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Determination of the tear properties of thermoplastic polyester elastomers (TPEEs) using essential work of fracture (EWF) test method
Polymer Testing 28 (2009) 854–865
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Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Test Method
Determination of the tear properties of thermoplastic polyester elastomers (TPEEs) using essential work of fracture (EWF) test method Jeong-Moo Lee a, Byoung-Ho Choi b, *, Jong-Sin Moon a, Eon-Seok Lee a a b
Tech Center, LG Chem Ltd., 84 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea School of Mechanical Engineering, Korea University, 1 5-ga, Anam-dong, Sungbuk-gu, Seoul 136-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 May 2009 Accepted 25 July 2009
In this paper, three commercial thin thermoplastic polyester elastomer (TPEE) samples are prepared by the injection molding process. Essential work of fracture (EWF) tests, which have been extended to cover the ductile tearing of polymeric materials that neck before fracture, in both the machine direction (MD) and the transverse direction (TD) are executed based on the ESIS TC-4 protocol as well as conventional trouser tear tests. Through an examination of the EWF test results, the tear properties of TPEE samples can be determined quantitatively. Partitioning of the test results is attempted for identifying the contribution of yielding and necking/tearing on the overall tear properties of TPEE samples. In addition, the fracture surfaces of TPEE samples following EWF tests are investigated through scanning electron microscopy (SEM). The root causes of the differences in tear properties are studied using the morphology of the fracture surfaces of TPEE samples. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Thermoplastic polyester elastomer Essential work of fracture Tear Injection molding Orientation
1. Introduction The market for thermoplastic elastomers (TPEs) is expanding because TPEs have good moldability, like thermoplastics, as well as elastic properties, like rubbers. Especially among TPEs, thermoplastic polyester elastomers (TPEEs) have been used for many automotive applications, such as constant velocity joint (CVJ) boots, door latches, etc., because TPEEs have good mechanical properties. During the running of automobiles, various complications, such as a rise in temperature, chemical attack, abrasion, fatigue, etc., can affect CVJ boots. Resistance to cracking is one of the important properties of CVJ boots because they should be replaced once their material cracks so that the CVJ is not functioning properly. For measuring the fracture toughness of thin TPEE ductile films, two practical teartests are popularly used in industry, viz., trouser tear (ISO
* Corresponding author. E-mail address: [email protected] (B.-H. Choi). 0142-9418/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2009.07.008
6383-1, ASTM D624) and Elmendorf tear (ISO 6383-2, ASTM D1922). These practical tear-tests can be used to rank samples but it is not possible to quantify the fracture toughness by applying these tests to TPEE samples. When a regular method of testing the fracture toughness, such as the critical stress intensity factor (KIc) test, is applied to materials with high toughness and low yield strength it is very difficult to analyze the test results due to the violation of conventional linear elastic fracture mechanics (LEFM). In this case, other methods based on elasto-plastic fracture mechanics, such as crack-openingdisplacement (COD), the d-5 method, the J-integral, etc., have been proposed to obtain the fracture toughness of such materials. There have been a number of discussions [1,2] that the application of conventional fracture mechanics to rubbers is not appropriate due to the large deformation around the crack tip. Moreover, some polymeric materials such as TPEEs that are fabricated as thin films and sheets are not suitable for conventional fracture toughness tests that are based on fracture mechanics under plane-strain conditions describe above.
J.-M. Lee et al. / Polymer Testing 28 (2009) 854–865
a
855
P
Gate Process zone (We)
MD-Specimen
Plastic zone (Wp)
Melt flow direction
100 mm
TD-Specimen
Specimen for EWF test
Clamped zone
150 mm Ligament Fig. 2. Preparation of specimens in machine direction (MD) and transverse direction (TD) from the injection molded plaque (2 mm in thickness).
(L)
Clamped zone
based on EWF tests. The effect of the resin flow on the fracture toughness of TPEE materials was evaluated. The suitability of EWF tests for the evaluation of the fracture toughness of TPEE materials was studied. Finally, a methodology for ranking those materials based on the evaluated fracture toughness is proposed. The morphologies of the fractured specimens following EWF tests were analyzed.
P
plane stress/ plane strain transition plane stress
2. The concept of EWF tests
Fig. 1. Description of the concept of essential work of fracture (EWF) method, (a) Schematics of the deep double-edge notched tension (DDENT) specimen, (b) Determination of EWF parameters(we and bwp).
The essential work of fracture (EWF) concept was proposed by Broberg [3]. Some researchers (e.g., [4–6]) expanded this concept to various cases. Recently, the European Structural Integrity Society (ESIS) published a test protocol for EWF tests of thin polymeric films [7] to initiate the standardization of EWF tests. Due to a large amount of plastic deformation around the crack tip of polymeric thin films as soon as the tear load is applied, the tear process can be assumed as the propagation of a mode-I crack [8,9]. Therefore, the fracture toughness of polymeric thin films under tear loads can be estimated by EWF tests. There have been reports on the application of EWF tests for evaluating the fracture toughness of various polymeric thin films [10–14] but only limited studies (e.g., [15,16]) are available for the evaluation of the fracture toughness of thin TPE or elastomeric materials. In this study, three thin, commercial-grade TPEE materials for automotive uses, such as CVJ boots, are selected. The fracture toughness of those samples was evaluated
W f ¼ We þ W p
a Clamped Clamped zone zone
(1)
b
Clamped zone
Notch L
100 mm
Ligament length, L
60 mm
W 2
100 mm
3t
20 mm
Valid ligament length
50 mm
β wp
The concept of EWF test was well-described by Cottrell and Reddel [4]. Two types of specimen, i.e., single-edge notched tension (SENT) specimen and deep double-edge notched tension (DDENT) specimen, are commonly used for EWF tests. Fig. 1 presents the schematics of the concept of EWF tests for DDENT specimens. The total work of fracture (Wf) can be separated into two components, i.e., the essential work of fracture (We) and the non-essential work of fracture (Wp).
Initial cut (Notch)
w f = w e + βw p L
we
Specific total fracture work, wf
b
Clamped zone
30 mm
30 mm t= 2 mm
Fig. 3. Geometry of tested specimens used in trouser tear and EWF tests, (a) Trouser tear specimen, (b) Deep double-edge notched tension (DDENT) specimen for EWF tests.
J.-M. Lee et al. / Polymer Testing 28 (2009) 854–865
a
Trouser tear specimen
a Notch
P
Notch
P MD-Notch
P
b
TD-Notch Melt flow direction
DDENT specimen
Engineering stress, σ (MPa)
856
50
30
20
TD-Notch
MD-Notch
Fig. 4. Definition of the direction of specimens, i.e., MD and TD, for trouser tear and EWF tests based on the notching direction with respect to the resin melt flow direction (The lines denote oriented fibrous crystals).
The essential work of fracture is directly related to the energy for creating the surfaces of new cracks in the process zone. The non-essential work of fracture is related to the formation of the plastic zone around the process zone. Considering the size of the process zone and the volume of the plastic zone, Eq. (1) can be rewritten as below based on the area under the load-displacement curve:
Wf ¼ we Lt þ bwp L2 t
(2)
In Eq. (2), L is the ligament length of the specimen, t is the thickness of the specimen, and b is the shape factor that is related to the formation of the plastic zone. Also, we is the specific essential work of fracture and wp is the specific non-essential work of fracture. Eq. (2) can be rewritten by using the specific total work of fracture (wf), as below:
Wf ¼ we þ bwp L Lt
10
0
100
200
300
400
500
600
700
800
900
50 P
A TD
40
C B 30
20
P TD Specimen TPEE- A TPEE- B TPEE- C
10
0
0
100
200
300
400
500
600
700
800
900
Engineering strain, ε (%) Fig. 5. Engineering stress-strain curves of samples under tensile tests (Test speed: 50 mm/min), (a) MD tensile specimens, (b) TD tensile specimens.
Those two components can be rewritten by mimicking the representation of Eq. (3) as below:
wf ;y ¼ we;y þ by wp$y L; wf ;n ¼ we;n þ bn wp;n L
(5)
where the subscripts y and n represent yielding and necking, respectively.
