and WT
specimens.

Specimen Notch Radius Fmax Displacement @ Fmax

(mm) (kN) (mm)
TWT_5 14.28 ± 0.39 9.0 ± 0.2
TWT_10 13.70 ± 0.12 8.9 ± 0.2
WT_10 13.77 ± 0.31 12.4 ± 0.6

Type B TWT and WT specimens are illustrated in Figure 3b,e. The TWT specimen had
a shorter reduced parallel side (dimension C in Tables 1 and 2). In addition, the grip width
of the specimen end (dimension B) was 60 mm, whereas 100 mm width was used with the
WT specimen. The length of the reduced parallel side of the Type B TWT specimen was
decided by evaluating the parallel side length of 0, 10, and 20 mm on 315 SDR 11 (t = 28.6
mm) and 450 SDR 9 (t = 50 mm) sizes. The 0 mm parallel side length makes the same shape
as the Type A specimen with a 10 mm fusion zone notch radius (dimension R in Figure 3a,b,
and Table 1).
The load–displacement curves of TWT specimens with 0, 10, and 20 mm parallel side
lengths are given in Figures 8a and 9a for 315 SDR 11 and 450 SDR 9 sizes, respectively. The
corresponding modes of TWT specimen failure are also presented in Figures 8 and 9. All
load–displacement curves were obtained at a 5 mm/min crosshead speed.
For the 315 SDR 11 TWT specimen, the maximum load is seen to decrease with the
parallel side length increase from 0, 10, and 20 mm. The displacement at maximum load
remains about the same (Table 6), and the total displacement increases with increasing
length (Figure 8a). The 20 mm parallel side length TWT compares best with the WT
(25 mm) specimen in load–displacement behavior (Figure 8a and Table 6) and failure mode
(Figure 8d,e). The 10 mm length also exhibited ductile yielding failure (Figure 8c), while the
0 mm length specimen was shown to have much reduced ductility at failure (Figure 8b).
 length (Figure 8a). The 20 mm parallel side length TWT compares best with the WT (25
mm) specimen in load–displacement behavior (Figure 8a and Table 6) and failure mode
(Figure 8d,e). The 10 mm length also exhibited ductile yielding failure (Figure 8c), while the 0
mm length specimen was shown to have much reduced ductility at failure (Figure 8b).
The results show that the Type A TWT specimen (i.e., Type B with 0 mm length) is
Polymers 2022, 14, 1187
unsuitable for the specimen thickness larger than 25 mm (e.g., 315 SDR 11 and 450 SDR 10 of 23
9), and the Type B specimen with a parallel side length of 10 or 20 mm radius needs to be
utilized.

Polymers 2021, 13, x FOR PEER REVIEW 11 of 24

(b) (c)

TWT_20 18.09 ± 0.15 9.6 ± 0.1

WT_25 17.56 ± 0.04 12.7 ± 0.9

Figure 9 illustrates the load–displacement curve and the failure appearance of TWT
specimens taken from the 450 SDR 9 pipe BF joints. The 0 mm parallel side length is not
tested as the failure occurred with lesser ductility when made from the 315 SDR 11 BF
joint (Figure 8a,b). The maximum force decreases going from 10 to 20 mm parallel side.
However, the corresponding specimen displacement remained the same. In addition, the
total displacement increases with increasing length. The 20 mm parallel side TWT speci-
men compares best with the WT (25 mm) in load–displacement behavior and failure mode
(Figure 9c,d). Although the displacement at maximum load is still smaller by about 30%
(a) the WT specimen (Table 7). Ten millimeters
than (d) length also exhibited (e) ductile yielding
failure;
Figure 8.however, with
Results of 315 SDRmuch-reduced ductility,
11 (Type B specimens) asjoint
pipe BF shown
(unit: in Figure
mm): 9b. The results show
(a) load–displacement
Figure Results
8.(b)
curves; TWT)_0
of 315 SDR 11 (Type B specimens) pipe BF joint (unit: mm): (a)parallel
load–displacement
that a parallel sidemm; (c) TWT_10
length of 20 mmmm; is
(d)most
TWT_20 mm; (e)
suitable forWT_25
Typemm B TWTreduced
specimens. side
curves;
length.(b) TWT)_0 mm; (c) TWT_10 mm; (d) TWT_20 mm; (e) WT_25 mm reduced parallel side length.

Table 6. Maximum load and corresponding displacement of Type B, 315 SDR 11 TWT, and WT
specimens.

Specimen Parallel Side Fmax Displacement @ Fmax

(mm) (kN) (mm)
TWT_0 20.11 ± 0.12 9.9 ± 0.4
TWT_10 18.65 ± 0.12 9.6 ± 0.8

(b)

(a) (c) (d)

Figure 9. Results of 450 SDR 9 (Type B specimens) pipe BF joint (unit: mm): (a) load–displacement
Figure 9. Results of 450 SDR 9 (Type B specimens) pipe BF joint (unit: mm): (a) load–displacement
curves; (b) TWT_10 mm; (c) TWT_20 mm; (d) WT_25 mm reduced parallel side length.
curves; (b) TWT_10 mm; (c) TWT_20 mm; (d) WT_25 mm reduced parallel side length.

The results show that the Type A TWT specimen (i.e., Type B with 0 mm length) is
unsuitable for the specimen thickness larger than 25 mm (e.g., 315 SDR 11 and 450 SDR 9),
Polymers 2022, 14, 1187 11 of 23

and the Type B specimen with a parallel side length of 10 or 20 mm radius needs to be
utilized.

Table 6. Maximum load and corresponding displacement of Type B, 315 SDR 11 TWT, and WT
specimens.

