Elongation at Break

Definition and Classification

Elongation at break (also called strain at break or fracture elongation) describes how much a material elongates in a tensile test until it tears. It is given as the percentage change in length referenced to a defined initial gauge length; the unit is %. In technical data sheets, the English term “elongation at break” is frequently used.

In the everyday usage of materials and sealing technology, elongation at break and strain at break are mostly used synonymously. In each case, what is meant is the elongation value at the exact moment of fracture. The parameter therefore states how deformable a material is until failure.

For interpretation, the following rule of thumb applies: high elongation at break often stands for tough, highly extensible behavior. Low elongation at break rather points to brittle behavior, in which the material absorbs only little elongation before it breaks.

Measurement Principle in the Tensile Test and Calculation

In the tensile test, a standardized specimen (test specimen) is clamped between two grips at a defined test speed and then pulled apart. In the process, the test machine records force and change in length up to the fracture point. From the progression, a force-elongation curve arises, and elongation at break is the elongation value at the end of this curve.

Particularly with polymers, what matters is how elongation is measured. An extensometer measures elongation directly on the gauge length of the specimen. The machine travel (crosshead travel) additionally includes compliances of the machine and can therefore lead to different values. With some plastics, necking also occurs — that is, a local cross-section reduction that strongly changes the elongation distribution.

What Is Actually Measured?

Two quantities are measured that are recorded up to fracture: the tensile force and the change in length of the specimen. Elongation at break refers to the moment when the specimen fails. As a result, it practically answers the question: how far can the material be deformed under tension before it breaks?

How Is Elongation at Break Calculated (Formally)?

Elongation at break is the percentage change in length referenced to the initial gauge length:

$${\epsilon }_B=\frac{L_B–L_0}{L_0}·100\%$$

Here, L₀ is the initial gauge length and L_B is the gauge length at fracture. For comparability, it is decisive that measurement method and standard match the stated value, because extensometer and machine-travel measurement can deliver different results.

Standards and Material Reference (Elastomers vs. Plastics)

Elongation at break is only cleanly comparable when standard, specimen geometry, and test conditions are named with the value. In data sheets, elongation at break is therefore frequently stated together with tensile strength and the standard reference. This matters in sealing technology, because sealing materials react very differently depending on material class.

Elastomers (Rubber) in Sealing Technology

For elastomers (rubber materials, as frequently used in O-rings, molded seals, and diaphragms), ISO 37 and, widespread in Europe, DIN 53504 are central standards. Both typically capture tensile strength and elongation at break on standardized specimen shapes. As a result, a common basis is created to meaningfully compare compounds within a material family.

Plastics and Necking as a Point of Interpretation

For plastics, DIN EN ISO 527 is frequently used. With these materials, necking can locally concentrate elongation strongly. As a result, the reported elongation at break depends more strongly on where and with which method elongation is measured. For practice, this means: elongation at break of plastics is useful but should always be read with a view to measurement method and specimen behavior.

Significance for Seals and Typical Causes of Differing Data Sheet Values

In sealing technology, elongation at break is particularly relevant when a seal experiences strain peaks during installation or operation. This happens, for example, when pulling over edges, with local over-stretching, or when the material is pulled into gaps (extrusion / gap loading). Sufficient elongation at break can help to tolerate short-term overloads in such situations before tearing occurs.

Nevertheless, elongation at break describes only part of the suitability. For the function of a seal, other parameters are frequently equally important or more important — for example compression set (permanent deformation after pressure loading), abrasion, and media and temperature resistance. Elongation at break is therefore a building block for material selection but not a sole quality proof.

Relationship with Tensile Strength and Modulus

Elongation at break primarily answers the question “how far?”. Tensile strength describes “how strongly?” — that is, the maximum tensile stress that a material reaches in the test. The modulus additionally helps (for example the 100% modulus) — that is, the stress at a defined elongation, because it reflects stiffness in the relevant elongation range.

Only the combination of these values delivers a more reliable picture, because a material can be both extensible and not very load-bearing — or vice versa.

Why Data Sheet Values Can Vary

Deviations between data sheets are frequent and usually explainable. Typical influencing factors are:

• Specimen geometry and gauge length of the standard specimen

• Test speed (polymers often react in a speed-dependent way)

• Conditioning, storage, or aging before testing

• Recipe and batch differences for compounds, particularly with elastomers

For comparisons, it should therefore always be checked under which standard measurement was carried out and whether the test conditions are similar. When this is not the case, numerical values quickly become spuriously precise.

In the end, a brief consultation with a specialized material or sealing advisor can be sensible when elongation at break becomes safety- or function-critical.