Trouser tear tests of two thin polymer films

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Trouser tear tests of two thin polymer films

Eskil Andreasson, Nasir Mehmood, Sharon Kao-Walter

13th International Conference on Fracture

2013

ICF13

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Trouser tear tests of two thin polymer films

Eskil Andreasson

1,2,*

, Nasir Mehmood

2

, Sharon Kao-Walter

2,3

1

Tetra Pak Packaging Solutions, SE 221 86, Lund, Sweden

2

Dept. of Mech. Eng. School of Eng., Blekinge Institute of Technology, SE 371 79, Karlskrona, Sweden

3

Faculty of Mech. & El. Eng., Shanghai Second Polytechnic University, 201209, Shanghai, China * Corresponding author: eskil.andreasson@tetrapak.com

Abstract Trouser tear testing has been concerned in this research work. A polypropylene film and a low

density polyethylene film used in the packaging industry are considered. The experimental trouser tear tests showed different results for both materials when they were subjected to load in different material directions. Therefore the hypothesis was verified, that the in-plane material orientation/alignment induced during manufacturing, hence creating anisotropic in-plane mechanical properties, also affects the tearing behavior. A brittle-like failure was shown in the polypropylene film while the low density polyethylene presented a highly ductile behavior. The two polymer films can be classified as one low-extensible and one high-extensible material according to the test method utilized. Material parameters in the principal material directions i.e. manufacturing direction and cross direction were extracted from the experimental tests for further numerical studies. Scanning electron microscope was used for micromechanical and fractographical analysis of the crack tip and crack surfaces created during the tests. The methods discussed will help classify different groups of materials and can be used as a predictive tool for the crack initiation and crack propagation path in packaging material, especially thin polymer films.

Keywords Anisotropic, thin polymer film, crack propagation, fracture mechanics, SEM

1. Introduction

Polymer films are extensively used in food packaging industry due to their beneficial mechanical properties, i.e. the combination of stiffness, strength and ductility. During transportation, handling and usage of packages, polymer films are exerted to different loading conditions. Polymers and rubber-like materials have previously been extensively studied experimentally in various fracture modes [1-3]. For the case of tearing, the experimental and theoretical analysis has been performed in [4-8]. This work will focus and extend the analysis on the experimental trouser tear tests in three different material directions for two types of polymer films used in packaging industry.

Fracture properties related to the specific material parameters such as critical fracture toughness, energy release rate, fracture energy and crack propagation resistance can be determined using a fracture mechanical test method. In brittle material this procedure is well known but for ductile material it is less developed. The two important fracture modes involved in the trouser tear test, mode I – in-plane opening mode and mode III – anti-plane shearing mode together with the mixed mode - trouser tear test are depicted in Fig. 1.

Figure 1. Three loading modes of cracked specimens: a) mode I b) mode III and c) mixed mode trouser tear test [9]

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13th International Conference on Fracture June 16–21, 2013, Beijing, China

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2. Materials

Two types of polymer films with different mechanical behavior were tested and analyzed in this work. One oriented polypropylene (PP) film and one low density polyethylene (LDPE) film were experimentally tested. The mechanical in-plane material properties for these two thin packaging materials have thoroughly been examined in previous works with slightly different scopes and interests [6-7,10-13]. In-plane elastic anisotropic material behavior is shown in the PP film according to (Table 1). To be able to distinguish the principal material directions a naming convention is used i.e. manufacturing direction (MD), cross direction (CD), and 45 degrees to the manufacturing direction (45). These abbreviations are further on used to indicate in which direction the load has been applied. In-plane material properties for the PP and LDPE are presented in (Table 1). In-plane material properties primarily dominate the mechanical behavior in the two polymer films studied. This is due to the thin thickness of the polymer films, hence plane stress assumption is valid. Therefore, out of plane properties are disregarded in this work.

Table 1. In-plane mechanical material properties for thin polymer films [10], [13] Material Thickness Material

orientation Young’s modulus Yield strength Poisson’s ratio [µµµµm] [-] [MPa] [MPa] [−−−−] PP 18 CD MD 45 5100 2200 2800 29 28 28 0.43 0.25 0.30 LDPE 27 CD MD 45 140 140 140 5.1 5.1 5.1 0.4 0.4 0.4

Manufacturing of polymer films involves several processing steps. During these different steps, polymer chains are aligned or enforced to orient in the manufacturing/rolling direction (MD) or stretching direction (MD or CD). The degree of orientation in the polymer chains vary in different polymer types and mixture of polymers. Temperature, thickness, chemical structure, polymer chain lengths, number of cross-links, entanglements and rate of crystallinity are all parameters affecting the final mechanical properties in the material. Anisotropy, different mechanical behavior in different directions, is therefore most often the case for many polymers. Due to anisotropy, polymer films tend to follow different preferred crack directions and find the lowest resistance path for crack propagation. Initial direction of crack extension depends of loading scheme and type of material. Brittle materials, as PP in this study, usually fracture by mode III defined in Fig. 1. Ductile materials, as LDPE in this study, usually fracture by mode I and mode III defined in Fig. 1 when exerted to a trouser tear test. If a crack is introduced into a specimen, such as in the trouser tear test, the stress distribution is no longer constant and homogenous within the material. The stress will vary and this variation is due to size and shape of crack and geometry of specimen. In fact, the geometry and the type of loading also have a significant influence on the crack propagation behavior. In brittle materials the process zone will be very local and in the vicinity of the crack tip, all the energy dissipates and new crack surfaces are created. On the other hand in a ductile material where a lot of plastic deformation occurs the process zone and active zone is a rather large area surrounding the crack tip. In this case a lot of energy is consumed in the plastic flow and for the trouser tear test in substantial leg deformation.