Specimen Parallel Side Fmax Displacement @ Fmax

(mm) (kN) (mm)
TWT_0 20.11 ± 0.12 9.9 ± 0.4
TWT_10 18.65 ± 0.12 9.6 ± 0.8
TWT_20 18.09 ± 0.15 9.6 ± 0.1
WT_25 17.56 ± 0.04 12.7 ± 0.9

Figure 9 illustrates the load–displacement curve and the failure appearance of TWT
specimens taken from the 450 SDR 9 pipe BF joints. The 0 mm parallel side length is
not tested as the failure occurred with lesser ductility when made from the 315 SDR 11
BF joint (Figure 8a,b). The maximum force decreases going from 10 to 20 mm parallel
side. However, the corresponding specimen displacement remained the same. In addition,
the total displacement increases with increasing length. The 20 mm parallel side TWT
specimen compares best with the WT (25 mm) in load–displacement behavior and failure
mode (Figure 9c,d). Although the displacement at maximum load is still smaller by about
30% than the WT specimen (Table 7). Ten millimeters length also exhibited ductile yielding
failure; however, with much-reduced ductility, as shown in Figure 9b. The results show
that a parallel side length of 20 mm is most suitable for Type B TWT specimens.

Table 7. Maximum load and corresponding displacement of Type B, 450 SDR 9 TWT, and WT
specimens.

Specimen-Parallel Side Fmax Displacement @ Fmax

(mm) (kN) (mm)
TWT_10 37.37 ± 0.40 8.8 ± 0.6
TWT_20 35.13 ± 0.09 9.6 ± 0.3
WT_25 34.50 ± 0.87 12.1 ± 1.0

Therefore, based on the experimental results given above, a fusion zone notch radius
of 10 mm (Type A, Figure 3a) and the parallel side length of 20 mm (Type B, Figure 3b) is
most suitable to use for TWT specimens with a wall thickness of ≤25 mm and >25mm,
respectively. These and other dimensions of Type A and Type B TWT specimens are given
in Table 1.

3.1.4. Test Speed Determination

The effect of crosshead speed was investigated to determine the optimum test speed
for the TWT specimen. Type A (110 SDR 9, 225 SDR 11) with fusion zone notch radius of
10 mm, and Type B (315 SDR 11, 450 SDR 9) having 20 mm parallel side length specimens
were tested. Two crosshead speeds were used, 5 mm/min, and 10 mm/min. First, 5
mm/min was chosen as this speed is used in testing WT specimens per ISO 13953. Next,
10 mm/min was tested to see if the test time could be reduced. The higher speeds were
not considered as 25 mm/min, and 50 mm/min noticeably reduced the elongation of the
110 SDR 9 TWT specimen, and much of the post-necking behavior differed compared to
the 5 and 10 mm/min speed shown in Figure 6. However, the load–displacement curves
of all sized TWT specimens are similar at 5 and 10 mm/min test speeds (Figure 10). In
addition, although some reduced elongation with 10 compared to 5 mm/min speed, most
of the post-necking behavior was retained to be similar at both speeds. Therefore, for the
TWT dimensions given in Table 1, test speeds of 5 or 10 mm/min are suitable.
 not considered as 25 mm/min, and 50 mm/min noticeably reduced the elongation of the
110 SDR 9 TWT specimen, and much of the post-necking behavior differed compared to
the 5 and 10 mm/min speed shown in Figure 6. However, the load–displacement curves
of all sized TWT specimens are similar at 5 and 10 mm/min test speeds (Figure 10). In addi-
tion, although some reduced elongation with 10 compared to 5 mm/min speed, most of
Polymers 2022, 14, 1187 the post-necking behavior was retained to be similar at both speeds. Therefore, for 12 the
of 23
TWT dimensions given in Table 1, test speeds of 5 or 10 mm/min are suitable.

Figure
Figure10.
10.Effect
Effectofofcrosshead
crossheadspeed
speedon
onload–displacement
load–displacementbehavior
behaviorof
ofTWT
TWTspecimens.
specimens.

3.2.FE
3.2. FEAnalysis
AnalysisofofTWT
TWTSpecimen
Specimen

Polymers 2021, 13, x FOR PEER REVIEW

To better understand the
To better understand thedeformation
deformationbehavior
behaviorofofTWT
TWT specimens, a finite
specimens, element
a finite element
13 of
(FE)
24
analysis
(FE) was performed
analysis using commercial
was performed finite element
using commercial softwaresoftware
finite element ABAQUS.ABAQUS.
WT specimensWT
are also analyzed
specimens are alsofor comparison.
analyzed The specimens
for comparison. were modeled
The specimens wereusing a four-node
modeled linear
using a four-
tetrahedral element (C3D4 in ABAQUS) as illustrated in Figures 11 and 12.
node linear tetrahedral element (C3D4 in ABAQUS) as illustrated in Figures 11 and 12.

(a) (b)
Figure
Figure11.
11.Specimen
Specimenand
andgrip
gripassembly
assemblydimensions
dimensionsfor
forFE
FEanalysis
analysis(unit:
(unit:mm):
mm):(a)
(a)TWT;
TWT;(b)
(b)WT.
WT.
Polymers 2022, 14, 1187
(a) (b) 13 of 23
Figure 11. Specimen and grip assembly dimensions for FE analysis (unit: mm): (a) TWT; (b) WT.

Figure 12.
Figure 12. 1/4
1/4Finite
Finiteelement
elementmodels
models and
and data
data collection
collection lines
lines along
along the
the longitudinal
longitudinal direction
direction from
from the center of the BF joint to the tip of the specimen (unit: mm): (a) Type
the center of the BF joint to the tip of the specimen (unit: mm): (a) Type A model A model
(110(110
SDRSDR
9); 9);
(b)
(b) Type B model (315 SDR 11); (c) points of interest.
Type B model (315 SDR 11); (c) points of interest.

