The mechanical properties of polymers differ significantly from those of metals, and so do the methods of testing for those properties.
In this first part of a two-part series on plastic properties, we are going to look at some of the key mechanical properties of polymers and the ASTM standards that guide how these properties are tested.
Tensile Strength (ASTM D 638)
The tensile strength of a polymer is tested per ASTM D 638. Basically, a 1/8 thick polymer specimen that has either been machined or injection molded to specified dimensions is placed within a pair of grips to firmly hold the specimen. One set of grips is attached to a fixed head, while the other set is attached to a moveable head. The specimen is stretched at a constant rate of motion with force and deflection being measured. The test continued until the specimen is pulled apart.
This is similar to the testing performed on metals, but the rate at which the test is performed is different. And, because most plastics stretch far more than metals, a different means of measuring elongation is used.
One of the key properties sought in this test is the tensile strength of the material. Ultimate tensile strength refers to the force per unit area necessary to cause a material to fail under tension.
The second major property obtained from this test is the elongation. The elongation refers to how much the specimen stretched before it failed. This is usually expressed as a percentage of the original length, and for polymers may well have a value greater than 100%.
The combination of tensile strength and elongation give a good indication the toughness of the material: a polymer with a high tensile strength and little elongation is not as tough as a polymer with a high tensile strength and a significant elongation.
Compressive Strength (ASTM D 695)
Compressive strength tests are performed according to ASTM D 695. The test specimen is ½ x ½ x 1 and is mounted between a pair of testing machine heads, one of which is stationary and the other which applies a compressive load to the specimen at a constant rate of 0.05 in/min.
The ultimate compressive strength is the amount of force per unit area required to rupture the test specimen. Sometimes the polymer being tested will not rupture, in which case the compressive strength at a specific deformation is recorded. The last key property obtained from a compressive strength test is the compressive yield strength, which is the stress at the point of permanent yield. This can be found by observing the stress-strain curve for the test and occurs at the point of zero slope.
Flexural Strength (ASTM D 790)
The flexural strength test uses beam theory to estimate the resistance of the material to bending under load. A specimen with dimensions of 1/8 x ½ x 5 is placed on two supports. A load is applied at the center point between the two supports. The flexural strength of the specimen is its load at yield. Yield is defined as the stress at 5% deformation/strain, or the loading required to stretch the outer surface of the specimen by 5%.
Hardness (ASTM D 785 or ASTM D 2240)
In a nutshell, hardness represents a resistance to indentation. There are two accepted test methods for measuring the hardness of a polymer. ASTM D 785 is based on the Rockwell hardness test and is recommended for materials that do not exhibit significant creep. ASTM D 2240 works for materials that do exhibit creep and for softer materials. ASTM D 2240 uses a durometer for measuring hardness.
Keep in mind that the hardness reading from a Rockwell hardness test cannot be compared to a reading from a durometer test. It is best to only compare readings from the same scale.
Toughness and Impact Strength (ASTM D 256 and ASTM D 1822)
Impact strength, or impact resistance, is a measure of how much rapidly applied energy a material is able to absorb. This value also gives a measure of toughness. The two most popular tests for measuring impact strength in polymers are the ASTM D 256 test, known as the Izod Impact test, and ASTM D 1822, known as the tensile impact test.
In the Izod test, a notched specimen is is secured in an anvil so that the specimen acts as a cantilevered beam. The pendulum arm strikes the specimen, and continues to travel in the same direction but with less energy. The resulting loss of energy is recorded as the Izod impact strength. It is recorded in units of energy.
The tensile impact test uses a setup similar to the Izod test, but the specimen is different. In this test, the specimen is a tensile bar that is mounted on the pendulum. As the pendulum swings, the front end of the specimen moves freely through the anvil but the back end of the specimen is stopped by the anvil, inducing a high-speed tensile load. The loss of energy experienced by the pendulum is a measure of how much energy is needed to pull the specimen apart under a rapidly applied tensile load the tensile impact strength of the material.
Mechanical properties like tensile strength, compressive strength, flexural strength, hardness, and impact tests are obtained using ASTM standardized test methods specific for polymers.
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