ASTM D882

What the test does

A rectangular strip cut from film or thin sheet is clamped at both ends at a set grip separation. The crosshead moves at a constant rate and pulls the strip along its length until it breaks, while force and crosshead travel are recorded continuously. There is no narrow waist and no dumbbell — the specimen is a plain parallel-sided strip, so it may break anywhere along the free length.

What it measures, and why it matters

The method reports tensile strength, elongation at break, modulus of elasticity, tensile energy to break and breaking factor. Strength and elongation are the routine lot-release pair for packaging film — a drop in elongation flags degraded resin or a drift in orientation. Modulus feeds stiffness and web-handling calculations. Breaking factor, force divided by width, is used where thickness cannot be gauged reliably. Tensile energy to break, the area under the curve, ranks toughness between grades that share a similar peak load.

Specimen

Specimens are plain strips no more than 1.0 mm thick, cut with clean parallel edges from film or sheet. Width is chosen so that the width-to-thickness ratio is at least eight, which keeps edge flaws from dominating the break; 25.4 mm is a common width. The initial grip separation is the reference length and is a set value — 125, 100 or 50 mm depending on expected elongation at break, and 250 mm for modulus, reduced to 100 mm where 250 mm is impracticable. Condition at least 40 h at 23 °C and 50 % relative humidity to ASTM D618 Procedure A, and test in the same atmosphere. Discard and replace any specimen that breaks at the jaw line or at a visible edge nick, rather than averaging it in.

What the machine must be capable of

Loads are low. A 25 mm strip of packaging polyethylene breaks well under 100 N, so a 500 N to 1 kN cell on a bench frame covers most of this work and resolution at the bottom of the scale matters more than capacity. The widest, thickest specimen the method admits — 25.4 mm at the full 1.0 mm in a strong oriented polyester — would demand several kN, so capacity is chosen against the material rather than against the method. Force indication must be verified to ASTM E4.

Speed is set by expected elongation, not by preference: an initial strain rate of 0.1 min⁻¹ at 125 mm separation (12.5 mm/min) below 20 % elongation, 0.5 min⁻¹ at 100 mm (50 mm/min) from 20 to 100 %, and 10.0 min⁻¹ at 50 mm (500 mm/min) above 100 %. The rate must hold within 5 % of the set value, so the frame needs genuine speed control at 500 mm/min under load. Modulus is run separately at 250 mm separation.

No extensometer is required — strain comes from grip separation. Polyolefin film routinely runs past 500 % though, so a high-elongation or non-contact device gives a cleaner total elongation than crosshead travel does.

Grips must minimise slippage and spread clamping stress evenly. Pneumatic vice-action or wedge grips with flat rubber faces suit fragile specimens, line-contact faces suit slippery ones, and roller grips — the strip wrapped round a rubber-faced drum against a compression bar — suit thin film specifically. Faces must be wider than the strip. Serrated steel faces are wrong here: they nick the film and start the failure at the jaw line.

What goes wrong in practice

Jaw-line tears are the dominant invalid result — the film fails where the faces bite instead of in the free length, so the number is a grip artefact. Slippage is the other half of the same problem: the strip creeps out under load, extension reads long and modulus reads low. An edge nick from a blunt cutter initiates the break early and depresses strength by a wide margin. Thickness variance across a web is the quiet one — stress is calculated from a measured thickness, so a gauge taken at one point misprices the whole specimen.

Related and equivalent standards

ISO 527-3 is the nearest counterpart and covers films and sheets, but specimen types, speeds and strain evaluation differ, so results do not transfer between the two. ASTM D638 takes over above 1.0 mm thickness, where a dumbbell specimen is used instead of a plain strip. ASTM D618 supplies the conditioning procedure and standard atmosphere, and ASTM E4 supplies the force verification these results depend on.