(3) 3. Specimen preparation and experiments
As shown in Eq. (3), the specific total work of fracture and the ligament length are linearly related. Here, we is defined as the intercept point in the y-axis of the wf L curve and bwp is defined as the slope of the wf L curve (Fig. 1(b)). Hence, the fracture toughness of the sample under planestress conditions can be measured by finding the intercept point, and the slope of the curve can be used to determine the resistance to crack propagation. Some scholars (e.g., [17,18]) reported that the work of fracture obtained from EWF tests of polymeric thin films can be separated again to reveal two distinct behaviors, i.e., yielding and necking. Therefore, the specific total work of fracture can be divided into two components:
wf ¼ wf ;y þ wf ;n
A B
C
P
Engineering strain, ε (%)
Engineering stress, σ (MPa)
P
TPEE- A TPEE- B TPEE- C
MD
Notch
b
wf ¼
MD Specimen
40
0 Notch
P
(4)
where wf,y is the specific total work of energy for yielding and wf,n is the specific total work of energy for necking.
3.1. Test materials Three commercial-grade thermoplastic polyester elastomers (TPEEs) that are block copolymers composed of polyether (soft segments) and polyester (hard segments) were selected; these are TPEE-A, TPEE-B, and TPEE-C. These types of TPEE are commonly used for fabricating constant velocity joint (CVJ) boots and have a hardness around 39w42 on the Shore D scales. 3.2. Test specimens The test specimens were prepared in the machine (MD) and transverse (TD) directions of 2 mm-thick injectionmolded plates (made through a 150-ton injection-molding machine from Engel), as shown in Fig. 2. Standard dumbbell
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Table 1 Mechanical properties of the materials obtained with the tensile tests at 50 mm/min. Properties
Unit
@ 3 ¼ 5% @ 3 ¼ 10% @ 3 ¼ 50%
Tensile stress
TPEE-A
MPa MPa MPa MPa %
Tensile strength Elongation to break
MD
TD
MD
TD
3.4 5.1 8.3 22.0 524
3.8 5.5 8.3 42.1 850
3.7 5.5 8.8 20.3 578
3.8 5.6 8.5 33.8 836
4.2 5.9 10.1 18.9 349
4.5 5.9 8.5 36.4 812
3.3. Test conditions Tensile tests were executed following ASTM D638 (50 mm/min and 23 C). The tear strengths of the samples
were tested based on ASTM D624 (50 mm/min and 23 C). EWF tests were performed as given in the test protocol of the European Structural Integrity Society (ESIS) (50 mm/min and 23 C) [5]. All mechanical tests were carried out on an Instron 3365 universal materials testing machine (5 kN).
3.4. Observation of specimens The tested samples were collected for microscopic observation. Optical microscopy was used to observe the variation of the deformation up to final failure during the
a
250 (d) (e)
300 TPEE-A, B , C : Straight crack path
Load, P (N)
200
Type T specimen TPEE-A, TD-Notch
(g) (h)
(b) 150
Necking + Tearing
Yielding
100
150
(f)
(c)
200
250
Load, P (N)
TPEE-C
TD
shaped specimens with 25 mm gauge length and 4 mm width were used for the tensile tests. For fracture tests, two types of specimens were used (Fig. 3): (a) trouser-tear (Type T) specimens for out-of-plane tear tests and (b) deep double-edge notched tension (DDENT) specimens for EWF tests. Type T specimens have a long notch (50 mm) and DDENT specimens have various notch lengths, viz., 6, 8, 10, 12 and 14 mm, for evaluating EWF properties. Notches were introduced by a razor blade and the direction of each notch was defined according to the direction of resin flow (Fig. 4).
a
TPEE-B
MD
(a) TPEE-A TD-Notch L10, S-2 v=50 mm/min
50
100 TD-Notch TPEE-A TPEE-B TPEE-C
50 0
0
50
100
150
200
250
300
0
5
10
15
20
350
b
Clamped zone
P
300 MD-Notch
TPEE-A : Straight crack path
TPEE-A TPEE-B TPEE-C
TPEE-B, C : Curved crack path
250
Load, P (N)
0
Displacement, δ (mm)
Displacement, δ (mm)
b
(i)
200
Notch
Type T specimen
P
TPEE-B, MD-Notch
(a)
(b)
(c )
(d)
(f )
(g)
(h)
(i)
30 mm
150 100 TPEE-C, MD-Notch
50 0
(e) 0
50
100
150
200
250
300
350
Displacement, δ (mm) Fig. 6. Tear test results obtained from the trousers tear specimens, (a) TDnotch specimens, (b) MD-notch specimens.
Fig. 7. Example of the aspects of deformation with increasing loads under EWF tests. (TPEE-A, TD-Notch, Ligament length ¼ 10 mm), (a) The relationship between the load and the displacement, (b) Optical photos of the deformation of the DDENT specimen.
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300
300
250
250
200
L14
150 100 L10
50 0
Load, P (N)
Load, P (N)
a
L6
0
5
10
15
20
TPEE-A TD-Notch L6 L8 L10 L12 L14
25
200 150 L14
100 L10
50 0
30
L6
0
5
Displacement, δ (mm)
300
Load, P (N)
200
L14
0
5
10
15
20
L14 L10 L6
25
0
30
0
5
10
TPEE-C TD-Notch L6 L8 L10 L12 L14
Load, P (N)
250 200
20
25
30
TPEE-C MD-Notch L6 L8 L10 L12 L14
300 250
Load, P (N)
300
15
Displacement, δ (mm)
Displacement, δ (mm)
c
30
150
50
L6
0
200
100
L10
50
25
TPEE-B MD-Notch L6 L8 L10 L12 L14
250
150 100
20
300
TPEE-B TD-Notch L6 L8 L10 L12 L14
250
15
Displacement, δ (mm)
Load, P (N)
b
10
TPEE-A MD-Notch L6 L8 L10 L12 L14
150 L14
100
200 150 100
L10
50
50
L6
L10
L6
0
0
5
10
15
20
25
Displacement, δ (mm)
30
0
0
5
10
15
20
L14
25
30
35
Displacement, δ (mm)
Fig. 8. The relationship between the load and the displacement of DDENT specimens for TD-notch and MD-notch directions, (a) TPEE-A sample, (b) TPEE-B sample, (c) TPEE-C sample.
EWF tests. A scanning electron microscope (SEM), Hitachi S-3500 N, was used to observe the fracture surface of specimens after EWF tests. 4. Test results and discussion 4.1. Tensile tests The stress–strain relationship of the test samples is shown in Fig. 5. The summary of tensile tests is presented in Table 1. For all samples, the tensile strength and the elongation at break in TD tensile specimens are better
than those in MD tensile specimens. For MD tensile specimens, all samples show similar tensile strength but the ductility varies across the samples. However, for TD tensile specimens, samples can be differentiated by the tensile strength rather than the ductility. In summary, for MD tensile specimens, the tensile properties of TPEE-A and TPEE-B are similar and the ductility of TPEE-C is less than for the other samples by about 40%. For TD tensile specimens, the tensile properties of TPEE-A are better than for other samples but the difference between samples in the mechanical properties is not that significant.
J.-M. Lee et al. / Polymer Testing 28 (2009) 854–865
859
Specific work of fracture, wf (kJ/m2)
Specific work of fracture, wf (kJ/m2)
a
Total work of Fracture
250
TD-Notch T PEE-A T PEE-B
200
T PEE-C
150
100
50 0
2
4
6
8
10
12
14
Total work of Fracture
250
MD-Notch T PEE-A T PEE-B T PEE-C
200
150
100
50
0
16
0
2
4
Ligament length, L (mm)
6
8
10
12
14
16
12
14
16
12
14
16
Ligament length, L (mm)
120
Specific work of fracture, wf,y (kJ/m2)
Specific work of fracture, wf,y (kJ/m2)
b Total work of Yielding TD-Notch 100
TPEE-A TPEE-B TPEE-C
80 60 40 20 0
0
2
4
6
8
10
12
14
16
120
Total work of Yielding MD-Notch
100
T PEE-A T PEE-B T PEE-C
80 60 40 20 0
0
2
Ligament length, L (mm)
4
6
8
10
Ligament length, L (mm)
180
Specific work of fracture, wf,n (kJ/m2)
Specific work of fracture, wf,n (kJ/m2)
c Total workof Necking/Tearing
160
TD-Notch
140
TPEE -A TPEE -B TPEE -C
120 100 80 60 40 20 0
0
2
4
6
8
10
12
14
16
Ligament length, L (mm)
180
Total workof Necking/Tearing
160
MD-Notch
140
TPEE-A TPEE-B TPEE-C
120 100 80 60 40 20 0
0
2
4
6
8
10
Ligament length, L (mm)
Fig. 9. The relationship between the specific work of fracture and the ligament length under EWF tests, (a) Total work of fracture, (b) Total work of yielding, (c) Total work of necking/tearing.
