Industrial Activities:
Pendulum Impact Activities
Fatigue Resistance Activities
Rebound Activity
Rebound Height Data Sheet
Click
herefor more advanced versions of these activities on the original
Hands on Plastics™ site - designed for students with more advanced equipment
like bunsen burners.
Pendulum Impact Activities
A variety of tests have been developed to determine plastic's or plastic
products' ability to withstand a variety of impacts. Different plastic
products are designed to withstand different impacts
- Car bumpers need to absorb energy upon collision.
- Plastic foodware should not break if dropped.
- A notebook should open without the spine cracking.
Different kinds of impact tests involve doing different things to samples:
- Tensile tests pull samples apart.
- Compression tests squeeze and push samples together.
- Flex tests bend samples.
- Torsion tests twist samples.
The Izod Impact test is a very popular impact test that requires the
use of a pendulum.
Before the pendulum is released, the hammer represents full potential
energy (PE). Potential energy is the stored energy that occurs as a result
of the relative positions of objects in a system. When the pendulum is
released, the potential energy changes to kinetic energy (KE). Kinetic
energy is the energy of a moving object resulting from its motion. The
PE is completely changed to KE at the bottom of the arc, assuming there
is no friction.
The pendulum will continue to swing to the other side to the same height
as it started at which point all the kinetic energy is changed to potential
energy. The pendulum swings back and forth unless something interrupts
the swing.
What happens when a plastic test sample interrupts the swing?
- The pendulum doesn't swing as high.
- The sample is broken.
- There is a sound on impact.
- There are vibrations on impact.
Activity 1
Using a child's toy golf club as the pendulum, compare the differences
between a variety of rigid plastic foam samples (Rigid foam samples
are available at home construction and supply stores. Thickness may
vary.)
Cut samples approximately 20cm x 5cm.
Clamp approximately half of the sample, keeping it in a vertical position.
Swing the golf club so that it impacts the top of the sample. Briefly
describe the amount of energy it takes to break the sample.
Did you hear the sample break? What happened? (Energy at impact was
transformed into sound energy.)
Did you feel vibrations when the pendulum hit the sample? What happened?
(Energy at impact was transformed into vibrations.)
Activity 2
Compare the impact resistance of samples cut from different types
of foamed material.
Activity 3
Compare the impact resistance of samples with varied depths, maintaining
the 20cm height and 5cm width.
Activity 4
Compare the impact resistance of samples cut with the grain of the
foam sheet to samples cut against the grain (Foam sheeting with a grain
will appear to have fine lines running the length of the sheet.)
Activity 5
Compare the impact resistance of notched samples versus unnotched
samples.
Activity 6
Impact the sample on
the face with the notch (Izod A), a tensile test.
Compare this to impacting the sample on the face opposite the notch
(Izod E), a compression test.
Keep a journal of all tests, procedures and results, including diagrams
of test samples.
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Fatigue Resistance Activities
Plastics in some applications need to withstand flexing, stretching,
compressing and twisting.
For example, a plastic chair is stressed each time someone sits in it
or leans back in it.
When the stress is repeated over and over, the material needs to be
resistant to "fatigue". The fatigue life of a material is described
as the number of cycles or times the sample is deformed.
A product or material that does not have good fatigue resistance may
not withstand normal usage and will crack, whiten, discolor, or break.
A material's fatigue resistance is very important in many applications.
List some products you can think of that need good fatigue resistance.
For the following set of activities, you will need several rectangular
pieces of plastic, consistenly the same thickness and size. We recommend
samples to be about 2mm thick and about 3cm by 15cm in size.
Consider using lids from butter containers, lids from coffee cans, or
lids from food storage containers. You can also use solid plastic binders.
You can also use plastic plates either solid or foam plates, as students
may understand better a plate holding up to the weight of food.
Activity 1
Develop a test to determine the number of cycles needed to break a
sample when flexing the sample back and forth.
- Describe what various loads do to a sample under stress (e.g.,
color change or whitening, a break, permanent change in shape).
- Describe how these results could affect the looks or performance
of a product, such as a chair, box lid, food container, or a car bumper.
Different plastic behaves differently at various temperatures.
Using samples chilled in the freezer, refrigerator, or in ice water,
develop a test to determine the number of cycles needed to break a sample
when flexing the sample back and forth.
Using samples that have been heated in warm or boiling water, develop
a test to determine the number of cycles needed to break a sample when
flexing the sample back and forth.
How does varying the temperature affect the fatigue resistance of
the plastic sample?
Activity 2
Develop a test to determine the number of cycles needed to break a
sample when twisting it.
- Describe what various loads do to a sample under stress (e.g.,
color change or whitening, a break, permanent change in shape).
- Describe how these results could affect the looks or performance
of a product, such as a chair, box lid, food container, or a car bumper.
Different plastic behaves differently at various temperatures.
Using samples chilled in the freezer, refrigerator, or in ice water,
develop a test to determine the number of cycles needed to break a sample
when twisting it.
Using samples that have been heated in warm or boiling water, develop
a test to determine the number of cycles needed to break a sample when
twisting it.
How does varying the temperature affect the fatigue resistance of
the plastic sample?
Activity 3
Develop a test that applies a constant stress (load) that deflects
(bends) a sample.
- Describe what various loads do to a sample under stress (e.g.,
color change or whitening, a break, permanent change in shape).
- Describe how these results could affect the looks or performance
of a product, such as a chair, box lid, food container, or a car bumper.
Using samples chilled in the freezer, refrigerator, or in ice water,
develop a test that applies a constant stress (load) that deflects (bends)
a sample.
Using samples that have been heated in warm or boiling water, develop
a test that applies a constant stress (load) that deflects (bends) a
sample.
How does varying the temperature affect the fatigue resistance of
the plastic sample?
Different plastic behaves differently at various temperatures.
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Rebound Activity
You work for the AJAX Ball Company and you have been asked
to test their product line of balls. There have been complaints about
the rebound height of some of the products and you need to determine which
balls are defective. Keep in mind that different materials and different
construction will affect the rebound height of the ball, therefore a low
rebound height may be appropriate in certain situations. You will also
need to calculate the Percent Energy Transferred.
What types of energy transfers occur in the second example?
You will test the rebound height of each sample and then determine if
the rebound height is appropriate for the use.
Procedure:
- Tape the meter tape such that the “zero” mark is flush
to the floor and the tape extends up the wall past the 100 cm mark.
- Place the ball to be tested so the bottom of the ball is even with
the 100 cm mark.
- Release the ball and note the height the bottom of the ball rebounds
to + 0.1 cm
- Repeat the test 4 more times (total of 5).
- Average the results.
- Determine the Percent Energy Transferred for the sample tested:
(Height Dropped – Average Rebound Height)
X 100 = Percent Energy Transferred
Height Dropped
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Rebound Height Data Sheet
Describe the sample you are going to test.
Data
1. _________________________ Calculate the Percent Energy Transferred
2. _________________________
3. _________________________
4. _________________________
5. _________________________
Average Rebound Height __________________________
Analysis of Data:
Your Recommendation:
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