ISO 527-2

What the test does

A waisted dumb-bell of moulded or machined plastic is clamped by its wide shoulders in two grips set a fixed distance apart. The crosshead moves at a constant speed and stretches the specimen along its axis until the narrow centre yields and breaks. Force is recorded against extension throughout, and extension over the gauge length is taken from an extensometer rather than from crosshead travel.

What it measures, and why it matters

The method reports tensile modulus, yield stress and yield strain, tensile strength, and stress and strain at break. Modulus is the number that goes straight into a stiffness calculation or a finite-element model of a moulded part. Yield stress sets the load at which the part deforms permanently; strain at break separates a ductile polyolefin from a brittle filled grade and exposes degraded regrind or a dried-out batch. Because these figures shift with rate and temperature, they serve material selection and lot release, not service-life prediction.

Specimen

Part 2 fixes the geometries for moulding and extrusion compounds. Type 1A is injection-moulded directly to shape and is the preferred multipurpose specimen; type 1B is machined from sheet or plate to the same 75 mm gauge length. Scaled versions — 1BA at 25 mm gauge, 1BB at 10 mm — exist for scarce material, and type 5A at 20 mm gauge for thin sections. Edges must be smooth and free of machining marks. Condition for at least 16 h at 23 °C and 50 % relative humidity to ISO 291 and test in that same atmosphere. At least five specimens are needed for a valid mean, more for anisotropic grades, and any specimen that breaks at a visible flaw or outside the narrow section is discarded and replaced.

What the machine must be capable of

An unfilled thermoplastic type 1A dumb-bell has a 10 mm by 4 mm section — 40 mm² — and breaks at roughly 1 to 3 kN. Glass-filled polyamides and similar high-strength grades reach about 8 to 10 kN, so a 10 kN frame covers the part. Elastomeric grades and scaled 1BA or 1BB specimens break in the tens of newtons and want a lower-capacity load cell rather than the bottom percent of a 10 kN one. Force indication must meet ISO 7500-1 Class 1.

Speed is not a free choice: 1 mm/min for the tensile modulus of types 1A and 1B, 5 or 50 mm/min for yield stress and tensile strength, and 50 mm/min upward to 500 mm/min for elongation at break, drawn from the ISO 527-1 nominal series that runs from 0.125 to 500 mm/min. Plastics are rate-sensitive, so a modulus taken at the wrong speed is a different number, not a noisier one.

Modulus is a chord between 0.05 % and 0.25 % strain, which crosshead travel cannot resolve — an extensometer to ISO 9513 Class 1 is required, and ductile grades running past 400 % strain need a long-travel or non-contact device. Grips clamp the shoulders at a fixed 115 ± 1 mm separation for types 1A and 1B, with jaw faces at least 20 mm wide. Self-tightening serrated wedges suit rigid and filled grades; pneumatic side-action jaws at regulated constant pressure suit soft, thin or notch-sensitive ones. Testing outside 23 °C and 50 % RH needs a conditioned chamber.

What goes wrong in practice

Jaw breaks are the standing problem: over-clamped or misaligned wedges crush the shoulder and the specimen fails at the grip line, so the result is discarded. Slippage is the mirror image — the curve looks compliant and the modulus reads low. A toe region from initial take-up corrupts the 0.05–0.25 % chord unless it is corrected or measured on the specimen itself. Conditioning drift is invisible in the data: an underconditioned polyamide gives plausible, wrong numbers.

Related and equivalent standards

ISO 527-1 carries the general principles, the speed series and the modulus definition; Part 2 supplies only the conditions for moulding and extrusion compounds, so the two are read together. Other parts cover films, composites and other forms. ASTM D638 is the nearest counterpart and covers the same physical test, but specimen geometries, speeds and modulus evaluation differ, so results do not transfer between them. ISO 291 governs conditioning, ISO 7500-1 force verification and ISO 9513 extensometer classification.