Elongation at break is one of the most widely measured mechanical properties for evaluating the ductility, toughness, and formability of plastic materials. This article presents a comprehensive technical guide to the determination of elongation at break of plastics, covering the testing principle, specimen requirements, apparatus, procedures, calculation methods, and international standards such as ISO 527, ASTM D638, ASTM D882, and GB/T 1040, serving as a practical reference for material engineers, quality-control specialists, and laboratory technicians.
Technical Overview
Elongation at break (εb), also known as tensile elongation at rupture, is the percentage increase in the gauge length of a plastic specimen at the moment it fractures during a tensile test, relative to its initial gauge length. It is a fundamental index of material ductility: materials with high elongation at break, such as polyethylene and certain thermoplastic elastomers, can undergo large plastic deformation before failure, while brittle materials such as unfilled polystyrene or thermosetting resins typically exhibit very low values.
The parameter is calculated from the initial gauge length L0 and the gauge length at break Lb using the following relationship:
Elongation at break εb = (Lb − L0) / L0 × 100%
Elongation at break is determined together with other tensile properties—such as tensile strength, yield stress, and tensile modulus—using a universal testing machine. The test is governed by a family of internationally recognized standards, the most important of which include:
- ISO 527-1: Plastics — Determination of tensile properties — Part 1: General principles
- ISO 527-2: Test conditions for moulding and extrusion plastics
- ISO 527-3: Test conditions for films and sheets
- ASTM D638: Standard Test Method for Tensile Properties of Plastics
- ASTM D882: Standard Test Method for Tensile Properties of Thin Plastic Sheeting
- GB/T 1040 series: Chinese national standards, technically aligned with ISO 527
- JIS K 7113 and related JIS standards for plastics tensile testing
Testing Samples
Rigid and Semi-Rigid Moulding and Extrusion Plastics
For moulding and extrusion plastics with a thickness of 1 mm or greater, dumbbell-shaped (dog-bone) specimens are used. Under ISO 527-2, the preferred specimen types are 1A and 1B, both with a parallel-sided gauge section of 10 mm width and a gauge length of 50 mm; type 1A is normally injection moulded directly, while type 1B is commonly machined from sheets, plates, or finished parts. Under ASTM D638, Type I specimens with a 50 mm gauge length and 13 mm width are the default for rigid plastics with thicknesses between 1 mm and 14 mm, with Types II through V used for materials that cannot be prepared in the standard thickness. Typical specimen thicknesses are 4 mm for ISO specimens and 3.2 mm for ASTM specimens.
Plastic Films and Thin Sheets
For flexible films and sheets thinner than 1 mm, ISO 527-3 specifies specimen types 2, 3, and 5 (parallel-strip specimens of various widths), as well as type 1B dumbbells for stiffer sheet materials. ASTM D882 applies to strips cut from thin plastic sheeting, typically 10 mm to 25.4 mm wide with an initial grip separation of 50 mm to 250 mm depending on elongation capability. Film testing generally requires lower-capacity load cells, pneumatic or soft-face grips, and higher test speeds to capture the very large elongations—often several hundred percent—exhibited by polyolefin and elastomeric films.
Finished Products and Special Forms
Elongation at break can also be measured on specimens machined or cut from finished goods such as injection-moulded components, extruded profiles, pipes (per ISO 6259), cable insulation and sheathing compounds (per IEC 60811-506), geosynthetics, and additively manufactured (3D-printed) parts. In these cases, specimen geometry, orientation relative to processing direction, and surface condition must be documented because they strongly influence the measured elongation.
Testing Items
Elongation at break is determined as part of a full tensile characterization. The parameters typically measured and reported in a single test campaign include:
- Elongation at break (εb) — the primary item of this test
- Tensile strength (stress at break) and tensile stress at yield
- Elongation at yield, where applicable
- Tensile modulus (Young's modulus), determined from the initial linear portion of the stress–strain curve using an extensometer
- Nominal elongation, based on grip separation rather than gauge-length extension, where extensometer measurement is impracticable
- Failure mode and break location (within or outside the gauge section)
- Test conditions: temperature, humidity, crosshead speed, specimen type, and conditioning history
Testing Methods
Specimen Preparation and Conditioning
Specimens shall be free from visible defects, voids, and notch-like surface imperfections, since stress concentrations can dramatically reduce measured elongation. Machined specimens should be prepared per ISO 2818, with machined edges smoothed to remove machining marks. Unless otherwise specified, specimens are conditioned in the standard atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 or ASTM D618. For hygroscopic materials such as polyamide, extended conditioning (up to 88 hours or longer, or conditioning to equilibrium moisture content) may be required by the material specification, because moisture content has a pronounced effect on elongation.
