Elongation at break is a fundamental mechanical property that quantifies the ductility of plastics, rubbers, films, textiles, and elastomers before fracture occurs. This technical specification summarizes the internationally recognized testing standards, specimen requirements, equipment conditions, procedural steps, and calculation methods used by accredited third-party laboratories to determine elongation at break with accuracy and repeatability. Engineers, quality managers, and procurement specialists can use this guide to understand how valid test results are obtained and reported.
Technical Overview
Elongation at break, commonly denoted as εb, is defined as the percentage increase in gauge length of a test specimen between the start of tensile loading and the moment of rupture. It is calculated by dividing the increase in length at break by the original gauge length and multiplying by 100. As a core ductility indicator, elongation at break reflects the ability of a material to undergo plastic deformation before failure, which is critical for applications involving stretching, forming, impact, or thermal expansion.
The test is performed on a universal testing machine (UTM) equipped with suitable grips and, where required, a contact or non-contact extensometer. During the test, the specimen is stretched at a constant crosshead speed until it fractures, while the force-elongation curve is recorded. Depending on the material type, elongation may be measured either with an extensometer attached to the gauge section or derived from crosshead displacement relative to the initial separation between grips.
Key elements of a technically valid elongation at break determination include standardized specimen geometry, controlled environmental conditioning, calibrated force and displacement measurement, an appropriate test speed, and clearly defined criteria for accepting or rejecting fractured specimens. Deviations in any of these elements can significantly bias the reported result.
Testing Samples
Rigid and Semi-Rigid Plastics
Molded or machined dumbbell-shaped specimens (dog-bone Type 1A and 1B per ISO 527-2; Types I through V per ASTM D638; corresponding geometries in GB/T 1040.2) are tested. Dumbbell geometry concentrates deformation within a defined gauge section, ensuring that rupture occurs away from the grips and that elongation is measured over a known reference length.
Rubber and Thermoplastic Elastomers
Vulcanized and thermoplastic rubber materials are tested using dumbbell or ring specimens in accordance with ISO 37, ASTM D412, and GB/T 528. Because rubbers can exhibit elongations exceeding several hundred percent, larger grip separations and high-travel extensometer systems or crosshead displacement measurement are typically used.
Plastic Films and Thin Sheets
Flexible films below approximately 1 mm in thickness are tested as rectangular strip specimens per ISO 527-3, ASTM D882, and GB/T 1040.3. Wide, thin strips require careful grip alignment and sufficient initial grip separation to minimize measurement error from grip slippage.
Textiles, Yarns, and Fabrics
Yarns are evaluated by the single-strand method per ISO 2062, ASTM D2256, and GB/T 14344, while woven and knitted fabrics are tested per ISO 13934-1, ASTM D5034 and D5035, and GB/T 3923.1. Results are frequently expressed as elongation at maximum force or elongation at rupture alongside breaking force or tenacity.
Adhesives, Sealants, and Coated Fabrics
Structural sealants and building joint products are tested per ASTM C1135, ISO 8339, and GB/T 13477. Coated fabrics and geomembranes are evaluated per ISO 1421, ASTM D751, and ISO 10319, using strip or grab procedures appropriate to the product configuration.
Fiber-Reinforced Composites
Flat rectangular coupons with bonded end tabs are tested per ASTM D3039 and ISO 527-4/527-5. Elongation at break for composites is usually low, so high-resolution extensometers or bonded strain gauges are used to capture strain to failure accurately.
Testing Items
A typical tensile test program designed around elongation at break also yields the following related properties from the same force-elongation record:
- Elongation at break (εb) and nominal elongation at break
- Elongation at yield, where applicable for ductile plastics
- Tensile strength and tensile stress at break
- Tensile stress at yield
- Tensile modulus (Young's modulus or 1% secant modulus)
- Breaking force, breaking tenacity, and elongation at maximum force for yarns and fabrics
Testing Methods
Specimen Preparation and Conditioning
Specimens must be free of visible defects, voids, and machining marks. Dumbbell specimens are die-cut with sharp, maintained cutting dies or machined per the relevant standard, and dimensions are verified with calibrated micrometers or thickness gauges. Prior to testing, specimens are conditioned in a standard atmosphere, typically 23 °C ± 2 °C and 50% ± 10% relative humidity for plastics per ISO 291 and ASTM D618, with the conditioning duration specified by the governing product standard.
