ISO 37

What the test does

A waisted strip of vulcanised or thermoplastic rubber — a dumb-bell — is held at its wide end tabs and pulled along its axis at a constant rate of traverse until it ruptures. Force and the separation of two reference marks inside the narrow test length are recorded throughout. The alternative ring test piece is not gripped at all: it runs on two pulleys that draw apart, one of them driven so the ring rotates as it stretches.

What it measures, and why it matters

The method reports tensile strength, elongation at break, and stress at a given elongation — the figures a compounder uses to release a batch, to confirm that a mix reached its intended state of cure, and to compare candidate compounds at the specification stage. The same three numbers measured before and after heat or fluid ageing quantify degradation, and a drop in elongation at break is the usual first evidence in a failure investigation. Rubber is strongly rate- and temperature-sensitive, so these values rank materials rather than predict service life.

Specimen

Five dumb-bell types are defined, distinguished by test length: 25 mm for type 1, 20 mm for types 1A and 2, and 10 mm for the small types 3 and 4. Pieces are die-cut from moulded or milled sheet, and the cutting direction relative to the grain left by milling or calendering is fixed and recorded, because that bias changes the result. Type 1 presents roughly 12 mm² of cross-section, type 4 only about 2 mm². Ring pieces are cut instead where a dumb-bell cannot be taken: a type A ring has a 44.6 mm bore and carries load through two cross-sections of about 16 mm² each. Conditioning runs at 23 ± 2 °C for at least 3 h without humidity control, or at least 96 h with humidity control for latex-derived samples. A piece that ruptures outside the test length does not represent the material and is replaced.

What the machine must be capable of

Forces are small and the span between test types is wide. A type 1 dumb-bell usually breaks between roughly 50 N and 500 N, a type 4 miniature piece below 50 N, while a type A ring on a high-strength compound can approach 1 kN — so one load cell sized for the ring will resolve the miniature dumb-bell's early curve poorly, and interchangeable cells are the practical answer. Force indication must meet ISO 5893 Class 1.

Rate of traverse is fixed by test piece: 500 ± 50 mm/min for types 1, 1A and 2, 200 mm/min for types 3 and 4, 500 mm/min for type A rings and 100 mm/min for type B, so the frame must as a minimum provide 100, 200 and 500 mm/min and hold them steadily under almost no load.

Extension is measured on the test length, not from crosshead travel: ISO 5893 Class D for types 1, 1A and 2, Class E for types 3 and 4. Strains run from zero to rupture — commonly 100 % to 800 %, beyond 1000 % for some thermoplastic rubbers — which on a 25 mm test length is up to about 250 mm of travel, so a long-travel or non-contact device is required rather than a clip-on gauge. Grips must hold at near-zero load, since a small prestress is applied to stop the piece bending. Rings need the twin-pulley rig, one pulley free-running with very low friction and the other driven at 10–15 r/min. Tests away from 23 ± 2 °C need a thermostatically controlled chamber fitted to the frame.

What goes wrong in practice

Soft compounds creep out of the jaws; the trace stays smooth, so slippage reads as extra elongation and depresses the calculated stress at a given elongation. Grip pressure or too short an overall length puts the break in the shoulder radius, which understates strength. Above a few hundred per cent, reference marks thin and smear and a non-contact tracker can lose them near rupture, truncating the strain record. Ring results sit below dumb-bell results on the same compound, because the bore strains more than the outer wall.

Related and equivalent standards

ASTM D412 is the nearest counterpart and covers the same property on the same materials, but the die geometries, test lengths and conditioning rules differ, so results are not interchangeable and the method used must be reported with the figures. ISO 5893 is not an alternative test but the supporting standard that defines the machine's force class and the extensometer classes this method calls up. ISO 37:2017 remains in circulation in older specifications and should be cited by edition where it applies.