POLYMER TENSILE TEST ANALYSIS PDF

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EXPERIMENT 1 Tensile Test 1. OBJECTVE 1.1 To emphasis the use of tensile using Instron machine to the polymer materials 1.2 To find the differences in mechanical properties on HDPE, LDPE, PP, and PS 2. INTRODUCTION One of way to find out a mechanical properties of materials is using the tensile test...

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EXPERIMENT 1 Tensile Test

1. OBJECTVE 1.1

To emphasis the use of tensile using Instron machine to the polymer materials

1.2

To find the differences in mechanical properties on HDPE, LDPE, PP, and PS

2. INTRODUCTION One of way to find out a mechanical properties of materials is using the tensile test. The tensile test itself is used to find out and evaluate the strength of material that being tested. By definition, Tensile testing is the measurement of the ability of a material to overcome forces pulling the sample apart and the extent it stretches before breaking [1]. Before running the test, we must formed specimens in such a way as to form a dumbbell specimen of a certain size, in this experiment we refer to ASTM D 638 (plastics) and ASTM D 412 (rubber). After that, the specimen is placed in the testing machine and force applied (figure 2.1). Load and elongation within the sample will be controlled via a monitor, and it will stop when the specimen was fractured. Then, all the mechanical data will be compiled in one diagram called stress-strain diagram. From this diagram (figure 2.2), it can be seen that some basic mechanical properties we can evaluate on tensile test , such as stress, strain, elongation, modulus elasticity, proportional limit, yield point, yield strength, ultimate strength, fracture, etc[1]. In solid polymers, there are three types of stress-strain curves (figure 2.3). For crystalline polymer, most of them show viscoelastic properties. Normally, stress-strain curves start with straight line when the strain increasing, and the stress at which slip becomes noticeable and significant [2]. This point is called yield point. After yield point, stress will constant with increasing strain, this situation is called cold drawing. At this phase, the sample does not become gradually thinner but suddenly becomes thinner at one point. At the same moment, the specimen starts to has a whitening effect, which means the chemical structure of the specimen were pulled and their secondary bond starts to break, in this process also known as necking (figure 2.4) [3]. While the stretching is continued, the strain hardening will occur the expenses of undrawn polymer become totally necking. As the result the polymer specimen will rapture.

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The typical stress-strain curve for polymers (figure 2.5) are showing their tensile properties [4] such as : 

Tensile stress: tensile load/unit area of minimum original cross section. It is expressed in force per unit area. It indicates the relationship between stress and strain in the deformation of a solid body[1]. 𝑠𝑡𝑟𝑒𝑠𝑠

𝜎

E=𝑠𝑡𝑟𝑎𝑖𝑛 = 𝜀 

Tensile strength: It describes the stress to break the sample. It also mentions the maximum tensile stress supported by specimen during test.



Tensile strength at break: tensile strength at the moment of rupture of the sample.



Elongation: increase in length produced by a tensile load. It is expressed in units of length, commonly as percentage.



Elongation at break: elongation at the moment of rupture of the sample.



Strain: ratio of elongation to the gage length of the sample; that is the change in length per unit of original length



Yield point: first point on the stress-strain curves at which an increase in strain occurs without an increase in stress.



Young’s modulus: initial slope of the stress-strain curves.



Toughness: area under the stress-strain curves. This area has the units of energy per unit volume and is the work expended in deforming the materials.

Figure 2.1 : The specimen and machine during applying force

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Figure 2.2 : stress-strain curve

Figure 2.3 : stress-strain curve for crystalline polymer

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Figure 2.4 : Sample during tensile test

Figure 2.5 : Tensile properties in polymer

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3. COMPONENT AND EQUIPMENT 3.1

Instron machine (figure 3. 1)

3.2

Vernier caliper (figure 3.2)

3.3

Dumbell cutter (figure 3.4)

3.4

Specimens (figure 3.4, 3.5, 3.6, 3.7)

Figure 3.1 : Instron Machine

Figure 3.2 : Vernier Calliper

Figure 3.3 : Dumbell cutter

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Figure 3.4 : HDPE

Figure 3.5 : LDPE

Figure 3.6 : PP

Figure 3.7 : PS

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4. METHODOLOGY Using a dumbbell cutter sample sheets were cut into dumbbell shape

Width and thickness of plastics were measured using a dial thickness gauge and vernier caliper

The gap between the upper jaw and lower jaw of the of the instron machine at 6.4 cm were adjusted and set

Cross head speed of instron machine at 3, 5, 10 mm/min were set for HDPE and PS. 10, 20, 30mm/mins were set for LDPE and PP

