ISO 527-3

What the test does

A rectangular strip of film or thin sheet is clamped at both ends between flat jaws, with no waisting anywhere along its length. The crosshead moves at a constant speed and pulls the strip along its own axis until it breaks. Force and extension are recorded throughout, with strain taken from an extensometer or from **gauge marks**, so the complete curve is captured rather than the breaking load alone.

What it measures, and why it matters

The method reports tensile strength at yield and at break, elongation at yield and at break, and tensile modulus, each quoted separately for the **machine direction** and the **transverse direction**. Strength and elongation govern lot release on a converting line and decide whether a gauge can be reduced without bags splitting. Modulus feeds web-tension and stiffness calculations for winding and printing. The gap between the two directions quantifies orientation from the die or casting line, so a drifting process shows up in the numbers before it shows up in complaints.

Specimen

The **type 2 specimen** is a plain rectangular strip, 10 to 25 mm wide and below 1 mm thick, with no waisting — a shoulder would force the break to a chosen point rather than let the strip fail at its weakest. Strips are cut from the web with a sharp die or blade so the edges are clean and square, and cut in both principal directions because film is anisotropic. Gauge length is 50 mm, grip separation 100 ± 5 mm, reducible to 50 mm where elongation is large. At least five valid specimens per direction are needed. Condition and test in the ISO 291 atmosphere, 23 °C and 50 % RH, for at least 16 h. Discard and replace any strip that tears at the jaw line or breaks from a visible edge flaw.

What the machine must be capable of

Film parts at very low loads — a 25 mm strip of blown polyethylene 25 µm thick breaks well under 100 N — so a 500 N to 1 kN load cell is the working range and a single-column bench frame carries the work. The method's own envelope reaches far higher: the widest, thickest specimen it admits, a 25 mm strip of strongly oriented polyester at the full 1 mm limit, approaches 5 kN. The cell is therefore chosen for the material, not for the standard. Force indication must meet ISO 7500-1 Class 1.

Speed follows the property. Tensile modulus is run at 1 mm/min, the speed selected to give a strain rate as near as possible to 1 % of gauge length per minute; strength and elongation use a nominal speed from the ISO 527-1 series — 5, 50, 100, 200, 300 or 500 mm/min. Plastics are rate-sensitive, so the speed used is part of the result.

Strain measurement is required, not optional. The modulus chord runs between 0.05 % and 0.25 % strain over 50 mm, which demands an ISO 9513 Class 1 device, and a small **preload** to remove slack so the curve starts from zero. Elongation then runs to break: 0–50 % for oriented polyester and rigid sheet, but commonly 500 % and beyond for cast and blown polyolefin, which rules out a clip-on and calls for a **long-travel extensometer** or a **video extensometer**. Grips are parallel-closing and pneumatic at regulated low pressure, with soft rubber or line-contact faces wider than the strip; roller grips spread the clamping load for very extensible or slippery film. Serrated faces tear film.

What goes wrong in practice

Jaw-line tearing is the signature failure: the clamp raises a local stress at the jaw edge and the strip parts there instead of in the gauge length, understating both strength and elongation. Slippage is the opposite error — the strip creeps out of the jaws, the curve flattens and modulus falls. A burr or nick from a blunt blade initiates the break at the edge early. Clip-on extensometers slip on thin film, so strain reads low and modulus reads high.

Related and equivalent standards

ISO 527-1 supplies the general principles, nominal speeds and modulus evaluation that Part 3 applies to film. ISO 527-2 covers moulding and extrusion plastics at 1 mm and above, where a waisted dumbbell replaces the plain strip. ASTM D882 is the nearest counterpart for thin sheeting, but widths, grip separations and speed selection differ, so results do not transfer between the two. ISO 291 fixes the conditioning atmosphere; ISO 7500-1 and ISO 9513 classify force and strain measurement.