4.2. Trouser-tear tests For trouser-tear tests, the energy for tearing is represented as a function of the force that is applied to the leg (F) and the thickness of the specimen (t), i.e.,
T ¼
2F t
(6)
where the force applied to the leg should be taken from the stabilized force from the load-displacement curves and the energy for tearing is independent of the length of the notch. Fig. 6 shows the tear strengths of all samples, as measured by the trouser-tear tests. In the case of TD-notch specimens, all samples have a straight crack path and it can be easily observed that the tear properties of TPEE-A are clearly much better than those of the TPEE-B and TPEE-C samples.
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Table 2 Calculated EWF parameters under EWF tests.
TPEE-A TPEE-B TPEE-C
Specimen type
Notch type
we (kJ/m2)
MD TD MD TD MD TD
TD MD TD MD TD MD
71.35 69.27 60.70 51.87 51.51 33.97
b wp (MJ/m3) 10.59 6.94 4.88 5.04 6.13 12.56
R-square 0.995 0.988 0.991 0.990 0.989 0.863
However, in the case of MD-notch specimens, only the TPEE-A sample shows a straight crack path; the crack path is severely curved (J-tear) for the TPEE-B and TPEE-C samples. Due to the non-uniform crack paths of the TPEE-B and TPEE-C samples, the quantitative evaluation of the tear properties of MD-notch specimens is rarely achieved. Moreover, a quantitative analysis of the energy for tearing is not possible because of the curved crack paths as well as the unstablized maximum loads.
4.3. EWF tests In Fig. 7, a typical curve of the applied load against displacement under EWF tests and the actual deformation
Specific essential work of fracture, we (kJ/m2)
a
Sample ID
Specimen type
Notch type
we,y (kJ/m2)
by wp,y (MJ/m3) R-square
TPEE-A
MD TD MD TD MD TD
TD MD TD MD TD MD
27.07 13.34 17.09 11.37 17.70 12.86
5.73 5.11 4.70 4.73 5.68 4.02
Sample ID
Specimen type
Notch type
we,n (kJ/m2)
by wp,n (MJ/m3) R-square
TPEE-A
MD TD MD TD MD TD
TD MD TD MD TD MD
44.28 55.93 43.28 40.50 33.81 21.10
4.86 1.83 0.18 0.30 0.45 8.54
TPEE-B TPEE-C
TPEE-B TPEE-C
TD-Notch
70
TPEE-A : Black TPEE-B : Red TPEE-C : Blue
60 50 40 30 20 10 0
we
we,y
we,n
MD-Notch TPEE-A : Black TPEE-B : Red TPEE-C : Blue
70 60 50 40 30 20 10 0
we
we,y
we,n
d 14
14 MD-Notch
TD-Notch TPEE-A : Black TPEE-B : Red TPEE-C : Blue
10 8 6 4 2 0
TPEE-A : Black TPEE-B : Red TPEE-C : Blue
12
Slope, βwp (MJ/m3)
12 3
0.976 0.879 0.496 0.480 0.359 0.761
b
c
Slope, βwp (MJ/m )
0.986 0.992 0.993 0.992 0.983 0.985
of some interesting points from the curve are shown (a TPEE-A sample notched in TD and 10 mm of ligament). As noted from Fig. 7, two phases, i.e., yielding and necking/ tearing, can be identified based on the point of the maximum load (point (e) in Fig. 7). The load-displacement curves for all samples are shown in Fig. 8 and the shape of the load-displacement curves
Specific essential work of fracture, we (kJ/m2)
Sample ID
Table 3 Calculated EWF parameters for yielding and necking processes by partitioning EWF test results.
10 8 6 4 2
βw p
βywp,y
βnwp,n
0
βwp
βywp,y
βnwp,n
Fig. 10. EWF parameters of yielding and necking behaviors by partitioning EWF test results, (a) Essential work of fracture (we) of TD-notch specimens, (b) Essential work of fracture (we) of MD-notch specimens, (c) Slope (bwp) of TD-notch specimens, (d) Slope (bwp) of MD-notch specimens.
J.-M. Lee et al. / Polymer Testing 28 (2009) 854–865
Fig. 11. Comparison of the pattern of fractured DDENT specimens under EWF (Ligament length : 10 mm).
remains the same when the length of the ligament increases. Thus, the application of EWF tests for those samples is feasible. The maximum load and displacement of TD-notch specimens are larger than those of MD-notch specimens for both the TPEE-A and TPEE-B samples for the same ligament length. Therefore, it can be observed that the necking and tearing processes of MD-notch specimens are faster than those of TD-notch specimens. For TPEE-C specimens, the maximum load is larger than that of MD-notch specimens but the maximum displacement of TD-notch specimens is
861
less than that of MD-notch specimens. Also, the shapes of the load-displacement curves of the MD-notch and TDnotch specimens are different. Further, the necking and tearing behaviors of MD-notch specimens are unstable when compared with those of TD-notch specimens. Based on the load-displacement curves in Fig. 8, three kinds of specific fracture energies, i.e., the specific total work of energy (wf), the specific total work of energy for yielding (wf,y) and the specific total work of energy for necking (wf,n), can be calculated. In Fig. 9, the relationship between the three kinds of specific total work of energy and the various ligaments is shown for all samples. As shown in Eq. (3), the specific total work of fracture and the ligaments have a linear relationship. As can be seen from Fig. 9(a), the specific total work of fracture of the TPEE-A sample is much higher than those of the TPEE-B and TPEE-C samples in TD-notch specimens, which coincides with the results from the trouser tear tests. Meanwhile, the specific total work of fracture of the TPEE-C sample is higher than those of the TPEE-A and TPEE-B samples in MD-notch specimens. However, in comparison with the TPEE-A and TPEE-B samples, the measured data of the TPEE-C sample have considerable scatter. In comparison with the TPEE-A and TPEE-C samples, the TPEE-B sample shows a rather balanced tear-behavior of both MDnotch and TD-notch specimens. Hence, it can be seen that the TPEE-B sample is more isotropic under the given injection-molding conditions. In Fig. 9(b), the relationship between the specific total work of energy for yielding and the ligaments is shown. In
Fig. 12. SEM photos for the fracture surface of TPEE-A sample, (a) TD-Notch specimen, (b) MD-Notch specimen.
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Fig. 13. Comparison of SEM photos taken from the fracture surfaces of TD-notch specimens of TPEE-A and TPEE-B samples. (Crack propagated from left to right in picture), (a) TPEE-A sample, (b) TPEE-B sample.
general, TPEE-A shows higher energy consumption during yielding than TPEE-B and TPEE-C but the differences between the samples are not that significant. In Fig. 9(c), the relationship between the specific total work of energy for necking and tearing and the ligaments is shown. Clearly, TPEE-A is superior in comparison with TPEE-B but TPEE-C shows extremely anisotropic behavior during necking and tearing. Moreover, unlike Fig. 9(b), TPEE-C with MD-notch shows a large scatter of the specific total work of energy for necking and tearing. Hence, it can
be seen that large scatter of the specific total work of energy of TPEE-C sample are directly related to the unstable necking and tearing behavior during EWF tests. 4.4. Determination of EWF parameters As depicted in Eqs. (3) and (5), two key parameters, i.e., the intercept between the specific total work of fracture and the ligaments (we: essential work of fracture) and the slope of the linear fit between the specific total work of
J.-M. Lee et al. / Polymer Testing 28 (2009) 854–865
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Fig. 14. Comparison of SEM photos taken from the fracture surfaces of MD-notch specimens of TPEE-A and TPEE-C samples (Crack propagated from left to right in picture), (a) TPEE-A sample, (b) TPEE-C sample.
