lab report - tensile testing

MCET 211

Group 2 Tuesday 11 AM

Tensile Testing

Section: 01

MCET 211 Materials in Engineering Design Lap

Tensile Testing

By

Dustyn Crowley Matt Eckert Patricia Delph

For Prof. Michael J. Parthum Jr. Group 2

Dustyn Crowley, Matt Eckert, Patricia Delph Rochester Institute of Technology

Date Performed:

2/13/17

Date Submitted:

2/21/17

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MCET 211 Section: 01


Tensile Testing Table of Contents

1.0 ABSTRACT……………… ABSTRACT…………………………………………… ……………………………………………………… ……………………………... …... 3

2.0 INTRODUCTION......... INTRODUCTION...................... ......................... ......................... .......................... .......................... .......................... ....................... .......... 3 2.1 Background………………………………………………………………………... 3 2.1.1 Test………………………………… Test……………………………………………………………… ……………………………………..... ………..... 3 2.1.2 Materials………………………… Materials……………………………………………………… …………………………………………. ……………. 3

2.1.3 Structure - Property Relationship (theory)........................... (theory)........................................ ................ ... 4 2.2 Goals and Objectives………………………………………………………..….... 4 3.0 DESCRIPTION OF TEST…………………………………………………………… TEST…………………………………………………………… 5

3.1 Procedure………………………………………………………………………….. Proce dure………………………………………………………………………….. 5 3.2 Apparatus………………………………………………………………………...... 5 4.0 RESULTS AND ANALYSIS…………… ANALYSIS………………………………………… …………………………………………….6 ……………….6

4.3 Data Products……………………………………………………………………....8 5.0 DISCUSSION……………… DISCUSSION………………………………………… ……………………………………………………… ……………………………….13 ….13 6.0 CONCLUSION………………… CONCLUSION……………………………………………… …………………………………………………….. ……………………….. 15 7.0 APPENDIX…………… APPENDIX………………………………………… ……………………………………………………… …………………………………. ………. 16

Raw Data…...…………..…………………………………………………………….... 16 Reference…….………..……………………...……………………………………….. 19 Group Activity Report….…………………………………………………………….... 19


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MCET 211


Tensile Testing

Section: 01

1.0 ABSTRACT 1.0 ABSTRACT Abstract

In this experiment, three materials (HDPE, LDPE, and HIPS) were tested to determine their tensile properties using an Instron Materials Testing Machine per ASTM D 638. The materials were also tested at two different strain rates (2 in/min and 20 in/min) to evaluate the difference, if any, strain rate makes. The material with the highest ultimate tensile strength, yield strength, and elastic modulus was the Hyrene (HIPS). All comparisons between the materials were done at the 2 in/min strain rate since that is the standard. One of the reasons HIPS had the best mechanical properties of the three materials is because it also has the greatest density. The more dense the material, the greater the mechanical properties will be due to how tightly the polymers are able to pack. Comparing strain rates, all three materials generally showed an increase in yield strength and ultimate tensile strength. This follows published data that states that at higher strain rates materials will exhibit higher strength properties 9.

2.0 INTRODUCTION: 2.1 Background 2.1.1 Test

Polymers will have different strength properties depending upon chain length, density, bonds, and many other factors. To test for the difference in these properties, samples are placed into a Materials Testing Machine. This machine pulls the sample apart at a given rate and calculates the yield strength, ultimate tensile strength, % elongation, and % reduction in area. For this experiment, three different materials (HDPE, LDPE, and HIPS) were tested and compared. Another test that was run in this experiment experiment was different different strain strain rates on each of of the materials. Samples were first pulled at a rate of 2 in/min according to ASTM D 638. After pulling at the standard rate, samples were then pulled at 20 in/min. The goal of this test was to determine the strain rate effect. When samples are pulled at a low strain rate, the standard in this case, the toughness of the material is more apparent. When the strain rate increases, the material's strength and stiffness become more apparent 9. 2.1.2 Materials

The materials used for Tensile Testing were HDPE (High-density polyethylene), LDPE (Low-density polyethylene) and HIPS (High impact polystyrene). The HDPE plastic is a polyethylene thermoplastic that is made of petroleum. It has high density, high strength, high impact resistance, lightweight, can last for a long time, and resists the weather as well. It is widely used for bottles and containers for foods, drinks, personal care products such as shampoo, and household products. The LDPE plastic is a thermoplastic that is made of monomer ethylene. It is low density, high impact resistance, high moisture resistance, high chemical resistance, lightweight, and strong. It is widely used for shopping bags, toys, germ-free packages, insulations of cables and wires, and plastic film.


