Lab 5
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Your Name:
Ulises Corona
Name of the Course:
MAE2160 Materials Science
Lab Date:
April 3, 2009
Report Date:
April 14, 2009
LAB REPORT #5: “POLYMERIC MATERIALS”
In this lab we observed qualitatively the behavior of polymeric materials under creep, temperature and impact loads. We used bouncing putty, PVC, and acrylic glass samples for respective tests. Polymers show particular responses under the tested mechanical and thermal stimuli that are not found in metallic crystalline materials. Polymeric materials demonstrate higher susceptibility to failure when exposed to creep, temperature and impact loads than metallic materials.
Lab Report
I
I NTRODUCTION We are interested in how polymeric materials react under sustained static forces along time (creep), and instantaneous stresses due to impact forces. Also, we are testing the incidence of temperature macroscopically in thermoplastics once is passed the transition temperature Tg. These experiments will form a reference for contrasting distinct mechanical behavior between crystalline metals and polymers. We need to understand first some basic concepts upon the performed tests are based on: Creep. Creep is high temperature progressive deformation at constant stress. "High temperature" is a relative term dependent upon the materials involved. Creep rates are used in evaluating materials for boilers, gas turbines, jet engines, ovens, or any application that involves high temperatures under load. Understanding high temperature behavior of metals is useful in designing failure resistant systems. A creep test involves a tensile specimen under a constant load maintained at a constant temperature. Measurements of strain are then recorded over a period of time. Thermoplastics. Polymers that turn to liquid when heated, and freeze to a very glassy state when cooled sufficiently. Thermoplastic polymers exhibit the phenomenon of glass transition temperature (Tg). It is the temperature at which the polymer chains are excited enough to be able to rotate and twist, especially at their free-ends. At this temperature the material becomes soft or rubbery. Most thermoplastic materials must be designed based on this temperature rather than the melting temperature. We have used a PVC sample and heated it up just above its Tg and observed the changes in its properties. Impact toughness. The amount of energy that a material can absorb from a sudden, sharp blow before it breaks or fracture.
To measure toughness, we perform the impact toughness test, which makes use of a pendulum-testing machine. The specimen is broken by a single overload event due to the impact of the pendulum. A stop pointer is used to record how far the pendulum swings back up after fracturing the specimen. The impact toughness of a sample is determined by measuring the energy absorbed in the fracture of the specimen. This is simply obtained by noting the height at which the pendulum is released and the height to which the pendulum swings after it has struck the specimen. The height of the pendulum times the weight of the pendulum produces the potential energy and the difference in potential energy of the pendulum at the start and the end of the test is equal to the absorbed energy. Toughness is greatly affected by temperature. Then the test for toughness is often repeated numerous times with each specimen tested at a different temperature. This produces a graph of impact toughness for the material as a function of temperature. At high temperatures the material is more ductile and impact toughness is higher. The transition temperature is the boundary between brittle and ductile behavior and this temperature is often an extremely important consideration in the selection of a material.
II
E XPERIMENT PROCEDURE
The cool PVC sample (prismatic bar) should be put in the oven and be heated up to 87 deg C. The PVC bar is easily bent when it is close to its melting temperature, unlike when it is at room temperature. For the impact toughness test, we make use of a hammer pendulum machine. This concerns a wedge-shaped formed load, which is hung up at a free running pendulum. The machine will sense the breaking energy and shear strength of the sample. To test the PMMA and Lynon samples for breaking energy, we put the given sample in the platform of the machine and hold it tight. The hammer is then loaded, and the breaking energy will be higher depending on how the sensors of the machine measure the bouncing of the hammer. The last experiment performed was testing the silly putty cylinder for creep. Using a ruler and a timer we can know the deformation of the cylinder along time as a tensile force causes tensile strain (maybe the weight of the silly putty itself, which was what we used).
III
R ESULT AND DISCUSSION (i)
Description of the creep test and the results obtained with a graph of the strain vs. time. Describe the creep phenomenon in amorphous materials.