fracture and the ligaments (bwp), are important for characterizing the fracture properties of test samples. In Tables 2 and 3, the two key parameters that are obtained from Fig. 9 are summarized for the specific total work of fracture, the specific total work of fracture for yielding, and the specific total work of fracture for necking and tearing. As shown in Figs. 10(a) and 10(b), the essential work of fracture of all samples with TD-notch is higher than that with MD-notch. The essential work of fracture can be
determined as the fracture toughness under plane-stress conditions for thin materials; therefore, in general, the fracture toughness of all samples with TD-notch is higher than that with MD-notch. This is reasonable because the direction of the resin flow is generally aligned with the direction of MD notch during the injection-molding process. By separating the essential work of fracture into two components, i.e., yielding and necking/tearing, the contribution of necking and tearing on the essential work
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of fracture is revealed to be larger than that of yielding with both MD-notch and TD-notch specimens. Thus, it can be observed that the essential work of fracture is more likely controlled by the load-displacement behavior after the yield point. However, it can be also observed that the contribution of yielding on the essential work of fracture of TD-notch specimens is relatively larger than that of MDnotch specimens. The TPEE-A sample has the highest essential work of fracture for both TD-notch and MD-notch specimens. The variation of the essential work of fracture of TPEE-A and TPEE-B samples with TD-notch and MD-notch is relatively smaller than that of the TPEE-C sample. Therefore, it can be observed that the TPEE-C sample has the worst and most unbalanced fracture toughness. In Figs. 10(c) and 10(d), the slopes of the loaddisplacement curves are compared for all samples; two components of the slopes, i.e., yielding and necking/ tearing, are also shown. The slopes of the load-displacement curves are related mainly to the resistance to the propagation of cracks. TPEE-A sample show the largest slope for TD-notch specimens and TPEE-C show the largest slope for MD-notch specimens. In general, the impact of yielding on the slope is relatively large except for the TPEEC sample, unlike the case of the essential work of fracture. Especially, the contribution of necking and tearing to the slope of TPEE-B is very little and the variation of the slope of TPEE-B with both TD-notch and MD-notch is very small, i.e., TPEE-B is relatively isotropic. 4.5. Fractographic analysis of failed samples from EWF tests In Fig. 11, the features of fracture after EWF tests of the TPEE-A, TPEE-B, and TPEE-C samples with 10 mm ligaments are shown. A clear variation in the fracture patterns according to the samples and flow directions can be observed. In the case of all TD-notch specimens, the crack propagates across the resin flow; hence, the localized thinning around the crack tip is a major mechanism of crack propagation. However, in the case of the MD-notch specimens of the TPEE-A and TPEE-B samples, the direction of resin flow and the crack path are the same; hence, the crack can move to its neighbor relatively easily during the crack propagation if there is a weak point close to the projected crack path (see the arrows in Fig. 11). The MDnotch specimens of the TPEE-C sample show a triangular shape of failure (see the circle in Fig. 11), which is similar to the case of ductile thermoplastic films. Many discontinuous traces along the triangle support the existence of unstable crack propagation (multiple changes of the crack path during propagation). This may be related to the microstructure of the TPEE-C sample, which is also a root cause of the large scatter in EWF data. In Fig. 12, the top views of the fracture surface of TDnotch and MD-notch specimens of the TPEE-A sample are shown. In the case of TD-notch specimens, two stages of failure can be identified, i.e., crack propagation and final rupture. Many wavy ripples can be observed on the fracture surface at the stage of crack propagation. A sudden rupture occurs when the ligament of the specimen is shorter than a critical length representing the instability of the specimen. For all samples, wavy ripples are observed on the fracture
surface at the stage of crack propagation though their number and size vary, which may be related to the geometrical mismatch that arises from the large amount of plastic deformation during crack propagation. Mouzakis et al. [15] and Marissen et al. [19] reported the formation of wavy ripples following tear tests. In the case of MD-notch specimens, the surface of the crack is relatively smooth for a couple of crack surfaces and the wavy ripples are not very visible at this scale. That is because the crack path is along the resin flow for MD-notch specimens. However, for the TPEE-C sample, multiple steps are observed in the crack surfaces, as described above. Marisesen et al. [19] observed the formation of fine ripples at the fracture surface that are roughly parallel to the specimen surface and parallel to the crack front. They noted that the development of steps in the direction of crack propagation is caused by the initiation of individual cracks at several places. In Fig. 13 and Fig. 14, the magnified photos of the fracture surfaces for TD-notch and MD-notch specimens are shown respectively. In the case of TD-notch specimens, the TPEE-A sample (Fig. 13(a)) shows larger-scale wavy ripples in comparison with the TPEE-B sample (Fig. 13(b)). Interestingly, the fracture surface of the TPEE-B sample is less elongated; this should be related to the low-tear property of TPEE-B in comparison with TPEE-A. In the case of MDnotch specimens, the size of the wavy ripples of the TPEE-A sample (Fig. 14(a)) is much smaller than that for TD-notch specimens (Fig. 13(a)). This characteristic is related with the resistance to crack propagation, i.e., the slope, because the essential work of fracture of the TPEE-A sample is almost the same for both MD-notch and TD-notch specimens. The TPEE-C sample (Fig. 14(a)) shows a larger size of wavy ripples in comparison with the TPEE-A sample (Fig. 14(b)), which is a good reason for the large slope of the TPEE-C sample with MD-notch.
5. Conclusions In the present study, the tear properties of commercial thermoplastic polyester elastomers (TPEEs) that are designed for constant velocity joints (CVS) were investigated through the essential work of fracture (EWF) method of testing as well as conventional trouser-tear tests. Two directions for specimens, i.e., the machine direction (MD) and the transverse direction (TD), which are based on the resin flow during the injection-molding process, were selected for this study. Also, scanning electron microscope (SEM) was applied to observe the fracture surface after EWF tests. The results obtained from this study are as follows:
(1) In the case of samples with severely curved J-tears, conventional trouser-tear tests are not appropriate for the quantitative analysis of tear properties, and the application of EWF tests is effective for evaluating the tear properties of TPEE materials. (2) The essential work of fracture of all samples with TDnotch is higher than that with MD-notch. Further, by the separation of the essential work of fracture into two
J.-M. Lee et al. / Polymer Testing 28 (2009) 854–865
components, i.e., yielding and necking/tearing, the contribution of necking and tearing to the essential work of fracture is found to be larger than that of yielding for both MD-notch and TD-notch specimens. (3) The root causes of the variability of the essential work of fracture and the slope of the load-displacement can be identified by the partitioning method. The large scatter of the measured data for MD-notch specimens of the TPEE-C sample is related mainly to the abnormally large portion of the slope of necking and tearing. (4) In the case of TD-notch specimens, two stages can be identified in failure, i.e., crack propagation and final rupture. Many wavy ripples can be observed on the fracture surface at the stage of crack propagation. A sudden rupture occurs when the ligament of the specimen is shorter than a critical length. In the case of MDnotch specimens, the surface of the crack is relatively smooth for a couple of crack surfaces and the wavy ripples are not very visible at this scale. It is found that the size and shape of wavy patterns can be related to the tear properties of TPEE samples. References [1] P.E. Bretz, R.W. Hertzberg, J.A. Manson, Correlation between crack growth rate and fracture mode transitions in low density polyethylene, Polymer 22 (1981) 575. [2] H. Aglan, A. Chudnovsky, A. Moet, T. Fleischman, Crack layer analysis of fatigue crack propagation in rubber compounds, International Journal of Fracture 44 (3) (1990) 167. [3] K.B. Broberg, On stable crack growth, Journal of Mechanics and Physics of Solids 23 (1975) 215. [4] B. Cotterell, J.K. Reddel, The essential work of plane stress ductile fracture, International Journal of Fracture 13 (3) (1977) 267.
