Strain at break is one of the most important mechanical parameters for evaluating the ductility, brittleness, and ultimate load-bearing performance of plastic materials. This article provides a comprehensive technical guide to the determination of strain at break for plastics in accordance with ISO 527, ASTM D638, and GB/T 1040 standards, covering specimen preparation, equipment requirements, test procedures, calculation methods, and typical industrial applications.
Technical Overview
Strain at break (εB), also referred to as tensile strain at break or nominal strain at break, is defined as the tensile strain of a plastic specimen at the moment of rupture, expressed as a percentage of the initial gauge length between reference marks. It quantifies how much permanent and elastic deformation a plastic can sustain before failing, making it a fundamental indicator of material ductility versus brittleness.
The determination of strain at break is performed as part of a uniaxial tensile test, in which a standardized dumbbell-shaped or strip specimen is pulled at a constant crosshead speed until fracture. The principal international and national standards governing this measurement include:
- ISO 527-1 and ISO 527-2 – Determination of tensile properties of plastics, general principles and test conditions for moulding and extrusion plastics
- ASTM D638 – Standard Test Method for Tensile Properties of Plastics
- GB/T 1040.1 / GB/T 1040.2 – Chinese national standards, technically aligned with ISO 527
- JIS K 7161 / JIS K 7162 – Japanese industrial standards harmonized with ISO 527
- ISO 527-3 and ASTM D882 – Tensile testing of plastic films and thin sheets
Strain at break values are highly sensitive to material grade, molecular weight, filler content, processing conditions, specimen geometry, test speed, temperature, and humidity. Therefore, strict adherence to standardized procedures is essential to obtain reproducible and comparable results, whether testing is performed in-house or by accredited third-party laboratories.
Testing Samples
Injection-Molded Dumbbell Specimens (ISO 527-2 Type 1A / 1B)
The preferred specimen for rigid and semi-rigid plastics is the ISO 527-2 Type 1A dumbbell with an overall length of 170 mm and a gauge length of 75 mm (initial distance between grips of 115 mm). Type 1B specimens (overall length 150 mm) are acceptable when full-size molds are impractical. Specimens should be injection molded under conditions representative of the material datasheet to avoid residual stress, voids, or orientation effects that can bias the strain at break.
ASTM D638 Specimens (Type I through Type V)
ASTM D638 specifies five specimen types. Type I (overall length 165 mm, width of narrow section 13 mm, thickness 3.2 mm) is the standard for rigid plastics. Type IV is commonly used for testing without extensive machining, while Type V (a miniaturized specimen) is suited to limited material quantities or small parts.
Machined or Die-Cut Specimens
For sheets, extruded profiles, compression-molded plaques, or finished parts, specimens may be machined or routed from the raw material. All edges must be smooth and free of machining marks, notches, or cracks, since surface defects act as stress concentrators and dramatically reduce measured strain at break, particularly in ductile materials.
Film and Thin-Sheet Specimens
For plastic films below 1 mm in thickness, strip specimens cut per ISO 527-3 or ASTM D882 are used. These require low-force load cells, pneumatic or film-specific grips, and long-travel or video extensometers due to the very large strains (often exceeding several hundred percent) exhibited by polyolefin films.
Specimen Conditioning and Dimensional Measurement
Unless otherwise specified, specimens shall be conditioned in accordance with ISO 291 or ASTM D618 at 23 °C ± 2 °C and 50 % ± 10 % relative humidity for a minimum of 40 hours (88 hours for certain hygroscopic materials). Before testing, the width and thickness of the narrow parallel section of each specimen shall be measured at a minimum of three positions using calibrated instruments, and the cross-sectional area shall be calculated from the mean values.
Testing Items
A complete tensile characterization program typically includes strain at break together with the following related parameters, all derived from the same stress–strain curve:
- Strain at break (εB) – the tensile strain at the moment of rupture, the core item of this test
- Tensile stress at break (σB) – the stress at the point of rupture
- Tensile strength (σM) – the maximum tensile stress sustained during the test
- Strain at tensile strength (εM) – the strain at which maximum stress occurs
- Yield strain (εY) and yield stress (σY) – the strain and stress at the yield point, where applicable
- Tensile modulus (Et) – the secant modulus between two specified strain points, typically 0.05 % and 0.25 %, measured at 1 mm/min
- Percent elongation at break – the corresponding elongation-based expression of failure ductility
Testing Methods
Test Equipment and Calibration Requirements
Testing shall be carried out using a universal testing machine (tensile tester) meeting the following requirements:
- Force measurement system calibrated in accordance with ISO 7500-1 or ASTM E4, typically Class 0.5 or Class 1 accuracy
- Load cell capacity selected so that the breaking force falls within the calibrated measurement range, generally between 15 % and 85 % of full scale
- An extensometer with sufficient travel to record the full strain at break, verified per ISO 9513 or ASTM E83; contact, long-travel, automatic contacting, or non-contact video extensometers are all acceptable
- Grips (wedge-action or pneumatic) with faces matched to the specimen material to prevent slippage while avoiding premature failure in the grip region
- An environmental chamber when testing at non-ambient temperatures is required
Test Procedure
The standard test sequence for determining strain at break is as follows:
- Verify specimen dimensions and mark the initial gauge length L0 on the narrow parallel section (typically 50 mm for ISO specimens and ASTM Type I specimens)
- Clamp the specimen in the grips, ensuring precise axial alignment to eliminate bending stresses
- Attach the extensometer to the gauge section and apply a small preload (typically not exceeding 0.01 times the expected maximum force or the standard-specified value) to remove slack
- Select and set the crosshead speed according to the material type: ISO 527-2 assigns preferred speeds of 1, 5, 10, 20, 50, 100, 200, and 500 mm/min, while ASTM D638 lists testing speeds of 1 to 500 mm/min; rigid thermoplastics are commonly tested at 5 mm/min or 50 mm/min, and the chosen speed must be reported
- Start the test and record force and extension continuously until the specimen ruptures
- Record the extension ΔLB at the instant of break and calculate the strain at break
- Repeat the test on a minimum of five valid specimens; ISO 527 requires reporting the median of valid results, whereas ASTM D638 reports the arithmetic mean
Calculation of Strain at Break
The strain at break is calculated using the following relationship:
εB = (ΔLB / L0) × 100 %, where ΔLB is the increase in the distance between the gauge marks at the moment of rupture and L0 is the initial distance between the gauge marks (initial gauge length). For materials exhibiting very large strains, ISO 527 permits removal of the extensometer before break, provided the remaining strain is captured via crosshead separation or that an alternative method such as a long-travel or video extensometer is validated for the full range.
