Determination of Elongation at Break at Room Temperature: Standards, Test Methods, and Applications

Elongation at break at room temperature is one of the most fundamental mechanical property tests used to evaluate the ductility and deformation capacity of rubber, plastics, and cable insulation and sheathing materials before fracture. This article provides a comprehensive technical guide to the determination of elongation at break under ambient laboratory conditions, covering international and national standards such as ISO 37, ISO 527, ASTM D412, ASTM D638, GB/T 528, GB/T 1040, and IEC 60811-501, along with specimen preparation, testing procedures, calculation methods, and typical quality-control applications.

Technical Overview

Elongation at break, also referred to as breaking elongation or tensile elongation at rupture, quantifies the ability of a material to deform under uniaxial tensile loading before fracture occurs. It is defined as the increase in the distance between two gauge marks on a test piece at the moment of rupture, expressed as a percentage of the initial distance between those marks. Together with tensile strength, it constitutes the core group of tensile stress-strain properties specified in most material and product standards.

The determination is carried out using the following relationship:

Elongation at break (Eb) = (Lu - L0) / L0 × 100%

• L0 = initial gauge length, i.e., the distance between the gauge marks before loading
• Lu = gauge length at the moment of rupture
• Eb = elongation at break, expressed as a percentage

In this context, "room temperature" testing means that the test is performed in a standard laboratory atmosphere, typically 23 ± 2 °C with a relative humidity of 50 ± 10 % (for example, the class 23/50 atmosphere defined in ISO 291 and GB/T 2918). Some specifications permit a wider tolerance of 23 ± 5 °C when humidity control is not critical to the result. Specimens must be conditioned in this atmosphere before testing, for example for at least 16 hours for rubber materials in accordance with ISO 23529 and GB/T 2941, and typically no less than 24 hours for plastics.

The result serves several important purposes: it verifies conformity with material specifications, provides an indicator of ductility and flexibility, supports incoming and outgoing quality control, and serves as the reference value for calculating elongation retention after heat aging, weathering, or chemical exposure.

Testing Samples

Rubber and Elastomer Materials

Vulcanized rubbers and thermoplastic elastomers, including natural rubber (NR), styrene-butadiene rubber (SBR), ethylene-propylene rubber (EPDM), nitrile rubber (NBR), silicone rubber, and thermoplastic polyurethane (TPU), are typically tested as dumbbell specimens die-cut from flat sheets with a preferred thickness of 2.0 ± 0.2 mm in accordance with GB/T 528 and ISO 37.

Plastic Materials

Rigid and semi-rigid plastics, engineering resins, films, and sheets are tested using machined or injection-molded multipurpose specimens (such as type 1A and 1B test pieces) or rectangular strips for films, as specified in GB/T 1040.2, ISO 527-2, ISO 527-3, and ASTM D638.

Wire and Cable Insulation and Sheathing Compounds

Polyvinyl chloride (PVC), cross-linked polyethylene (XLPE), polyethylene, halogen-free flame-retardant compounds, and other cable materials are sampled directly from finished cables. Dumbbell specimens are prepared when the wall thickness permits; otherwise, tubular (ring-shaped) test pieces are tested in accordance with GB/T 2951.11 and IEC 60811-501.

Films, Sheets, and Geosynthetic Products

Packaging films, geomembranes, protective sheets, adhesive tapes, and coated fabrics are tested in strip form to evaluate their stretchability and rupture behavior under tensile loading.

Other Polymer Products

Seals, gaskets, hoses, footwear components, medical gloves, and elastomeric parts can all be evaluated for room-temperature elongation at break as part of material qualification, lot release testing, or failure analysis.

Testing Items

A room-temperature elongation at break determination is normally performed as part of a complete tensile test program, which may include the following items:

• Elongation at break at room temperature (23 ± 2 °C, 50 ± 10 % relative humidity)
• Tensile strength at break
• Tensile stress at specified elongation, for example 100 % or 300 % modulus for rubber compounds
• Yield stress and yield strain for plastics that exhibit a yield point
• Retention of elongation at break after air-oven aging or other conditioning, calculated as the ratio of the aged value to the unaged room-temperature value

Testing Methods

Test Apparatus

The test requires a calibrated universal tensile testing machine equipped with suitable grips, a force-measuring system, and a means of measuring extension, such as grip separation measurement, a crosshead displacement readout, or preferably a contact or video extensometer attached to the gauge marks. Auxiliary equipment includes dumbbell cutting dies and a cutting press, a dial or digital thickness gauge, a gauge-marking device, and a conditioning chamber or temperature-controlled laboratory.

Rubber Test Method: GB/T 528 / ISO 37 / ASTM D412 / JIS K 6251

Dumbbell specimens of type 1, 2, 3, or 4 are die-cut from standardized sheets. For the commonly used type 1 specimen, the gauge length is 25 mm, the width of the narrow section is 6.0 mm, and the preferred thickness is 2.0 ± 0.2 mm. Thickness is measured at three points within the narrow section, and the median value is used in the stress calculation. The test is performed at a crosshead speed of 500 ± 50 mm/min. Five specimens are normally tested, and the median of the individual elongation values is reported as the result.

