Tensile Strength

What is Tensile Strength?

Tensile strength refers to a material’s ability to resist fracturing under maximum load. This characteristic is applicable to various materials, including metals, composites, natural materials like timber and stone, and even rubber. When a load is applied that is below the material’s tensile strength, it may experience deformation or stretching, but should generally regain its original size and shape. As the applied load approaches the material’s maximum tensile strength, it may exhibit a phenomenon known as “necking,” where permanent deformation becomes evident. With further increase in load, the necked region will eventually fracture.

Thermoset Rubber

Thermoset rubber undergoes a curing process where it is heated and molded into a specific shape. Curing involves a permanent chemical reaction that enhances the strength and durability of thermoset rubber products. Examples of items made from thermoset rubbers include tires, hockey pucks, circuit breakers, electrical housings, and motor components.

Thermoplastic elastomers (TPE)

Thermoplastic elastomers (TPE) are versatile materials that encompass polymers, polymer blends, or compounds. Typically, they start as pellets and transform into a liquid state when heated, allowing them to be easily injected into molds. As the thermoplastic cools and solidifies, it assumes the desired shape. Interestingly, this process can be reversed by reheating the material, enabling flexibility and reusability. TPE finds diverse applications such as drive belts, shock absorbers, breathing tubes, catheters, and shoe soles.

Calculating Tensile Strength

Calculating TS for a material is accomplished using:

TS = F(BE)/A)

where:

TS = tensile strength, the stress at rupture, MPa (lbf/in.2)
F(BE) = the force magnitude at rupture, MN (lbf)
A = cross-sectional area of unstrained specimen, m2(in.2)

Objective of the Tensile Test on Elastomers According to ASTM D412

ASTM D412 outlines the requirements for the testing equipment, test specimens (including their shape and size), as well as the test procedure and suitable methods to be employed. The primary goal of this test is to ascertain a range of material characteristic values.
These values are subsequently utilized in the development of innovative products, achieving product performance objectives, or for research and quality control purposes.

Conducting the ASTM D412 Test: Evaluating Material Performance and Quality

The ASTM D412 standard encompasses two test methods: Method A and Method B.
Method A utilizes dumbbell and straight section specimens, whereas Method B employs cut ring specimens.
Tests conducted in accordance with ASTM D412 aim to assess the elasticity of a material when subjected to tensile load, as well as its behavior after the load has been removed.The test is performed using a universal testing machine, commonly known as a tensile testing machine, at a controlled speed of 500 ± 50 mm/min until the specimen reaches failure.
While ASTM D412 enables the measurement of various tensile properties, some of the key characteristic values include:

Characteristic values determined according to ASTM D412

Definitions as per ASTM D1566 – Terminology Related to Rubber:
Tensile stress: The stress applied to a specimen to induce stretching.
Tensile stress at a given elongation: The stress required to stretch the uniform cross-section of a test specimen to a specific elongation.
Tensile strength: The maximum tensile stress applied to a specimen until it reaches the point of rupture.
Yield point: The point on the stress-strain curve, prior to ultimate failure, where the rate of stress with respect to strain reaches zero and may become negative.
Ultimate elongation: The elongation of a specimen at the point of rupture.
Tensile set: The residual extension that remains after a specimen has been stretched and then allowed to retract in a specified manner. It is expressed as a percentage of the original length.

Specimens and dimensions in accordance with ASTM D412

ASTM D412 provides specifications for conducting tensile tests on rubber and elastomers. It outlines six types of dumbbell specimens and two types of cut ring specimens for this purpose.
Among these specimens, the C-shape dumbbell specimen (ASTM D412 Type C) is the most commonly used. It has an overall length of 115 mm (4.5 inches), a gauge length of 25 mm (1 inch), a measurement width of 6 mm (0.25 inches), and a thickness of 3 mm. The thickness measurement is performed at three different points, and the median of these measurements is used to calculate the cross-sectional area.