This document summarizes an experiment that tested the time-dependent deformation properties of polymers through creep testing and impact testing. Creep testing of a Silly Putty sample found that its strain rate increased over time under a constant load as its cross-sectional area decreased. Impact testing showed that an unnotched HDPE sample absorbed nearly all impact energy without breaking, while an unnotched PMMA sample broke after absorbing less than half the energy. The experiment demonstrated methods for determining a polymer's resistance to time-dependent deformation and impact forces.

1. MAE 2060 Material Science
Postlab Report #5
Polymer Materials
Brandi Homer

2. Abstract:
The following experiment uses a creep test to determine the time-dependent deformation
properties of a highly viscous polymer, a Blue Lundberg Oven to observe the physical
behavior of a PVC specimen at a temperature just above its glass transition temperature,
and an Izod Impact Tester to determine the possible impact loading of three polymer
specimens. The experiment results are consistent with current creep predictions and with
previously know impact-loading capabilities. The experiment focuses on properties that
may be determined from time-dependent deformation and on fracture toughness.
Introduction:
This laboratory experiment examines the viscoelastic creep of the highly viscous polymer
“Silly Putty” in order to examine the time-dependent deformation under a constant load.
This test useful in that the information gathered is used to determine deformation to
failure, steady-state strain rate, time to rupture, and the time-dependent modulus. The
properties are especially important to engineers in design considerations. The experiment
also includes the testing of three polymer specimens in an Izod Impact Tester in order to
determine fracture toughness. Since fracture toughness for polymers is highly dependent
upon the temperature of the test specimen relative to its glass transition temperature, a
PVC specimen was observed just above this temperature. Fracture toughness is an
important design consideration because proper material must be used when a specific
amount of energy needs to be absorbed during use.
Approach:
The sample of Silly Putty was molded into a square rod approximately 4 inches long and
0.25 in on each side. The sample had an apparently negligible number of voids or cracks.
A mass of material was left on the end of the rod in order to attach it to the edge of the
table. The sample was suspended at the edge of the table with the reference point being
the lower surface of the table. The length of the rod as viscoelastic creep ensued was
recorded every 15 seconds until failure.

3. The sample was remolded into another rod of approximately the same dimensions and the
experiment was performed again for a second set of data points. Upon completion of the
second test performance, the rod was remolded again without the additional mass at the
end in order to be weighed. The weight of the rod was halved then converted from g to
lbm and that was used to calculate the load on the rod. Using the recorded data, a graph
of the strain versus time was constructed for both Engineering strain and True strain. The
time-dependent creep modulus versus time graph was also calculated. The following
equations (Callister 2003) were used for the calculations.
εe = li – l0 Eq. 1
l0
εT = ln
Ec(t) =
li
-- Eq. 2
l0
σ0
--- Eq. 3
ε(t)
Where εe is engineering strain, εT is true strain, li is the instantaneous length, l0 is the
original length, Ec(t) is the time-dependent creep modulus, σ0 is the constant applied
stress, and ε(t) is the time dependent strain.
The Lab TA preheated the Blue Lundberg Oven to a temperature just above the Tg of
PVC (190°F). A sample of PVC tubing was inserted into the oven and allowed to reach
its glass transition temperature. Using asbestos gloves, the tubing was removed and
manipulated in order to observe the behavior of the material.
An impact test was performed on three different polymer materials, one unnotched high-
density polyethylene (HDPE) specimen, one notched HDPE specimen, and one

4. unnotched polymethyl methacrylate (PMMA), with an Izod Impact Tester. The potential
energy of the impact tester was recorded in order to compare and contrast the results of
the three specimens. Table 15.1 (Callister 2003) was used in the comparison.
Results:
The weight of the silly putty rod was measured at 6.3 g. Half of this is 3.15 g. This was
converted to 0.00694 lbm then used to find the force of 0.2233 lbf on the specimen.
During the creep test, the length in inches of the specimens from the first and second test
were recorded in columns two and three at the given time interval of fifteen seconds
provided in column one. Using Eq. 1 and Eq. 2, columns four through seven for the
engineering strain and true strain for the first and second test were tabulated.
Table 1 – Compilation of test and calculated data for creep test
Time (m:s) Test 1
(in)
Test 2
(in)
εe Test 1 εe Test 2 εT Test 2 εT Test 2
0:15 4 4.125 0 0.03125 0 0.03077
0:30 4.125 4.25 0.03125 0.0625 0.03077 0.06062
0:45 4.25 4.35 0.0625 0.0875 0.06062 0.08388
1:00 4.3 4.4 0.075 0.1 0.07232 0.09531
1:15 4.4 4.5 0.1 0.125 0.09531 0.11778
1:30 4.5 4.62 0.125 0.155 0.11778 0.14410
1:45 4.625 4.63 0.15625 0.1575 0.14518 0.14623
2:00 4.75 4.75 0.1875 0.1875 0.17185 0.17185
2:15 5 4.87 0.25 0.2175 0.22314 0.19680
2:30 5.125 4.9 0.28125 0.225 0.24783 0.20294
2:45 5.25 5 0.3125 0.25 0.27193 0.22314
3:00 5.375 5.125 0.34375 0.28125 0.29546 0.24783
3:15 5.5 5.25 0.375 0.3125 0.31845 0.27193