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[5] Y.-M. Mai, B. Cotterell, Effect of specimen geometry on the essential work of plane stress ductile fracture, Engineering Fracture Mechanics 21 (1) (1985) 123. [6] Y.-M. Mai, B. Cotterell, On the essential work of ductile fracture in polymers, International Journal of Fracture 32 (2) (1986) 105. [7] European Structural Integrity Society (ESIS) TC-4, Essential Work of Fracture Test Protocol Ver. 6 (2000). [8] A.C. Chang, T. Inge, L. Tau, A. Hiltner, E. Baer, Tear strength of ductile polyolefin films, Polymer Engineering and Science 42 (11) (2002) 2202. [9] D.F. Isherwood, J.G. Williams, Some observations on the tearing of ductile materials, Engineering Fracture Mechanics 10 (4) (1978) 887. [10] D. Ferrer-Balas, M.Ll Maspoch, A.B. Martinez, E. Ching, R.K.Y. Li, Y.-M. Mai, Fracture behaviour of polypropylene films at different temperatures: assessment of the EWF parameters, Polymer 42 (6) (2001) 2665. [11] M.Ll. Maspoch, J. Gamez-Perez, A. Gordillo, M. Sanchez-Soto, J.I. Velasco, Characterisation of injected EPBC plaques using the essential work of fracture (EWF) method, Polymer 43 (15) (2002) 4177. [12] R.S. Yamakawa, C.A. Razzino, C.A. Correa, E. Hage Jr., Influence of notching and molding conditions on determination of EWF parameters in polyamide 6, Polymer Testing 23 (2) (2004) 195. [13] G. Gong, B.-H. Xie, W. Yang, Z.-M. Li, W.-Q. Zhang, M.-B. Yang, Essential work of fracture (EWF) analysis for polypropylene grafted with maleic anhydride modified polypropylene/calcium carbonate composites, Polymer Testing 24 (4) (2005) 410. [14] K. Saminathan, P. Selvakumar, N. Bhatnagar, Fracture studies of polypropylene/nanoclay composite. Part I: effect of loading rates on essential work of fracture, Polymer Testing 27 (3) (2008) 296. [15] D.E. Mouzakis, M. Gahleitner, J. Karger-Kocsis, Toughness assessment of elastomeric polypropylene (ELPP) by the essential work of the fracture method, Journal of Applied Polymer Science 70 (5) (1998) 873. [16] M.Ll. Maspoch, O.O. Santana, J. Grando, D. Ferrer, A.B. Martinez, The essential work of fracture of a thermoplastic elastomer, Polymer Bulletin 39 (2) (1997) 249. [17] D. Ferrer-Balas, M.Ll. Maspoch, A.B. Martinez, O.O. Santana, On the essential work of fracture method: energy partitioning of the fracture process in iPP films, Polymer Bulletin 42 (1) (1999) 101. [18] M.Ll. Maspoch, J. Gamez-Perez, J. Karger-Kocsis, Effects of thickness, deformation rate and energy partitioning on the work of fracture parameters of uPVC films, Polymer Bulletin 50 (4) (2003) 279. [19] R. Marissen, R.F.M. Lange, S. Bibels, P. Hinkel, H. Nowack, Scanning electron microscope visualisation of crack initiation and propagation under static and fatigue loading on thermoplastic elastomers, International Journal of Fatigue 27 (1) (2005) 71.
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Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Test Method
Determination of the tear properties of thermoplastic polyester elastomers (TPEEs) using essential work of fracture (EWF) test method Jeong-Moo Lee a, Byoung-Ho Choi b, *, Jong-Sin Moon a, Eon-Seok Lee a a b
Tech Center, LG Chem Ltd., 84 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea School of Mechanical Engineering, Korea University, 1 5-ga, Anam-dong, Sungbuk-gu, Seoul 136-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 May 2009 Accepted 25 July 2009
In this paper, three commercial thin thermoplastic polyester elastomer (TPEE) samples are prepared by the injection molding process. Essential work of fracture (EWF) tests, which have been extended to cover the ductile tearing of polymeric materials that neck before fracture, in both the machine direction (MD) and the transverse direction (TD) are executed based on the ESIS TC-4 protocol as well as conventional trouser tear tests. Through an examination of the EWF test results, the tear properties of TPEE samples can be determined quantitatively. Partitioning of the test results is attempted for identifying the contribution of yielding and necking/tearing on the overall tear properties of TPEE samples. In addition, the fracture surfaces of TPEE samples following EWF tests are investigated through scanning electron microscopy (SEM). The root causes of the differences in tear properties are studied using the morphology of the fracture surfaces of TPEE samples. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Thermoplastic polyester elastomer Essential work of fracture Tear Injection molding Orientation
1. Introduction The market for thermoplastic elastomers (TPEs) is expanding because TPEs have good moldability, like thermoplastics, as well as elastic properties, like rubbers. Especially among TPEs, thermoplastic polyester elastomers (TPEEs) have been used for many automotive applications, such as constant velocity joint (CVJ) boots, door latches, etc., because TPEEs have good mechanical properties. During the running of automobiles, various complications, such as a rise in temperature, chemical attack, abrasion, fatigue, etc., can affect CVJ boots. Resistance to cracking is one of the important properties of CVJ boots because they should be replaced once their material cracks so that the CVJ is not functioning properly. For measuring the fracture toughness of thin TPEE ductile films, two practical teartests are popularly used in industry, viz., trouser tear (ISO
* Corresponding author. E-mail address: [email protected] (B.-H. Choi). 0142-9418/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2009.07.008
6383-1, ASTM D624) and Elmendorf tear (ISO 6383-2, ASTM D1922). These practical tear-tests can be used to rank samples but it is not possible to quantify the fracture toughness by applying these tests to TPEE samples. When a regular method of testing the fracture toughness, such as the critical stress intensity factor (KIc) test, is applied to materials with high toughness and low yield strength it is very difficult to analyze the test results due to the violation of conventional linear elastic fracture mechanics (LEFM). In this case, other methods based on elasto-plastic fracture mechanics, such as crack-openingdisplacement (COD), the d-5 method, the J-integral, etc., have been proposed to obtain the fracture toughness of such materials. There have been a number of discussions [1,2] that the application of conventional fracture mechanics to rubbers is not appropriate due to the large deformation around the crack tip. Moreover, some polymeric materials such as TPEEs that are fabricated as thin films and sheets are not suitable for conventional fracture toughness tests that are based on fracture mechanics under plane-strain conditions describe above.
J.-M. Lee et al. / Polymer Testing 28 (2009) 854–865
a
855
P
Gate Process zone (We)
MD-Specimen
Plastic zone (Wp)
Melt flow direction
100 mm
TD-Specimen
Specimen for EWF test
Clamped zone
150 mm Ligament Fig. 2. Preparation of specimens in machine direction (MD) and transverse direction (TD) from the injection molded plaque (2 mm in thickness).
(L)
Clamped zone
based on EWF tests. The effect of the resin flow on the fracture toughness of TPEE materials was evaluated. The suitability of EWF tests for the evaluation of the fracture toughness of TPEE materials was studied. Finally, a methodology for ranking those materials based on the evaluated fracture toughness is proposed. The morphologies of the fractured specimens following EWF tests were analyzed.
P
plane stress/ plane strain transition plane stress
2. The concept of EWF tests
Fig. 1. Description of the concept of essential work of fracture (EWF) method, (a) Schematics of the deep double-edge notched tension (DDENT) specimen, (b) Determination of EWF parameters(we and bwp).
The essential work of fracture (EWF) concept was proposed by Broberg [3]. Some researchers (e.g., [4–6]) expanded this concept to various cases. Recently, the European Structural Integrity Society (ESIS) published a test protocol for EWF tests of thin polymeric films [7] to initiate the standardization of EWF tests. Due to a large amount of plastic deformation around the crack tip of polymeric thin films as soon as the tear load is applied, the tear process can be assumed as the propagation of a mode-I crack [8,9]. Therefore, the fracture toughness of polymeric thin films under tear loads can be estimated by EWF tests. There have been reports on the application of EWF tests for evaluating the fracture toughness of various polymeric thin films [10–14] but only limited studies (e.g., [15,16]) are available for the evaluation of the fracture toughness of thin TPE or elastomeric materials. In this study, three thin, commercial-grade TPEE materials for automotive uses, such as CVJ boots, are selected. The fracture toughness of those samples was evaluated
W f ¼ We þ W p
a Clamped Clamped zone zone
(1)
b
Clamped zone
Notch L
100 mm
Ligament length, L
60 mm
W 2
100 mm
3t
20 mm
Valid ligament length
50 mm
β wp
The concept of EWF test was well-described by Cottrell and Reddel [4]. Two types of specimen, i.e., single-edge notched tension (SENT) specimen and deep double-edge notched tension (DDENT) specimen, are commonly used for EWF tests. Fig. 1 presents the schematics of the concept of EWF tests for DDENT specimens. The total work of fracture (Wf) can be separated into two components, i.e., the essential work of fracture (We) and the non-essential work of fracture (Wp).
Initial cut (Notch)
w f = w e + βw p L
we
Specific total fracture work, wf
b
Clamped zone
30 mm
30 mm t= 2 mm
Fig. 3. Geometry of tested specimens used in trouser tear and EWF tests, (a) Trouser tear specimen, (b) Deep double-edge notched tension (DDENT) specimen for EWF tests.
J.-M. Lee et al. / Polymer Testing 28 (2009) 854–865
a
Trouser tear specimen
a Notch
P
Notch
P MD-Notch
P
b
TD-Notch Melt flow direction
DDENT specimen
Engineering stress, σ (MPa)
856
50
30
20
TD-Notch
MD-Notch
Fig. 4. Definition of the direction of specimens, i.e., MD and TD, for trouser tear and EWF tests based on the notching direction with respect to the resin melt flow direction (The lines denote oriented fibrous crystals).
