The tensile strain at break test is a fundamental mechanical evaluation that measures how much a plastic material can elongate before fracturing under uniaxial tension. Governed by internationally recognized standards such as ISO 527 and ASTM D638, this test quantifies the ductility of polymers and provides essential data for material selection, quality control, and product design. This article explains the underlying experimental principle, specimen requirements, test methods, and typical applications of tensile strain at break measurement for plastics.
Technical Overview
The tensile strain at break, also referred to as elongation at break or nominal tensile strain at break, is the engineering strain exhibited by a plastic specimen at the instant of fracture during a uniaxial tensile test. It is defined as the change in length of the gauge section divided by the original gauge length, expressed as a percentage. A higher strain at break indicates a more ductile material, while a low value indicates brittleness. The experimental principle rests on applying a continuously increasing tensile load to a standardized specimen at a constant crosshead separation speed until the specimen fails, while simultaneously recording force and extension to construct the stress-strain curve.
Stress-Strain Behavior of Plastics
When a plastic specimen is stretched, its molecular chains uncoil, orient, and slide relative to one another before ultimate failure occurs. Depending on the polymer structure, temperature, and strain rate, the stress-strain curve typically passes through several characteristic regions: an initial near-linear elastic region where deformation is reversible; a yield point marking the onset of plastic deformation; possible strain softening and cold drawing in ductile materials, where molecular chains align and necking propagates along the gauge length; strain hardening as the oriented chains resist further extension; and finally fracture, where chain scission, craze coalescence, or void growth leads to separation. Brittle polymers such as general-purpose polystyrene fail at low strain with little or no yielding, whereas ductile polymers such as polyethylene or polypropylene may sustain several hundred percent strain before breaking.
Strain Measurement Principle
Strain at break is calculated as the ratio of elongation at the moment of fracture to the initial gauge length, multiplied by one hundred. Elongation may be measured by a contact extensometer attached directly to the gauge section, by a non-contact video extensometer that tracks gauge marks, or in some cases by crosshead displacement or grip separation. Extensometer-based measurement is strongly preferred for accuracy because grip separation includes machine compliance, grip slip, and deformation outside the gauge section, all of which overstate the true strain, especially for rigid materials with small break strains.
Governing Standards
- ISO 527-1: general principles for determining tensile properties of plastics.
- ISO 527-2: test conditions for moulding and extrusion plastics, including the Type 1A and 1B dumbbell specimens.
- ISO 527-3: test conditions for films and sheets.
- ASTM D638: tensile properties of plastics using dumbbell-shaped specimens.
- ASTM D882: tensile properties of thin plastic sheeting and films.
- GB/T 1040 series: Chinese national standards technically aligned with ISO 527.
Testing Samples
Injection-Moulded and Machined Dumbbell Specimens
For rigid and semi-rigid moulding and extrusion plastics, ISO 527-2 specifies the Type 1A specimen with a gauge length of 50 mm, an overall length of 170 mm, and a width of 20 mm at the shoulders, together with the smaller Type 1B specimen for materials or processes where the larger geometry is impractical. ASTM D638 defines five specimen types (Type I through Type V) selected according to material thickness, with Type I being the standard 3.2 mm thick dumbbell for rigid plastics. Specimens may be injection moulded, compression moulded, or machined and die-cut from sheets, plates, or finished parts, provided the surface is free of notches, scratches, and voids that could induce premature failure.
Film and Sheet Strip Specimens
Flexible films and thin sheets are tested as rectangular strips in accordance with ISO 527-3 or ASTM D882. These specimens are cut with clean, parallel edges using a sharp die, since any edge imperfection concentrates stress and reduces the measured strain at break. Specimen width and gauge length are chosen based on film thickness as prescribed by the relevant standard.
Specimen Conditioning
Prior to testing, specimens are conditioned in a controlled atmosphere, typically 23 °C and 50 percent relative humidity for at least 24 hours as specified in ISO 291 or ASTM D618, unless the material standard dictates otherwise. Thickness and width of each specimen are measured at several points within the gauge section, and the cross-sectional area is computed from the minimum measured values for stress calculation.
