Was ist die Bruchdehnung und warum ist sie wichtig?

Die Bruchdehnung ist ein Maß dafür, wie weit sich ein Werkstoff dehnen lässt, bevor er bricht. Sie wird in Prozent der Ausgangslänge angegeben. Sie ist wichtig, weil sie die Duktilität und Flexibilität eines Werkstoffs zeigt, besonders dort, wo Werkstoffe Verformungen ausgesetzt sind, etwa bei der Montage eines O-Rings.

Wie wird die Bruchdehnung gemessen?

Die Bruchdehnung wird im Zugversuch gemessen. Eine Probe wird in eine Prüfmaschine eingespannt und gedehnt, bis sie bricht. Aus der Längenänderung beim Bruch im Verhältnis zur Ausgangslänge ergibt sich die Bruchdehnung in Prozent. Die Formel lautet ε = ((L₁ − L₀) / L₀) × 100 %, mit L₀ als Ausgangslänge und L₁ als Länge nach dem Bruch.

Was ist der Unterschied zwischen Bruchdehnung und Zugfestigkeit?

Die Bruchdehnung beschreibt, wie weit sich ein Werkstoff dehnen lässt, bevor er bricht, und ist ein Maß für die Duktilität. Die Zugfestigkeit beschreibt die maximale Spannung, die ein Werkstoff unter Zug aushält, bevor er bricht, und ist ein Maß für die Festigkeit. Beide Kennwerte stammen aus demselben Zugversuch.

Unterscheidet sich die Bruchdehnung zwischen Werkstoffen?

Ja, die Bruchdehnung variiert stark. Elastomere wie Naturkautschuk erreichen sehr hohe Werte und lassen sich weit dehnen, bevor sie brechen. Harte, spröde Werkstoffe wie manche Kunststoffe und Metalle weisen deutlich geringere Bruchdehnungen auf.

Welche Rolle spielt die Bruchdehnung bei O-Ringen?

Bei O-Ringen ist die Bruchdehnung vor allem bei der Montage wichtig. Beim Aufziehen über Wellen oder Kanten wird der Ring gedehnt. Ein Werkstoff mit ausreichender Bruchdehnung reißt dabei nicht ein. Im Betrieb sollte die dauerhafte Dehnung jedoch weit unterhalb der Bruchdehnung bleiben, damit Dichtkraft und Lebensdauer erhalten bleiben.

Nach welcher Norm wird die Bruchdehnung von O-Ring-Werkstoffen geprüft?

Für Elastomere, die bei O-Ringen verwendet werden, ist ASTM D412 der gängige Standard für den Zugversuch. Daneben gibt es ISO 37 für Elastomere sowie weitere Normen für Kunststoffe (ASTM D638, ISO 527) und Metalle (ASTM E8/E8M, ISO 6892).

Elongation at break | Simply explained

Key takeaways

Elongation at break is the elongation of a material up to fracture, expressed as a percentage of the original length.
It is a measure of ductility: high values indicate tough, readily stretchable materials, low values indicate brittle behaviour.
It is measured in a tensile test in accordance with standards such as ASTM D412 for elastomers.
For O-rings, installation matters most: enough extensibility so that the ring does not tear as it is fitted.
Consider elongation at break, tensile strength and hardness together, not in isolation.

Last updated on 22 June 2026 · Author: Luke Williams, "Lord Of The O-Rings"

Basics of elongation at break

Elongation at break, also known as elongation at rupture or tensile elongation, is a measure of how far a material stretches up to the moment it fractures under tensile load. It is expressed as a percentage and indicates how much a specimen elongates relative to its original length before it tears.

Elongation at break is therefore an indicator of a material's ductility and toughness. It is one of the parameters stated in technical data sheets and material test reports. For O-rings, it tells you how far an elastomer can be stretched before it fails. This is particularly relevant during installation, when the ring is pulled over component edges.

What elongation at break depends on

Several factors influence the value:

Material type: elastomers, plastics and metals show very different elongations at break.
Temperature: high or low temperatures change a material's ductility.
Processing and treatment: the compound formulation, additives and cross-linking affect the extensibility.

Definition

Elongation up to fracture, expressed as a percentage of the original length.

Ductility

High values indicate a tough, readily stretchable material.

Brittleness

Low values indicate brittle behaviour with no reserve.

Measurement in the tensile test

Elongation at break is determined in a tensile test. A material specimen is clamped in a testing machine and stretched under controlled conditions until it breaks. Force and elongation are recorded continuously in the process. Three steps are decisive:

Preparing the specimen: the specimen is prepared to a defined standard with specified dimensions so that the results are comparable.
Carrying out the tensile test: the specimen is stretched at a steady rate while force and change in length are measured.
Evaluation: elongation at break is obtained from the change in length at fracture relative to the original length, expressed as a percentage.

The formula

Elongation at break is often denoted by the symbol ε and calculated as follows:

ε = ((L₁ − L₀) / L₀) × 100 %

Here L₀ is the original length of the specimen before the test and L₁ is the length after fracture. The result indicates by what percentage the specimen elongated relative to its original length before it tore.