The essential work of fracture is directly related to the energy for creating the surfaces of new cracks in the process zone. The non-essential work of fracture is related to the formation of the plastic zone around the process zone. Considering the size of the process zone and the volume of the plastic zone, Eq. (1) can be rewritten as below based on the area under the load-displacement curve:
Wf ¼ we Lt þ bwp L2 t
(2)
In Eq. (2), L is the ligament length of the specimen, t is the thickness of the specimen, and b is the shape factor that is related to the formation of the plastic zone. Also, we is the specific essential work of fracture and wp is the specific non-essential work of fracture. Eq. (2) can be rewritten by using the specific total work of fracture (wf), as below:
Wf ¼ we þ bwp L Lt
10
0
100
200
300
400
500
600
700
800
900
50 P
A TD
40
C B 30
20
P TD Specimen TPEE- A TPEE- B TPEE- C
10
0
0
100
200
300
400
500
600
700
800
900
Engineering strain, ε (%) Fig. 5. Engineering stress-strain curves of samples under tensile tests (Test speed: 50 mm/min), (a) MD tensile specimens, (b) TD tensile specimens.
Those two components can be rewritten by mimicking the representation of Eq. (3) as below:
wf ;y ¼ we;y þ by wp$y L; wf ;n ¼ we;n þ bn wp;n L
(5)
where the subscripts y and n represent yielding and necking, respectively.
(3) 3. Specimen preparation and experiments
As shown in Eq. (3), the specific total work of fracture and the ligament length are linearly related. Here, we is defined as the intercept point in the y-axis of the wf L curve and bwp is defined as the slope of the wf L curve (Fig. 1(b)). Hence, the fracture toughness of the sample under planestress conditions can be measured by finding the intercept point, and the slope of the curve can be used to determine the resistance to crack propagation. Some scholars (e.g., [17,18]) reported that the work of fracture obtained from EWF tests of polymeric thin films can be separated again to reveal two distinct behaviors, i.e., yielding and necking. Therefore, the specific total work of fracture can be divided into two components:
wf ¼ wf ;y þ wf ;n
A B
C
P
Engineering strain, ε (%)
Engineering stress, σ (MPa)
P
TPEE- A TPEE- B TPEE- C
MD
Notch
b
wf ¼
MD Specimen
40
0 Notch
P
(4)
where wf,y is the specific total work of energy for yielding and wf,n is the specific total work of energy for necking.
3.1. Test materials Three commercial-grade thermoplastic polyester elastomers (TPEEs) that are block copolymers composed of polyether (soft segments) and polyester (hard segments) were selected; these are TPEE-A, TPEE-B, and TPEE-C. These types of TPEE are commonly used for fabricating constant velocity joint (CVJ) boots and have a hardness around 39w42 on the Shore D scales. 3.2. Test specimens The test specimens were prepared in the machine (MD) and transverse (TD) directions of 2 mm-thick injectionmolded plates (made through a 150-ton injection-molding machine from Engel), as shown in Fig. 2. Standard dumbbell
J.-M. Lee et al. / Polymer Testing 28 (2009) 854–865
857
Table 1 Mechanical properties of the materials obtained with the tensile tests at 50 mm/min. Properties
Unit
@ 3 ¼ 5% @ 3 ¼ 10% @ 3 ¼ 50%
Tensile stress
TPEE-A
MPa MPa MPa MPa %
Tensile strength Elongation to break
MD
TD
MD
TD
3.4 5.1 8.3 22.0 524
3.8 5.5 8.3 42.1 850
3.7 5.5 8.8 20.3 578
3.8 5.6 8.5 33.8 836
4.2 5.9 10.1 18.9 349
4.5 5.9 8.5 36.4 812
3.3. Test conditions Tensile tests were executed following ASTM D638 (50 mm/min and 23 C). The tear strengths of the samples
were tested based on ASTM D624 (50 mm/min and 23 C). EWF tests were performed as given in the test protocol of the European Structural Integrity Society (ESIS) (50 mm/min and 23 C) [5]. All mechanical tests were carried out on an Instron 3365 universal materials testing machine (5 kN).
3.4. Observation of specimens The tested samples were collected for microscopic observation. Optical microscopy was used to observe the variation of the deformation up to final failure during the
a
250 (d) (e)
300 TPEE-A, B , C : Straight crack path
Load, P (N)
200
Type T specimen TPEE-A, TD-Notch
(g) (h)
(b) 150
Necking + Tearing
Yielding
100
150
(f)
(c)
200
250
Load, P (N)
TPEE-C
TD
shaped specimens with 25 mm gauge length and 4 mm width were used for the tensile tests. For fracture tests, two types of specimens were used (Fig. 3): (a) trouser-tear (Type T) specimens for out-of-plane tear tests and (b) deep double-edge notched tension (DDENT) specimens for EWF tests. Type T specimens have a long notch (50 mm) and DDENT specimens have various notch lengths, viz., 6, 8, 10, 12 and 14 mm, for evaluating EWF properties. Notches were introduced by a razor blade and the direction of each notch was defined according to the direction of resin flow (Fig. 4).
a
TPEE-B
MD
(a) TPEE-A TD-Notch L10, S-2 v=50 mm/min
50
100 TD-Notch TPEE-A TPEE-B TPEE-C
50 0
0
50
100
150
200
250
300
0
5
10
15
20
350
b
Clamped zone
P
300 MD-Notch
TPEE-A : Straight crack path
TPEE-A TPEE-B TPEE-C
TPEE-B, C : Curved crack path
250
Load, P (N)
0
Displacement, δ (mm)
Displacement, δ (mm)
b
(i)
200
Notch
Type T specimen
P
TPEE-B, MD-Notch
(a)
(b)
(c )
(d)
(f )
(g)
(h)
(i)
30 mm
150 100 TPEE-C, MD-Notch
50 0
(e) 0
50
100
150
200
250
300
350
Displacement, δ (mm) Fig. 6. Tear test results obtained from the trousers tear specimens, (a) TDnotch specimens, (b) MD-notch specimens.
Fig. 7. Example of the aspects of deformation with increasing loads under EWF tests. (TPEE-A, TD-Notch, Ligament length ¼ 10 mm), (a) The relationship between the load and the displacement, (b) Optical photos of the deformation of the DDENT specimen.
858
J.-M. Lee et al. / Polymer Testing 28 (2009) 854–865
300
300
250
250
200
L14
150 100 L10
50 0
Load, P (N)
Load, P (N)
a
L6
0
5
10
15
20
TPEE-A TD-Notch L6 L8 L10 L12 L14
25
200 150 L14
100 L10
50 0
30
L6
0
5
Displacement, δ (mm)
300
Load, P (N)
200
L14
0
5
10
15
20
L14 L10 L6
25
0
30
0
5
10
TPEE-C TD-Notch L6 L8 L10 L12 L14
Load, P (N)
250 200
20
25
30
TPEE-C MD-Notch L6 L8 L10 L12 L14
300 250
Load, P (N)
300
15
Displacement, δ (mm)
Displacement, δ (mm)
c
30
150
50
L6
0
200
100
L10
50
25
TPEE-B MD-Notch L6 L8 L10 L12 L14
250
150 100
20
300
TPEE-B TD-Notch L6 L8 L10 L12 L14
250
15
Displacement, δ (mm)
Load, P (N)
b
10
TPEE-A MD-Notch L6 L8 L10 L12 L14
150 L14
100
200 150 100
L10
50
50
L6
L10
L6
0
0
5
10
15
20
25
Displacement, δ (mm)
30
0
0
5
10
15
20
L14
25
30
35
Displacement, δ (mm)
Fig. 8. The relationship between the load and the displacement of DDENT specimens for TD-notch and MD-notch directions, (a) TPEE-A sample, (b) TPEE-B sample, (c) TPEE-C sample.
EWF tests. A scanning electron microscope (SEM), Hitachi S-3500 N, was used to observe the fracture surface of specimens after EWF tests. 4. Test results and discussion 4.1. Tensile tests The stress–strain relationship of the test samples is shown in Fig. 5. The summary of tensile tests is presented in Table 1. For all samples, the tensile strength and the elongation at break in TD tensile specimens are better
than those in MD tensile specimens. For MD tensile specimens, all samples show similar tensile strength but the ductility varies across the samples. However, for TD tensile specimens, samples can be differentiated by the tensile strength rather than the ductility. In summary, for MD tensile specimens, the tensile properties of TPEE-A and TPEE-B are similar and the ductility of TPEE-C is less than for the other samples by about 40%. For TD tensile specimens, the tensile properties of TPEE-A are better than for other samples but the difference between samples in the mechanical properties is not that significant.