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3. Experimental Procedure

Preparation and cutting of specimens were performed after pr 23°C and 50% RH for 40 hours prior to test in accordance standard ASTM D618

sharp medical knife and errors, such as

operator. Mounting and handling of the polymer film

material and edges. Trouser tear test specimen geometry and dimensions the specimen is mounted

the name of the test method directions to create tearing action. is fixed and the other

extension is measured standard ASTM 1938 European standard propagate

Several experimental test

setup to characterize the mechanical orientations.

Figure

According to Fig. 2 a experimental equipment

pre-made crack will continue to grow. one-color area is t

and sensitive introduced

is important that the grippers are rigid and can cause significant deviation in results and this Hence the gripping equipment

clamps in the other directions than the stretching direction.

Experimental Procedure

Preparation and cutting of specimens were performed after pr C and 50% RH for 40 hours prior to test in accordance standard ASTM D618

sharp medical knife and errors, such as edge effects

. Mounting and handling of the polymer film

material and edges. Trouser tear test specimen geometry and dimensions the specimen is mounted

the name of the test method directions to create tearing action. is fixed and the other

extension is measured ASTM 1938

European standard ISO 6383

propagate a crack in a trouser tear test in plastic/polymer films with a thickness Several experimental test

to characterize the mechanical orientations.

Figure 2. Trous According to Fig. 2 a experimental equipment

crack will continue to grow.

color area is the material subjected to load sensitive to stress

introduced when mounting

is important that the grippers are rigid and can cause significant deviation in results and this Hence the gripping equipment

Experimental Procedure

Preparation and cutting of specimens were performed after pr C and 50% RH for 40 hours prior to test in accordance standard ASTM D618-08 [16]. Sample

sharp medical knife and it is recommended to

edge effects, the specimens were cut in the same way every time with the same . Mounting and handling of the polymer film

material and edges. Trouser tear test specimen geometry and dimensions the specimen is mounted in the tensile test equipment it looks like a the name of the test method. The ‘legs’ of the trouser

directions to create tearing action. is fixed and the other one is moved

extension is measured by grip separation. Th

ASTM 1938-08 [15], was used for calculation of the ISO 6383-1:1983

a trouser tear test in plastic/polymer films with a thickness Several experimental tests, minimum five for each material direction,

to characterize the mechanical

. Trouser tear test specimen geometry, According to Fig. 2 a pre-made

experimental equipment. During the test, when the legs are separated and thus extended, the crack will continue to grow.

he material subjected to load

to stress and to avoid crack initiation prior to the test when mounting. The registered forces in the experimental test is important that the grippers are rigid and

can cause significant deviation in results and this Hence the gripping equipment was

Experimental Procedure

Preparation and cutting of specimens were performed after pr C and 50% RH for 40 hours prior to test in accordance

. Sample cutting ecommended to

he specimens were cut in the same way every time with the same . Mounting and handling of the polymer film

material and edges. Trouser tear test specimen geometry and dimensions in the tensile test equipment it looks like a

. The ‘legs’ of the trouser directions to create tearing action. One of the grips

moved at a constant rate by grip separation. Th

, was used for calculation of the 1:1983 [17]. These two methods

a trouser tear test in plastic/polymer films with a thickness , minimum five for each material direction,

to characterize the mechanical behavior

er tear test specimen geometry, made crack is

During the test, when the legs are separated and thus extended, the crack will continue to grow. In the figure the

he material subjected to load

and to avoid crack initiation prior to the test he registered forces in the experimental test is important that the grippers are rigid and un

can cause significant deviation in results and this was adjusted

clamps in the other directions than the stretching direction. Preparation and cutting of specimens were performed after pr

C and 50% RH for 40 hours prior to test in accordance cutting of the two ecommended to frequently

he specimens were cut in the same way every time with the same . Mounting and handling of the polymer film was

. The ‘legs’ of the trouser

One of the grips in the tensile test m a constant rate

by grip separation. The test method utilized in this work, the American , was used for calculation of the

. These two methods

behavior of each material

er tear test specimen geometry, crack is introduced

During the test, when the legs are separated and thus extended, the In the figure the

he material subjected to load, where the tearing action takes place and to avoid crack initiation prior to the test

he registered forces in the experimental test unable to move during the test can cause significant deviation in results and this external

adjusted to be ultimately stiff to prohibit any movement of the clamps in the other directions than the stretching direction.