Sample 1 in between of jaws were clipped tightly

The instron machine started up the test after start button were pressed. The upper jaw proceeds to move upwards at the rate set. When failure occurred, the results and stress-strain graph is automatically recorded

Return button of the instron machine to allow upper jaw was pressed to allow upper jaw return to its original position

For the next plastic samples, steps 3-7 were repeated

For each sample steps 1-7 were repeated at different cross head speed e.g 50 mm/min, 100 mm/min, and 200 mm/min

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Steps 1-9 were repeated for vulcanized rubber samples but with different cross head speed e.g 50 mm/min, 100 mm/min, 200 mm/min, 30 and 300 mm/min

For more details, ASTM D 638 (plastic) and ASTM D 412 (rubber) were our reference our experiment

5.

RESULT AND DISCUSSION

Ultimate tensile strength

Breaking strength point

Yield strength

Modulus of elasticity, E

Figure 5.1 : Strain-stress curve on LDPE at 10 mm/min

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Ultimate tensile strength

Yield strength

Modulus of elasticity, E

Figure 5.2 : Strain-stress curve on LDPE at 20 mm/min

Yield strength Ultimate tensile strength

Breaking strength point

Modulus of elasticity, E

Figure 5.3 : Strain-stress curve on LDPE at 30 mm/min

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Ultimate tensile strength

Yield strength

Modulus of elasticity, E

Figure 5.4 : Strain-stress curve on PP at 10 mm/min

Breaking strength point Ultimate tensile strength Yield strength

Modulus of elasticity, E

Figure 5.5 : Strain-stress curve on PP at 20 mm/min

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Ultimate tensile strength

Yield strength

Breaking strength point

Modulus of elasticity, E

Figure 5.6 : Strain-stress curve on PP at 30 mm/min

Ultimate tensile strength Breaking strength point

Yield strength

Modulus of elasticity, E

Figure 5.7 : Strain-stress curve on HDPE at 3 mm/min

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Ultimate tensile strength

Breaking strength point

Yield strength

Modulus of elasticity, E

Figure 5.8 : Strain-stress curve on HDPE at 5 mm/min

Breaking strength point Ultimate tensile strength

Yield strength

Modulus of elasticity, E

Figure 5.9 : Strain-stress curve on HDPE at 10 mm/min

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Ultimate tensile strength

Breaking strength point

Yield strength

Modulus of elasticity, E

Figure 5.10 : Strain-stress curve on PS at 3 mm/min

Ultimate tensile strength

Breaking strength point

Yield strength

Modulus of elasticity, E

Figure 5.11 : Strain-stress curve on PS at 5 mm/min

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Breaking strength point Yield strength

Modulus of elasticity, E

Figure 5.12 : Strain-stress curve on PS at 10 mm/min

Figure 3.1-3.12 were the stress-strain curves of LDPE, PP, HDPE and PS with different of strain rate which are 10, 20, 30mm/mins for LDPE and PP, 3, 5, 10 mm/mins respectively. From those curves we obtained that different specimens give different figure of curves. Meaning that each curves give a characteristic of each specimen in term of their mechanical properties. It can be seen on table 3.1-3.6 which give some properties of these specimens, such as young modulus, yield stress, yield strain, stress at break, strain at break and ultimate tensile strength.

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Table 5.1: Tensile properties of LDPE and PP at 10 mm/mins

Sample

Modulus of

Yield

elasticity,

stress,

MPa

MPa

Yield strain, %

Stress at the break, MPa

Ultimate Strain at

tensile

break, %

strength, MPa

𝑺𝒕𝒓𝒆𝒔𝒔,𝝈

E =𝒔𝒕𝒓𝒂𝒊𝒏,𝜺 LDPE

=

𝟑−𝟐.𝟐

7.24

18.2

7.92

97.5

8.2

23.5

9

33

200

33.5

𝟕−𝟐

=0.16 𝑺𝒕𝒓𝒆𝒔𝒔,𝝈

E =𝒔𝒕𝒓𝒂𝒊𝒏,𝜺 PP

𝟐𝟐−𝟎

= 𝟏𝟎−𝟎 =2.2

Table 5.2: Tensile properties of LDPE and PP at 20 mm/mins

Sample

Modulus of

Yield

elasticity,

stress,

MPa

MPa

Yield strain, %

Stress at the break, MPa

Ultimate Strain at

tensile

break, %

strength, MPa

𝑺𝒕𝒓𝒆𝒔𝒔,𝝈

E =𝒔𝒕𝒓𝒂𝒊𝒏,𝜺 LDPE

𝟐𝟏−𝟎

= 𝟏𝟎−𝟎

22.5

10

27

200

28

6.5

16

7.5

104.5

7.7

= 2.1 𝑺𝒕𝒓𝒆𝒔𝒔,𝝈

E =𝒔𝒕𝒓𝒂𝒊𝒏,𝜺 PP

=

𝟐.𝟖−𝟐.𝟐 𝟓−𝟑

= 0.3

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Table 5.3: Tensile properties of LDPE and PP at 30 mm/mins