J.-M. Lee et al. / Polymer Testing 28 (2009) 854–865
859
Specific work of fracture, wf (kJ/m2)
Specific work of fracture, wf (kJ/m2)
a
Total work of Fracture
250
TD-Notch T PEE-A T PEE-B
200
T PEE-C
150
100
50 0
2
4
6
8
10
12
14
Total work of Fracture
250
MD-Notch T PEE-A T PEE-B T PEE-C
200
150
100
50
0
16
0
2
4
Ligament length, L (mm)
6
8
10
12
14
16
12
14
16
12
14
16
Ligament length, L (mm)
120
Specific work of fracture, wf,y (kJ/m2)
Specific work of fracture, wf,y (kJ/m2)
b Total work of Yielding TD-Notch 100
TPEE-A TPEE-B TPEE-C
80 60 40 20 0
0
2
4
6
8
10
12
14
16
120
Total work of Yielding MD-Notch
100
T PEE-A T PEE-B T PEE-C
80 60 40 20 0
0
2
Ligament length, L (mm)
4
6
8
10
Ligament length, L (mm)
180
Specific work of fracture, wf,n (kJ/m2)
Specific work of fracture, wf,n (kJ/m2)
c Total workof Necking/Tearing
160
TD-Notch
140
TPEE -A TPEE -B TPEE -C
120 100 80 60 40 20 0
0
2
4
6
8
10
12
14
16
Ligament length, L (mm)
180
Total workof Necking/Tearing
160
MD-Notch
140
TPEE-A TPEE-B TPEE-C
120 100 80 60 40 20 0
0
2
4
6
8
10
Ligament length, L (mm)
Fig. 9. The relationship between the specific work of fracture and the ligament length under EWF tests, (a) Total work of fracture, (b) Total work of yielding, (c) Total work of necking/tearing.
4.2. Trouser-tear tests For trouser-tear tests, the energy for tearing is represented as a function of the force that is applied to the leg (F) and the thickness of the specimen (t), i.e.,
T ¼
2F t
(6)
where the force applied to the leg should be taken from the stabilized force from the load-displacement curves and the energy for tearing is independent of the length of the notch. Fig. 6 shows the tear strengths of all samples, as measured by the trouser-tear tests. In the case of TD-notch specimens, all samples have a straight crack path and it can be easily observed that the tear properties of TPEE-A are clearly much better than those of the TPEE-B and TPEE-C samples.
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Table 2 Calculated EWF parameters under EWF tests.
TPEE-A TPEE-B TPEE-C
Specimen type
Notch type
we (kJ/m2)
MD TD MD TD MD TD
TD MD TD MD TD MD
71.35 69.27 60.70 51.87 51.51 33.97
b wp (MJ/m3) 10.59 6.94 4.88 5.04 6.13 12.56
R-square 0.995 0.988 0.991 0.990 0.989 0.863
However, in the case of MD-notch specimens, only the TPEE-A sample shows a straight crack path; the crack path is severely curved (J-tear) for the TPEE-B and TPEE-C samples. Due to the non-uniform crack paths of the TPEE-B and TPEE-C samples, the quantitative evaluation of the tear properties of MD-notch specimens is rarely achieved. Moreover, a quantitative analysis of the energy for tearing is not possible because of the curved crack paths as well as the unstablized maximum loads.
4.3. EWF tests In Fig. 7, a typical curve of the applied load against displacement under EWF tests and the actual deformation
Specific essential work of fracture, we (kJ/m2)
a
Sample ID
Specimen type
Notch type
we,y (kJ/m2)
by wp,y (MJ/m3) R-square
TPEE-A
MD TD MD TD MD TD
TD MD TD MD TD MD
27.07 13.34 17.09 11.37 17.70 12.86
5.73 5.11 4.70 4.73 5.68 4.02
Sample ID
Specimen type
Notch type
we,n (kJ/m2)
by wp,n (MJ/m3) R-square
TPEE-A
MD TD MD TD MD TD
TD MD TD MD TD MD
44.28 55.93 43.28 40.50 33.81 21.10
4.86 1.83 0.18 0.30 0.45 8.54
TPEE-B TPEE-C
TPEE-B TPEE-C
TD-Notch
70
TPEE-A : Black TPEE-B : Red TPEE-C : Blue
60 50 40 30 20 10 0
we
we,y
we,n
MD-Notch TPEE-A : Black TPEE-B : Red TPEE-C : Blue
70 60 50 40 30 20 10 0
we
we,y
we,n
d 14
14 MD-Notch
TD-Notch TPEE-A : Black TPEE-B : Red TPEE-C : Blue
10 8 6 4 2 0
TPEE-A : Black TPEE-B : Red TPEE-C : Blue
12
Slope, βwp (MJ/m3)
12 3
0.976 0.879 0.496 0.480 0.359 0.761
b
c
Slope, βwp (MJ/m )
0.986 0.992 0.993 0.992 0.983 0.985
of some interesting points from the curve are shown (a TPEE-A sample notched in TD and 10 mm of ligament). As noted from Fig. 7, two phases, i.e., yielding and necking/ tearing, can be identified based on the point of the maximum load (point (e) in Fig. 7). The load-displacement curves for all samples are shown in Fig. 8 and the shape of the load-displacement curves
Specific essential work of fracture, we (kJ/m2)
Sample ID
Table 3 Calculated EWF parameters for yielding and necking processes by partitioning EWF test results.
10 8 6 4 2
βw p
βywp,y
βnwp,n
0
βwp
βywp,y
βnwp,n
Fig. 10. EWF parameters of yielding and necking behaviors by partitioning EWF test results, (a) Essential work of fracture (we) of TD-notch specimens, (b) Essential work of fracture (we) of MD-notch specimens, (c) Slope (bwp) of TD-notch specimens, (d) Slope (bwp) of MD-notch specimens.
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Fig. 11. Comparison of the pattern of fractured DDENT specimens under EWF (Ligament length : 10 mm).
remains the same when the length of the ligament increases. Thus, the application of EWF tests for those samples is feasible. The maximum load and displacement of TD-notch specimens are larger than those of MD-notch specimens for both the TPEE-A and TPEE-B samples for the same ligament length. Therefore, it can be observed that the necking and tearing processes of MD-notch specimens are faster than those of TD-notch specimens. For TPEE-C specimens, the maximum load is larger than that of MD-notch specimens but the maximum displacement of TD-notch specimens is
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less than that of MD-notch specimens. Also, the shapes of the load-displacement curves of the MD-notch and TDnotch specimens are different. Further, the necking and tearing behaviors of MD-notch specimens are unstable when compared with those of TD-notch specimens. Based on the load-displacement curves in Fig. 8, three kinds of specific fracture energies, i.e., the specific total work of energy (wf), the specific total work of energy for yielding (wf,y) and the specific total work of energy for necking (wf,n), can be calculated. In Fig. 9, the relationship between the three kinds of specific total work of energy and the various ligaments is shown for all samples. As shown in Eq. (3), the specific total work of fracture and the ligaments have a linear relationship. As can be seen from Fig. 9(a), the specific total work of fracture of the TPEE-A sample is much higher than those of the TPEE-B and TPEE-C samples in TD-notch specimens, which coincides with the results from the trouser tear tests. Meanwhile, the specific total work of fracture of the TPEE-C sample is higher than those of the TPEE-A and TPEE-B samples in MD-notch specimens. However, in comparison with the TPEE-A and TPEE-B samples, the measured data of the TPEE-C sample have considerable scatter. In comparison with the TPEE-A and TPEE-C samples, the TPEE-B sample shows a rather balanced tear-behavior of both MDnotch and TD-notch specimens. Hence, it can be seen that the TPEE-B sample is more isotropic under the given injection-molding conditions. In Fig. 9(b), the relationship between the specific total work of energy for yielding and the ligaments is shown. In
Fig. 12. SEM photos for the fracture surface of TPEE-A sample, (a) TD-Notch specimen, (b) MD-Notch specimen.