Preparation and cutting of specimens were performed after pre-C and 50% RH for 40 hours prior to test in accordance with

of the two types of frequently change blades.

he specimens were cut in the same way every time with the same was carefully done in order to

. The ‘legs’ of the trouser specimen are then pulled in opposite in the tensile test m

a constant rate (10mm/min)

e test method utilized in this work, the American , was used for calculation of the tear resistance and is similar to the

. These two methods

of each material

er tear test specimen geometry, illustration by Carl Nordenskjöld introduced in each specimen before mounting in During the test, when the legs are separated and thus extended, the

In the figure the grip area

where the tearing action takes place and to avoid crack initiation prior to the test

he registered forces in the experimental test able to move during the test

external noise has to

to be ultimately stiff to prohibit any movement of the clamps in the other directions than the stretching direction.

-conditioning of the material with the test procedure defined in the types of polymer films were done

change blades. To minimize

he specimens were cut in the same way every time with the same carefully done in order to

material and edges. Trouser tear test specimen geometry and dimensions are shown in in the tensile test equipment it looks like a pair of trouser

specimen are then pulled in opposite in the tensile test machine, holding the specimen,

(10mm/min) during the test

e test method utilized in this work, the American tear resistance and is similar to the . These two methods calculate the force necessar a trouser tear test in plastic/polymer films with a thickness

, minimum five for each material direction, were performed

of each material and for different material

lustration by Carl Nordenskjöld in each specimen before mounting in During the test, when the legs are separated and thus extended, the

grip area is marked (hatched) and the where the tearing action takes place

and to avoid crack initiation prior to the test a small he registered forces in the experimental tests ar

able to move during the tests. Even a small vibration noise has to be controlled and

to be ultimately stiff to prohibit any movement of the

(dimensions in mm)

conditioning of the material the test procedure defined in the

polymer films were done o minimize uncontrolled he specimens were cut in the same way every time with the same

carefully done in order to not damage the shown in Fig. 2

trousers, which explains specimen are then pulled in opposite , holding the specimen, during the test.

e test method utilized in this work, the American tear resistance and is similar to the

the force necessar a trouser tear test in plastic/polymer films with a thickness less than

were performed for each test and for different material

lustration by Carl Nordenskjöld in each specimen before mounting in During the test, when the legs are separated and thus extended, the

is marked (hatched) and the where the tearing action takes place. PP is

small slack (2 mm are low and

. Even a small vibration controlled and

to be ultimately stiff to prohibit any movement of the

(dimensions in mm)

conditioning of the materials at the test procedure defined in the polymer films were done with a uncontrolled he specimens were cut in the same way every time with the same

damage the Fig. 2. When which explains specimen are then pulled in opposite , holding the specimen,

. Specimen e test method utilized in this work, the American tear resistance and is similar to the the force necessary to less than 250 µm. for each test and for different material

lustration by Carl Nordenskjöld in each specimen before mounting in the

During the test, when the legs are separated and thus extended, the is marked (hatched) and the . PP is brittle 2 mm) was therefore it . Even a small vibration controlled and minimized. to be ultimately stiff to prohibit any movement of the

at the test procedure defined in the with a uncontrolled he specimens were cut in the same way every time with the same damage the When which explains specimen are then pulled in opposite , holding the specimen,

Specimen e test method utilized in this work, the American tear resistance and is similar to the y to . for each test and for different material

the

During the test, when the legs are separated and thus extended, the is marked (hatched) and the brittle was it . Even a small vibration minimized. to be ultimately stiff to prohibit any movement of the

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4. Experimental trouser tear testing results

In the test method, ASTM D1938 PP is a low

response graphs

Fig. 3. Low extensible extensible films

the tearing energy. deformation.

a)

Figure 3. Load vs. time

The force needed measured in the

tear propagation resistance of various plastic extension

Fig.4 for PP and in Fig.5 for LDPE. an idea of

registered in the PP significant difference

Figure

. Experimental trouser tear testing results

n the test method, ASTM D1938 low extensible or non

response graphs from trouser tear test Low extensible

extensible films, i.e. LDPE, tearing energy. T deformation.

a)

Load vs. time

needed to propagate

in the laboratory at Tetra Pak in Lund. tear propagation resistance of various plastic extension were recorded during loading a Fig.4 for PP and in Fig.5 for LDPE.

mechanical

registered in the PP-film, as shown in significant difference

Figure 4. Trouser tear test in material direction

. Experimental trouser tear testing results

n the test method, ASTM D1938

extensible or non-extensible film from trouser tear test

Low extensible films, i.e. PP,

, i.e. LDPE, the deformation energy of Tearing of highly extensible film

Load vs. time for trouser tear test

to propagate the pre

aboratory at Tetra Pak in Lund. tear propagation resistance of various plastic

were recorded during loading a Fig.4 for PP and in Fig.5 for LDPE.

mechanical behavior and film, as shown in significant difference of registered force in

