Plastic Elongation at Break Testing Standards: A Complete Guide to ISO 527, ASTM D638, and GB/T 1040

Elongation at break is one of the most widely reported mechanical indicators for evaluating the ductility and toughness of plastic materials. This guide explains the plastic elongation at break testing standards that govern tensile testing worldwide—including ISO 527, ASTM D638, GB/T 1040, and JIS K 7161—covering specimen requirements, test conditions, calculation methods, and typical industrial applications.

Technical Overview

Elongation at break, usually written as εB or Eb, is the percentage increase in gauge length of a plastic tensile specimen measured at the exact moment of rupture. It is calculated as: εB = (L − L0) / L0 × 100, where L0 is the initial gauge length (typically 50 mm) and L is the gauge length at break. The property quantifies ductility: brittle materials such as general-purpose polystyrene may show values below 2 %, while ductile polyethylene films can exceed 500 %.

Plastic elongation at break testing is standardized internationally by the ISO 527 series, which has been adopted as the Chinese national standard GB/T 1040 series and the Japanese industrial standard JIS K 7161 and JIS K 7162. In North America, ASTM D638 is the equivalent tensile test method for rigid plastics, while ASTM D882 applies to thin plastic films. Because ISO and ASTM specimens and test speeds differ, results from the two systems are not directly interchangeable.

• ISO 527-1 / GB/T 1040.1 — General principles for determining tensile properties of plastics
• ISO 527-2 / GB/T 1040.2 — Test conditions for moulding, extrusion and cast plastics (Types 1A and 1B specimens)
• ISO 527-3 / GB/T 1040.3 — Test conditions for films and sheets
• ASTM D638 — Standard test method for tensile properties of plastics (Types I through V specimens)
• ASTM D882 — Tensile properties of thin plastic sheeting (under 1 mm thick)
• ISO 291 / ASTM D618 — Standard conditioning atmospheres for specimen conditioning

Testing Samples

ISO Multipurpose Specimens (Type 1A and Type 1B)

Type 1A is the preferred specimen for directly injection-moulded material, while Type 1B is intended for specimens machined from sheets, compression-moulded plaques or finished parts (machined per ISO 2818). Both have a 50 mm gauge length, a 10 mm narrow-section width, and a preferred thickness of 4 mm. Type 1A has an overall length of 170 mm with 115 mm grip separation; Type 1B is 150 mm long with 100 mm grip separation. Multipurpose specimens are defined in ISO 3167.

ASTM D638 Specimens (Type I through Type V)

Type I is the standard rigid-plastic specimen: 165 mm overall length, 13 mm narrow-section width, 50 mm gauge length, 115 mm grip separation and a nominal thickness of 3.2 mm. Types II, III and IV cover special material forms such as non-rigid plastics, while the miniature Type V specimen (approximately 63.5 mm long with a gauge length of about 7.9 mm) is used when only a small amount of material is available, for example from small moulded parts or 3D-printed coupons.

Film and Thin-Sheet Specimens

For flexible films and sheets thinner than 1 mm, ASTM D882 and ISO 527-3 use rectangular strip specimens, typically 10 mm to 25 mm wide, tested with an initial grip separation of 100 mm. Elongation is commonly determined from crosshead travel because the strip has no reduced gauge section.

Specimen Preparation and Quantity

Specimens should be moulded under controlled conditions (ISO 294 or equivalent) or carefully machined to avoid notches, scratches and heat damage that act as stress concentrators. ISO 527-1 requires at least five valid specimens per condition; ASTM D638 requires at least five specimens, and five in each principal direction when the material is anisotropic (for example, filled or fibre-reinforced grades).

Testing Items

A standard plastic tensile test performed under ISO 527 or ASTM D638 yields a set of related properties, of which elongation at break is one item on the report:

• Elongation at break (εB) — strain at the moment of rupture, expressed as a percentage
• Tensile strength — the maximum tensile stress sustained during the test
• Tensile stress at yield (σY) and elongation at yield (εY) — the stress and strain at the yield point, where applicable
• Tensile stress at break (σB) — the stress at rupture
• Tensile modulus (ET) — Young's modulus in tension, determined over the initial linear region
• Nominal elongation at break (εtB) — calculated from grip separation when an extensometer cannot be used or its range is exceeded
• Stress at 50 % strain (σ50) — frequently reported for films and very ductile materials that do not break within the extensometer range

For quality control purposes, elongation at break is often the most closely watched item, because it responds sharply to processing defects, contamination, moisture and degradation.