Sample

Modulus of

Yield

elasticity,

stress,

MPa

MPa

Yield strain, %

Ultimate

Stress at the break, MPa

Strain at

tensile

break, %

strength, MPa

𝑺𝒕𝒓𝒆𝒔𝒔,𝝈

E =𝒔𝒕𝒓𝒂𝒊𝒏,𝜺 LDPE

=

𝟑.𝟕−𝟐.𝟓

7.6

15

8

120

8.5

32

5

3.4

36

10

𝟓−𝟐.𝟓

= 0.48 𝑺𝒕𝒓𝒆𝒔𝒔,𝝈

E =𝒔𝒕𝒓𝒂𝒊𝒏,𝜺 PP

=

𝟐𝟏−𝟒 𝟑−𝟎

= 5.66667

Chart 5.1 : Ultimate tensile stress (MPa) against different strain rate between LDPE and PP sample. 40 35 30 25 20 LDPE 15

PP

10 5 0 10

20

30

strain rates mm/min

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Chart 5.2 : Modulus of elasticity (MPa) against different strain rate between LDPE and PP sample. 6 5 4 3 LDPE PP

2 1 0 10

20

30

strain rate (mm/min

Chart 5.3 : elongation at the break (%) against different strain rate between LDPE and PP sample. 250

200

150 LDPE

100

PP

50

0 10

20

30

Strain Rates (mm/min)

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These tables (5.1, 5.2, 5.3) are showing the actual mechanical properties of LDPE and PP which can be measured from tensile test that we conducted, such as yield strength, ultimate strength, young modulus, breaking strength point, etc, and the data is grouped in three groups based on the strain rates applied in the experiment. On the other hand, chart 5.1, 5.2 and 5.3 are showing the differences on LDPE and PP in ultimate tensile strength, modulus elasticity, and their elongation at break against their strain rates respectively. From these data we can simply conclude that there is a difference between samples properties on difference applied strain rates. On chart 5.1, it had shown that the ultimate tensile strength (UTS) of LDPE and PP are fluctuating, at 10 mm/min PP has greater UTS than LDPE at 33.5 MPa than 8.2 MPa respectively. However, on 20mm/min LDPE has greater UTS than PP at 28 than 7 respectively. Meaning that the applied strain rate were effecting much on their reading. Moreover, it also applies on chart 5.2 and 5.3 as well. On 10 mm/min the modulus elasticity of LDPE was on 0.16 MPa and PP on 2.1 MPa. Table 5.4: Tensile properties of HDPE and PS at 3 mm/mins

Sample

Modulus of

Yield

elasticity,

stress,

MPa

MPa

Yield strain, %

Stress at the break, MPa

Ultimate Strain at

tensile

break, %

strength, MPa

𝑺𝒕𝒓𝒆𝒔𝒔,𝝈

E =𝒔𝒕𝒓𝒂𝒊𝒏,𝜺 HDPE

𝟗−𝟑

= 𝟏−𝟎

16.5

2.16

23.5

5.16

24

34.3

2.05

37.5

2.7

40

=6 𝑺𝒕𝒓𝒆𝒔𝒔,𝝈

E =𝒔𝒕𝒓𝒂𝒊𝒏,𝜺

PS

= 𝟏𝟖.𝟖−𝟏.𝟐 𝟎.𝟗𝟕𝟓−𝟎.𝟎𝟓

= 19.03

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Table 5.5: Tensile properties of HDPE and PS at 5 mm/mins