Equipment Requirements
The universal testing machine shall be verified in accordance with ISO 7500-1 or ASTM E4, typically to accuracy class 0.5 or better. Extensometers shall comply with ISO 9513 or ASTM E83. Grip selection depends on the material: wedge-action grips for rigid plastics, pneumatic or capstan grips for films and yarns, and serrated or roller grips for elastomers. The machine must offer a crosshead speed range covering the slow speeds used for modulus determination and the higher speeds used for elongation measurement.
Test Procedure
- Measure and record the width and thickness of each specimen at the gauge section and calculate the cross-sectional area.
- Set the gauge length or initial grip separation exactly as specified by the standard, for example 50 mm for ISO 527-2 Type 1A and 115 mm for ASTM D638 Type I.
- Mount the specimen symmetrically in the grips, apply the specified preload if required, and attach the extensometer when direct strain measurement is mandated.
- Select the test speed defined by the standard, such as 1 mm/min for modulus determination and 50 mm/min for tensile strength and elongation in ISO 527-2.
- Run the test until complete rupture, recording the force-elongation curve and the extension at the breaking point.
- Inspect the fracture location; breaks occurring within the grips are generally invalidated and retested.
Calculation and Reporting
Elongation at break is calculated as the extension at break divided by the initial gauge length, expressed as a percentage. For measurement based on crosshead travel, nominal elongation is calculated using the initial distance between grips. The standard requires a minimum number of valid results, typically five specimens for plastics and three for rubbers, and the report shall include the mean value, standard deviation where required, specimen type, gauge length, test speed, conditioning details, and any deviations from the standard.
Factors Affecting Result Validity
Test speed strongly influences measured elongation because most polymers are strain-rate sensitive; faster speeds generally reduce measured elongation in ductile materials. Temperature and humidity also alter ductility, particularly for moisture-sensitive polyamides. Specimen preparation quality, nicks at die-cut edges, grip slippage, misalignment, and using crosshead displacement on short gauge lengths all introduce systematic bias. Interlaboratory comparisons should therefore always state the full test conditions alongside the numeric result.
Applications
Elongation at break data generated under these technical specifications support a wide range of industrial and regulatory activities:
- Quality control and release testing of plastic compounds, rubber parts, films, and geosynthetics during serial production
- Incoming material inspection and supplier qualification against datasheet limits
- Material selection and design verification for packaging, automotive components, medical devices, wire and cable insulation, and construction products
- Research and development, including formulation comparison, filler and plasticizer optimization, and aging or heat-exposure studies
- Failure analysis, where abnormally low elongation indicates degradation, contamination, or improper processing
- Compliance demonstration for product certification against ISO, ASTM, EN, JIS, UL, and GB standard requirements
FAQ
What does a high elongation at break value indicate?
A high value indicates a highly ductile material capable of substantial deformation before fracture, which is desirable for stretch films, seals, flexible tubing, and impact-resistant parts. A low value indicates a brittle material that fails with little deformation.
How is elongation at break different from elongation at yield?
Elongation at yield is the strain at the point where the material begins to deform plastically, occurring before fracture. Elongation at break is the total strain achieved at rupture and is always greater than or equal to elongation at yield in ductile materials.
Which standard should be selected for testing?
Standard selection depends on the material and the market: ISO 527 or ASTM D638 for molded plastics, ISO 37 or ASTM D412 for rubbers, ISO 527-3 or ASTM D882 for films, ISO 13934-1 or ASTM D5034 for fabrics, and GB standards for conformity assessment in the Chinese market. Because ISO and ASTM procedures differ in specimen geometry and speed, results are not directly interchangeable.
Why do ISO and ASTM results for the same material differ?
Different dumbbell geometries, gauge lengths, and crosshead speeds produce different stress states and strain rates, which measurably affect ductility results. Reports should therefore always identify the exact standard and specimen type used.
What happens if a specimen breaks inside the grips?
Such breaks are normally considered invalid because stress concentration at the grip distorts both force and extension readings. The result is discarded and an additional specimen is tested until the required number of valid fractures is obtained.
How many specimens are required for a valid result?
Most plastics standards require a minimum of five valid specimens, while rubber standards typically require three. Coefficients of variation and outlier criteria defined in each standard govern the final reported mean or median value.
Can elongation at break be measured without an extensometer?
Yes, for highly extensible materials such as rubbers and films, elongation is commonly derived from crosshead displacement, and the result is then reported as nominal elongation. For low-elongation rigid plastics and composites, a calibrated extensometer is strongly recommended because grip and machine compliance can otherwise dominate the measurement.