- Pendulum Impact Tests
- Fatigue Resistance Tests

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Pendulum Impact Tests

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 a 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). The PE is completely changed to KE at the bottom of the arc, assuming there is no friction. Kinetic energy is the energy of a moving object resulting from its motion.

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.
  

The pendulum does not swing as high because the energy was transferred from the pendulum to the sample.

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. The height and width of samples should remain constant.

Clamp approximately half of the sample as shown, keeping it in a vertical position. To build a clamp, you may use two 'v'-shaped pieces of wood. Glue, nail or screw these pieces into a sturdy surface. This step requires adult supervision.

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. To make a notch, indent a "V" wedge the entire width of the sample.

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.

Fatigue Resistance Tests

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, consistently the same thickness and size. We recommend that the samples be about 2mm thick and about 3cm by 15cm in size.

Consider using lids from butter containers, coffee cans, or from food storage containers.

Activity 1

Develop a test to determine the number of cycles needed to break a sample when flexing the sample back and forth.

Activity 2

Develop a test to determine the number of cycles needed to break a sample when twisting it.

Activity 3

Develop a test that applies a constant stress (load) that bends a sample. How long does it take before the sample whitens, cracks, or breaks?

• Compare various loads with the same material.
• Compare how the same load affects different materials.
• Describe what various loads do to a sample under stress (e.g., color change or whitening, a break, or 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 plastics behave differently at various temperatures.

Some materials may become brittle in very cold temperatures while others remain very flexible (such as ice cube trays).

Some plastics can be made into food trays and can be heated in the microwave, while another plastic would melt under the same conditions.

Activity 4

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 it back and forth.

How does varying the temperature affect the fatigue resistance of the plastic sample?

Activity 5

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.

How does varying the temperature affect the fatigue resistance of the plastic sample?

Activity 6

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.

How does varying the temperature affect the fatigue resistance of the plastic sample?

Activity 7

Important safety information to follow when heating plastics:

• Extreme care must be taken when heating plastics. Boiling water can cause severe burns.
• This should be done with adult supervision and proper ventilation.
• Use tongs to hold any plastics you are heating.
• Use heat resistant gloves or pads when handling materials.
• Do not heat samples to the point of melting or degradation.
  

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 8

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 9

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?