Trouser tear test in material direction Bold lines represent

. Experimental trouser tear testing results

n the test method, ASTM D1938-08, [15] two different types of behavior is classified; extensible film

from trouser tear tests, for the two , i.e. PP, exhibit

the deformation energy of earing of highly extensible film

for trouser tear tests in a) low

the pre-made aboratory at Tetra Pak in Lund. tear propagation resistance of various plastic

were recorded during loading and Fig.4 for PP and in Fig.5 for LDPE. Five different

behavior and statistical variation film, as shown in Fig. 4

of registered force in all the three material directions

Trouser tear test in material direction

Bold lines represent mean curves for each material direction. -4-

. Experimental trouser tear testing results

two different types of behavior is classified; extensible film and LDPE

, for the two different classes of materials exhibit a constant load during trouser testing. the deformation energy of the

earing of highly extensible film

b)

in a) low-extensible and b)

made crack in a aboratory at Tetra Pak in Lund. The utilized tear propagation resistance of various plastic/polymer

nd tearing of the specimen different specimens

statistical variation

Fig. 4, was low (note the unit mN on the y all the three material directions

Trouser tear test in material direction 45°, MD

mean curves for each material direction.

two different types of behavior is classified; LDPE is a highly

different classes of materials a constant load during trouser testing.

the specimen legs is significantly higher than earing of highly extensible films is accompanied by significant plastic

extensible and b) highly

a polymer film specimen The utilized test method

/polymer films of comparable thickness. tearing of the specimen

specimens for each

in the two types of polymer films , was low (note the unit mN on the y

all the three material directions

MD and CD for PP

13th International Conference on Fracture June 16

two different types of behavior is classified;

highly extensible film. The generic different classes of materials

a constant load during trouser testing.

specimen legs is significantly higher than s is accompanied by significant plastic

highly-extensible

film specimen test method can be used films of comparable thickness. tearing of the specimens. The results are

each direction

in the two types of polymer films , was low (note the unit mN on the y

all the three material directions.

for PP-film, force vs. extension. mean curves for each material direction.

two different types of behavior is classified; in this study extensible film. The generic different classes of materials are displayed a constant load during trouser testing.

specimen legs is significantly higher than s is accompanied by significant plastic

extensible polymer

film specimen was experimentally can be used for

films of comparable thickness. The results are

direction were studied to get in the two types of polymer films

, was low (note the unit mN on the y-axis). There was a

film, force vs. extension. mean curves for each material direction.

13th International Conference on Fracture Beijing, China

in this study extensible film. The generic are displayed in a constant load during trouser testing. For highly specimen legs is significantly higher than s is accompanied by significant plastic

polymer film, [15]

was experimentally for rating the films of comparable thickness. Force and The results are shown in

were studied to get in the two types of polymer films. The force There was a

orce vs. extension. 13th International Conference on Fracture

Beijing, China

in this study extensible film. The generic in For highly specimen legs is significantly higher than s is accompanied by significant plastic

was experimentally rating the and in were studied to get The force There was a

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For PP it is possible to

A noticeable high peak is shown in the chemical bonds in

than the average force for the

energy crack path direction. Lowest alignment in CD

that the highest force PP is depicted in Appendix propagation paths noticeable fluctuations

the experimental equipment but rather due to the many polymers

to the morphology and alignment,

However, systematic understand the the force values is depicted in Appe

For LDPE the

total extension from tearing is only 50 mm, elongation of the two legs.

enable a significant stretching

rearrangement, plastic work, elongation of the legs

until the circle shown in Fig. 5, similar behavior is presented in all three material directions CD, MD and 45 in LDPE.

of the legs tearing distance B, and for

For PP it is possible to

A noticeable high peak is shown in the

chemical bonds in-between the polymer chains or the crystallites. The force is significantly higher the average force for the

crack path direction. Lowest alignment in CD, therefore

highest force PP is depicted in Appendix propagation paths noticeable fluctuations in the force

the experimental equipment but rather due to the

many polymers [8]. The frequency and amplitude of these small fluctuations mos to the morphology and

alignment, arrangement of crystallites, However, systematic

understand the “stick-the force values during tests

is depicted in Appendix B at different loading stages.

Figure 5.

For LDPE the total extension

total extension from tearing is only 50 mm, elongation of the two legs.

a significant stretching

until the circle shown in Fig. 5, similar behavior is presented in all three material directions CD, and 45 in LDPE.

of the legs and the total extension is tearing distance. Test results

for low extensible films

For PP it is possible to clearly distinguish the three different material orientations as shown in Fig A noticeable high peak is shown in the

between the polymer chains or the crystallites. The force is significantly higher the average force for the continuous

crack path direction. Lowest , therefore the crack path highest force is registered in these

PP is depicted in Appendix A at different loading stages. propagation paths noticeable as depicted above in Fig. 4

in the force values during tearing the experimental equipment but rather due to the

. The frequency and amplitude of these small fluctuations mos to the morphology and micro mechanism

arrangement of crystallites, However, systematic micro structural

-slip” behavior

during tests as shown in Fig. 5 ndix B at different loading stages.