The true
The true stress–strain
stress–straindata dataofofPE100
PE100utilized
utilizedforforthe
the
FEFE analysis
analysis is shown
is shown in Figure
in Figure 13.
13. The
The truetrue stress–strain
stress–strain data data
up toup thetoonset
the onset
neckingnecking was estimated
was estimated from the from the nominal
nominal stress–
stress–strain
strain data Equations
data using using Equations
(1) and(1)(2).
andAn(2). An exponent
exponent valuevalue
of n =of1.25
n = was
1.25 chosen
was chosen
as it
as it gave
gave an optimal
an optimal curvecurve
passingpassing
through through thestress
the true true and
stress and data
strain strain data (open
points pointscircles)
(open
circles) obtained
obtained from thefrom the real-time
real-time experimental
experimental measurement
measurement of the cross-sectional
of the cross-sectional area andareathe
and the length change during the tensile deformation, as shown
length change during the tensile deformation, as shown in Figure 13. The independent in Figure 13. The inde-
pendent
elastic elastic constants
constants (i.e.,GPa,
(i.e., E = 1.1 E = 1.1
σy =GPa,
18.5σMPa,
y = 18.5 MPa,
εy = εy = 0.03037,
0.03037, and νwere
and ν = 0.45) = 0.45) were
used to
used to describe
describe the elastic thepart.
elastic
Thepart. Thepart
plastic plastic
afterpart
the after
yieldthe yield was
strength strength was tabulated
tabulated from the
from the
fitted truefitted true stress–strain
stress–strain curve. curve.
(
σnom (1 + ε nom ),  σnom ≤ σy
σtrue = 
n (1)
σy 1 + ε nom − ε y , σnom > σy

Z l  
l
ε true = dl/l = ln = ln(1 + ε nom ) (2)
l0 l0
where σnom , εnom , σtrue , εtrue, σy, εy , and n are the nominal stress and strain, true stress and
strain, true yield stress and strain, and plastic exponent, respectively. l0 is the initial gauge
length set at 50 mm, and l is the distance between gauge marks at any time.
The true stress–strain data of PE100 was further converted into appropriate elastic-
plastic data for FE analysis using ABAQUS/Standard. The plastic strain is obtained by
subtracting the elastic strain from the total strain, as shown in Equations (3) and (4).

ε total = ε true = ε e + ε p (3)

ε p = ln(1 + ε nom ) − ln 1 + σy /E , ε nom ≥ ε y (4)
where εtotal , εe , εp , and E are the total strain, elastic strain, plastic strain, and elastic modulus,
respectively.
Figure 14 illustrate the contours of von Mises stress (σv ), axial displacement (u1 ), and
transverse deflection (u3 ) at the maximum reaction force, Fmax . The axial stress (σ1 ), σv
and u1 of 110 SDR 9, 225 SDR 11, 315 SDR 11, and 450 SDR 9 TWT and WT specimens at
Fmax are also shown in Figures 15–18, respectively. Tables 8 and 9 provide selected σv , σ1,
u1 , and u3 values at different locations along the specimen axis, as shown in Figure 12.
Polymers
Polymers 2021,
2022, 13,
14, x1187
FOR PEER REVIEW 1414ofof24
23

Polymers 2021, 13, x FOR PEER REVIEW 15 of 24

Figure 13.
Figure True stress–strain
13. True stress–strain curve
curve of
of PE
PE 100
100 pipe
pipe grade
grade resin.
resin.

𝜎 1 𝜀 ), 𝜎 𝜎
𝜎 (1)
𝜎 1 𝜀 −𝜀 , 𝜎 𝜎

𝜀 𝑑𝑙 ⁄𝑙 𝑙𝑛 𝑙 𝑙 𝑙𝑛 1 𝜀 ) (2)

where σnom, εnom, σtrue, εtrue, σy, εy, and n are the nominal stress and strain, true stress and
strain, true yield stress and strain, and plastic exponent, respectively. l0 is the initial gauge
length set at 50 mm, and l is the distance between gauge marks at any time.
The true stress–strain data of PE100 was further converted into appropriate elastic-
plastic data for FE analysis using ABAQUS/Standard. The plastic strain is obtained by
subtracting the elastic strain from the total strain, as shown in Equations (3) and (4).
𝜀 𝜀 𝜀 𝜀 (3)
𝜀 𝑙𝑛 1 𝜀 ) − 𝑙𝑛 1 𝜎 ⁄𝐸 , 𝜀 𝜀 (4)
where εtotal, εe, εp, and E are the total strain, elastic strain, plastic strain, and elastic modulus,
respectively.
Figure 14 illustrate the contours of von Mises stress (𝜎 ), axial displacement (𝑢 ), and
transverse deflection (𝑢 ) at the maximum reaction force, Fmax. The axial stress (𝜎 ), 𝜎 and
𝑢 of 110 SDR 9, 225 SDR 11, 315 SDR 11, and 450 SDR 9 TWT and WT specimens at Fmax
are also shown in Figures 15–18, respectively. Tables 8 and 9 provide selected 𝜎 , 𝜎 , 𝑢 ,
and 𝑢 values at different locations along the specimen axis, as shown in Figure 12.

Figure
Figure 14.
14. FE
FE analysis
analysis results
results at
at Fmax (a)𝜎σv;;(b)
max: :(a) (b) 𝑢u1;; (c)
(c) u𝑢3 contours.
contours.

Typical behavior of 𝜎 and 𝜎 of WT specimens (Figures 15–18 and Table 8) shows

higher values at the traction hole (Position WP4 in Figure 12c), BF beadroll contact (Posi-
tion WP2), and at the center of the BF bead (Position WP1). The magnitude of 𝜎 , and 𝜎
depends on the Type and size of the WT specimen. On the other hand, 𝑢 is always the
highest at the traction hole upper end (WP5). Similarly, as shown in the same figures and
Table 9, in TWT specimens, 𝜎 and 𝜎 are always larger at the BF beadroll contact (Posi-
tion NP2 in Figure 12c) than at the BF bead center (Position NP1), independent of the
specimen type and size. In addition, 𝜎 and 𝜎 continue to decrease and 𝑢 increases
towards the specimen end without interruption (Figures 15–18). Furthermore, 𝑢 at NP2
and NP3 increases with the specimen size (Table 9).

