ISO 527-1

What the test does

A plastics specimen is clamped at both ends and pulled along its long axis at a constant crosshead speed until it breaks. Force is taken from the load cell; extension is taken over a marked gauge length, by extensometer rather than by crosshead travel. The recorded output is a continuous stress–strain curve, so the whole deformation history is captured, not a single breaking figure.

What it measures, and why it matters

The method defines tensile modulus, yield stress and yield strain, stress and strain at break, and nominal strain for cases where no extensometer follows the gauge. Modulus is a direct design input, feeding stiffness calculations and finite-element models. Yield stress bounds the load a moulded part carries before it deforms permanently. Strain at break separates ductile grades from brittle ones and exposes degraded regrind, so it does most of the work in incoming-lot release and in failure investigation.

Specimen

Part 1 carries the principles; the geometry belongs to the daughter part. ISO 527-2 governs moulding and extrusion compounds, tested as dumb-bells over a gauge length of 50 or 75 mm; ISO 527-3 governs films and sheets, tested as strips over a 50 mm gauge length. Specimens are moulded or cut with clean, notch-free edges and marked at the gauge ends. Conditioning and testing both happen in the ISO 291 standard atmosphere, 23 °C and 50 % relative humidity, with 27 °C and 65 % as the subtropical alternative. Replicate count and the grounds for rejection come from the daughter part and the material specification rather than from Part 1; a specimen that breaks at the jaw line or outside the marked gauge has not been loaded uniaxially, so it is replaced rather than averaged in.

What the machine must be capable of

Part 1 covers the whole plastics range, so force demand follows the daughter part. A thin film strip breaks at a few newtons, an unfilled moulding dumb-bell at roughly 1 to 3 kN, and a glass-filled or high-performance grade at about 10 kN. One 10 kN frame with interchangeable low-capacity load cells covers the family, provided each cell holds ISO 7500-1 Class 1 over the range actually used — a 10 kN cell reading a 30 N film break is not Class 1 there.

Speeds are drawn from a nominal series running 0.125, 0.25, 0.5, 1, 2, 5, 10, 20, 50, 100, 200, 300 and 500 mm/min. Plastics are rate-sensitive, so the speed used is part of the result. Modulus is stricter: the crosshead must give a strain rate as near as possible to 1 % of gauge length per minute, which is 1 mm/min for the type 1A and 1B dumb-bells.

Modulus is evaluated between 0.05 % and 0.25 % strain. That interval rules out crosshead travel and requires an extensometer to ISO 9513 Class 1, while the same test then has to follow elongation to break — under 2 % for a filled thermoset, beyond 500 % for a ductile polyolefin or film. Grips must hold the specimen with its long axis on the line of pull, so self-tightening serrated wedges suit rigid dumb-bells and pneumatic side-action jaws suit soft or thin material where clamping force must stay constant; jaw faces should be at least as wide as the specimen.

What goes wrong in practice

Slippage is the quiet failure: the specimen creeps through the jaws, the crosshead logs extension the gauge never saw, and modulus reads low while strength looks near-normal. Over-clamping does the opposite damage — the specimen fails at the jaw line and the number belongs to the grip, not the material. Off-axis clamping puts a bending moment into the gauge and scatters the set. Conditioning drift leaves no trace in the curve; polyamides take up moisture within hours outside the standard atmosphere and simply read softer.

Related and equivalent standards

ASTM D638 is the nearest counterpart and covers the same physical test, but specimen geometries, speed selection and the modulus interval differ, so results do not transfer between the two and the standard used must be reported. Part 1 is not a complete method on its own: ISO 527-2 supplies moulding and extrusion compounds, ISO 527-3 films and sheets. ISO 7500-1 classifies the force measurement, ISO 9513 the extensometer and ISO 291 the atmosphere. The European adoption EN ISO 527-1:2019, published in Britain as BS EN ISO 527-1:2019, is technically identical.