Following the loading strain ε0, the creep rate (slope of strain vs time curve), is high but decreases as the material deforms during the primary creep stage. At sufficiently large strains, the material creeps at a constant rate. This is called the secondary or steadystate creep stage. Ordinarily this is the most important stage of creep since the time to failure tf is determined primarily by the secondary creep rate s. In the case of tension creep, the secondary creep stage is eventually interrupted by the onset of tertiary creep, which is characterized by internal fracturing of the material, creep acceleration, and finally failure. The creep rate is usually very temperature-dependent. At low temperatures or applied stresses the time scale can be thousands of years or longer. At high temperatures the entire creep process can occur in a matter of seconds. The mechanism of creep invariably involves the sliding motion of atoms or molecules past each other. In amorphous materials such as glasses, almost any atom or molecule within the material is free to slide past its neighbor in response to a shear stress. In plastics, the long molecular chains can slide past each other only to a limited extent. Such materials typically show large anelastic creep effects. Position Time (mm) Strain (sec) 103 0.03 15 105 0.05 30 106 0.06 45 107 0.07 60 108 0.08 75 109 0.09 90 109 0.09 105 109 0.09 120 109 0.09 135 109 0.09 150 110 0.1 165 111 0.11 180 112 0.12 195 113 0.13 210 114 0.14 225 115 0.15 240 116 0.16 255 116 0.16 270 117 0.17 285 118 0.18 300 119 0.19 315 119 0.19 330
119 120 120 121 122 123 125 126 127 127 128 128 128 128 129 130 131 132 133 133 133 134 136
0.19 0.2 0.2 0.21 0.22 0.23 0.25 0.26 0.27 0.27 0.28 0.28 0.28 0.28 0.29 0.3 0.31 0.32 0.33 0.33 0.33 0.34 0.36
345 360 375 390 405 420 435 450 465 480 495 510 525 540 555 570 585 600 615 630 645 660 675
137 138 139 140 141 142 143 145 146 147 149 150 153 154 155 157 160 163 165 167 173 177 180
0.37 0.38 0.39 0.4 0.41 0.42 0.43 0.45 0.46 0.47 0.49 0.5 0.53 0.54 0.55 0.57 0.6 0.63 0.65 0.67 0.73 0.77 0.8
690 705 720 735 750 765 780 795 810 825 840 855 870 885 900 915 930 945 960 975 990 1005 1020
185 194 200 225 260
0.85 0.94 1 1.25 1.6
1035 1050 1065 1080 1095
Position (0) = 100 (L0) Epsilon = (L L0)/L
(ii)
Describe the glass transition temperature experiment and discuss your observations. Explain why thermoplastic materials exhibit Tg.
PVC cannot be bent cool. Increasing the temperature above the transition temperature made the sample more ductile since PVC is a thermoplastic. This macroscopic behavior can be explained in terms of the internal structure of the sample. As temperature
increases the kinetic energy of the particles increases as well, the vibrations make unstable weak molecular bonding. Then, the polymeric carbon chains begin to slide one against the other more easily than when the temperature is low, in comparison to the melting point of the sample (~100 deg C). The Glass transition temperature, Tg, is the temperature at which an amorphous solid, such as glass or a polymer, becomes brittle on cooling, or soft on heating. The glass transition is a phase transition in which a supercooled melt yields, on cooling, a glassy structure and properties similar to those of crystalline materials e.g. of an isotropic solid material. Tg is usually applicable to wholly or partially amorphous solids such as common glasses and plastics (organic polymers), although there is an analogous phenomenon in crystalline metals called the ductile-brittle transition temperature. Below the glass transition temperature, Tg, amorphous solids are in a glassy state and most of their joining bonds are intact. In inorganic glasses, with increased temperature more and more joining bonds are broken by thermal fluctuations so that broken bonds begin to form clusters. Above Tg these clusters become large facilitating the flow of material. In organic polymers, secondary, non-covalent bonds between the polymer chains become weak above Tg. Above Tg glasses and organic polymers become soft and capable of plastic deformation without fracture. This behavior is one of the things which make most plastics useful. (iii)
Report the results of the impact test for the two test materials in a table. Compare the toughness levels and explain the differences.
Impact Toughness Test Material PMMA Polytetrafluoroethylene
BE (breaking energy) 26.368 in-lbf 48.602 in-lbf
The impact toughness for lynon doubles that for PMMA. PMMA is a brittle material when loaded with impact forces.
IV
C ONCLUSION Creep, ductility and impact strength are all three functions of
temperature. The macroscopic behavior of the given material sample reflects the internal structure of such material. Polymeric materials are of amorphous structure, so unlike metals, they show greater creep under similar loads, lower impact toughness, and show lower melting points.