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MCET 211


Tensile Testing

Section: 01

Figure 1: The Structure of Polyethylene (used in both HDPE and LDPE). 1

The HIPS plastic is a polystyrene thermoplastic that is made of rubber and general purpose polystyrene (GPPS). It has high impact resistance, and low strength. It is widely used for television, appliance parts, bicycle trailer, hot and cold drink cups, instruments panels and gasoline tanks.

Figure 2: The Structure of Polystyrene (HIPS). 4

2.1.3 Structure – Property Property relationship (theory)

Tensile strength of a polymer varies with density. The more dense a polymer, the higher the tensile strength will be. If a polymer is branched, then the density will decrease since branching doesn’t allow for tight packing or folding. Linear polymers are much denser since there is nothing in the way of them getting close to each other. The only exception to this is cross-linking. Though the density of the material may be lower, the cross-linked covalent bonds provide the polymers with strength when under load.

2.2 Goals and Objectives:

The goal of this experiment was to compare tensile properties of three different materials (HDPE, LDPE, HIPS) and at two different loading rates (2 in/min and 20 in/min). The properties were also compared to published data, only for the 2 in/min per ASTM D 638. Another goal was to determine what relationship existed between the materials and their differing tensile properties.


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MCET 211


Tensile Testing

Section: 01

3.0 DESCRIPTION OF TEST 3.1 Procedure

The two samples of each HDPE, LDPE, and HIPS was picked to be tested in this experiment and used the system of ASTM D 638. One sample of each materials were tested at the loading rate of 2 in/min and the other samples were tested at the loading rate of 20 in/min. An Instron Material Testing Testing Machine Machine was used used for this experiment experiment.. The samples samples were loaded into the machine, the load rate was set and the test was run. The machine collected all of the data for later analysis while the samples were pulled by the load. A sample of HDPE was tested first with the loading rate of 2 in/min, then the data was saved into the flash-drive. A sample of LDPE was tested second with the loading rate of 2 in/min and the data was saved. A sample of HIPS was tested third with the loading rate of 2 in/min and the data was saved. The second samples of each material was repeated but with the loading rate of 20 in/min and the data was saved each time into the flash-drive.

3.2 Apparatus

Figure 3: Tensile Machine: Instron

The tensile machine, Instron, is used to determine the Ultimate Tensile Strength, Modulus of Elasticity, Yield Offset, Elongation, and the breaking point. This specific machine is used for plastics and non-metallic polymers.


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MCET 211


Tensile Testing

Section: 01

4.0 RESULTS AND RESULTS AND ANALYSIS

Table 1: Summary Table for important data collected by the tensile tester


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Tensile Testing

Table 2: Differences between experimental and published data for Yield Strength and % Elongation at break


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MCET 211


Tensile Testing

Section: 01

4.3 Data Products

Graph 1: Stress-Strain Stress-Strai n diagram diagram for the six plastic samples. Per the data supplied by the tensile testing device, the ultimate tensile strength for HDPE at an extension rate of 2 inches per minute was 21.84 MPa. The ultimate tensile strength of LDPE at an extension rate of 2 inches per minute was 14.97 MPa. The ultimate tensile strength of HIPS at an extension rate of 2 inches per minute was 28.68 MPa. For an extension rate of 20 inches per minute, the ultimate tensile strength for HDPE is 42.79 MPa. At an extension rate of 20 inches per minute, the ultimate tensile strength for LDPE was 15.76 MPa. At an extension rate of 20 inches per minute, the ultimate tensile strength for HIPS was 34.38 MPa.


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Tensile Testing

Graph 2: HDPE stress-strain graph at an extension speed of 2 inches per minute Per the data displayed in Graph 2, the ultimate tensile strength for the specimen of HDPE at an extension rate of 2 inches per minute was 21.84 MPa. The yield strength was roughly 16 MPa, the modulus of elasticity as calculated using the linear elastic region was 9.3 MPa, and the % elongation of the part reached 1012% before the tensile tester ran out of travel.

