Your Name: Ulises Corona

Name of the MAE2160 Materials Science

Course:
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 steady-
state 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 119 0.19 345 137 0.37 690

(mm) Strain (sec) 120 0.2 360 138 0.38 705
103 0.03 15 120 0.2 375 139 0.39 720
105 0.05 30 121 0.21 390 140 0.4 735
106 0.06 45 122 0.22 405 141 0.41 750
107 0.07 60 123 0.23 420 142 0.42 765
108 0.08 75 125 0.25 435 143 0.43 780
109 0.09 90 126 0.26 450 145 0.45 795
109 0.09 105 127 0.27 465 146 0.46 810
109 0.09 120 127 0.27 480 147 0.47 825
109 0.09 135 128 0.28 495 149 0.49 840
109 0.09 150 128 0.28 510 150 0.5 855
110 0.1 165 128 0.28 525 153 0.53 870
111 0.11 180 128 0.28 540 154 0.54 885
112 0.12 195 129 0.29 555 155 0.55 900
113 0.13 210 130 0.3 570 157 0.57 915
114 0.14 225 131 0.31 585 160 0.6 930
115 0.15 240 132 0.32 600 163 0.63 945
116 0.16 255 133 0.33 615 165 0.65 960
116 0.16 270 133 0.33 630 167 0.67 975
117 0.17 285 133 0.33 645 173 0.73 990
118 0.18 300 134 0.34 660 177 0.77 1005
119 0.19 315 136 0.36 675 180 0.8 1020
119 0.19 330
185 0.85 1035
194 0.94 1050
200 1 1065
225 1.25 1080
260 1.6 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
BE (breaking
Material energy)
PMMA 26.368 in-lbf
Polytetrafluoroethylene 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.