. Trouser tear test in material direction C Bold lines represent

extension of the trouser total extension from tearing is only 50 mm, elongation of the two legs. The un

a significant stretching

until the circle shown in Fig. 5, similar behavior is presented in all three material directions CD, and 45 in LDPE. For low extensible film

total extension is

Test results for highly extensible films, i.e. L low extensible films, i.e.

distinguish the three different material orientations as shown in Fig A noticeable high peak is shown in the

PP-between the polymer chains or the crystallites. The force is significantly higher continuous crack propagation when the material has found the lowest crack path direction. Lowest tearing

the crack path is

registered in these specimen at different loading stages. as depicted above in Fig. 4 values during tearing

the experimental equipment but rather due to the

. The frequency and amplitude of these small fluctuations mos micro mechanism

arrangement of crystallites, and distribution micro structural and fractographic

behavior. The LDPE as shown in Fig. 5 ndix B at different loading stages.

ear test in material direction C

Bold lines represent mean curves for each material direction.

of the trouser total extension from tearing is only 50 mm,

he un-bundled polymer

a significant stretching in CD. Therefore a lot of energy rearrangement, plastic work, elongation of the legs

until the circle shown in Fig. 5, similar behavior is presented in all three material directions CD, extensible film

total extension is therefore 50 mm highly extensible films, i.e. L , i.e. PP in Appendix A

distinguish the three different material orientations as shown in Fig -CD samples which probably indicates the breakage of between the polymer chains or the crystallites. The force is significantly higher crack propagation when the material has found the lowest tearing resistance path for

is not orthogonal specimens. at different loading stages. as depicted above in Fig. 4 values during tearing, present in all PP the experimental equipment but rather due to the “stick

. The frequency and amplitude of these small fluctuations mos

of the polymer material, such as the polymer chain and distribution

and fractographic

LDPE-specimens don’t show

as shown in Fig. 5. The experimental trouser tear test results for LDPE ndix B at different loading stages.

ear test in material direction C

of the trouser test is 90 mm

total extension from tearing is only 50 mm, hence a significant part bundled polymer

Therefore a lot of energy rearrangement, plastic work, elongation of the legs and heat generation

until the circle shown in Fig. 5, similar behavior is presented in all three material directions CD, extensible films such as PP

therefore 50 mm highly extensible films, i.e. L

in Appendix A.

distinguish the three different material orientations as shown in Fig samples which probably indicates the breakage of between the polymer chains or the crystallites. The force is significantly higher crack propagation when the material has found the lowest

resistance path for

orthogonal to the stretching direction The experiment

at different loading stages. In Appendix A is the three different crack as depicted above in Fig. 4. It is important to note that the small

esent in all PP-graphs in Fig. stick-slip” behavior

. The frequency and amplitude of these small fluctuations mos

of the polymer material, such as the polymer chain and distribution of crystalline and amorphous phases. and fractographical characterization is needed to fully

specimens don’t show

The experimental trouser tear test results for LDPE

ear test in material direction CD, MD and 45 mean curves for each material direction.

90 mm in CD hence a significant part

bundled polymer chains, with the majority Therefore a lot of energy

and heat generation

until the circle shown in Fig. 5, similar behavior is presented in all three material directions CD, such as PP on the contrary

therefore 50 mm, the same length as the minimum possible highly extensible films, i.e. LDPE

distinguish the three different material orientations as shown in Fig samples which probably indicates the breakage of between the polymer chains or the crystallites. The force is significantly higher crack propagation when the material has found the lowest resistance path for PP-45 is along the material

to the stretching direction The experimental trouser tear test

In Appendix A is the three different crack It is important to note that the small

graphs in Fig.

behavior observed during fracture in . The frequency and amplitude of these small fluctuations mos

of the polymer material, such as the polymer chain of crystalline and amorphous phases. al characterization is needed to fully specimens don’t show these

The experimental trouser tear test results for LDPE

and 45° for LDPE

in CD and 60 mm

hence a significant part of the LDPE with the majority

Therefore a lot of energy is dissipated in material and heat generation. However, the initial part, until the circle shown in Fig. 5, similar behavior is presented in all three material directions CD, on the contrary, there is no deformation , the same length as the minimum possible DPE is depicted visually in Appendix distinguish the three different material orientations as shown in Fig

samples which probably indicates the breakage of between the polymer chains or the crystallites. The force is significantly higher crack propagation when the material has found the lowest is along the material to the stretching direction. This means

al trouser tear test

In Appendix A is the three different crack It is important to note that the small graphs in Fig. 4, are not noise from observed during fracture in . The frequency and amplitude of these small fluctuations most probably relates of the polymer material, such as the polymer chain of crystalline and amorphous phases. al characterization is needed to fully

these small fluctuations The experimental trouser tear test results for LDPE

for LDPE-film. mean curves for each material direction.