Sample

Modulus of

Yield

elasticity,

stress,

MPa

MPa

Yield strain, %

Stress at the break, MPa

Ultimate Strain at

tensile

break, %

strength, MPa

𝑺𝒕𝒓𝒆𝒔𝒔,𝝈

E =𝒔𝒕𝒓𝒂𝒊𝒏,𝜺 HDPE

=

𝟏𝟏−𝟎

17.5

2

23.75

4.95

24.25

38

2

35

2.19

40.5

𝟏−𝟎

= 11 𝑺𝒕𝒓𝒆𝒔𝒔,𝝈

E =𝒔𝒕𝒓𝒂𝒊𝒏,𝜺 PS

𝟐𝟎−𝟓

= 𝟎.𝟗−𝟎.𝟐𝟓 = 23.08

Table 5.6: Tensile properties of HDPE and PS at 10 mm/mins

Sample

Modulus of

Yield

elasticity,

stress,

MPa

MPa

Yield strain, %

Stress at the break, MPa

Ultimate Strain at

tensile

break, %

strength, MPa

𝑺𝒕𝒓𝒆𝒔𝒔,𝝈

E =𝒔𝒕𝒓𝒂𝒊𝒏,𝜺 HDPE

𝟏𝟑.𝟓−𝟎

= 𝟏.𝟏𝟔−𝟎

21.5

2

26.25

8.48

27.125

46

2.05

46.05

2.95

46.5

= 11.64 𝑺𝒕𝒓𝒆𝒔𝒔,𝝈

E =𝒔𝒕𝒓𝒂𝒊𝒏,𝜺 PS

𝟐𝟗−𝟓

= 𝟏.𝟐−𝟎.𝟐𝟓 = 25.26

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Chart 5.4 : Ultimate tensile stress (MPa) against different strain rate between HDPE and PS sample 50 45 40 35 30 25 HDPE

20

PS

15 10 5 0 3

5

10

strain rate (mm/min)

Chart 5.5 : Modulus of elasticity (MPa) against different strain rate between LDPE and PP sample 50 45 40 35 30 25

HDPE

20

PS

15 10 5 0 3

5

10

strain rate mm/min

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Chart 5.6 : Elongation at the break (%) against different strain rate between LDPE and PP sample. 9 8 7 6 5 HDPE

4

PS 3 2 1 0 3

5

10

Strain rate (mm/min)

Table 5.4, 5.5, 5.6 are showing the actual mechanical properties of HDPE and PS which were measured from tensile test that we conducted and the data is grouped in three groups based on the strain rates applied in the experiment. On the other hand, chart 5.4, 5.5 and 5.6 are showing the differences on HDPE and PS in ultimate tensile strength, modulus elasticity, and their elongation at break against their strain rates respectively. From these data also we can simply conclude that there is a difference between samples properties on difference applied strain rates. On chart 5.4, it had shown that the ultimate tensile strength (UTS) of HDPE and PS are tend to rise, at 3 mm/min PP has greater UTS than HDPE at 40 MPa than 24 MPa respectively. On the other hand, at 5 mm/min PS still leading with 40.5 and HDPE with 24.5. According to this data, the higher strain rate applied the higher UTS amount we can get from this sample. On chart 5.4, similarly with UTS, the modulus elasticity also tends to rise up in line with the increasing of strain rate. At 3 mm/min the modulus elasticity of PS is higher than HDPE with 19.3 MPa and 6 MPa respectively. At 10 mm/min the modulus elasticity of PS and HDPE are proportionally increase with 25.26 MPa and 11.64 MPa respectively. However, for the elongation data of HDPE and PS on chart 5.6 is fluctuating, where at 3 mm/min tends to increase when we compared with 10 mm/min, inversely when we compare with 5 mm/min which is showing that it tends to decrease. TENSILE TEST

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Actually the data that we measured cannot be considered as valid data. It because the fluctuating data on reading that we collected. Especially on chart 5.1, 5.2, 5.3, and 5.6. There are several factors that made our data not valid, such as human error, tools error, and so on. On human error, we might error on one or more on methodology. We could wrong on the specimen dimension measurement or the technical error while using instron machine and so on. Also, there are certain factors that influence the mechanical properties of polymers, such as molecular weight, degree of crystallinity, heat treating and so on [6].On the other hand also the tools might be giving some error on reading, such as the tensile machine did not worked well or maybe we need more accurate test to measure the tensile properties of polymeric materials, such as Split Hopkins Pressure Bar [5]. According to [5], the strain rate gives direct effect to the mechanical properties of polymers during the tensile test. Supposedly, the stress-strain curve will significantly change when different strain rate applied. The ultimate tensile strength and the modulus elasticity must be increase proportional to the increasing strain rate. On the other hand, the elongation at break will be reduce since increasing strain rate applied. The Effect of Different Polymer on Mechanical Properties. Since we conducted 4 specimens on the experiment, for sure there is a big difference on their mechanical properties. Because for each specimen, they have their own chain structures which differentiate between one and another. LDPE are thermoplastic materials which has a branch-chain structure [1]. It has lower degree of crystalinity also the density compared to the HDPE. The branch-chain structure (fig. 5.13) reduces the intermolecular bonding forces, hence it lowers the...