ISO 34-1

What the test does

A die-cut strip of vulcanised or thermoplastic rubber is clamped in two tensile grips and pulled apart at constant jaw separation. The tear starts at a slit or a razor nick already cut into the piece, so the machine is not breaking the rubber but driving an existing tear along it. Force is recorded continuously against time, and the trace is jagged rather than smooth.

What it measures, and why it matters

The result is tear strength: the force to propagate the tear divided by the test piece thickness, reported in kilonewtons per metre. It is the property that decides whether a seal survives demoulding, whether a hose sleeve resists a nick growing into a split, and how much a tyre or conveyor cover tolerates cuts in service. Compounders use it to judge filler loading and cure state, so it works as a batch-release check and as a formulation input rather than as a service-life prediction.

Specimen

Three geometries are covered — trouser, angle and crescent — each die-cut from a sheet whose preferred thickness is 2.0 mm. The trouser piece is slit along its length so the two legs can be clamped separately; the crescent piece is nicked on its concave edge and the angle piece torn from its apex, with or without a nick. The nick is made with a guided razor cutter, since its depth sets where the tear begins. Sheets are conditioned for at least three hours and tested at a standard laboratory temperature of 23 ± 2 °C or 27 ± 2 °C to ISO 23529. Thickness is measured on the test piece itself, because it is the divisor in the result. Pieces that tear away from the intended path are discarded.

What the machine must be capable of

This is a low-force test. With the preferred 2.0 mm sheet, ordinary rubbers tear at roughly 10 N to 250 N and only the toughest reinforced grades approach 500 N, so a frame is chosen for fine resolution at the low end rather than for capacity. The force system must meet ISO 5893 Class 1, and it must be low in inertia: tear peaks arrive fast, and a heavy load train lags them and reads low.

Speed is fixed by geometry — 100 ± 10 mm/min jaw separation for the trouser piece, 500 ± 50 mm/min for the angle and crescent pieces. No strain or elongation measurement is required and no extensometer is needed. The demand is travel instead: the trouser tear must be followed along the whole remaining ligament while soft rubbers add substantial leg extension, and ISO 5893 warns that highly extensible materials can need over a metre of traverse.

Grips must tighten as tension rises and press uniformly across the widened end, with positioning that puts each piece symmetrically on the machine axis; wedge or pneumatic self-closing types are named for flexible materials. No dedicated tear fixture is called for — the geometry lives in the cutting die and the nick cutter. Ambient testing suffices unless another ISO 23529 temperature is chosen, which then needs a chamber.

What goes wrong in practice

Knotty tear is the common one: the tear advances in stick-slip jumps and the trace turns saw-toothed, so a median force to ISO 6133 must be read rather than a single peak. Leg extension in soft compounds eats crosshead travel and can end the test before the ligament is consumed. An inconsistent nick depth moves the tear initiation point and scatters results between operators. Grip slippage lets a leg creep, flattening the trace and understating the force.

Related and equivalent standards

ASTM D624 is the nearest counterpart and uses related geometries, but die shapes, speeds and result reporting differ, so figures do not transfer between the two. ISO 34-2 covers small Delft test pieces for the same property, again not interchangeable. ISO 23529 supplies conditioning and standard laboratory temperatures, ISO 5893 the machine specification, and ISO 6133 the rule for reading a median force off a jagged trace.