60 mm in MD and 45 of the LDPE extension with the majority oriented in MD,

dissipated in material However, the initial part, until the circle shown in Fig. 5, similar behavior is presented in all three material directions CD, , there is no deformation , the same length as the minimum possible depicted visually in Appendix distinguish the three different material orientations as shown in Fig. 4

samples which probably indicates the breakage of between the polymer chains or the crystallites. The force is significantly higher crack propagation when the material has found the lowest is along the material This means al trouser tear test results for In Appendix A is the three different crack It is important to note that the small , are not noise from observed during fracture in t probably relates of the polymer material, such as the polymer chain of crystalline and amorphous phases. al characterization is needed to fully fluctuations in The experimental trouser tear test results for LDPE

and 45. The extension is oriented in MD, dissipated in material However, the initial part, until the circle shown in Fig. 5, similar behavior is presented in all three material directions CD, , there is no deformation , the same length as the minimum possible depicted visually in Appendix 4. samples which probably indicates the breakage of between the polymer chains or the crystallites. The force is significantly higher crack propagation when the material has found the lowest is along the material This means for In Appendix A is the three different crack It is important to note that the small , are not noise from observed during fracture in t probably relates of the polymer material, such as the polymer chain of crystalline and amorphous phases. al characterization is needed to fully in The experimental trouser tear test results for LDPE

The is oriented in MD, dissipated in material However, the initial part, until the circle shown in Fig. 5, similar behavior is presented in all three material directions CD, , there is no deformation , the same length as the minimum possible depicted visually in Appendix

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In addition to the well known fracture mechanical parameters, such as stress intensity factor and J-integral defined in [14], Rivlin and Thomas defined the critical fracture energy from a trouser tear test. This quantity is also known as tearing energy, which is the energy spent per unit thickness per unit increase in crack length. Tearing energy includes surface energy, energy dissipated in plastic flow processes, and energy dissipated irreversibly in viscoelastic processes [1]. The equations described below are derived from the trouser tear test based on theoretical analysis of crack growth behavior [5]. This can simplify the description of the tearing energy from the experimental results. The equation for calculation of tearing energy was derived with experimental test of rubber-like materials and is also applicable for polymers. The tear strength equation to calculate the critical tear energy, , of a propagated crack in LDPE in this study is

= − (1)

is the tear propagation force, is the initial width of specimen, is the thickness of specimen,

is the strain energy density. For LDPE the strain energy density can be calculated using the area

under stress-strain curve from an ordinary tensile test. It was found that the strain energy for LDPE is, = 2.8 / . is the extension ratio of the legs, current length of specimen divided by initial length, which is normally 1 except for some materials which have high extension of legs as LDPE-CD ( = 1.8). In case where high stretching of legs is visible, then Eq. 1 will be used to calculate the critical tear energy. Strain energy density, , becomes zero in materials with no leg

extension, in this study for PP, resulting in the general equation used widely for calculation of critical tearing energy in brittle materials,

= (2)

The relationship between rate of tearing and strain energy release rate is a material characteristic that is independent of test specimen geometry, when tested low extensible materials [4]. The extension in the specimen legs is negligible and ignored for such cases. It can be confirmed from Eq. 2 that the critical tearing energy is independent of the initial sample geometry and crack length. This assumption is valid only if the specimen undergoes mode III dominated failure. Critical tear energy for the PP & LDPE is calculated using the above equation. Crack propagation for PP is a completely mode III phenomenon so its crack propagation is a complete tearing process, while LDPE has plastic flow and deformation of legs in addition to tearing which is generating a mixed mode I and mode III failure. Tearing or crack propagation force, tearing work, tearing energy & tear extension for PP and LDPE are summarized in (Table 2).

Table 2. Trouser tear test results for two thin polymer films; PP and LDPE Material Thickness Material

orientation Tearing Force Critical Tearing energy [µµµµm] [-] [mN] [N/m] PP 18 CD MD 45 21 50 68 2330 5560 7560 LDPE 27 CD MD 45 2500 750 750 333330 55560 55560

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The force applied in a trouser tear test Fig. 6. Both loading and un

extension to start a tear

growing, point F indicates the initial force required to start a crack

indicates the force needed to propagate the crack which is constant for PP and increasing non-linear

Area below

required to start a crack), area Arrow 3 indicates

curves, prior to tearing and during unloading, correspond to stored strain energy in the legs of the specimen.

Figure

From the fracture surfaces shown in Fig. 7 for PP and LDPE mechanical behavior and processe

deformation is developed in the LDPE material leading to localized thinning material has no plastic deformation in the crack tip

presenting a wavy shaped geometry in the LDPE and representing a straight line area that needs more thorough understan

behavior and also the fracture mechanical behavior technology and process settings, what polymers that composition

increase the knowledge and understanding of the

PP 18µµµµ

he force applied in a trouser tear test Both loading and un

extension to start a tear

, point F indicates the initial force required to start a crack

indicates the force needed to propagate the crack which is constant for PP and increasing for LDPE.

below arrow 1 indicates the s required to start a crack), area

indicates the stored energy in the legs at the end of test. The non

curves, prior to tearing and during unloading, correspond to stored strain energy in the legs of the

Figure 6. Trouser tear test, loading and unloading

behavior and also the fracture mechanical behavior technology and process settings, what polymers that

composition. This subject has to be addressed separately and finding technologies to be able to increase the knowledge and understanding of the