Testing Items
Although this article focuses on the strain at break principle, the same tensile experiment simultaneously yields a complete tensile property profile. Typical reported results include:
- Tensile strain at break (nominal elongation at break).
- Tensile stress at break and tensile strength at yield.
- Tensile strain at yield.
- Tensile modulus (Young's modulus) measured with an extensometer between strains of 0.05 percent and 0.25 percent.
- Poisson's ratio, when transverse strain is also recorded.
Testing Methods
The test is performed on a calibrated universal testing machine equipped with appropriately rated load cells and grips suited to the specimen geometry, such as wedge-action grips for rigid dumbbells or pneumatic side-action grips with padded faces for films. The general procedure is as follows:
- Condition the specimens and measure gauge dimensions, then mark the gauge length reference points.
- Mount the specimen in the grips, ensuring axial alignment to avoid bending stresses, and apply a small preload to remove slack.
- Attach the extensometer or activate the video extensometer to track the gauge marks.
- Set the crosshead speed specified by the standard, for example 1 mm/min for modulus determination or 50 mm/min for the standard 1A specimen, and run the test to complete fracture.
- Record the force-extension curve continuously and capture the extension value at the moment of break.
- Calculate strain at break as the elongation at break divided by the initial gauge length, multiplied by one hundred.
For ductile materials that neck extensively, the extensometer must have sufficient travel, or the strain may be tracked by grip separation with a stated correction. A test is considered valid only if fracture occurs within the gauge length; fractures at the shoulders or in the grips are invalid and must be repeated. A minimum of five valid specimens is normally tested, and the arithmetic mean, standard deviation, and coefficient of variation are reported along with specimen type, conditioning, test speed, temperature, humidity, and the strain measurement method used.
Applications
Strain at break data serve a wide range of engineering and commercial purposes:
- Material datasheets and grade comparison for selecting polymers with appropriate ductility.
- Incoming inspection and statistical process control in plastic compounding, injection moulding, and extrusion plants.
- Verification of the effect of additives, fillers, recycled content, plasticizers, and processing conditions on toughness.
- Design inputs for packaging films, geomembranes, pipes, medical devices, and flexible components where stretchability is critical.
- Failure analysis and studies of ageing, UV exposure, chemical contact, and thermal degradation, all of which typically reduce strain at break.
- Compliance demonstration against product specifications referencing ISO 527, ASTM D638, GB/T 1040, or EN standards, often verified by accredited third-party laboratories.
FAQ
What is the difference between strain at break and elongation at break?
The two terms are used interchangeably in most contexts. Strain at break is the dimensionless ratio expressed as a percentage of the original gauge length, whereas elongation at break refers to the same quantity and is reported in percent. Nominal strain is calculated from the change in gauge length divided by the initial gauge length.
Why is an extensometer preferred over crosshead displacement?
Crosshead displacement includes machine frame deflection, load cell compliance, grip movement, and specimen deformation outside the gauge section. These contributions are negligible for highly extensible films but significant for rigid plastics whose break strain may be only a few percent. An extensometer measures deformation directly in the gauge region and therefore provides accurate results for modulus and strain at break.
How do test speed and temperature affect the result?
Plastics are viscoelastic, so their mechanical response depends on strain rate and temperature. Increasing the test speed generally raises the measured strength and lowers the strain at break, while higher temperatures increase ductility. This is why standards prescribe fixed speeds and conditioning environments, and why test conditions must always accompany reported values.
What are typical strain at break values for common plastics?
Values vary widely by grade and processing history. General-purpose polystyrene and some filled or glass-reinforced compounds typically show strain at break below 5 percent, ABS often falls between 10 and 50 percent, while polyethylene and polypropylene can exceed 300 percent. Thin films of linear low-density polyethylene may reach several hundred percent before failure. Actual values must always be confirmed by testing under standardized conditions.