Tensile strain at break is a fundamental ductility parameter that quantifies how much a plastic material can deform before fracturing under uniaxial tensile loading. This article provides a professional technical overview of the standard experiment for determining the tensile strain at break of plastics in accordance with ISO 527 and ASTM D638, covering specimen preparation, apparatus requirements, test procedures, calculation methods, validity criteria, and typical industrial applications.
Technical Overview
Tensile strain at break (symbol: εB) is defined as the strain of a specimen at the tensile stress at break, i.e., the relative extension of the gauge length at the instant the specimen ruptures. It is normally expressed as a percentage and is one of the most widely reported indicators of ductility for polymeric materials. Because the measurement reference differs between methods, ISO 527-1 additionally defines the nominal tensile strain at break (εtB), which is calculated from the change in grip separation relative to the initial distance between grips rather than from an extensometer gauge length. Distinguishing εB from εtB is essential when comparing datasheets, because the two values are numerically different for the same material.
The standard experiment is governed internationally by the following documents:
- ISO 527-1 and ISO 527-2 — general principles and test conditions for moulding and extrusion plastics, with corresponding national series GB/T 1040.1 / GB/T 1040.2 and JIS K 7161 / JIS K 7162.
- ASTM D638 — standard test method for tensile properties of unreinforced and reinforced plastics.
- ISO 527-3 — tensile properties of films and sheets less than 1 mm thick; ASTM D882 is the equivalent thin-film method.
- ISO 527-4 and ISO 527-5 — isotropic and orthotropic fibre-reinforced plastic composites.
The test is performed on a calibrated universal testing machine fitted with an extensometer or non-contact video extensometer. A complete stress–strain curve is recorded to rupture, from which tensile strain at break, stress at break, tensile strength, yield behavior, and tensile modulus can be determined in a single experiment.
Testing Samples
Rigid and Semi-Rigid Moulding and Extrusion Plastics
The preferred specimen for rigid thermoplastics and thermosets is the ISO 527-2 type 1A dumbbell, with a gauge length of 50 mm, a parallel-section width of 10 mm, and a preferred thickness of 4 mm, normally produced by injection moulding. The type 1B dumbbell, with the same gauge length but shorter overall dimensions, is used for machined or compression-moulded sheet. The closest ASTM D638 equivalent is the Type I specimen (50 mm gauge length, 3.2 mm nominal thickness); because the ISO and ASTM dumbbell geometries are not identical, results from the two methods are not directly interchangeable.
Flexible Films and Sheets
For materials thinner than 1 mm, ISO 527-3 specifies rectangular strip specimens (typically 10 mm to 25 mm wide) tested with gauge lengths of 50 mm or 100 mm, while ASTM D882 covers strip specimens for thin plastic sheeting and film. These specimens are die-cut cleanly, without nicks or edge defects that would trigger premature failure.
Fibre-Reinforced Plastics and Composites
For reinforced and composite materials, ISO 527-4 and ISO 527-5 specify rectangular specimens, frequently with bonded end tabs to prevent grip-induced failure, and testing in both the principal fibre direction and the transverse direction to characterize anisotropy.
Specimen Preparation and Conditioning
Specimens should be injection moulded in accordance with ISO 294 or ISO 10724, or machined from sheet or finished parts in accordance with ISO 2818, and must be free of flash, voids, notches, and surface scratches. Unless otherwise specified, specimens are conditioned at 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 (or ASTM D618 Procedure A) for a minimum of 88 hours, or 40 hours respectively. Hygroscopic materials such as polyamides may require conditioning to a defined moisture state, for example using the accelerated humidification described in ISO 1110.
Testing Items
The following properties are determined from the same standardized tensile experiment:
- Tensile strain at break (εB) — rupture strain measured with an extensometer on the gauge length.
- Nominal tensile strain at break (εtB) — rupture strain calculated from crosshead movement referenced to the initial grip separation.
- Tensile stress at break (σB) and tensile strength (σM) — reported together with the strain at break.
- Tensile strain and stress at yield (εY, σY) — for materials that exhibit a yield point.
- Tensile modulus (ET) — determined at 1 mm/min as the secant modulus between 0.05 % and 0.25 % strain in accordance with ISO 527-1.