Plastics Test Method: GB/T 1040 / ISO 527 / ASTM D638

Multipurpose specimens (type 1A or 1B, with a gauge length of 50 mm) or film strips are tested at a crosshead speed selected from the standard series of 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min according to the material specification. An extensometer is used whenever accurate strain measurement is required, particularly for modulus determination. At least five valid specimens are tested, and the arithmetic mean, together with the standard deviation where required, is reported.

Cable Material Test Method: GB/T 2951.11 / IEC 60811-501

Insulation and sheath samples are removed from finished cables. Where the thickness permits, dumbbell specimens are punched from the tubular material after slitting and flattening; otherwise the test is performed directly on tubular pieces. Gauge marks are applied 20 mm apart in the narrow section. The test is generally conducted at a speed of 250 ± 50 mm/min unless the product standard specifies otherwise. Elongation at break is calculated from the extension at rupture, and the results are averaged as specified in the applicable cable product standard.

General Test Procedure

• Condition the specimens in the standard laboratory atmosphere for the period specified by the relevant standard.
• Measure and record the thickness (and width, for rectangular or film specimens) at multiple points within the gauge region.
• Apply gauge marks accurately and symmetrically using a marking device that does not damage the specimen.
• Mount the specimen in the grips, ensuring axial alignment and avoiding any pre-strain.
• Select the crosshead speed and full-scale force range specified for the material and specimen type.
• Extend the specimen until rupture and record the distance between the gauge marks at break.
• Calculate the elongation at break using the formula above and report the statistical value (median or mean) required by the standard.

Result Validity

A specimen is considered invalid if fracture occurs outside the narrow parallel section, at the edge of a poorly cut die, at a visible flaw, or exactly at the grip interface, provided the recorded value falls below the requirement. Invalid results are discarded and replacement specimens are tested so that the minimum number of valid results is obtained.

Applications

Determination of elongation at break at room temperature is applied extensively across polymer-related industries:

• Wire and cable manufacturing, for conformity assessment of insulation and sheathing compounds against product standards such as the IEC 60227, IEC 60245, and GB/T 5023 series, where minimum elongation requirements commonly range from about 125 % to 350 % depending on the compound family
• Rubber product manufacturing, including seals, hoses, belts, vibration isolators, and tires, for compound qualification and routine quality control
• Plastics processing, covering pipes, fittings, films, injection-molded parts, and packaging materials
• Automotive, aerospace, construction, and rail transit industries, where material datasheets and customer specifications define minimum ductility limits
• Reliability and aging studies, in which the room-temperature value serves as the baseline for calculating retention after heat aging, with acceptance criteria frequently expressed as a retention of 70 % to 80 % or as a maximum permitted variation
• Type testing and certification programs conducted by accredited third-party laboratories on behalf of manufacturers, buyers, and regulatory bodies

FAQ

What temperature counts as "room temperature" for this test?

Room temperature normally means a standard laboratory atmosphere of 23 ± 2 °C with 50 ± 10 % relative humidity. Some standards allow 23 ± 5 °C where humidity is not a significant factor. Testing outside these tolerances can noticeably shift the measured elongation, so temperature and humidity should always be recorded.

How is elongation at break calculated?

It is calculated as (Lu - L0) / L0 × 100 %, where L0 is the initial gauge length between the marks and Lu is the distance between the marks at the instant of rupture. For example, if a 25 mm gauge length extends to 125 mm before breaking, the elongation at break is 400 %.

Which standard should be used for my material?

Use ISO 37, GB/T 528, ASTM D412, or JIS K 6251 for vulcanized and thermoplastic rubber; GB/T 1040, ISO 527, or ASTM D638 for plastics and films; and GB/T 2951.11 or IEC 60811-501 for cable insulation and sheathing materials. The product specification usually nominates the governing method.

How many specimens are required?

Rubber standards typically require five specimens and report the median value. Plastics standards require at least five valid specimens and report the arithmetic mean. Cable standards specify the number of pieces in the product standard, commonly three to five, with the results averaged.

Why do specimens sometimes break outside the gauge marks?

Fracture outside the narrow section or at the grips is usually caused by die-cutting defects, material inclusions, incorrect grip alignment, or excessive clamping force. Such results are treated as invalid and the affected specimens are replaced and retested.

What factors influence the measured result?

Crosshead speed, specimen thickness and geometry, gauge length, test temperature, conditioning time, specimen preparation quality, and grip alignment all influence elongation at break. Strict adherence to the specified conditions is essential for reproducible and comparable results.

How does room-temperature elongation relate to aging tests?

The room-temperature value is the unaged reference. After heat aging or other conditioning, the test is repeated and the retention is calculated as the aged elongation divided by the unaged elongation, expressed as a percentage. A significant drop in retention indicates degradation such as cross-linking, chain scission, or plasticizer loss.