Table 8. 𝜎 , 𝜎 , 𝑢 , and 𝑢 values at selected positions (Figure 12c) along the longitudinal direction
from the center of the BF joint to the tip of the WT specimens at Fmax.
Position WP1 Position WP2 Position WP4 Position (WP5-WP6)
 Polymers2022,
Polymers 2021,14,
2021, 13,1187
13, xxFOR
FORPEER
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REVIEW 17
17
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24
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Polymers 2021, 13, x FOR PEER REVIEW 17 of 24

(a)
(a) (b)
(b)
(a) (b)
Figure15.
Figure 15.FEA
FEAresults
resultsofofType
TypeA,
A,110110SDR
SDR99TWTTWTand
andWT
WTspecimens
specimensatatFFmax (a) 𝜎𝜎 and
max::(a) and 𝜎𝜎 ;;(b)
(b) 𝑢𝑢
Figure 15.FEA
Figure15. FEAresults
resultsofofType
TypeA,A,110
110SDR
SDR9 9TWT
TWTand
andWT specimensatatFmax
WTspecimens (a)σv𝜎 and
Fmax: :(a) σ1 ;𝜎(b)
and u1 . 𝑢
; (b)

(a)
(a) (b)
(b)
(a) (b)
Figure 16.
Figure 16. FEA
FEA results
results of
of Type
Type A,
A, 225
225 SDR
SDR 11
11 TWT
TWT and
and WT
WT specimens
specimens at
at FFmax (a) 𝜎𝜎 and
max:: (a) and 𝜎𝜎 ;; (b)
(b)
Figure16.
Figure 16.FEA
FEAresults
resultsofof Type
Type A,A,
225225
SDR SDR
11 11
TWTTWT
andand
WTWT specimens
specimens at Fat Fmax: (a) 𝜎 and 𝜎 ; (b)
max : (a) σv and σ1 ; (b) u1 .
𝑢𝑢 ..
𝑢 .

(a)
(a) (b)
(b)
(a) (b)
Figure17.
Figure 17.FEA
FEAresults
resultsof
ofType
TypeB,
B,315
315SDR
SDR11
11TWT
TWTand
andWT
WTspecimens
specimensat
atFFmax (a) 𝜎𝜎 and
max::(a) and 𝜎𝜎 ;;(b)
(b) 𝑢𝑢 ..
Figure 17.FEA
Figure17. FEAresults
resultsofofType
TypeB,B,315
315SDR
SDR11
11TWT
TWTand
andWT
WTspecimens atFFmax
specimensat (a) σ𝜎v and
max: (a) andσ1𝜎; (b)
; (b)u1𝑢. .
 Polymers
Polymers 2021,
2022, 14,13, x FOR PEER REVIEW
1187 18ofof2324
16

(a) (b)
Figure18.
Figure 18.FEA
FEAresults
resultsofofType
TypeB,B, 450
450 SDR
SDR 9 TWT
9 TWT and
and WTWT specimens
specimens atat (a)σ 𝜎and
Fmax: :(a)
Fmax σ1 ;𝜎(b)
and u1 . 𝑢 .
; (b)
v

Table 8.The
σv , transverse
σ1, u1 , and udeflection (𝑢 ) of the 110 SDR 9 WT specimen is shown in Figure 19.
3 values at selected positions (Figure 12c) along the longitudinal direction
𝑢
from the center of the BF joint to the tip specimen
causes the neutral plane of the to bend during
of the WT specimens at Fmaxthe
. tensile test and is the larg-
est with the 110 SDR 11 size (Position WP5 in Table 8). When the size is increased to 225
Position WP1 Position WP2 Position WP4 Position (WP5-WP6)
WT Specimen SDR 11, |𝑢 is seen to decrease by about 50% and approximately maintained in larger
σv (MPa) σ1 (MPa) |u 3 max (MPa) (MPa) |u1 |max |u3 |max σ (MPa) σ (MPa) |u1 |max |u3 |max
specimens, σ v σ
(mm) as shown in Figure 19.1 (mm)On the(mm) v 1
other hand, with the TWT(mm) specimen,(mm)𝑢 is
110 SDR 9 22.51 23.44 3.08 26.56 33.88 1.12 3.07 30.81 38.51 7.61 4.53
225 SDR 11 23.68 smallest
46.90 with
2.37 the 110
29.42 SDR 9 size
36.13 and increases
1.50 2.40incrementally
29.85 as the
29.45 size gets
6.34 larger.
1.20
315 SDR 11 26.84 45.49 By2.09
comparing 19 (𝑢 ) with
29.84Figure41.30 3.79 Figure ), one can
2.1120 (𝑢 26.78 observe 6.67
33.14 that the excessive
2.11
450 SDR 11 23.87 39.70 2.50 29.16 47.88 3.47 2.55 26.78 28.45 6.81 2.55
transverse deflection in 110 SDR 9 WT specimen brings about large specimen displacement.
This is speculated to be due to the lower stiffness of the thinner 110 SDR 9 size compared to
Table 9. σv , σ1, u1 , and u3 values at selected positions (Figure 12c) along the longitudinal direction
other larger sizes. However, with the TWT specimen, such large transverse deflection is not
from the center of the BF joint to the tip of the TWT specimens at Fmax .
observed in 110 SDR 9 size, which indicates the stability of the specimen under the tensile test.
As the
Position specimen gets larger, the difference
NP1 Position 𝑢 (Figure 19) and 𝑢 (Figure
inNP2 20) between
Position NP3 TWT
TWT Specimen σv (MPa) σ1 (MPa) |u3 |max (mm) σv (MPa) σ1 (MPa) |u1 |max (mm) |u3 |max (mm) |u1 |max (mm) |u3 |max (mm)
110 SDR 9 21.44 and WT gets
29.67 1.09 smaller, indicating
26.60 that a comparable
34.05 1.52 load–displacement
1.08 behavior to F0.51
2.39 max can be
225 SDR 11 24.06 expected to
30.23 occur. This
1.56 is experimentally
29.41 40.03 shown
2.63 in Figure 1.5221b, where2.85the difference
0.51in dis-
315 SDR 11 25.99 33.77 1.88 29.26 34.91 3.12 1.91 3.34 0.51
450 SDR 11 22.34 placement2.51
32.28 at Fmax becomes28.00 smaller38.62 with larger
3.27 specimen 2.55sizes. 3.47 0.25
Comparing 110 SDR 9 and 225 SDR 11 WT specimens (Figures 15a and 16a, and Table 8), the maximum σ1 at
the traction hole (WP4) decreases by 9.06 MPa while increasing at the BF beadroll contact (WP2) by 2.25 MPa,
going from 110 SDR 9 to 225 SDR11 size. Therefore, in 225 SDR 9 specimen while σv is somewhat maintained
approximately the same at WP4 (29.85 MPa) and WP2 (29.42 MPa) locations, the σ1 is now the largest at WP2
(36.13 MPa) compared to 29.45 MPa at WP4. The location of the maximum u1 is maintained at WP5; however, it
was lowered from 7.61 mm (110 SDR) to 6.34 mm (225 SDR 11), as shown in Figures 15b and 16b, and Table 8). The
increase of σv and σ1 at the BF beadroll (WP2) and decrease at the traction hole (WP4) when going from 110 SDR 9
to 225 SDR 11 size is due to the increased width (60–80 mm) and the thickness (12.0–20.5 mm) of the unreduced
portion of the specimen containing the traction hole.