Figure Fracture surface

PP

µµµµm

he force applied in a trouser tear test

Both loading and un-loading is presented in the graphs. extension to start a tear, overcome the threshold

indicates the force needed to propagate the crack which is constant for PP and increasing Arrow 3 indicates the final retraction of specimen as applied force is removed. arrow 1 indicates the s

required to start a crack), area below

the stored energy in the legs at the end of test. The non

Trouser tear test, loading and unloading

behavior and also the fracture mechanical behavior technology and process settings, what polymers that

. This subject has to be addressed separately and finding technologies to be able to increase the knowledge and understanding of the

Figure 7. SEM pictures of Fracture surface profile

he force applied in a trouser tear test for PP and LDPE is

loading is presented in the graphs. , overcome the threshold

indicates the force needed to propagate the crack which is constant for PP and increasing rrow 3 indicates the final retraction of specimen as applied force is removed. arrow 1 indicates the strain energy stored in specimen before crack

below arrow 2 indicates

From the fracture surfaces shown in Fig. 7 for PP and LDPE

mechanical behavior and processes are different in the two materials. deformation is developed in the LDPE material leading to localized thinning material has no plastic deformation in the crack tip

presenting a wavy shaped geometry in the LDPE and representing a straight line

area that needs more thorough understanding and knowledge for future studies. The mechanical behavior and also the fracture mechanical behavior

technology and process settings, what polymers that

. This subject has to be addressed separately and finding technologies to be able to increase the knowledge and understanding of the

. SEM pictures of the fracture surface profile in profile

for PP and LDPE is

loading is presented in the graphs.

, overcome the threshold value of force needed to start the , point F indicates the initial force required to start a crack

indicates the force needed to propagate the crack which is constant for PP and increasing rrow 3 indicates the final retraction of specimen as applied force is removed.

train energy stored in specimen before crack arrow 2 indicates

From the fracture surfaces shown in Fig. 7 for PP and LDPE

are different in the two materials. deformation is developed in the LDPE material leading to localized thinning material has no plastic deformation in the crack tip

ding and knowledge for future studies. The mechanical behavior and also the fracture mechanical behavior

technology and process settings, what polymers that are

. This subject has to be addressed separately and finding technologies to be able to increase the knowledge and understanding of the micro mechanical behavior is important.

the fracture surface profile in

LDPE

27µµµµ

30µµµµm

for PP and LDPE is plotted loading is presented in the graphs.

value of force needed to start the , point F indicates the initial force required to start a crack

train energy stored in specimen before crack arrow 2 indicates the energy

Trouser tear test, loading and unloading for PP From the fracture surfaces shown in Fig. 7 for PP and LDPE

are different in the two materials. deformation is developed in the LDPE material leading to localized thinning material has no plastic deformation in the crack tip for tearing fracture presenting a wavy shaped geometry in the LDPE and representing a straight line

ding and knowledge for future studies. The mechanical behavior and also the fracture mechanical behavior are strongly coupled to the manufacturing are used and also the morphology and chemical . This subject has to be addressed separately and finding technologies to be able to

micro mechanical behavior is important.

the fracture surface profile in Fracture surface

LDPE

µµµµm

plotted versus extension of the clamps loading is presented in the graphs. Arrow 1 indicates the initial

value of force needed to start the , point F indicates the initial force required to start a crack (crack

train energy stored in specimen before crack

the energy released during crack extension the stored energy in the legs at the end of test. The non-linear segments of t curves, prior to tearing and during unloading, correspond to stored strain energy in the legs of the

and

LDPE-From the fracture surfaces shown in Fig. 7 for PP and LDPE it is evident that the fracture are different in the two materials.

deformation is developed in the LDPE material leading to localized thinning of the cross section. PP for tearing fracture

ding and knowledge for future studies. The mechanical strongly coupled to the manufacturing used and also the morphology and chemical . This subject has to be addressed separately and finding technologies to be able to

the fracture surface profile in PP and LDPE. Fracture surface profile

extension of the clamps Arrow 1 indicates the initial value of force needed to start the pre-made crack (crack-initiation), arrow 2 indicates the force needed to propagate the crack which is constant for PP and increasing

rrow 3 indicates the final retraction of specimen as applied force is removed. train energy stored in specimen before crack growth (energy released during crack extension

linear segments of t curves, prior to tearing and during unloading, correspond to stored strain energy in the legs of the

-film in MD

it is evident that the fracture are different in the two materials. Substantial plastic of the cross section. PP for tearing fracture. Fracture

presenting a wavy shaped geometry in the LDPE and representing a straight line in PP.

ding and knowledge for future studies. The mechanical strongly coupled to the manufacturing used and also the morphology and chemical . This subject has to be addressed separately and finding technologies to be able to

LDPE.