Graph 3: LDPE stress-strain graph at an extension speed of 2 inches per minute Per the data displayed in Graph 3, the ultimate tensile strength for the specimen of LDPE at an extension rate of 2 inches per minute is 14.97 MPa. The yield strength was roughly


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Tensile Testing

7 MPa, the modulus of elasticity as calculated using the linear elastic region was 1.8 MPa, and the % elongation of the part reached 240% before the specimen broke.

Graph 4: HIPS stress-strain graph at an extension speed of 2 inches per minute Per the data displayed in Graph 4, the ultimate tensile strength for the specimen of HIPS at an extension rate of 2 inches per minute was 28.68 MPa. The yield strength was roughly the same, the modulus of elasticity as calculated using the linear elastic region was 21.26 MPa, and the % elongation of the part reached 63.64% before the specimen broke.


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Tensile Testing

Graph 5: HDPE stress-strain graph at an extension speed of 20 inches per minute Per the information displayed in Graph 5, the ultimate tensile strength for the specimen of HDPE at an extension rate of 20 inches per minute was 42.79 MPa. The yield strength as calculated using the 0.2% offset line was roughly 41 MPa, the modulus of elasticity was 19.08 MPa, and the % elongation was 35.12%.

Graph 6: LDPE stress-strain graph at an extension speed of 20 inches per minute As displayed displayed on Graph Graph 6, the ultimate tensile tensile strength strength for the specimen of LDPE at an extension rate of 20 inches per minute was 15.48 MPa. The yield strength as calculated using Dustyn Crowley, Matt Eckert, Patricia Delph Rochester Institute of Technology

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Tensile Testing

the 0.2% offset line was roughly 9 MPa, the modulus of elasticity was 2.68 MPa, and the % elongation was 212.24%.

Graph 7: HIPS stress-strain graph at an extension speed of 20 inches per minute Per the data displayed in Graph 7, the ultimate tensile strength for the specimen of HIPS at an extension rate of 2 inches per minute was 34.38 MPa. The yield strength was roughly the same as the ultimate tensile strength, the modulus of elasticity as calculated using the linear elastic region was 19.01 MPa, and the % elongation of the part reached 65.83%

5.0 DISCUSSION


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Tensile Testing

Tensile testing was performed using three different plastics, each at two different extension speeds. The first plastic was high density polyethylene DMDA 8904 NT7, the second plastic tested was low density polyethylene NA960000, and the last was Entec Hyrene® PS-HI 8/2 High Impact Polystyrene. Each plastic type was run at two different tensile extension speeds: 2 inches per minute, and 20 inches per minute. Tensile testing was performed per ASTM D638 D638 standards. standards. Of the three plastics tested, HIPS had the highest ultimate and yield tensile strength, ~28 MPa for both, at 2 in./min. At an extension rate of 20 in./min, HDPE had the highest ultimate tensile strength, 42.79 MPa, while HIPS had the highest yield strength, 34.38 MPa. The greatest elongation noted for any of the samples was 1012% for HDPE at 2 in./min, while the greatest elongation at 20 in./min was significantly less at 212% for LDPE (see Table #1). Of the three plastics tested, LDPE was the only plastic that displayed brittle material characteristics, i.e. no peak at the ultimate tensile strength point followed by a drop. This held true for LDPE at both 2 in./min and 20 in./min extension rates (see Graphs #3 & #6). LDPE acts as a brittle material due to the structure of the polymer chains, which are branched. Polymers with branched chains tend to stretch poorly, because the branches wrap around other chains, stiffening and strengthening the polymer while also reducing its ability to stretch. Both HDPE and HIPS at 2 in./min and 20 in./min displayed characteristics of ductile materials. This means that their stress-strain graphs had an initial spike at ultimate tensile strength, followed by a rapid drop in strength and they a gradual increase in strength until the specimen broke. The only exception to this was HDPE, which did not break before the tensile machine ran out of travel. These plastics act as ductile materials due to the polymer chain structure type, which are linear. Linear polymers stretch well, due to a lack of branching or crosslinking. The three different plastics all displayed higher yield and ultimate tensile strength at the higher extension rate, while HDPE and LDPE displayed greater % elongation at the lower extension rate. HIPS responded very similarly in both cases. The reason why all the plastics displayed a higher yield and ultimate tensile strength at the higher extension rate is due to the inability of the polymers to stretch. Very little stretching occurred for in the polymer structures, which caused the cross-sectional area to remain roughly the same, ultimately causing the yield and tensile strength to go up. At a lower extension speed, however, the polymer chains were able to stretch, reducing the cross-sectional area. Because of this, the % elongation for LDPE and HDPE were greater at the lower speed, but their yield and tensile strengths were also less than at the higher extension rate. Additionally, both HDPE and LDPE have significant percentages of crystalline structures, 90% and 40% respectively, which will uncoil when placed under stress, thereby increasing the % elongation of the specimens. HIPS responded similarly to both extension rates because, unlike the other two plastics, HIPS has a phenyl group within the monomer structure. This phenyl group impacts the material properties in two different ways. Firstly, the phenyl groups cause increased polymer stiffness, Dustyn Crowley, Matt Eckert, Patricia Delph Rochester Institute of Technology