Typical behavior of σv and σ1 of WT specimens (Figures 15–18 and Table 8) shows

higher values at the traction hole (Position WP4 in Figure 12c), BF beadroll contact (Position
WP2), and at the center of the BF bead (Position WP1). The magnitude of σv , and σ1 depends
on the Type and size of the WT specimen. On the other hand, u1 is always the highest at
(a) (b) (c)
the traction hole upper end (WP5). Similarly, as shown in the same figures and Table 9, in
Figure
TWT 19. Transverse
specimens, deflection
σv and σ1 areofalways
110 SDR 9 WTat
larger specimen: (a) Experimental;
the BF beadroll contact (b) FEA result
(Position NP2 at in
Fmax
(unit: mm); (c) Comparison of 𝑢 between TWT and WT specimens at F .
Figure 12c) than at the BF bead center (Position NP1), independent of the specimen type
max

and size. In addition, σv and σ1 continue to decrease and u1 increases towards the specimen
end without interruption (Figures 15–18). Furthermore, u1 at NP2 and NP3 increases with
the specimen size (Table 9).
In Type A TWT specimens (110 SDR 9 and 225 SDR 11), σv , σ1 , and u1 distribution is
less complex than the WT specimen (Figures 15a and 16a, and Table 9). σv , and σ1, continue
to decrease, and u1 increases from NP2 to the specimen end as shown. The highest σv and
σ1 are always at the BF beadroll contact (NP2), and the BF bead center (NP1) has lower
Polymers 2022, 14, 1187 17 of 23

values (Table 9). σv and σ1 at NP2 increase by 2.81 MPa and 5.98 MPa, going from 110 SDR
9 to 225 SDR size, and is attributed to the thickness increase (no width increase for the TWT
specimen-Figure 3).
For Type B WT specimens (Figures 17a and 18a, and Table 8), σv , and σ1 at the traction
hole (WP4) were always less than those values at the BF beadroll contact (WP2). σv was
10.3% and 8.2% lower, and σ1 was 19.8% and 40.6% lower for 315 SDR 11 and 450 SDR
9 models, respectively. On the other hand, the u1 was always higher at WP5. The lower
stresses at WP4 were due to the wider width of the unreduced portion of the specimen (100
mm) containing the traction hole and the increased thickness of the specimen (see Figure 3).
With Type B TWT specimens, σv and σ1 were kept the largest at the BF beadroll contact
(NP2) for all sizes as shown in Figures 17a and 18a and Table 9.
In Type A specimens, σv , σ1 , and u1 are seen to be lower in WT than TWT specimen in
length between the region just after the BF beadroll contact (<10 mm) to the beginning of
the traction hole (35 mm), as shown in Figures 15 and 16. This is expected due to the shorter
distance from the BF beadroll (WP2) to the free surface of the traction hole (WP3), giving
rise to a higher stress gradient (faster drop) than in the longer Type B TWT specimen. In
addition, the larger taper of the TWT (30◦ ) waist compared to WT (0◦ ) has caused a higher
u1 in the TWT specimen in the same region, as shown in Figures 15b and 16b.
In Type B specimens, σv , σ1 , and u1 are higher in the WT than the TWT specimen in
length between the region around the BF beadroll (~20 mm) to the beginning of the traction
hole at 70 mm Figures 17 and 18). σv , σ1 , and u1 are higher because the stress gradient is
lower and u1 higher due to the length of this region being longer than the total length of
the TWT specimen and WT having a longer parallel side length (20 versus 25 mm).
For BF beadroll contacts (WP2 and NP2), σv is similar between WT and TWT specimens
(Figures 15–18) for all sizes. That is, similar load behavior up to the point of onset necking
(Fmax ) is expected between each size WT and TWT specimens. The experimental force–
displacement curves shown in Section 3.3.1 demonstrate that similar Fmax values are reached
on each size of WT and TWT specimens. However, the displacements at Fmax are shown to
be different due to the traction hole deformation in WT specimens.
Therefore, from the σ1 and u1 behavior, while a larger portion of the displacement in
the Type A WT specimen arises from the traction hole deformation (84% for 110 SDR 11
size), the percentage decreases to 42% in the Type B specimen. However, with the TWT
specimen, the u1 comes from the specimen displacement, independent of the specimen
type. Since the rate of u1 increase is higher from the bead center to the taper end portion of
the specimen (Figures 15–18), a good part of the specimen deformation is expected to come
from this portion of the TWT specimen.
The transverse deflection (u3 ) of the 110 SDR 9 WT specimen is shown in Figure 19.
u3 causes the neutral plane of the specimen to bend during the tensile test and is the largest
with the 110 SDR 11 size (Position WP5 in Table 8). When the size is increased to 225 SDR
11, u3 is seen to decrease by about 50% and approximately maintained in larger specimens,
as shown in Figure 19. On the other hand, with the TWT specimen, u3 is smallest with the
110 SDR 9 size and increases incrementally as the size gets larger.
By comparing Figure 19 (u3 ) with Figure 20 (u1 ), one can observe that the excessive
transverse deflection in 110 SDR 9 WT specimen brings about large specimen displacement.
This is speculated to be due to the lower stiffness of the thinner 110 SDR 9 size compared to
other larger sizes. However, with the TWT specimen, such large transverse deflection is not
observed in 110 SDR 9 size, which indicates the stability of the specimen under the tensile
test. As the specimen gets larger, the difference in u3 (Figure 19) and u1 (Figure 20) between
TWT and WT gets smaller, indicating that a comparable load–displacement behavior to
Fmax can be expected to occur. This is experimentally shown in Figure 21b, where the
difference in displacement at Fmax becomes smaller with larger specimen sizes.
 This is speculated to be due to the lower stiffness of the thinner 110 SDR 9 size compared to
other larger sizes. However, with the TWT specimen, such large transverse deflection is not
observed in 110 SDR 9 size, which indicates the stability of the specimen under the tensile test.
As the specimen gets larger, the difference in 𝑢 (Figure 19) and 𝑢 (Figure 20) between TWT
and WT gets smaller, indicating that a comparable load–displacement behavior to Fmax can be
Polymers 2022, 14, 1187 expected to occur. This is experimentally shown in Figure 21b, where the difference in dis- 18 of 23

placement at Fmax becomes smaller with larger specimen sizes.