30

extension of the clamps in Arrow 1 indicates the initial made crack initiation), arrow 2 indicates the force needed to propagate the crack which is constant for PP and increasingly

rrow 3 indicates the final retraction of specimen as applied force is removed. growth (energy released during crack extension linear segments of the curves, prior to tearing and during unloading, correspond to stored strain energy in the legs of the

in MD.

it is evident that the fracture Substantial plastic of the cross section. PP Fracture edges are

in PP. This is an ding and knowledge for future studies. The mechanical strongly coupled to the manufacturing used and also the morphology and chemical . This subject has to be addressed separately and finding technologies to be able to

30µµµµm

in Arrow 1 indicates the initial made crack initiation), arrow 2 ly rrow 3 indicates the final retraction of specimen as applied force is removed. growth (energy released during crack extension he curves, prior to tearing and during unloading, correspond to stored strain energy in the legs of the

it is evident that the fracture Substantial plastic of the cross section. PP are This is an ding and knowledge for future studies. The mechanical strongly coupled to the manufacturing used and also the morphology and chemical . This subject has to be addressed separately and finding technologies to be able to

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5. Conclusions and discussion

Experimental trouser tear tests were performed in this research work according to the American standard ASTM 1938-08 [15]. Two polymer films with different fracture mechanisms and micro structural composition were studied, PP and LDPE. Repeatable and reproducible experimental results were obtained after adjustments of the experimental equipment. Non-compliant test equipment was used due to the low forces registered in the tests. For both materials different responses were measured in the three material directions MD, CD and 45.

Trouser tear test results for the highly extensible polymer film, LDPE in this study, show: - Fracture is governed by a mixed mode material behavior (mode I and mode III).

- The tearing energy is directly proportional to the deformation of the plastic yielded zone at the fracture edge, hence creating increasing deformation zone with increasing force. Thus deformation and strain energy rate is continuously increasing showing higher tearing energy. - One of the legs elongates when the crack tip exhibit both mode I and mode III failure, which is

clearly shown in the case of loading in CD material direction.

Trouser tear test results for the low extensible polymer film, PP in this study, show: - Fracture is solely governed by mode III material behavior.

- There is no pronounced yielded zone, hence all strain energy is consumed and dissipated into local plastic flow, crack tip growth, polymer chain orientation and heat generation.

- Low covalent bonding forces and voids present in the material gives a knotty or shaky tear graph. Knotty tear is due to that the crack path follows these small voids which result in small variation in forces.

It was found that, the low-extensible PP film requires only a small force to fracture, almost negligible compare to the highly-extensible film, LDPE. If the material fractures in a brittle fashion, PP in this case, the result is independent of the shape of the test specimen and the manner in which the deformation is applied. An almost constant tearing force is needed in brittle materials to propagate the crack in different material directions. In this type of material the local deformation in the surroundings of the crack tip is determining the global response. However, if the material is ductile the behavior is much more complicated. The plastic flow at the crack tip is not directly involved in the fracture process and hence the deformation doesn’t only take place locally in the vicinity of the crack tip. The test specimen size and geometry influence the result and therefore it is hard to find a material parameter governing ductile tearing. To separate the leg extension, the plastic flow and the actual tearing force is therefore challenging. It should be noticed that tearing force is also influenced to a large extent by type of polymer, temperature, material anisotropy and loading rate which has not been tested/discussed in this work. Finally, as seen in the SEM pictures, it is possible to distinguish a low-extensible and a highly extensible material by studying at the fracture surfaces in samples. In the highly extensible material the fractured surface is presenting a wave-shaped geometry. Low-extensible material shows a very sharp crack surface and hence a straight line is created during the trouser tear tests.

Acknowledgements

This research work was carried out at Tetra Pak Packaging Solutions AB in Lund, Sweden with the cooperation of the Department of Mechanical Engineering, Blekinge Institute of Technology, Karlskrona, Sweden.

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References

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Appendix A a ) initial trouser b ) continuous c ) final trouser d ) finalize A – Polypropylene trouser tearing b ) continuous trouser trouser tearing

lized trouser tear test Polypropylene (PP)

tearing

trouser tearing

tearing, edge effects

tear test

(PP) during trouser tear test,

edge effects may come into consideration

-10-

during trouser tear test,

may come into consideration

during trouser tear test, ASTM D1938

may come into consideration

13th International Conference on Fracture June 16

ASTM D1938-02

direction of crack

direction of crack propagation crack propagation path

propagation crack propagation path

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Appendix B

a ) initial trouser tearing

b ) continuous trouser tearing

c ) final trouser tearing

d ) finalized

e ) finalized

Appendix B – Low density polyethylene (LDPE) a ) initial trouser tearing

final trouser tearing

ized trouser tearing

ized trouser tear test

density polyethylene (LDPE) a ) initial trouser tearing

final trouser tearing for MD (edge effects)

trouser tearing for MD,

trouser tear test for MD,

density polyethylene (LDPE)

(edge effects)

for MD, continued

density polyethylene (LDPE) during trouser tear test,

(edge effects), extension of

continued extension of legs in CD

continued extension of legs in CD during trouser tear test,

, extension of legs in CD

extension of legs in CD

during trouser tear test, ASTM D1938

legs in CD extension of legs in CD extension of legs in CD direction of crack deformation of leg ASTM D1938-08

direction of crack propagation crack propagation path deformation of leg

08

propagation crack propagation path