Tensile strain at break is one of the most informative single-point properties for characterizing the ductility and failure behavior of plastics. This article explains how the numerical determination of tensile strain at break is performed in accredited third-party laboratories, covering the governing standards such as ISO 527 and ASTM D638, specimen types, strain measurement techniques, data evaluation, and typical industrial applications.
Technical Overview
Tensile strain at break, commonly denoted as εB, is the increase in gauge length of a plastic specimen at the moment of rupture, expressed as a percentage of the initial gauge length. The fundamental relationship used for its numerical determination is defined 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 result quantifies how much a material can be stretched before final failure and serves as a direct indicator of ductility versus brittleness.
The numerical determination of strain at break is primarily governed by ISO 527-1 and ISO 527-2 for moulding, extrusion, and compression-moulded plastics, ISO 527-3 for films and sheets, ISO 527-4 and ISO 527-5 for fibre-reinforced composites, ASTM D638 as the dominant North American method, GB/T 1040 as the Chinese national equivalent of ISO 527, and JIS K 7161 and JIS K 7162 as the corresponding Japanese standards. A landmark change introduced by ISO 527-1:2019 is that strain shall be measured directly with a calibrated extensometer within the gauge length, rather than being derived from grip or crosshead separation. This change significantly improves the accuracy and interlaboratory comparability of strain-at-break values, because machine compliance, grip slippage, and frame deflection no longer distort the measured elongation.
Modern numerical determination relies on a fully traceable measurement chain: a calibrated force frame, a class-verified extensometer or non-contact optical system, synchronized digital data acquisition, and standardized statistical evaluation. For ductile materials that neck before failure, the distinction between global engineering strain and local true strain, obtainable through digital image correlation, becomes an important part of a rigorous numerical assessment.
Testing Samples
Rigid and Semi-Rigid Thermoplastics
Materials such as ABS, PC, PP, PE, PA, POM, PET, PMMA, and PBT are tested using the multipurpose specimens defined in ISO 527-2, principally type 1A with an overall length of 170 mm, a width of 10 mm in the parallel section, a thickness of 4 mm, and a 50 mm gauge length, or the slightly smaller type 1B. Under ASTM D638, the equivalent geometry is the type I specimen with an overall length of 165 mm, a width of 13 mm, and a thickness of 3.2 mm. Specimens are normally injection moulded in accordance with ISO 294 or compression moulded in accordance with ISO 293 to ensure consistent morphology and crystallinity.
Thermosets and Machined Sheet Materials
Thermosets such as epoxy, phenolic, and unsaturated polyester, as well as stock shapes and semi-finished sheets, are machined to the required geometry in accordance with ISO 2818 or the corresponding ASTM D638 specimen types. Careful milling and polishing of edges is essential, because machining marks and edge defects act as stress concentrators and can severely depress the measured strain at break.
Films and Sheets
Flexible packaging films and thin sheets are tested in strip form in accordance with ISO 527-3 or ASTM D882. Because these materials frequently exhibit elongations of several hundred percent, long-travel or non-contact extensometers are mandatory for accurate numerical determination.
Fibre-Reinforced Plastics and Composites
Unidirectional and multidirectional laminates are evaluated under ISO 527-4 and ISO 527-5 using tabbed rectangular specimens cut at defined angles, typically 0° and 90°. Strain gauges or biaxial extensometers are attached to capture the low and highly anisotropic failure strains characteristic of reinforced systems.
Specimens from Finished Parts and Additive Manufacturing
When parts cannot be moulded directly into test bars, coupons are cut from components using ISO 2818 machining, with the note that surface condition, weld lines, and orientation may alter the measured value. Additively manufactured coupons are increasingly tested along and perpendicular to the build direction, since layer adhesion dominates the failure strain of printed polymers.
Testing Items
A complete tensile characterization program built around the numerical determination of strain at break typically includes the following measurands:
- Tensile strain at break (εB), the primary quantity, expressed in percent
- Tensile stress at break (σB), reported alongside strain at break for full failure characterization
- Tensile strain at yield (εY) and tensile stress at yield (σY) for ductile materials that yield before rupture
- Tensile modulus (Et), calculated as the secant between strain limits of 0.05% and 0.25% per ISO 527-1
- Nominal tensile strain values, where required for comparison with legacy datasets based on grip separation
- Optional local true strain at break determined by digital image correlation for necking materials
Testing Methods
Equipment and Calibration
Testing is performed on a universal testing machine meeting the requirements of ISO 5893 or an equivalent frame class, with force verification in accordance with ISO 7500-1. Strain is measured with a contact extensometer, a video extensometer, or a full-field digital image correlation system calibrated in accordance with ISO 9513 or ASTM E83. Extensometers used for modulus determination must achieve the accuracy class specified by ISO 527-1, while strain-at-break measurements on ductile materials require devices with sufficient travel range. For brittle materials, high resolution at small strains is the governing requirement. Contact devices that would be damaged at rupture are automatically released before break, whereas video and optical systems measure through the entire test without any risk of specimen interference.