(a) (b) (c)

Figure
Polymers 2021, 13, x FOR PEER REVIEW 19. Transverse
Figure deflection
19. Transverse of 110
deflection SDRSDR
of 110 9 WT specimen:
9 WT (a) Experimental;
specimen: (b) FEA
(a) Experimental; result
(b) FEA at19
result Fat
ofF 24
max
max
(unit: mm); (c) Comparison of 𝑢 between TWT and WT specimens at Fmax.
(unit: mm); (c) Comparison of u3 between TWT and WT specimens at Fmax .

Polymers 2021, 13, x FOR PEER REVIEW 20 of 24

Figure20.
Figure 20.Comparison of 𝑢u1along
Comparisonof alongthe
thespecimen
specimencenterline
centerline(see
(seeFigure
Figure12c).
12c).

It is noted that based on the FE analysis, the 20 mm gauge length was selected for
TWT and WT tensile tests (Figure 2). As shown in Figure 20, the change in displacement
between the TWT and WT specimens is less than 5% for all specimen sizes when using a
20 mm (±10 mm) gauge length. Therefore, as expected, the experimental load–displace-
ment curves measured with the extensometer are practically identical between the TWT
and WT specimens at displacements up to 10 mm for the 110 SDR 9 size and up to 20 mm
for the larger size, as presented in 3.3.2. In addition, the EN 12814-7 standard requirement
to use an extensometer with a 50 mm (±25 mm) gauge length for WT specimens to avoid
traction hole displacement is not required with TWT specimens. This is due to a small dif-
ference (less than 1.0 mm) in displacement measured at the specimen length 25 mm and at
the specimen end from the BF bead center for all specimens, as shown in Figure 20.

3.3. Comparison of TWT and WT Specimens

3.3.1.
(a) Load–Displacement Behavior Based on Crosshead Displacement (b)
The tensile performance of TWT specimens with the shape and dimension (Table 1)
Figure21. 21.Comparison
Comparison betweenTWT TWTand
andWTWTspecimens:
specimens:(a)
(a)Load–displacement
Load–displacementcurves;
curves;(b)
(b)Onset
Onset
Figure
proposed is comparedbetween
necking displacement. to the WT specimen (ISO 13953) at 5 mm/min crosshead speed, as
neckingin
shown displacement.
Figure 21. The shape of the force–displacement curve is similar between the
same
3.3.2.Itsize and Type TWT and WT specimens.
Load-Displacement on However, some differences were observed
is noted that based Behavior
on the FEBased
analysis, Gage Length
the 20 of 20
mm gauge mm
length was selected for
in the total and onset necking (Fmax) displacements. With 110 SDR 9 size specimens, the
TWT and Per ISOWT13953,
tensilethe WT
tests BF fracture
(Figure 2). Asenergy
shownisintaken
Figurefrom
20,the
thetotal areainunder
change the load–
displacement
displacement at Fmax of the WT specimen was ~100% larger than the TWT specimen. For
between
displacementthe TWT andbased
curve WT specimens is less than
on the crosshead 5% for all specimen
displacement. Thus, thesizes
WTwhen
or TWTusing
BF afrac-
20
sizes larger than 225 SDR 11, the difference was about ~30%, as shown in Figure 21b. In
ture(±
mm 10 mm)
energy gauge length.
calculation basedTherefore, as expected,
on the total the grossly
area would experimental load–displacement
overestimate the fracture
addition, the onset necking displacement is shown to be the maximum with the 110 SDR
energy of the BF joint. Therefore, to better estimate the load–displacement behavior of the
9 WT specimen and largely decreases to a relatively constant value with larger size WT
BF joint under the tensile load, measurement was carried out using a 20 mm gage length
specimens. The displacement continues to increase in smaller increments with the increas-
extensometer with the BF bead at the center (Figure 2). The result of the load–displace-
ing size of the TWT specimen, as shown. Therefore, in all sizes, the onset necking displace-
ment curve using the extensometer is shown in Figure 22a for the TWT specimen, along
ment of the TWT specimen is smaller than that of the WT specimen. However, the total
Polymers 2022, 14, 1187 19 of 23

curves measured with the extensometer are practically identical between the TWT and WT
specimens at displacements up to 10 mm for the 110 SDR 9 size and up to 20 mm for the
larger size, as presented in 3.3.2. In addition, the EN 12814-7 standard requirement to use
an extensometer with a 50 mm (±25 mm) gauge length for WT specimens to avoid traction
hole displacement is not required with TWT specimens. This is due to a small difference
(less than 1.0 mm) in displacement measured at the specimen length 25 mm and at the
specimen end from the BF bead center for all specimens, as shown in Figure 20.

3.3. Comparison of TWT and WT Specimens

3.3.1. Load–Displacement Behavior Based on Crosshead Displacement
The tensile performance of TWT specimens with the shape and dimension (Table 1)
proposed is compared to the WT specimen (ISO 13953) at 5 mm/min crosshead speed,
as shown in Figure 21. The shape of the force–displacement curve is similar between the
same size and Type TWT and WT specimens. However, some differences were observed
in the total and onset necking (Fmax ) displacements. With 110 SDR 9 size specimens, the
displacement at Fmax of the WT specimen was ~100% larger than the TWT specimen. For
sizes larger than 225 SDR 11, the difference was about ~30%, as shown in Figure 21b.
In addition, the onset necking displacement is shown to be the maximum with the 110
SDR 9 WT specimen and largely decreases to a relatively constant value with larger size
WT specimens. The displacement continues to increase in smaller increments with the
increasing size of the TWT specimen, as shown. Therefore, in all sizes, the onset necking
displacement of the TWT specimen is smaller than that of the WT specimen. However,
the total displacement varies depending on the specimen type (Figure 21). With Type A
specimens (110 SDR 9 and 225 SDR 11), TWT elongation is higher. And lower when Type
B (315 SDR 11, 450 SDR 9) are compared. The difference is explained later with the FE
analysis, and it is due to the presence of a fusion zone notch (Type A) versus the parallel
side (Type B) of the TWT and WT specimens.