Apparatus
The test is performed on a constant-rate-of-extension universal testing machine whose force-measuring system complies with ISO 7500-1 or ASTM E4. Grips shall be self-aligning to minimize bending stresses; wedge-action, pneumatic, or soft-face grips are selected according to specimen rigidity to prevent slippage and premature breaks at the grips. Strain measurement for elongation at break may be performed by direct optical or contact extensometry on the gauge length (conforming to ISO 9513 or ASTM E83) or, for materials where grip-to-grip compliance effects are acceptable, by crosshead displacement referenced to the initial distance between grips. Gauge marks are applied to the specimen with a marker or template before testing.
Test Procedure
- Measure the width and thickness of each specimen at several points within the gauge section and compute the minimum cross-sectional area; for films, measure thickness to the nearest 1 µm or better.
- Mark the initial gauge length L0 (typically 50 mm for ISO 1A/1B and ASTM D638 Type I specimens) on the parallel section.
- Mount the specimen in the grips, ensuring axial alignment, and apply a small preload (typically less than 5 % of the expected yield or breaking force) to remove slack.
- Select the test speed specified by the material standard—common speeds include 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min; rigid plastics are often tested at 5 mm/min or 50 mm/min per ISO 527-2, while many ductile films are tested at 200–500 mm/min.
- Attach the extensometer or activate the video strain system, then start the test and record the force–extension curve continuously until the specimen ruptures.
- Note the extension (or gauge length Lb) at the moment of rupture and record the break location.
- Test at least five valid specimens; discard and replace results from specimens breaking within the grips or at an obvious flaw, as such breaks do not represent the true material behavior.
Calculation and Expression of Results
Elongation at break is calculated as (Lb − L0)/L0 × 100 % and reported as the arithmetic mean of valid specimens, together with the standard deviation and, where required, the coefficient of variation. If strain is derived from crosshead displacement, the result is reported as nominal elongation at break to distinguish it from extensometer-based values, because system compliance typically causes grip-separation strain to overestimate true gauge strain. It should also be noted that for materials exhibiting very high elongation, results are highly sensitive to test speed, and comparisons between laboratories are only meaningful when specimen geometry, conditioning, and speed are identical.
Applications
Determination of elongation at break is indispensable across the entire plastics value chain. Typical applications include:
- Material datasheet generation and grade comparison for resin producers and compounders
- Incoming inspection and statistical process control of raw polymers, masterbatches, and films
- Flexible packaging development, where film formability, puncture resistance, and sealing performance correlate with elongation
- Automotive and aerospace component qualification, including assessment of embrittlement after heat aging, UV exposure, or chemical contact
- Medical device and pharmaceutical packaging validation, where ductility retention demonstrates suitability for sterilization processes
- Wire and cable manufacturing, where insulation and sheathing compounds must retain minimum elongation at break after thermal aging
- Research and failure analysis, including the effects of fillers, recycled content, molecular weight changes, weld lines, and environmental stress cracking
Accredited third-party laboratories routinely perform elongation at break testing to support product certification, regulatory submissions, supplier qualification, and dispute resolution, providing results traceable to recognized international standards.
FAQ
What is elongation at break and how is it different from elongation at yield?
Elongation at break is the strain at the moment of specimen rupture, representing the total deformability of the material. Elongation at yield is the strain at the point where the material begins to deform plastically, marked by the first peak or plateau on the stress–strain curve. For ductile plastics, elongation at break is far larger than elongation at yield; for brittle materials, fracture may occur before any yield point is reached.
Which standard should be chosen: ISO 527 or ASTM D638?
Both standards are technically similar but use different specimen geometries, and results are not directly interchangeable. ISO 527-2 uses 1A/1B dumbbells with a 10 mm wide gauge section, while ASTM D638 Type I uses a 13 mm wide section, producing systematically different stress and strain values. The choice depends on the specification, the end market, and customer requirements; GB/T 1040 is aligned with the ISO approach.
Can elongation at break exceed 100 %?
Yes. Elongation at break is not bounded by 100 %. Highly ductile materials such as LDPE films, polyurethanes, and certain thermoplastic elastomers can reach elongations of 300 % to more than 800 % before rupture. Values above 100 % simply indicate that the specimen has more than doubled its gauge length.
What factors most strongly influence the measured elongation at break?
Key influences include specimen preparation quality and edge condition, molecular orientation introduced by processing (machine direction versus transverse direction in films), test speed and temperature, moisture conditioning for hygroscopic polymers, grip slippage, and system compliance when strain is measured from crosshead travel. Controlling these variables is essential for reproducible results.
How many specimens are required for a valid test?
Both ISO 527 and ASTM D638 require a minimum of five valid specimens per condition. Additional specimens should be tested when results show high scatter, and any specimen breaking in the grips or at a visible flaw should be discarded and replaced so that the reported mean reflects true material behavior.
Why do results sometimes differ between laboratories for the same material?
Small differences in specimen geometry, conditioning state, test speed, strain-measurement method (extensometer versus crosshead displacement), and grip alignment can shift elongation values significantly, especially for very ductile materials. Interlaboratory comparability is best achieved by specifying the complete test conditions—specimen type, speed, atmosphere, and strain measurement technique—in the test report.