Specimen Conditioning and Preparation
Specimens are conditioned at 23 °C ± 2 °C and 50% ± 10% relative humidity in accordance with ISO 291 or ASTM D618 Procedure A, unless a material standard dictates otherwise. Hygroscopic polymers such as polyamide are additionally dried or moisture-equilibrated to a defined state, because moisture content dramatically shifts the strain at break. Width and thickness are measured at several positions within the gauge section, and the smallest cross-sectional area is used for stress calculation. Flash, burrs, and visible flaws are cause for rejection before testing.
Test Procedure
The specimen is mounted with a grip separation of 115 mm for ISO type 1A and ASTM type I geometries, a small preload is applied to remove slack, and the extensometer gauge length is set to 50 mm. The test speed is selected from the standard tables: 1 mm/min is used for modulus determination, while speeds of 5, 50, 100, or 500 mm/min are applied for failure properties depending on the material. Because strain at break is strongly rate dependent, the chosen speed must be reported with the result. Data are sampled continuously until force drops sharply at rupture, and the elongation at that instant is captured for the strain calculation.
Numerical Evaluation and Validity Criteria
The strain at break is computed from the extensometer reading at the point of rupture using the gauge-length ratio, ideally averaging measurements on both faces of the specimen to eliminate bending effects. A failure is considered valid only when rupture occurs within the gauge length; breaks at grips, at machining defects, or outside the gauge section are retested according to the rejection rules of the standard. At least five valid specimens are required by ISO 527 for a statistically meaningful dataset. The reported result includes the arithmetic mean, standard deviation, and coefficient of variation, and for highly scattered ductile materials the individual values are also listed, since strain at break frequently follows a non-normal distribution. For necking materials, conversion to true strain using εtrue = ln(1 + εB) may be added for simulation-oriented reporting.
Influencing Factors and Measurement Uncertainty
Principal contributors to uncertainty include test speed, temperature, moisture conditioning, specimen preparation quality, edge surface condition, grip alignment, extensometer slippage, and operator judgment of the break point. Ductile thermoplastics commonly show coefficients of variation of 10% to 20% for strain at break, considerably larger than for strength values, which is why standardized preparation and direct strain measurement are decisive for reproducible numbers.
Applications
The numerical determination of tensile strain at break underpins material selection, quality control, and failure analysis across the plastics industry:
- Generation and comparison of material datasheets under ISO 10350-1, where strain at break at 23 °C is a mandatory single-point entry
- Incoming quality control of resins and verification of moulding process consistency in automotive, electrical, and electronics manufacturing
- Assessment of embrittlement after thermal aging, UV weathering, or chemical exposure, where a sharp drop in retained strain at break is often the earliest warning indicator
- Calibration of elastoplastic material models for finite element simulation of drop tests, crash events, and snap-fit assembly, where accurate failure strain limits are required
- Validation of recyclate content, biobased compounds, and additive formulations, where ductility retention is a key acceptance criterion
- Quality assurance of packaging films, pipes and fittings, medical components, and construction products against international product standards
FAQ
What is the difference between strain at break and elongation at break?
The two terms describe the same quantity: the extension at rupture divided by the initial gauge length. "Strain at break" is the terminology used in ISO 527 and related standards, while "elongation at break" appears frequently in ASTM and commercial datasheets. Both are expressed as a percentage, but values are only comparable when the measurement basis, specimen, and speed match.
Why is crosshead displacement no longer acceptable for reporting strain at break?
Crosshead and grip separation include machine compliance, grip slippage, and specimen deformation outside the gauge section, which systematically distorts the elongation value, particularly for stiff or low-elongation materials. ISO 527-1:2019 therefore mandates direct strain measurement with a calibrated extensometer, video system, or equivalent optical method within the gauge length.
Are ISO 527 and ASTM D638 results interchangeable?
No. The two standards differ in specimen geometry, thickness, preferred speeds, and strain measurement details, and reported strain-at-break values can differ noticeably, especially for ductile or rate-sensitive plastics. Comparison should only be made within one standard system, and the standard, specimen type, and speed must always accompany the numerical result.
How is strain at break measured for extremely ductile or extremely brittle plastics?
Very ductile films and elastomers, which may stretch several hundred percent, require long-travel or non-contact video extensometers that follow the specimen throughout necking. Brittle, highly filled, or reinforced plastics, whose failure strains may be below one percent, require high-resolution class-verified extensometers or strain gauges to resolve the small elongation reliably.
What happens if the specimen breaks outside the gauge length?
Such failures are invalid under the standard because the stress state near the grips is disturbed. The result is discarded and an additional specimen is tested until the minimum number of valid breaks, normally five, is achieved. Systematic grip-zone failures indicate preparation or gripping problems that must be corrected before reporting.
Which factors most strongly influence the numerical value?
Test speed, temperature, moisture conditioning, and specimen edge quality dominate the outcome. Higher speeds generally reduce strain at break, elevated temperature increases it, moisture raises the ductility of polyamides, and machining marks or weld lines can cut the measured value substantially. Consistent control of these parameters is essential for meaningful comparisons over time or between laboratories.