3.3.2. Load-Displacement Behavior Based on Gage Length of 20 mm

Per ISO 13953, the WT BF fracture energy is taken from the total area under the
load–displacement curve based on the crosshead displacement. Thus, the WT or TWT BF
fracture energy calculation based on the total area would grossly overestimate the fracture
energy of the BF joint. Therefore, to better estimate the load–displacement behavior of the
BF joint under the tensile load, measurement was carried out using a 20 mm gage length
extensometer with the BF bead at the center (Figure 2). The result of the load–displacement
curve using the extensometer is shown in Figure 22a for the TWT specimen, along with
the result obtained using the crosshead displacement. In all sizes, the onset-necking load
(Fmax ) and the shape of the post-onset necking load–displacement behavior are similar
between the extensometer and the crosshead measurements. However, the displacement at
Fmax is much reduced with the extensometer measurement. Furthermore, this difference
in displacement (crosshead versus extensometer) is seen to be maintained throughout the
post-onset necking to failure, indicating that the post-onset necking displacement is mostly
from the gage length region. Therefore, the pre-onset necking displacement of the crosshead
measurement is mostly from the total length of the specimen, and the difference to the
extensometer measurement becomes maximum at the onset necking load, as indicated in
Figure 22a. These behaviors are due to the necking initiating and propagating from the
point of contact between the BF beadroll and the pipe, where the stress concentration is
created, as shown in Figures 15–18.
Polymers 2021, 13, x FOR PEER REVIEW 21 of 24
Polymers2022,
Polymers 2021,14,
13,1187
x FOR PEER REVIEW 21 of 23
20 of 24

(a) (b)
(a) (b)
Figure 22. Extensometer versus crosshead measurements of TWT specimen; (a) Load–displacement
Figure22.22.Extensometer
Extensometerversus
versuscrosshead
crossheadmeasurements
measurementsof of TWTspecimen;
specimen;(a)
(a)Load–displacement
Load–displacement
Figure
curves; (b) Normalized energies calculated at Fmax (𝐸 ) andTWTthe energy difference (𝐸 −𝐸 ).
curves; (b) Normalized energies calculated at Fmax (𝐸 )
at Fmax (EFmax ) and the energy difference (𝐸
and the energy difference −𝐸
(ETotal − EFmax ).).
𝐸curves;is(b)theNormalized
normalizedenergies calculated
total fracture energy.
𝐸 is the normalized total fracture energy.
ETotal is the normalized total fracture energy.
In support of this, deformation energies from the load–displacement curves of the
In support
In support of ofthis,
this,deformation
deformation energies
energies from
fromthe theload–displacement
load–displacement curves curves of of the
the
crosshead versus extensometer are compared in Figure 22b. 𝐸 is the energy normal-
crosshead versus extensometer are compared in Figure 22b.
crosshead versus extensometer are compared in Figure 22b. EFmax is the energy normalized 𝐸 is the energy normal-
ized by the cross-sectional area of the TWT specimen, calculated at Fmax and 𝐸 is the
ized
by thebycross-sectional
the cross-sectional area of area
theofTWT
the TWT specimen,
specimen, calculated
calculated at Fmaxat and E 𝐸is the isarea
Fmax and the
area normalized total energy to failure. The energy of pre-onset neckingTotal (𝐸 ) from
area normalized
normalized total energy
total energy to failure.
to failure. The energy Theofenergy
pre-onset of pre-onset
necking (Enecking
F ) from(𝐸 )
crossheadfrom
crosshead displacement is about five times larger than the energy frommax extensometer dis-
crosshead displacement
displacement is about five is times
about larger
five times
thanlarger than the
the energy from energy from extensometer
extensometer displacement dis-
placement for all TWT specimens. In comparison, the mean post-necking energies are
placement
for all TWTfor all TWT specimens.
specimens. In comparison, In comparison, the mean post-necking
the mean post-necking energies areenergiespractically are
practically the same (within 1~4.5% difference). Hence, confirming
the source the source
and of pre and
practically
the same (withinthe same 1~4.5%(within 1~4.5% difference).
difference). Hence, confirming
Hence, confirming the
of source
pre of pre and
post-onset
post-onset
necking necking displacements
displacements as indicated asabove.
indicated above.
post-onset necking displacements as indicated above.
AA comparison of of load–displacementbetween betweenthe theTWT
TWTand andWT WTspecimens
specimens is is shown
Acomparison
comparison ofload–displacement
load–displacement between the TWT and WT specimens shown
is shown in
in Figure
Figure 23.
23. 23. They
They are
are are practically
practically identical
identical up
up up to
to the the point
point of onset
of onset necking.
necking. AndAnd similar
similar to
in Figure They practically identical to the point of onset necking. And similar
to
thethe crosshead
crosshead load–displacement
load–displacement behavior,
behavior, thethe total
total displacement
displacement measured
measured is is depend-
dependent
to the crosshead load–displacement behavior, the total displacement measured is depend-
ent
on on the specimen type. With Type A specimens (110 SDR 9 and 225 SDR 11), the TWT
entthe
on specimen
the specimen type. With
type. WithType
TypeA specimens
A specimens (110
(110SDRSDR 9 9and
and225225SDRSDR11),11),thetheTWT
TWT
displacements
displacements are larger than the WT displacements and smaller when Type B (315 SDR
displacementsare arelarger
largerthan
thanthetheWTWT displacements
displacements and andsmaller
smaller when
when TypeTypeB (315 SDR
B (315 11,
SDR
11,
450 450
SDR SDR 9) 9)
areare compared.
compared. TheThedeviation
deviationininthe theload–displacement
load–displacement curve curve after
after the
the onset
onset
11, 450 SDR 9) are compared. The deviation in the load–displacement curve after the onset
necking
necking is seen to occur at the smallest displacement for 110 SDR 9 (TWT-20 mm notch
necking isis seen
seen totooccur
occurat atthe
thesmallest
smallestdisplacement
displacement for for 110
110 SDR
SDR 99 (TWT-20
(TWT-20mm mmnotch notch
versus
versus WT-10 mm diameter notch), followed by 225 SDR 11 (both are 20 mm diameter
versus WT-10
WT-10mm mmdiameter
diameter notch),
notch), followed
followed by by 225
225 SDR
SDR 11 11 (both
(both are
are 20
20mm mmdiameter
diameter
notch).
notch). For Type B specimens, TWT (20 mm parallel side) and WT (25 mm parallel side)
notch). ForFor Type
TypeBBspecimens,
specimens, TWT TWT (20 (20 mm
mm parallel
parallel side)
side) and
and WT WT (25(25 mm
mm parallel
parallel side)
side)
showed
showed more or less the same behavior throughout the necking region up to aa 20 mm
showed more or less the same behavior throughout the necking region up to
more or less the same behavior throughout the necking region up to a 2020 mmmm
displacement.
displacement.
displacement.