Validity of Results and Common Error Sources
A test shall be considered invalid and the specimen retested if rupture occurs within the grips, if the specimen slips in the grips, or if the break occurs clearly outside the gauge section for reasons attributable to a defect rather than material behavior. Additional factors that must be controlled to ensure accurate strain at break values include:
- Crosshead separation must not be used as a substitute for extensometer measurement when accurate strain values are required, because system compliance and grip seating effects inflate the apparent strain
- Specimen preparation defects (notches, voids, weld lines, burn marks) artificially lower strain at break
- Testing speed significantly affects ductility: higher speeds generally reduce strain at break in ductile polymers
- Temperature and moisture content, especially in hygroscopic materials such as polyamides, strongly influence ductility and must be controlled and documented
- Insufficient extensometer travel or premature extensometer removal leads to truncated, non-comparable data
Applications
Determination of strain at break supports a wide range of engineering, quality, and research activities across the plastics industry:
- Material datasheets and material selection: Strain at break values allow designers to compare the ductility of candidate polymers, for example distinguishing ductile ABS and polypropylene from brittle polystyrene or highly filled compounds
- Incoming material and molded-part quality control: A reduction in strain at break is a sensitive early indicator of resin degradation, contamination, excessive moisture, or improper molding conditions
- Formulation and compounding development: Evaluating the effect of fillers, tougheners, plasticizers, impact modifiers, and recycled content on ductility
- Aging and durability studies: Strain at break is one of the most widely used metrics for tracking embrittlement caused by thermal aging, UV weathering, hydrolysis, chemical exposure, and gamma or electron-beam sterilization
- Weld line and anisotropy evaluation: Comparing strain at break parallel and perpendicular to flow direction, or across weld lines, reveals processing-induced weaknesses
- Failure analysis: Establishing whether a failed product exhibits the ductility expected of the virgin material, supporting root cause investigation
- Compliance verification: Demonstrating conformity of materials and components to product standards and customer specifications through accredited third-party laboratories
FAQ
What is the difference between strain at break and elongation at break?
Both parameters describe deformation at failure. Strain at break is the dimensionless ratio (expressed in percent) of extension at rupture to the initial gauge length, as defined in ISO 527. Elongation at break, the term used in ASTM D638 and many datasheets, is numerically identical when both are referenced to the same initial gauge length; the key practical requirement is to always report the gauge length and measurement method used.
Why is an extensometer preferred over crosshead displacement?
Crosshead displacement includes machine frame compliance, grip penetration, and specimen deformation outside the gauge section, which systematically overstates strain. A properly calibrated extensometer measuring directly on the gauge section provides traceable, accurate strain at break values that are comparable between laboratories.
What are typical strain at break values for common plastics?
Indicative values at 23 °C include: general-purpose polystyrene at approximately 1–2.5 %, glass-fiber-reinforced compounds at 1.5–4 %, ABS at 20–50 %, polycarbonate at 50–120 %, unfilled polyamide 6 (dry) at 20–60 %, high-density polyethylene at 600 % or more, and LDPE films often exceeding 300 %. Actual values depend strongly on grade, conditioning, and test speed.
How many specimens should be tested?
Both ISO 527 and ASTM D638 require a minimum of five valid specimens per material condition. For materials known to be anisotropic or heterogeneous, or when testing aged specimens with high scatter, additional specimens are recommended to improve statistical confidence.
What should be done if the specimen breaks in or near the grips?
Such tests are normally invalid. The specimen should be discarded and retested. If the problem recurs, check grip pressure, grip face material, specimen alignment, and the presence of stress-concentrating defects in the specimen.
Can strain at break be tested at elevated or sub-ambient temperatures?
Yes. ISO 527-1 permits testing at defined temperatures, typically from −40 °C to +150 °C or beyond, using an environmental chamber and temperature-rated extensometry. Because polymer ductility is strongly temperature dependent, the test temperature and conditioning time at temperature must always be reported alongside the result.