Testing Methods
Apparatus and Calibration
A universal testing machine verified in accordance with ISO 7500-1 or ASTM E4 is used, with a load cell capacity selected so that the rupture force lies well within the verified working range. Strain shall be measured with an extensometer of accuracy class 1 or better in accordance with ISO 9513, or class B-2 or better in accordance with ASTM E83; non-contact video extensometers are acceptable. Grips must be self-aligning to minimize bending stresses, and verified periodically as part of the laboratory quality system.
Test Procedure
- Condition the specimens, then measure width and thickness at the gauge length to the nearest 0.1 mm and 0.02 mm respectively, using the median of at least three measurements for cross-sectional area.
- Mark the gauge length L0 (50 mm for types 1A/1B and ASTM Type I) and attach the extensometer.
- Set the initial grip separation (115 mm for ISO type 1A, 100 mm for type 1B) and apply a small straightening preload.
- Select the test speed specified by the relevant material standard — commonly 50 mm/min for strain at break of ductile thermoplastics and 1 mm/min for modulus determination — and record it in the report.
- Run each specimen to complete rupture, capture the full force-extension curve, and test a minimum of five valid specimens per condition.
Calculation of Strain at Break
The tensile strain at break is calculated as εB = (ΔLB / L0) × 100 %, where ΔLB is the extensometer extension at rupture and L0 is the initial gauge length. The nominal strain at break is calculated as εtB = (ΔL / L) × 100 %, where ΔL is the increase in grip separation and L is the initial distance between grips. Because grip separation includes deformation of the shoulders and fillet radii, εtB is generally lower than the locally measured εB of the parallel section. The report shall include the arithmetic mean, standard deviation, test speed, specimen type, conditioning state, and the specific standard used.
Validity of Results
Only specimens that break within the gauge length are valid for εB determination; specimens failing in or near the grips, or outside the gauge length, shall be discarded and replaced. When scatter is high, additional specimens shall be tested. For anisotropic or direction-sensitive materials, specimens shall be taken and reported for both the principal and transverse directions.
Applications
- Incoming inspection and lot-release quality control against material datasheet specification limits.
- Material selection and design, matching ductility to forming, snap-fit assembly, thermal expansion, and energy-absorption requirements.
- Research and development of formulations, quantifying the effect of plasticizers, impact modifiers, fillers, recycled content, and compatibilizers on ductility.
- Durability assessment, tracking the reduction of strain at break after heat aging, UV and weathering exposure, hydrolysis, or chemical contact.
- Sector-specific uses including automotive trim and structures, pipes and fittings, packaging films, medical devices, wire and cable insulation, and consumer electronics housings.
- Compliance verification and independent data generation for regulatory submissions and customer specifications through accredited third-party laboratories.
FAQ
What is the difference between tensile strain at break and elongation at break?
They describe the same rupture event: elongation at break is the strain at break expressed as a percentage of the original reference length. The practically important distinction is between εB, measured with an extensometer on the gauge length, and nominal εtB, computed from grip separation; datasheets should always state which definition and which reference length were used.
Which test speed should be used?
The speed is defined by the relevant material standard. In ISO 527-2, the modulus is measured at 1 mm/min while strain at break for most rigid thermoplastics is measured at 50 mm/min. ASTM D638 lists several standard speeds, with 50 mm/min common for ductile materials and 5 mm/min for modulus or stiff materials. Because plastics are strain-rate sensitive, the speed must always be reported with the result.
How many specimens are required?
Both ISO 527-1 and ASTM D638 require a minimum of five valid specimen breaks per condition, and more specimens are recommended for heterogeneous, reinforced, or highly scattered materials.
Why is an extensometer required for εB?
Crosshead displacement includes machine and grip compliance as well as deformation of the specimen shoulders, which systematically distorts strain values. Direct strain measurement on the gauge length — with a contacting or video extensometer — is required for an accurate εB and is mandatory for tensile modulus determination.
What are typical strain-at-break values for common plastics?
Unmodified polystyrene and other brittle amorphous plastics typically fail below about 3 % strain, engineering materials such as polycarbonate and polyamide can exceed 50 % to well over 100 %, and polyolefins such as polyethylene and polypropylene can reach several hundred percent, usually reported as nominal strain at break.
Do ISO 527 and ASTM D638 produce identical results?
No. Differences in dumbbell geometry, specimen thickness, grip separation, and default test speeds lead to systematically different values. The test method must always be cited when results are compared, exchanged between suppliers and customers, or verified by accredited third-party laboratories.