Figure 23.
Figure Load–displacement
23.Load–displacement curves
curves of TWT
of TWT andand
WT WT specimens
specimens usingusing a 20gage
a 20 mm mmlength
gage exten-
length
Figure 23. Load–displacement curves of TWT and WT specimens using a 20 mm gage length exten-
someter.
extensometer.
someter.
Polymers 2022, 14, 1187 21 of 23

Therefore, the TWT specimen shape and dimension proposed in Table 1 gives practi-
cally the same load–displacement of the WT specimen in the BF joint area up and beyond
the onset necking. On the other hand, the crosshead displacement at the maximum load is
smaller with the TWT specimen. Hence, indicating that two specimens provide the same
deformation behavior at the BF joint area. At the same time, the TWT specimen provides
a better estimate of the BF fracture energy when determined from a more convenient
crosshead load–displacement curve. In addition, since the TWT specimens are smaller than
the WT specimens, the BF area to be tested is about 60% larger (Table 3). Furthermore, the
TWT specimen preparation is made simpler by removing tractions holes and having the
width of the unreduced part of the specimen fixed for all pipe-size BF joints (Figure 3).
Thus, a single-size surface angle grip is made to accommodate all-size TWT specimens
(Figure 5). In terms of the practicality of testing, TWT specimens are more stable (push-in
grips) (Figures 2a, 5 and 11a) than WT specimens (pin rods) (Figures 2b and 11b).

4. Conclusions
The tapered waist tensile (TWT) test was developed and applied to evaluate the
integrity of high-density polyethylene pipe butt fusion (BF) joints. Experimental and
numerical analyses of the TWT specimens were performed to obtain an optimum design
for the BF joint destructive test. As a result, the Type A TWT specimen has been shown
to be suitable for pipe thicknesses of 25 mm and less, and Type B has been suggested
for thicknesses greater than 25 mm. Type A TWT specimens were characterized by a
taper angle to a circular notch in the fusion zone, whereas Type B was designed with the
same taper angle to a parallel sided BF region. In addition to the taper angle created to
support the tensile loading, other important features of the TWT specimen include a 20 mm
diameter notch (or 20 mm parallel side length) in the BF area, a 60 mm unreduced width,
and a 25 mm reduced fusion zone width. The same dimensions apply to all pipe sizes and
standard dimensional ratios (SDR). Furthermore, no traction holes are used for loading the
specimen. The use of simple push-in taper-support grips with an angle of 30◦ was made to
minimize specimen contribution in calculating the BF fracture energy. Depending on the
test time requirement, the tensile test speeds of 5 mm/min and 10 mm/min can be used.
These features separate the TWT specimens from the WT specimens and reduce
specimen displacement by more than 100% (for the 110 SDR 9 size) and 30% (225 SDR
11 and larger) at the onset of necking load. Similarly, the stability of the TWT specimens
during tensile testing is significantly better than the WT specimens, as analyzed by the
development of lower transverse deflections. Additionally, TWT specimens can test at least
60% more portions of pipe BF joints than WT specimens. Therefore, compared to ISO 13953
waisted tensile (WT) specimens, TWT specimens offer significant improvements in BF
joint tensile testing in terms of simplicity, adaptability, stability, and accuracy in specimen
preparation, testing, and analysis. At the same time, the TWT specimens exhibited BF
fracture behavior equivalent to that of the WT specimens, as evidenced by similar load–
displacement curves obtained using a 20 mm gauge length extensometer centered on the
BF joint.
Finally, the proposed TWT specimen will be applied to evaluate the various flawed
high-density polyethylene BF joints. In addition, it can be used as an alternative to the WT
specimen of ISO 13953.

Author Contributions: Conceptualization, S.K., T.E. and S.C.; methodology, S.K., T.E. and W.L.;
formal analysis, S.K. and T.E.; investigation, S.K., T.E. and W.L.; resources, S.C.; data curation, S.K.,
T.E. and W.L.; writing—original draft preparation, S.K. and S.C.; writing—review and editing, S.C.;
supervision, S.C. All authors have read and agreed to the published version of the manuscript.
Funding: Nuclear Power Core Technology Development Program of the Korea Institute of Energy
Technology Evaluation and Planning (KETEP), Ministry of Trade, Industry and Energy, Republic of
Korea (Project no. 20181520102810).
Institutional Review Board Statement: Not applicable.
Polymers 2022, 14, 1187 22 of 23

Informed Consent Statement: Not applicable.

Data Availability Statement: The data that supports the findings of this study are available within
the article.
Acknowledgments: This work was supported by the Nuclear Power Core Technology Development
Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), Ministry of
Trade, Industry and Energy, Korea (Project No. 20181520102810).
Conflicts of Interest: The authors declare no conflict of interest.

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