Ulises Corona
Name of the Course:
MAE2160 Materials Science
Lab Date:
April 3, 2009
Report Date:
April 14, 2009
LAB REPORT #5: “POLYMERIC MATERIALS”
In this lab we observed qualitatively the behavior of polymeric materials under creep, temperature and impact loads. We used bouncing putty, PVC, and acrylic glass samples for respective tests. Polymers show particular responses under the tested mechanical and thermal stimuli that are not found in metallic crystalline materials. Polymeric materials demonstrate higher susceptibility to failure when exposed to creep, temperature and impact loads than metallic materials.
Lab Report
I
I NTRODUCTION We are interested in how polymeric materials react under sustained static forces along time (creep), and instantaneous stresses due to impact forces. Also, we are testing the incidence of temperature macroscopically in thermoplastics once is passed the transition temperature Tg. These experiments will form a reference for contrasting distinct mechanical behavior between crystalline metals and polymers. We need to understand first some basic concepts upon the performed tests are based on: Creep. Creep is high temperature progressive deformation at constant stress. "High temperature" is a relative term dependent upon the materials involved. Creep rates are used in evaluating materials for boilers, gas turbines, jet engines, ovens, or any application that involves high temperatures under load. Understanding high temperature behavior of metals is useful in designing failure resistant systems. A creep test involves a tensile specimen under a constant load maintained at a constant temperature. Measurements of strain are then recorded over a period of time. Thermoplastics. Polymers that turn to liquid when heated, and freeze to a very glassy state when cooled sufficiently. Thermoplastic polymers exhibit the phenomenon of glass transition temperature (Tg). It is the temperature at which the polymer chains are excited enough to be able to rotate and twist, especially at their free-ends. At this temperature the material becomes soft or rubbery. Most thermoplastic materials must be designed based on this temperature rather than the melting temperature. We have used a PVC sample and heated it up just above its Tg and observed the changes in its properties. Impact toughness. The amount of energy that a material can absorb from a sudden, sharp blow before it breaks or fracture.
To measure toughness, we perform the impact toughness test, which makes use of a pendulum-testing machine. The specimen is broken by a single overload event due to the impact of the pendulum. A stop pointer is used to record how far the pendulum swings back up after fracturing the specimen. The impact toughness of a sample is determined by measuring the energy absorbed in the fracture of the specimen. This is simply obtained by noting the height at which the pendulum is released and the height to which the pendulum swings after it has struck the specimen. The height of the pendulum times the weight of the pendulum produces the potential energy and the difference in potential energy of the pendulum at the start and the end of the test is equal to the absorbed energy. Toughness is greatly affected by temperature. Then the test for toughness is often repeated numerous times with each specimen tested at a different temperature. This produces a graph of impact toughness for the material as a function of temperature. At high temperatures the material is more ductile and impact toughness is higher. The transition temperature is the boundary between brittle and ductile behavior and this temperature is often an extremely important consideration in the selection of a material.
II
E XPERIMENT PROCEDURE
The cool PVC sample (prismatic bar) should be put in the oven and be heated up to 87 deg C. The PVC bar is easily bent when it is close to its melting temperature, unlike when it is at room temperature. For the impact toughness test, we make use of a hammer pendulum machine. This concerns a wedge-shaped formed load, which is hung up at a free running pendulum. The machine will sense the breaking energy and shear strength of the sample. To test the PMMA and Lynon samples for breaking energy, we put the given sample in the platform of the machine and hold it tight. The hammer is then loaded, and the breaking energy will be higher depending on how the sensors of the machine measure the bouncing of the hammer. The last experiment performed was testing the silly putty cylinder for creep. Using a ruler and a timer we can know the deformation of the cylinder along time as a tensile force causes tensile strain (maybe the weight of the silly putty itself, which was what we used).
III
R ESULT AND DISCUSSION (i)
Description of the creep test and the results obtained with a graph of the strain vs. time. Describe the creep phenomenon in amorphous materials.
