Elongation at break testing is one of the most widely performed mechanical tests for evaluating the ductility and stretchability of materials before fracture. This technical guide covers the definition and calculation of elongation at break, specimen preparation, internationally recognized test standards (ISO, ASTM, GB, JIS, IEC), step-by-step test procedures, and typical applications across plastics, rubber, films, textiles, metals, composites, and wire and cable products.
Technical Overview
Elongation at break, often denoted as Eb or εb, is the percentage increase in the gauge length of a specimen at the moment of rupture, relative to its original gauge length. It is a fundamental ductility indicator that reflects how much permanent and elastic deformation a material can withstand under uniaxial tensile loading before failing. Together with tensile strength and tensile modulus, elongation at break forms the core of nearly every tensile test report issued by accredited third-party laboratories and in-house quality control departments.
The elongation at break is calculated using the following relationship:
Elongation at break (%) = (Lb − L0) / L0 × 100, where L0 is the initial gauge length and Lb is the gauge length at the moment of rupture.
Testing is typically performed on a universal testing machine (UTM) equipped with an appropriate load cell, grips, and either a contact or non-contact extensometer for accurate strain measurement. Test conditions such as temperature, humidity, crosshead speed, and specimen geometry are strictly defined by the applicable international standards to ensure repeatability and comparability of results.
Testing Samples
Rigid and Semi-Rigid Plastics
Injection-molded or machined dumbbell-shaped specimens are tested according to ISO 527-2 (specimen types 1A and 1B), ASTM D638 (specimen Types I through V), or GB/T 1040. The standard specimen features a preferred gauge length of 50 mm, and the choice of specimen type depends on the thickness of the sheet or plate from which it is machined.
Rubber and Thermoplastic Elastomers
Vulcanized rubber and TPE materials are tested using die-cut dumbbell specimens per ISO 37 or ASTM D412 (commonly die C). Specimen thickness is measured at three points within the gauge length, and the narrow section width is used in cross-sectional area calculation. For rubber, results are normally reported as the median of at least three (preferably five) specimens.
Plastic Films and Thin Sheets
Flexible films below approximately 1 mm in thickness are tested as rectangular strips in accordance with ISO 527-3, ASTM D882, or GB/T 1040.3. Strip specimens are cut in both the machine direction and transverse direction to evaluate anisotropy caused by film orientation during processing.
Textiles, Yarns, and Fabrics
Woven and nonwoven fabrics are evaluated using strip or grab methods under ISO 13934-1, ASTM D5035, or ASTM D5034. Yarns and threads are tested as single strands per ISO 2062 or ASTM D2256, where elongation at break is reported alongside breaking force and tenacity. JIS L 1096 provides equivalent Japanese national requirements.
Metallic Materials
Percent elongation after fracture for metals is determined using proportional or non-proportional gauge lengths defined in ISO 6892-1, ASTM E8/E8M, and GB/T 228.1. Because measured elongation in metals depends on gauge length, results are reported with a subscript identifying the gauge ratio (for example, A5 or A50), and fractured specimen ends must be carefully fitted back together before final measurement.
Fiber-Reinforced Composites
Composite laminates are tested using straight-sided rectangular coupons per ASTM D3039 for tensile properties of polymer matrix composites, or ISO 527-4 and ISO 527-5 for multidirectional and unidirectional laminates. Bonded tabs are frequently used to prevent grip-induced premature failure.
Wire and Cable Insulation and Sheaths
Insulation and sheath compounds removed from finished cables are tested per IEC 60811-501, which specifies tensile strength and elongation at break requirements, including aged-condition testing after oven exposure to assess thermal aging resistance.
Testing Items
A standard tensile test program typically reports elongation at break together with the following related parameters:
- Elongation at break (%)
- Tensile strength and tensile stress at break
- Yield stress and elongation at yield
- Tensile modulus (Young's modulus) or 1% secant modulus
- Stress at specified elongation, such as 100% or 300% elongation for rubber compounds
- Breaking force and elongation for yarns, threads, and textile strips
- Percent elongation after fracture for metallic materials
- Complete stress-strain curve and tensile energy to break
- Elongation at break after thermal, UV, or chemical aging for durability assessment
Testing Methods
Specimen Conditioning
Unless otherwise specified, specimens are conditioned prior to testing in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10% relative humidity, as defined in ISO 291 and ASTM D618 for plastics. Rubber specimens are conditioned per ASTM D1349, and textiles per ISO 139, typically for a minimum conditioning period of 16 to 24 hours. Conditioning ensures that moisture content and temperature do not introduce uncontrolled variability into the results.
