Tensile strength in O-rings simply explained

Tensile strength: the basics

Tensile strength is an essential measure of how much load a material can withstand without breaking when it is stretched. It describes the maximum load that a material can endure without breaking or permanent damage.

It is defined as the maximum load a material can bear without breaking, divided by the original cross-sectional area of the specimen. Tensile strength is expressed as force per unit area, often in megapascals (MPa), and for very strong materials also in gigapascals (GPa). In Anglo-American practice, it is additionally given in psi (pounds per square inch).

If the applied load stays below the tensile strength, an elastic material returns fully or partly to its original shape and size. Once the tensile strength is reached, a ductile material begins to deform plastically. It forms a narrowed region, the so-called neck, where it eventually breaks. Other materials, by contrast, break abruptly as soon as their tensile capacity is reached.

How tensile strength is measured

A common method for determining tensile strength is the tensile test using a universal testing machine (UTM). It applies a controlled stress until the specimen breaks. An extensometer records the change in length of the specimen material.

For metals, the specimen shape for the tensile test is typically a round bar with a wide shoulder area at each end. This is where the machine grips the material while it applies the tensile stress. During pulling, the change in gauge length is recorded continuously.

Tensile strength is not only a measure of how much a material can bear under stress. It also provides information about its behavior under these conditions. Some materials show an abrupt break as soon as the tensile capacity is reached. Others deform or neck down before they break completely. In alloys, the composition influences the tensile strength, for example the carbon content in carbon steels.

The formula for tensile strength is:

Test methods and standards

Testing tensile strength in the tensile test is a critical step in examining the mechanical properties of materials. Several standards define the procedure so that the results are reliable and internationally comparable.

For elastomers, and thus for O-rings, ASTM D412 is the governing standard. It describes the tensile test on vulcanized rubber and thermoplastic elastomers and defines how tensile strength and elongation at break are determined on standardized test specimens.

For metals, ASTM E8 or ASTM E8M is among the most widely used standards. It describes testing at room temperature. ASTM E8 refers to inches and pounds, while ASTM E8M uses metric units. Both define the preparation of the specimens and the test sequence in order to minimize the influence of the machining process. For plastics, ASTM D638 applies.

In addition to the ASTM standards, there are ISO standards such as ISO 6892-1, which focuses on tensile testing of metals at room temperature. This standard specifies the determination of characteristic values such as yield strength, tensile strength and elongation at break. Separate standards exist for different temperature ranges and material types.

Choosing the right test equipment is crucial. It should be calibrated and have secure specimen holders, an extensometer to measure elongation and a data acquisition system to ensure accurate readings. If one provider's test equipment does not match a material supplier's machinery, this can lead to discrepancies. In that case, close coordination of the test methods and instruments is needed so that the results agree.

Tensile strength in a material comparison

Tensile strength varies widely depending on the material type. Please note that the following values are approximate. They can vary depending on composition, heat treatment and manufacturing process.

Graphene is considered one of the strongest known materials and clearly outperforms steel. Boron nitride nanotubes are suitable for aerospace applications thanks to their high tensile strength. Metals and alloys such as steel, titanium, tungsten and Inconel are among the materials with the highest tensile strength. Modern composite materials such as carbon fiber or aramids combine high strength with low weight.

Elastomers are at the lower end of this scale. O-rings made of FFKM, for example, reach a tensile strength of only around 10 to 20 MPa. That is not a disadvantage for a seal, because an O-ring is not meant to bear loads but to seal.

Why tensile strength matters for O-rings

An O-ring does not bear a load like a steel component. Even so, its tensile strength is relevant, especially during installation. When it is pulled over a shaft or placed into a groove, the ring is stretched briefly. If this stretch stays below the critical limit, the material returns to its initial shape and seals reliably.

If an O-ring is stretched beyond its load capacity, however, it can tear or become permanently deformed. In the plastics and rubber industry, tensile strength is therefore tested to determine whether a product has enough strength to withstand external physical stress.

Tensile strength and elongation at break in context

Tensile strength and elongation at break are determined in the same tensile test and are closely related. Tensile strength indicates the force per area a material can withstand. Elongation at break indicates how far it can be stretched until it breaks.

For O-rings, both parameters are meaningful. A material with high elongation at break can be stretched further during installation without tearing. Tensile strength describes the limit of the force it can bear. Anyone who wants to design an O-ring correctly considers both values together. Read more about elongation in our article on elongation at break.

Frequently asked questions