Following the loading strain ε0, the creep rate (slope of strain vs time curve), is high but decreases as the material deforms during the primary creep stage. At sufficiently large strains, the material creeps at a constant rate. This is called the secondary or steadystate creep stage. Ordinarily this is the most important stage of creep since the time to failure tf is determined primarily by the secondary creep rate s. In the case of tension creep, the secondary creep stage is eventually interrupted by the onset of tertiary creep, which is characterized by internal fracturing of the material, creep acceleration, and finally failure. The creep rate is usually very temperature-dependent. At low temperatures or applied stresses the time scale can be thousands of years or longer. At high temperatures the entire creep process can occur in a matter of seconds. The mechanism of creep invariably involves the sliding motion of atoms or molecules past each other. In amorphous materials such as glasses, almost any atom or molecule within the material is free to slide past its neighbor in response to a shear stress. In plastics, the long molecular chains can slide past each other only to a limited extent. Such materials typically show large anelastic creep effects. Position Time (mm) Strain (sec) 103 0.03 15 105 0.05 30 106 0.06 45 107 0.07 60 108 0.08 75 109 0.09 90 109 0.09 105 109 0.09 120 109 0.09 135 109 0.09 150 110 0.1 165 111 0.11 180 112 0.12 195 113 0.13 210 114 0.14 225 115 0.15 240 116 0.16 255 116 0.16 270 117 0.17 285 118 0.18 300 119 0.19 315 119 0.19 330
119 120 120 121 122 123 125 126 127 127 128 128 128 128 129 130 131 132 133 133 133 134 136
0.19 0.2 0.2 0.21 0.22 0.23 0.25 0.26 0.27 0.27 0.28 0.28 0.28 0.28 0.29 0.3 0.31 0.32 0.33 0.33 0.33 0.34 0.36
345 360 375 390 405 420 435 450 465 480 495 510 525 540 555 570 585 600 615 630 645 660 675
137 138 139 140 141 142 143 145 146 147 149 150 153 154 155 157 160 163 165 167 173 177 180
0.37 0.38 0.39 0.4 0.41 0.42 0.43 0.45 0.46 0.47 0.49 0.5 0.53 0.54 0.55 0.57 0.6 0.63 0.65 0.67 0.73 0.77 0.8
690 705 720 735 750 765 780 795 810 825 840 855 870 885 900 915 930 945 960 975 990 1005 1020
185 194 200 225 260
0.85 0.94 1 1.25 1.6
1035 1050 1065 1080 1095
Position (0) = 100 (L0) Epsilon = (L L0)/L
(ii)
Describe the glass transition temperature experiment and discuss your observations. Explain why thermoplastic materials exhibit Tg.
PVC cannot be bent cool. Increasing the temperature above the transition temperature made the sample more ductile since PVC is a thermoplastic. This macroscopic behavior can be explained in terms of the internal structure of the sample. As temperature
increases the kinetic energy of the particles increases as well, the vibrations make unstable weak molecular bonding. Then, the polymeric carbon chains begin to slide one against the other more easily than when the temperature is low, in comparison to the melting point of the sample (~100 deg C). The Glass transition temperature, Tg, is the temperature at which an amorphous solid, such as glass or a polymer, becomes brittle on cooling, or soft on heating. The glass transition is a phase transition in which a supercooled melt yields, on cooling, a glassy structure and properties similar to those of crystalline materials e.g. of an isotropic solid material. Tg is usually applicable to wholly or partially amorphous solids such as common glasses and plastics (organic polymers), although there is an analogous phenomenon in crystalline metals called the ductile-brittle transition temperature. Below the glass transition temperature, Tg, amorphous solids are in a glassy state and most of their joining bonds are intact. In inorganic glasses, with increased temperature more and more joining bonds are broken by thermal fluctuations so that broken bonds begin to form clusters. Above Tg these clusters become large facilitating the flow of material. In organic polymers, secondary, non-covalent bonds between the polymer chains become weak above Tg. Above Tg glasses and organic polymers become soft and capable of plastic deformation without fracture. This behavior is one of the things which make most plastics useful. (iii)
Report the results of the impact test for the two test materials in a table. Compare the toughness levels and explain the differences.
Impact Toughness Test Material PMMA Polytetrafluoroethylene
BE (breaking energy) 26.368 in-lbf 48.602 in-lbf
The impact toughness for lynon doubles that for PMMA. PMMA is a brittle material when loaded with impact forces.
IV
C ONCLUSION Creep, ductility and impact strength are all three functions of
temperature. The macroscopic behavior of the given material sample reflects the internal structure of such material. Polymeric materials are of amorphous structure, so unlike metals, they show greater creep under similar loads, lower impact toughness, and show lower melting points.
