What is the tensile strength of elastomers?
The tear resistance, also called tear resistance, is the force required to tear an existing incision in an elastomer test specimen relative to the thickness of the specimen. It is expressed in N/mm or kN/m.
| Characteristic | Indication |
|---|---|
| Key value | Further tear strength or further tear resistance (English:tear strength) |
| Formula symbols | Ts |
| Unity | N/mm or kN/m (1 N/mm = 1 kN/m) |
| Definition | Strength to tear apart an existing incision, based on the thickness of the test specimen |
| Testing standards | ISO 34-1:2022, DIN ISO 34-1:2024-12 |
| Test specimens | Strip sample, Angle sample, Bow sample |
| Test temperature | 23 °C |
| Value range of technical elastomers | about 5 to 100 N/mm, depending on the testing method |
How is the tear strength calculated?
The tensile strength is calculated from the force exerted during the tensile process and the thickness of the test specimen.
The tear strength Ts (in N/mm) results from the force F (in N), with which the incision is further torn apart, divided by the thickness d of the test specimen (in mm). Some data sheets indicate the tear strength in kN/m; the numerical value remains the same.
As an example: If a 2 mm thick test specimen continues to tear at 50 N, the tear strength is 50 N.
Unlike tensile strength, the tear resistance is not a stress; the force is only divided by the thickness and not by a surface area. Therefore, the tear resistance cannot be converted into MPa and cannot be directly compared with tensile strength.
How is the tensile strength of elastomers tested?
The tensile strength of elastomers is tested on a specimen that receives a cut or has a notched shape before the test. The specimen is pulled apart in a tensile testing machine at a constant speed while the force that propels the crack forward is recorded. ISO 34-1 describes three methods with different specimens:
| Procedure | Test specimens |
|---|---|
| A | Strip sample, also called trousers sample, with a long cut |
| B | Angle test, with or without incision |
| C | Arching sample with incision |
The achieved tensile strength depends not only on the elastomer mixture but also on the shape of the test specimen, the test speed, and the test temperature. Therefore, values from the three methods cannot be directly compared with each other, which is why the results always include a complete reference to the standard and method.
What is the tensile strength of elastomers?
Polyurethane and natural rubber have the highest tear strength among elastomers, and silicone rubber the lowest. Since the same base polymers can be combined in formulations with different additives and fillers, the tear strength for a base polymer can vary greatly, and the values mentioned are only approximate.
| Elastomer | Abbreviations | Further tear strength in N/mm (recommended value) |
|---|---|---|
| Polyurethane | AU, EU | 40–100 |
| Natural rubber | NR | 20–60 |
| Hydrated nitrile rubber | HNBR | 20–50 |
| Chloroprene rubber | CR | 20–50 |
| Nitrile rubber | NBR | 15–45 |
| Ethylene-propylene-diene rubber | EPDM | 10–40 |
| Fluoroplastic | FKM | 15–30 |
| Silicone rubber | VMQ | 10–25 |
What is the difference between tear strength and tear resistance?
The Tear resistance Describes how much tension an undamaged test specimen withstands before tearing. The tear strength, on the other hand, describes how much force is required for an already existing tear to tear further. The two values are determined in two different tests and with different test specimens. While the tensile strength is stated in MPa, the tear strength is measured in N/mm. Therefore, it is not possible to infer the tensile strength from the tear strength. The entire load accumulates at the tear tip in a very small area, so that an injected elastomer yields much earlier than an undamaged one. The effect of packaging films, which are only easily torn with a small notch at the edge, is known. An elastomer with high tear strength may therefore have a low tear strength.