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Fig. 13. Comparison of SEM photos taken from the fracture surfaces of TD-notch specimens of TPEE-A and TPEE-B samples. (Crack propagated from left to right in picture), (a) TPEE-A sample, (b) TPEE-B sample.
general, TPEE-A shows higher energy consumption during yielding than TPEE-B and TPEE-C but the differences between the samples are not that significant. In Fig. 9(c), the relationship between the specific total work of energy for necking and tearing and the ligaments is shown. Clearly, TPEE-A is superior in comparison with TPEE-B but TPEE-C shows extremely anisotropic behavior during necking and tearing. Moreover, unlike Fig. 9(b), TPEE-C with MD-notch shows a large scatter of the specific total work of energy for necking and tearing. Hence, it can
be seen that large scatter of the specific total work of energy of TPEE-C sample are directly related to the unstable necking and tearing behavior during EWF tests. 4.4. Determination of EWF parameters As depicted in Eqs. (3) and (5), two key parameters, i.e., the intercept between the specific total work of fracture and the ligaments (we: essential work of fracture) and the slope of the linear fit between the specific total work of
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Fig. 14. Comparison of SEM photos taken from the fracture surfaces of MD-notch specimens of TPEE-A and TPEE-C samples (Crack propagated from left to right in picture), (a) TPEE-A sample, (b) TPEE-C sample.
fracture and the ligaments (bwp), are important for characterizing the fracture properties of test samples. In Tables 2 and 3, the two key parameters that are obtained from Fig. 9 are summarized for the specific total work of fracture, the specific total work of fracture for yielding, and the specific total work of fracture for necking and tearing. As shown in Figs. 10(a) and 10(b), the essential work of fracture of all samples with TD-notch is higher than that with MD-notch. The essential work of fracture can be
determined as the fracture toughness under plane-stress conditions for thin materials; therefore, in general, the fracture toughness of all samples with TD-notch is higher than that with MD-notch. This is reasonable because the direction of the resin flow is generally aligned with the direction of MD notch during the injection-molding process. By separating the essential work of fracture into two components, i.e., yielding and necking/tearing, the contribution of necking and tearing on the essential work
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of fracture is revealed to be larger than that of yielding with both MD-notch and TD-notch specimens. Thus, it can be observed that the essential work of fracture is more likely controlled by the load-displacement behavior after the yield point. However, it can be also observed that the contribution of yielding on the essential work of fracture of TD-notch specimens is relatively larger than that of MDnotch specimens. The TPEE-A sample has the highest essential work of fracture for both TD-notch and MD-notch specimens. The variation of the essential work of fracture of TPEE-A and TPEE-B samples with TD-notch and MD-notch is relatively smaller than that of the TPEE-C sample. Therefore, it can be observed that the TPEE-C sample has the worst and most unbalanced fracture toughness. In Figs. 10(c) and 10(d), the slopes of the loaddisplacement curves are compared for all samples; two components of the slopes, i.e., yielding and necking/ tearing, are also shown. The slopes of the load-displacement curves are related mainly to the resistance to the propagation of cracks. TPEE-A sample show the largest slope for TD-notch specimens and TPEE-C show the largest slope for MD-notch specimens. In general, the impact of yielding on the slope is relatively large except for the TPEEC sample, unlike the case of the essential work of fracture. Especially, the contribution of necking and tearing to the slope of TPEE-B is very little and the variation of the slope of TPEE-B with both TD-notch and MD-notch is very small, i.e., TPEE-B is relatively isotropic. 4.5. Fractographic analysis of failed samples from EWF tests In Fig. 11, the features of fracture after EWF tests of the TPEE-A, TPEE-B, and TPEE-C samples with 10 mm ligaments are shown. A clear variation in the fracture patterns according to the samples and flow directions can be observed. In the case of all TD-notch specimens, the crack propagates across the resin flow; hence, the localized thinning around the crack tip is a major mechanism of crack propagation. However, in the case of the MD-notch specimens of the TPEE-A and TPEE-B samples, the direction of resin flow and the crack path are the same; hence, the crack can move to its neighbor relatively easily during the crack propagation if there is a weak point close to the projected crack path (see the arrows in Fig. 11). The MDnotch specimens of the TPEE-C sample show a triangular shape of failure (see the circle in Fig. 11), which is similar to the case of ductile thermoplastic films. Many discontinuous traces along the triangle support the existence of unstable crack propagation (multiple changes of the crack path during propagation). This may be related to the microstructure of the TPEE-C sample, which is also a root cause of the large scatter in EWF data. In Fig. 12, the top views of the fracture surface of TDnotch and MD-notch specimens of the TPEE-A sample are shown. In the case of TD-notch specimens, two stages of failure can be identified, i.e., crack propagation and final rupture. Many wavy ripples can be observed on the fracture surface at the stage of crack propagation. A sudden rupture occurs when the ligament of the specimen is shorter than a critical length representing the instability of the specimen. For all samples, wavy ripples are observed on the fracture
surface at the stage of crack propagation though their number and size vary, which may be related to the geometrical mismatch that arises from the large amount of plastic deformation during crack propagation. Mouzakis et al. [15] and Marissen et al. [19] reported the formation of wavy ripples following tear tests. In the case of MD-notch specimens, the surface of the crack is relatively smooth for a couple of crack surfaces and the wavy ripples are not very visible at this scale. That is because the crack path is along the resin flow for MD-notch specimens. However, for the TPEE-C sample, multiple steps are observed in the crack surfaces, as described above. Marisesen et al. [19] observed the formation of fine ripples at the fracture surface that are roughly parallel to the specimen surface and parallel to the crack front. They noted that the development of steps in the direction of crack propagation is caused by the initiation of individual cracks at several places. In Fig. 13 and Fig. 14, the magnified photos of the fracture surfaces for TD-notch and MD-notch specimens are shown respectively. In the case of TD-notch specimens, the TPEE-A sample (Fig. 13(a)) shows larger-scale wavy ripples in comparison with the TPEE-B sample (Fig. 13(b)). Interestingly, the fracture surface of the TPEE-B sample is less elongated; this should be related to the low-tear property of TPEE-B in comparison with TPEE-A. In the case of MDnotch specimens, the size of the wavy ripples of the TPEE-A sample (Fig. 14(a)) is much smaller than that for TD-notch specimens (Fig. 13(a)). This characteristic is related with the resistance to crack propagation, i.e., the slope, because the essential work of fracture of the TPEE-A sample is almost the same for both MD-notch and TD-notch specimens. The TPEE-C sample (Fig. 14(a)) shows a larger size of wavy ripples in comparison with the TPEE-A sample (Fig. 14(b)), which is a good reason for the large slope of the TPEE-C sample with MD-notch.
5. Conclusions In the present study, the tear properties of commercial thermoplastic polyester elastomers (TPEEs) that are designed for constant velocity joints (CVS) were investigated through the essential work of fracture (EWF) method of testing as well as conventional trouser-tear tests. Two directions for specimens, i.e., the machine direction (MD) and the transverse direction (TD), which are based on the resin flow during the injection-molding process, were selected for this study. Also, scanning electron microscope (SEM) was applied to observe the fracture surface after EWF tests. The results obtained from this study are as follows:
(1) In the case of samples with severely curved J-tears, conventional trouser-tear tests are not appropriate for the quantitative analysis of tear properties, and the application of EWF tests is effective for evaluating the tear properties of TPEE materials. (2) The essential work of fracture of all samples with TDnotch is higher than that with MD-notch. Further, by the separation of the essential work of fracture into two
J.-M. Lee et al. / Polymer Testing 28 (2009) 854–865
components, i.e., yielding and necking/tearing, the contribution of necking and tearing to the essential work of fracture is found to be larger than that of yielding for both MD-notch and TD-notch specimens. (3) The root causes of the variability of the essential work of fracture and the slope of the load-displacement can be identified by the partitioning method. The large scatter of the measured data for MD-notch specimens of the TPEE-C sample is related mainly to the abnormally large portion of the slope of necking and tearing. (4) In the case of TD-notch specimens, two stages can be identified in failure, i.e., crack propagation and final rupture. Many wavy ripples can be observed on the fracture surface at the stage of crack propagation. A sudden rupture occurs when the ligament of the specimen is shorter than a critical length. In the case of MDnotch specimens, the surface of the crack is relatively smooth for a couple of crack surfaces and the wavy ripples are not very visible at this scale. It is found that the size and shape of wavy patterns can be related to the tear properties of TPEE samples. References [1] P.E. Bretz, R.W. Hertzberg, J.A. Manson, Correlation between crack growth rate and fracture mode transitions in low density polyethylene, Polymer 22 (1981) 575. [2] H. Aglan, A. Chudnovsky, A. Moet, T. Fleischman, Crack layer analysis of fatigue crack propagation in rubber compounds, International Journal of Fracture 44 (3) (1990) 167. [3] K.B. Broberg, On stable crack growth, Journal of Mechanics and Physics of Solids 23 (1975) 215. [4] B. Cotterell, J.K. Reddel, The essential work of plane stress ductile fracture, International Journal of Fracture 13 (3) (1977) 267.
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