Stress-strain characteristics are test values used to demonstrate how an elastomer will behave when stretched. There are several common tests that can be used to show how the elastomer will perform in its final application environment.
This test value is also known as ultimate tensile strength. In this test, the elastomer specimen is pulled until it breaks, and the force required to break the material is simultaneously measured. The unit of measurement is usually pounds per square inch (psi) or megapascals (MPa). Elastomers with high ultimate tensile strength are less likely to break compared to those with lower test values.
This test value indicates how well the elastomer resists tearing. The tear strength test is similar to the ultimate tensile strength test, but the test specimen has a V-shaped notch on one side as an extension point. The test material is stretched until it is completely torn, and the force required to tear the specimen is also recorded. The unit of measurement is usually pounds per inch (psi) or kilonewtons per meter (kN/m).
In the tensile modulus test, the elastomer is stretched to various lengths, and the resistance to stretching is measured separately. This test value usually represents the tensile strength of the elastomer at various different percentages of its length compared to its original length, such as 50%, 100%, or 300%. The resistance of the elastomer to stretching may be very strong initially but may become weaker as it becomes elongated (known as "necking").
Elongation is not a measure of how difficult or easy it is to stretch the material but rather a measure of how long it can be stretched before breaking. The elongation at break is expressed as a percentage of its original length. Certain soft elastomers can be stretched to over 1000% of their original length before breaking. The elongation at break of soft TPE elastomers is typically much higher than that of hard, rigid materials.
The method of forming the test specimen and the direction of molten flow can both affect the test values of stress-strain characteristics. Therefore, for many elastomers, stress-strain characteristics must be measured in both the flow direction and the cross-sectional direction.
Like many other characteristics of elastomers, stress-strain characteristics are influenced by the orientation of polymer molecules during formation. Therefore, depending on whether stretching occurs along the flow direction of polymer formation or transverse to it, stress-strain characteristics can vary significantly.
Some tests are performed using specimens molded by injection molding, while others are performed using specimens molded by extrusion molding. Since test values for different types of specimens can differ significantly, it is important to compare test values only for the same type of specimen.