Tensiles: Strength, Modulus & Elongation

Meet The Tensiles: Strength, Modulus & Elongation

If you are like me, you often wonder how to assess a plastic material for an application based on its Tensile properties. What do they even mean? Let’s start with the word tensile, tensile refers to the capability of being stretched or drawn out, essentially it is a measurement of ductility. The tensile properties are measured in accordance with ASTM D638 for US-based manufacturers or ISO 527-2 for European and many Asian manufacturers. Tensile properties are measured by applying tensile or axial force using a tensometer to a dog bone-shaped specimen until the specimen breaks. Tensile properties are known to vary with specimen preparation as well as the environment such as temperature. Thus, the parameters of the test are carefully controlled by the requirements set in the test protocol. The resulting test yields a Stress-Strain curve which can be used to characterize the material for use in an application. Let’s look at three common tensile tests to understand a material’s potential.

TENSILE MODULUS

Also known as Young's Modulus, this measures the material’s resistance to deformation in the elastic phase before the strain hardening or drawing out of the polymer chain. This is represented on the graph as the slope of the straight line from the zero position to the Proportional Limit. If a material has a high tensile modulus, it resists deformation and is considered a rigid material. The value is expressed in pounds per square inch (psi) or megapascals (MPa).

TENSILE STRENGTH AT YIELD

Yield is the amount of stress a plastic material can withstand without undergoing plastic deformation. Tensile strength is exhibited on the graph as Ultimate Strength and is measured in units of force (psi or MPa). This UTS is the point of the end of Strain Hardening and the beginning of the necking of the specimen as well as where the slope of the stress-strain curve is zero. A practical example of tensile strength is adding weight to the end of a cable, the point the cable stretches beyond the ability to resist permanent deformation is the Ultimate Tensile Strength of the material.

TENSILE ELONGATION

This is measured by applying tensile or axial force to the specimen until it breaks. This point is depicted as Fracture Point on the stress-strain curve. Elongation is expressed as a percentage of change in length from original to fracture and expressed as a percentage. This provides a comparison of a material’s ductility or brittleness.

Understanding how to interpret tensile values on a data-sheet is critical to being able to select the optimal material for an application.

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