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Tensile Testing

giving HIPS greater impact resistance and relatively high strengths at lower extension rates. On the other hand, however, the phenyl groups do not allow the polymer chains to align themselves in crystal structures. Therefore, due to the polymer being stiff and amorphous, it does not stretch well. These results exactly match the behavior of HIPS as observed under tensile loading conditions. Marked differences were noted between experimental and published data. It is, however, important to note that published data could not be found to compare to all the experimental data, leaving potential errors unaccounted for in the experimental process. As such, the largest difference noted was a 53.6% difference between the experimental % elongation of LDPE at 2 in./min, 239% elongation, and the published value of 515% elongation. The smallest difference was a 9.6% difference between the experimental and published value for yield strength of HIPS. This significant error in % elongation can be attributed to two potential sources: 1) an incorrect value was used for published data, or 2) the tensile test specimen was of the wrong material. Major sources of error in this lab originate from several areas. First, user knowledge of the tensile testing machine, such as improper loading techniques. Next, with so many available tensile testing methods, finding data created following ASTM standard D638 and the correct extension rate posed several problems. As much data as possible was found from credible sources, and the rest of the experimental data was left uncompared. A third a very slight source of error may have been the dimensions entered for the tensile specimen, or the calibration of the machine itself.

Picture 1: Samples of HDPE, LDPE and HIPS, in respective order from top to bottom, at a loading rate of 2 in./min

Picture 2: Samples of HDPE, LDPE, and HIPS, in respective order from top to bottom, at a loading rate of 20 in/min