Elongation at break of different materials

Elongation at break varies considerably between material groups. An elastomer such as natural rubber reaches very high values, whereas brittle plastics and many metals lie well below. The following overview shows typical orders of magnitude.

Material	Typical elongation at break	Note
Natural rubber	800 %	Very high elasticity, ideal for products requiring great extensibility.
Polyurethane	600 %	High flexibility, depending on formulation and composition.
Polyethylene (high density)	300 %	Good extensibility, often used for containers and films.
Steel (low-alloy)	25 %	Higher ductility than many other metals, versatile in use.
Aluminium	20 %	Moderate ductility, often used in the construction industry.
Polystyrene	3 %	Very low elongation, a brittle material for rigid applications.

Typical elongation-at-break values for different materials. The values vary depending on formulation and treatment.

For plastics, the range extends from very brittle to extremely extensible. Polystyrene, for instance, has a very low elongation at break, whereas polyethylene can be stretched many times over. For metals, the value depends on the alloy and the heat treatment.

Relevance for installation and stretch of O-rings

For O-rings, elongation at break matters above all during installation. As the ring is fitted over a shaft, a groove or a component edge, it is briefly stretched. If the material's extensibility is insufficient, the ring can tear or be permanently damaged.

A material with a high elongation at break copes better with this load and offers more reserve. Even so, in operation an O-ring should never be loaded close to its elongation at break. The permanent stretch on installation stays well below it, so that the ring retains its sealing force and does not age prematurely.

Alongside elongation at break, other properties act together. Shore hardness determines how easily the ring deforms, and tensile strength indicates what force the material can withstand. Only in combination do they give a reliable picture for material selection.

Installation safety

Enough extensibility so that the ring does not tear as it is fitted.

Reserve in operation

Keep the permanent stretch well below the elongation at break.

Interplay

Consider elongation at break together with hardness and tensile strength.

Relationship with tensile strength

Elongation at break and tensile strength are determined in the same tensile test but say different things about a material.

Elongation at break: it describes how far a material can be stretched before it breaks. It is a measure of ductility, that is, the ability to deform under tension without tearing.
Tensile strength: it describes the maximum stress a material withstands under tension before it breaks. It is measured as force per area, for example in megapascals (MPa), and is a measure of strength.

Together, the two parameters describe the behaviour under tension more completely than either does on its own. A material may have high strength but low extensibility, or vice versa. For seals, a balanced combination is required: enough extensibility for installation and enough strength for the sealing force in operation.

Standards and test methods

So that measured values are comparable, elongation at break is tested in accordance with defined standards. These specify the specimen shape, test conditions and evaluation. Different standards apply to the various material groups.

Standard	Material group	Scope
ASTM D412	Elastomers	Tensile testing of vulcanized rubber and thermoplastic elastomers; the common standard for O-ring materials.
ASTM D638	Plastics	Tensile testing of plastics, including elongation at break.
ASTM E8/E8M	Metals	Method for tensile testing of metals, applied worldwide.
ISO 37	Elastomers	Determination of the tensile stress-strain properties of elastomers.
ISO 527	Plastics	Method for tensile testing of plastics, comparable to ASTM D638.

Key standards for measuring elongation at break, by material group.

Compliance with these standards ensures that results are comparable regardless of location and laboratory. This matters both for quality assurance in manufacturing and for material selection.

Frequently asked questions

What is elongation at break and why is it important?

Elongation at break is a measure of how far a material can be stretched before it breaks. It is expressed as a percentage of the original length. It is important because it shows a material's ductility and flexibility, especially where materials are subjected to deformation, such as during the installation of an O-ring.

How is elongation at break measured?

Elongation at break is measured in a tensile test. A specimen is clamped in a testing machine and stretched until it breaks. Elongation at break, as a percentage, is obtained from the change in length at fracture relative to the original length. The formula is ε = ((L₁ − L₀) / L₀) × 100 %, with L₀ as the original length and L₁ as the length after fracture.

What is the difference between elongation at break and tensile strength?

Elongation at break describes how far a material can be stretched before it breaks and is a measure of ductility. Tensile strength describes the maximum stress a material withstands under tension before it breaks and is a measure of strength. Both parameters come from the same tensile test.

Does elongation at break differ between materials?

Yes, elongation at break varies considerably. An elastomer such as natural rubber reaches very high values and can be stretched a long way before it breaks. Hard, brittle materials such as some plastics and metals have markedly lower elongations at break.

What role does elongation at break play in O-rings?

For O-rings, elongation at break matters above all during installation. As the ring is fitted over shafts or edges, it is stretched. A material with sufficient elongation at break does not tear in the process. In operation, however, the permanent stretch should remain well below the elongation at break, so that sealing force and service life are maintained.

To which standard is the elongation at break of O-ring materials tested?

For elastomers used in O-rings, ASTM D412 is the common standard for the tensile test. In addition, there is ISO 37 for elastomers, as well as further standards for plastics (ASTM D638, ISO 527) and metals (ASTM E8/E8M, ISO 6892).

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Luke Williams

Lord Of The O-Rings · NH O-RING Academy

"I firmly believe that we should share our knowledge. I hope this article answers your questions about the elongation at break of O-rings. If not, get in touch with us at any time; we are always happy to help."