5. The following graphs were constructed of the engineering strain and true strain using the
information calculated for Table 1. Series 1 was the first test and Series 2 was the second
test.
"
Graph 1 – Engineering Strain vs. time
3:30 5.62 5.375 0.405 0.34375 0.34003 0.29546
3:45 5.63 5.5 0.4075 0.35 0.34181 0.31845
4:00 5.75 5.625 0.4375 0.40625 0.36291 0.34093
4:15 5.875 5.75 0.46875 0.4375 0.38441 0.36291
4:30 6 5.875 0.5 0.46875 0.40546 0.38441
4:45 6.25 6.125 0.5625 0.53125 0.44629 0.42608
5:00 6.375 6.25 0.59375 0.5625 0.46609 0.44629
5:15 6.625 6.5 0.65625 0.625 0.50456 0.48551
5:30 6.75 6.75 0.6875 0.6875 0.52325 0.52325
5:45 7 7 0.75 0.75 0.55962 0.55962
6:00 7.375 7.375 0.84375 0.84375 0.66180 0.66180
6:15 7.625 7.875 0.90625 0.96875 0.64514 0.67740
6:30 8.5 8.75 1.125 1.1875 0.75377 0.78276
6:45 9.75 14 1.4375 2.5 0.89097 1.25276
6:56 15.25 N/A 2.8125 N/A 1.33829 N/A
EngineeringStrain
0
0.75
1.5
2.25
3
Time
0:15 1:00 1:45 2:30 3:15 4:00 4:45 5:30 6:15 6:56
Series1
Series2

6. "
Graph 2 – True Strain vs. time
Using true strain for the time-dependent strain, the following graph was created for the
time-dependent creep modulus vs. time. Series 1 was the first test and Series 2 was the
second test.
"
Graph 3 – Time-dependent creep modulus vs. time
This table presents the impact test results for the unnotched high-density polyethylene
(HDPE) and polymethyl methacrylate (PMMA) specimens and the notched HDPE
specimen.
TrueStrain
0
0.35
0.7
1.05
1.4
Time
0:15 1:00 1:45 2:30 3:15 4:00 4:45 5:30 6:15 6:56
Series1
Series2
Time-DependentCreep
Modulus
0
2
4
6
8
Time
0:15 1:00 1:45 2:30 3:15 4:00 4:45 5:30 6:15 6:56
Series1
Series2

7. Table 2 – Impact test results
Discussion:
The experimental results for the second creep test appear more accurate than those of the
first test. The non-uniformity in the first set of data could be caused by any number of
variables; i.e. voids or cracks in the material, non-uniform dimensions, lower
temperature, inaccuracy in measurements, or the lint that was visible in the specimen.
For results with a higher accuracy, the experiment should be performed in a controlled
environment with higher quality measuring equipment and higher purity material.
The strain rates changes over time because the load does not change relatively slowly for
the given material. The cross-sectional area where the load is applied shrinks and as it
shrinks, total load capacity decreases exponentially. With regard to the time-dependent
creep modulus, as indicated by the shape of the graph, less force is required to induce
deformation in the specimen.
The main difference between the unnotched and notched specimens of HDPE was the
type of break. The unnotched HDPE absorbed 99.8% of the potential energy of the Izod
Impact Tester preventing even the partial break of the notched HDPE.
The unnotched PMMA was capable of absorbing less than half of the energy the
unnotched HDPE, therefore causing it to fracture completely. Reasons for this breakage
of the PMMA include higher yield strength and tensile strength causing the material to be
less ductile than HDPE.
Material Potential Energy
(in-lb)
Break Energy
(Absorbed) (in-lb)
Type of Break
HDPE unnotched 48.694 48.606 Non Break
HDPE notched 48.694 48.570 Partial
PMMA unnotched 48.694 23.743 Complete

8. Conclusions:
Creep tests to failure are an accurate way to determine time-dependent deformation and
fracture under constant loading when the experiment is performed accurately.
Deformation to failure is not as difficult to predict as brittle fracture because creep rates
are relatively slow and materials tend to follow the same general trend. The Izod Impact
tester, when used appropriately is an accurate means of determining impact loading of
varying polymers. The determination of impact loading is highly relevant because
engineers are required to know if a particular material will be able to absorb a sufficient
amount of energy to prevent fracture. The experiment itself provided hands on
experience with the testing methods and equipment and the application of specific
concepts.