6.0 CONCLUSION


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Tensile Testing

In this lab, plastic tensile testing was performed to determine the material properties of high density polyethylene (HDPE) DMDA 8904 NT7, low density polyethylene (LDPE) NA960000, and Entec Hyrene® PS-HI 8/2 High Impact Polystyrene (HIPS). The material properties measured and calculated for this lab were ultimate and yield tensile strength, modulus of elasticity, and % elongation at break. Based on the testing performed, the traits of each plastic type were determined. At an extension speed of 2 in./min, the ultimate tensile strength of HDPE was 21.84 MPa, the yield strength was 16 MPa, the modulus of elasticity was 9.3 MPa, and the % elongation reached 1012% before the tensile tester ran out of travel. At 20 in./min, the ultimate tensile strength of HDPE was 42.79 MPa, the yield tensile strength was 41 MPa, the modulus of elasticity was 19.08 MPa, and the % elongation was 35.12%. For LDPE at an extension rate of 2 in./min, the ultimate tensile strength was 14.97 MPa, the yield strength was 7 MPa, the modulus of elasticity was 1.8 MPa, and the % elongation was 240%. At 20 in./min, the ultimate tensile strength was 15.79 MPa, the yield strength was 9 MPa, the modulus of elasticity was 2.68 MPa, and the % elongation was 212%. For HIPS at an extension rate of 2 in./min, the ultimate tensile strength was 28.68 MPa, the yield strength was 28.5 MPa, the modulus of elasticity was 21.26 MPa, and the % elongation was 63.64%. At 20 in./min, the ultimate tensile strength was 34.38 MPa, the yield strength was 34 MPa, the modulus of elasticity was 19.01 MPa, and the % elongation was 65.83%. See Table #1 for a summary of this data, as well as for HDPE and LDPE. Comparing the test results to published data proved to be an obstacle. Finding published data for exactly these plastics at exactly these feed rates using exactly the same ASTM test standard proved to be difficult. What data that could be found was used, however, at which point some considerable differences were noted between experimental and published data. The largest difference seen in yield strength difference was 31.6% difference between the experimental and published value for HDPE. For % elongation at break, the largest difference between experimental and published data was 53.6% for LDPE. From a structural standpoint, different traits in the plastics tested originate from differences in the polymer chain structures. These qualities relate to the type of chain structure, whether linear or branched, the level of crystallization within the polymer, and whether or not phenyl groups are present within the monomer. Error was an important factor in this lab report, concerning both the test data and the differences between test data and published data. Errors in the test data may have occurred due to lack of proper sample size and incorrect loading or operation of the tensile tester. Concerning the published data, differences are significant enough to lead to one of two conclusions. Either 1) the plastic samples tested were not made of the right material, or 2) incorrect data was used for comparison. Dustyn Crowley, Matt Eckert, Patricia Delph Rochester Institute of Technology

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MCET 211


Tensile Testing

Section: 01

7.0 APPENDIX Raw Data

See Attached Spreadsheet for Raw Data

Graph 8: HDPE with 2 in/min load rate

Graph 9: LDPE with 2 in/min load rate


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MCET 211


Tensile Testing

Section: 01

Graph 10: HIPS with with 2 in/min load rate

Graph 11: HDPE with 20 in/min load rate


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MCET 211


Tensile Testing

Section: 01

Graph 12: LDPE wiTh 20 in/min load rate

Graph 13: HIPS with 20 in/min load rate


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Tensile Testing

References

1. Chemical Structure of of Polyethylene from Google. http://www.tudosobreplasticos.com/Imagens/estruturaPE.jpg , Accessed February 16, 2017 2. Information on HDPE and LDPE from Plastic Make it Possible. https://www.plasticsmakeitpossible.com/ Accessed February 16, 2017 3. Information on HIPS from Polystyrene Package Council. http://www.polystyrenepackaging.co.za/et6-high-impact-polystyrene.htm Accessed February 16, 2017 4. How to Make Stress-Strain Stress-Str ain Graphs and Calculate Modulus and % Elongation https://www.ipc.org/TM/2.4.18.3.pdf 5. Chemical Structure of of Polystyrene from Google. https://www.researchgate.net/profile/Mahasin_AlKadhemy/publication/256442141/figure/fig2/AS:297970374332438@1448053081922/Fi g-2-Chemical-structure-of-PS-Mitchell-2004.png Accessed February 16, 16, 2017 6. HDPE Material Properties. http://www.matweb.com/search/datasheet.aspx?matguid=a1d6b5d64f8f4184b7708583b 1ed2980 Accessed 1ed2980 Accessed February 19, 19, 2017 7. LDPE Material Properties. http://www.matweb.com/search/datasheet.aspx?matguid=d205f4bf11aa446ebf7da105e 5cd9259&ckck=1,, https://www.plasticsintl.com/datasheets/LDPE.pdf Accessed February 5cd9259&ckck=1 19, 2017 8. HIPS Material Properties. Accessed February 19, 2017 http://www.matweb.com/search/datasheet.aspx?matguid=c49f4fb2a4c347aca55f94cdc1 26a545 9. The Strain Rate Effect, Accessed February 20, 2017 http://www.ptonline.com/columns/the-strain-rate-effect Group Activity Report

Dustyn Crowley did Table of Contents, Abstract, Test, Structure, Goals and Objectives, Discussion and Reference. Matt Eckert did Results and Analysis, Data Products, Discussion, Conclusion, Raw Data, and References. Patricia Delph did the first page of lab report, Table of Contents, Materials and Procedure, Apparatus, Discussion, References, and Group activities Report.


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lab report - tensile testing

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