Instrumentation and Setup
A calibrated universal testing machine is configured with a load cell of appropriate capacity so that the anticipated breaking force falls within the verified measurement range. Grip selection depends on the material: wedge mechanical grips for rigid plastics and metals, pneumatic or capstan grips for films and yarns, and self-tightening grips for elastomers. Strain measurement above approximately 10% elongation may be derived from crosshead displacement with grip separation correction, while contact or video extensometers conforming to ISO 9513 or ASTM E83 are recommended for precise modulus and low-elongation measurements.
Step-by-Step Test Procedure
- Verify machine calibration status and select the correct test method in the software.
- Measure specimen width and thickness at multiple points within the gauge length and calculate the minimum cross-sectional area.
- Mark the gauge length on the specimen using a benchmark gauge or non-permanent marker, or attach the extensometer.
- Mount the specimen symmetrically in the grips with the gauge marks aligned to the axis of loading, applying sufficient initial grip pressure to prevent slippage.
- Set the crosshead speed specified by the standard, for example 50 mm/min for ISO 527-2 1A specimens, 500 mm/min for ASTM D412 die C dumbbells, or the strain-rate-controlled regime of ISO 6892-1 for metals.
- Apply a small preload to remove slack, then start the test and continue loading at constant speed until the specimen ruptures.
- Record the elongation at the moment of rupture, along with force and displacement data throughout the test.
- Discard results from specimens that break in the grip region or outside the gauge length, and retest replacement specimens.
- Calculate elongation at break using the recorded gauge length change and the original gauge length.
Data Processing and Reporting
For plastics, the arithmetic mean and standard deviation of valid specimens are reported, and results are typically rounded to two significant figures. For rubber, the median value of the tested specimens is reported as required by ASTM D412 and ISO 37. Metallic materials require fitting the fractured ends together carefully before measuring the final gauge length. A complete report should identify the test standard, specimen type and dimensions, conditioning environment, crosshead speed, number of specimens, individual and statistical results, and any deviations or invalid break locations.
Applications
Elongation at break data supports material selection, product design, incoming inspection, and regulatory compliance across a broad range of industries:
- Polymer compounding and resin qualification for comparing ductility of plastic grades and formulations
- Automotive and industrial rubber components such as seals, gaskets, hoses, and vibration isolators
- Flexible packaging films where stretchability, puncture resistance, and orientation balance are critical
- Wire and cable insulation and sheathing, including mandatory aged elongation retention checks
- Geotextiles, technical textiles, and safety webbing where breaking elongation affects performance
- Metal forming operations where elongation indicates formability and bendability
- Composite structures in aerospace, wind energy, and sporting goods
- Medical devices such as gloves, tubing, and catheters requiring high extensibility
- Failure analysis and material degradation studies after thermal, UV, or chemical exposure
- Quality control release testing and comparison against supplier datasheet values
FAQ
What is the difference between elongation at break and elongation at yield?
Elongation at yield is the strain at which the material begins to deform plastically and the stress-strain curve shows a yield point, while elongation at break is the total strain reached at rupture. For ductile plastics, elongation at break can be many times larger than elongation at yield; for brittle materials, the two values may be very close.
Which standard should I use for my material?
Use ISO 527-2 or ASTM D638 for rigid plastics, ISO 37 or ASTM D412 for rubber, ASTM D882 or ISO 527-3 for thin films, ISO 13934-1 or ASTM D5035 for fabrics, ISO 6892-1 or ASTM E8 for metals, ASTM D3039 for composites, and IEC 60811-501 for cable insulation and sheaths. When targeting a specific market, the corresponding national or regional standards such as GB/T or JIS equivalents should be considered.
Why is an extensometer sometimes required?
Crosshead displacement includes grip slip, frame compliance, and grip-region deformation, which can significantly overstate strain, especially for stiff materials or modulus determination. Extensometers directly measure deformation within the gauge length and provide accurate strain data required for modulus and low-elongation reporting.
What causes test results to be discarded?
Specimens that fracture inside the grips, at grip marks, or outside the gauge length generally produce non-representative results and should be discarded and retested. Excessive grip slippage, specimen misalignment, and pre-test flaws such as nicks or voids are also common causes of invalid breaks.
How many specimens are required?
Most standards require a minimum of five valid specimens for plastics and at least three for rubber, with the median reported for rubber per ASTM D412. Metals typically require at least two or more replicate specimens depending on the product specification.
Can elongation at break testing be performed at elevated or low temperatures?
Yes. Environmental chambers fitted to universal testing machines allow testing across a wide temperature range, commonly from −70 °C to +300 °C, to simulate service conditions. Aging studies also compare elongation retention after oven conditioning as an indicator of long-term durability.