Tear Strength

Definition and Distinction

Tear strength (in many data sheets also called tensile strength for elastomers) describes which maximum tensile stress a material can withstand in a tensile test before it tears or breaks. A tensile stress is the force per area — that is, how strongly a material is pulled, referenced to its cross-section. Tear strength is therefore stated as a stress, usually in MPa (megapascals).

In sealing technology, this parameter helps to understand how load-bearing an elastomer is under tensile and elongation loading — for example during installation or during short-term over-stretching. Important is the classification: the value applies to an undamaged specimen under defined laboratory conditions.

Tear Strength vs. Tear Propagation Resistance

In practice, tear strength is frequently confused with tear propagation resistance. Tear propagation resistance describes how easily an existing crack or cut grows further. It is tested with pre-damaged specimens and is frequently stated in N/mm or kN/m, because the geometry of the crack co-determines the assessment.

Particularly for seals, tear propagation resistance is often more critical, because small notches, installation marks, or edge contacts can start a crack. Then a high tear strength alone says little about whether the seal resists crack propagation.

Testing and Standards (Elastomers in Sealing Technology)

For elastomers, tear strength is determined in the tensile test. A standardized specimen is stretched at a defined speed until it breaks. Frequently used standards are ISO 37 and ASTM D412. The results are only meaningfully comparable when standard, specimen shape, and test conditions match.

Why is this important? Elastomers react sensitively to test parameters. Even the test speed or the specimen shape can shift the measured maximum value. A data sheet should therefore name, alongside the numerical value, also the underlying standard.

What Exactly Is Evaluated?

In the tensile test, different points can be reported. Customary is the maximum tensile stress shortly before fracture. Some sources, instead, cite the stress at fracture, which depending on material behavior can be practically similar but is not always identical. For technical assessment, the following therefore counts: which definition was used and according to which standard was testing carried out? Without this information, numerical values are hard to classify.

Relationship with Elongation at Break and Further Tensile Parameters

For sealing materials, tear strength is almost always given together with the elongation at break. Elongation at break is the percentage change in length that a specimen reaches until it tears. Both values complement each other, because they describe different aspects of failure.

An elastomer can have a high tear strength and nevertheless react unfavorably when elongation at break is low. Then it tends to break brittly under elongation — that is, with little warning through deformation. Conversely, a high elongation at break with moderate tear strength can point to a material that can stretch strongly but fails earlier under high tensile stress.

From the same tensile test, further parameters frequently arise — for example the stress at a defined elongation (often referred to as modulus or stress at given elongation). These values help to classify stiffness at $100\%$ or $300\%$ elongation, which can be relevant for the installation behavior of a seal.

Practical Relevance for Seals and Influencing Factors

Tear strength is particularly relevant in sealing technology when a seal is over-stretched during installation — for example when pulling over edges, when inserting into grooves, or with component geometries that have unfavorable radii. In operation, pressure cycles and small relative motions additionally act and can generate local elongations. Whether a tear arises from this depends on several factors, but tear strength provides a first indication of the fracture reserve under tension.

The parameter is also state-dependent. Temperature, media contact, and aging can change the material — for example through plasticizer loss, post-cross-linking, or swelling. As a result, tear strength can rise or drop, and elongation at break frequently changes with it. In test reports, values before and after storage (heat aging, media storage) are therefore often compared in order to better estimate suitability for the planned environment.

For practical selection, the following applies: tear strength is helpful when the seal is mainly endangered by tension and elongation. As soon as notches, cuts, or crack initiation sites are likely, tear propagation resistance should be considered at least equally. With complex installation and media conditions, specialized material and sealing consultation can be sensible.