Testing Methods

Apparatus

Testing is performed on a calibrated universal testing machine (UTM) with a suitable load cell, so that the specimen failure force falls within a well-calibrated portion of the force range, ideally 20 % to 80 % of full scale. Machine force verification follows ISO 7500-1 or ASTM E4. Strain can be measured with a contact or video extensometer verified to ISO 9513 or ASTM E83. Pneumatic or wedge-action grips are recommended to prevent slippage on ductile specimens.

Conditioning

Unless otherwise specified by the material standard, specimens are conditioned at 23 °C ± 2 °C and 50 % ± 10 % relative humidity, per ISO 291 (typically at least 88 hours when no other requirement applies) or ASTM D618 Procedure A (commonly 40 hours). Testing is preferably carried out in the same atmosphere, since temperature and humidity strongly influence the ductility of many plastics.

Test Procedure

• Measure specimen width and thickness at several points within the gauge section and compute the minimum cross-sectional area.
• Set the gauge length to 50 mm (ISO Type 1A/1B or ASTM Type I), mount the specimen, and attach the extensometer.
• Select the test speed: for ISO 527-2 specimens, 1 mm/min is used for modulus determination and 50 mm/min is the typical speed for strength and elongation at break. For ASTM D638, the standard speed for Type I specimens is 5 mm/min (0.2 in./min), although other speeds such as 50 mm/min are frequently specified for ductile materials and small specimens.
• Apply the load at constant speed and record the force-extension curve until the specimen ruptures.
• Note the extension at break, check that the break occurred within the gauge length, and calculate εB.
• Test at least five valid specimens and report the arithmetic mean and standard deviation.

Calculation and Result Validity

Elongation at break is calculated from the extensometer reading at rupture divided by the initial gauge length and multiplied by 100. Under ISO 527, the tensile modulus is determined as a chord modulus between strains of 0.05 % and 0.25 % at 1 mm/min. Results must be discarded and the test repeated when the specimen breaks outside the gauge length, fails at a grip, or slips during loading. For very ductile materials that exceed the extensometer range, nominal elongation at break based on grip separation may be reported, clearly identified as such.

Applications

Material Qualification and Quality Control

Elongation at break is a core entry on virtually every plastic resin datasheet. Compounders, moulders and extruders use it for incoming-material inspection and lot release testing, and accredited third-party laboratories routinely perform ISO 527 and ASTM D638 tensile testing for certification purposes.

Packaging and Film Products

Manufacturers of PE, PP and PET films rely on elongation at break testing under ASTM D882 or ISO 527-3 to verify machine-direction and transverse-direction ductility, which governs formability, sealability and puncture resistance of packaging.

Pipes, Fittings and Construction Products

Tensile elongation data support the qualification of PE and PVC piping systems, where strip specimens taken from pipe walls are tested to confirm ductility after extrusion and jointing.

Wire and Cable

Elongation at break of insulation and sheathing compounds is specified in cable product standards and tested according to methods aligned with IEC 60811-501 and UL 2556, both before and after thermal aging.

Durability and Aging Studies

Because elongation at break degrades faster than strength when polymers are damaged, retained elongation after UV, heat or hydrolytic aging is one of the most sensitive indicators of long-term durability, widely used in automotive, aerospace and outdoor-product development.

FAQ

What is elongation at break in plastics?

Elongation at break is the percentage increase in the gauge length of a tensile specimen at the moment it ruptures, relative to its initial 50 mm gauge length. It measures how far a plastic can be stretched before failing, and is a direct indicator of ductility.

Which standard should I use: ISO 527 or ASTM D638?

Use the standard referenced by your customer specification or target market: ISO 527 (or its national adoptions GB/T 1040 and JIS K 7161/7162) is dominant in international and Asian markets, while ASTM D638 is common in North America. Because the specimen geometry and test speeds differ, the two methods produce different numbers and results should not be compared directly.

How many specimens are required?

Both ISO 527-1 and ASTM D638 require a minimum of five valid specimens per test condition, and five specimens in each principal direction for anisotropic or reinforced materials.

Why do my specimens keep breaking outside the gauge length?

Common causes include grip damage, slippage, misalignment, surface defects from machining, or internal voids. Such breaks are treated as invalid and the specimens must be replaced or the gripping method improved.

Does test speed affect elongation at break?

Yes. Plastics are strain-rate sensitive: higher crosshead speeds generally increase measured strength and reduce elongation at break. This is exactly why the standards fix the test speed and why speeds must never be changed without agreement between the parties.

How are thin plastic films tested?

Films thinner than 1 mm are tested as rectangular strips under ASTM D882 or ISO 527-3, usually with 100 mm initial grip separation, and elongation is calculated from crosshead displacement rather than a dog-bone gauge section.