Elongation at Break Determination: Step-by-Step Testing Procedure and Standards

Elongation at break is one of the most widely measured mechanical properties, quantifying how far a material can stretch before fracturing under tensile loading. This article provides a complete, step-by-step guide to elongation at break determination based on recognized standards such as ISO 527, ISO 37, ASTM D638, ASTM D412, and ISO 6892-1, covering specimen preparation, conditioning, test execution, and result calculation. Laboratory technicians, quality engineers, and material developers can follow this procedure to obtain accurate, repeatable results in accredited third-party laboratories or in-house testing facilities.

Technical Overview

Elongation at break, also referred to as breaking elongation or fracture strain, is the increase in length of a specimen, expressed as a percentage of its original gauge length, at the moment of fracture during a tensile test. It is a key indicator of material ductility and formability: brittle materials such as glassy thermoplastics or hardened steels exhibit low values, typically below 5%, while elastomers and flexible films may exceed 300%, 500%, or even 1000%.

The fundamental calculation is defined as follows: Elongation at break (%) = (Lb − L0) / L0 × 100, where L0 is the initial gauge length and Lb is the gauge length at the moment of break. The measurement is performed on a universal testing machine (tensile tester), and the extension at break may be captured either by a contact or video extensometer, or derived from crosshead displacement depending on the applicable standard and the required accuracy.

The test is significant for several reasons:

• It verifies that a material meets minimum ductility requirements in product specifications and material data sheets.
• It enables comparison of materials during selection and formulation development.
• It is a sensitive indicator of degradation, since thermal aging, UV exposure, and chemical attack typically reduce elongation at break before other properties change noticeably.
• It supports compliance demonstration for cables, construction products, packaging, and components covered by ISO, IEC, ASTM, EN, GB, UL, and JIS standards.

Testing Samples

The specimen geometry depends strongly on the material class. The most common sample types, together with their governing standards, are listed below.

Rigid and Semi-Rigid Plastics

Dumbbell-shaped (dog-bone) specimens machined or injection-molded to ISO 527-2 (specimen types 1A and 1B) or ASTM D638 (Types I through V). Preferred thickness is 4 mm for rigid plastics; sheet materials are tested at their natural thickness. Flat rectangular strips per ISO 527-4 may be used for fiber-reinforced composites.

Rubber and Elastomers

Dumbbell specimens die-cut from flat sheets to ISO 37 (types 1 to 4) or ASTM D412 (dies A through F). Preferred thickness is 2 mm, and specimens must be cut cleanly without nicks or edge defects, as these cause premature failure and artificially low results.

Plastic Films and Thin Sheets

Rectangular strips tested per ISO 527-3 or ASTM D882, with widths between 10 and 25 mm. Because thickness is very small (typically below 0.25 mm for ASTM D882), width rather than cross-sectional area governs stress calculation, and specimens must be free of wrinkles and creases.

Textiles, Yarns, and Webbing

Fabric strip or grab specimens per ISO 13934-1, ASTM D5034, or GB/T 3923.1; yarn specimens per ASTM D2256 or ISO 2062; webbing and straps per applicable EN or JIS product standards. Fraying of strip ends is prevented by raveling to the standard width.

Wire and Cable Insulation and Sheaths

Tubular or dumbbell specimens stripped from finished cables and tested per IEC 60811-501, UL 2556, or the corresponding GB standards. Elongation at break of insulation and sheath materials is a mandatory routine and type test for cable certification.

Metallic Materials

Proportional or non-proportional flat and round specimens per ISO 6892-1, ASTM E8/E8M, GB/T 228.1, or JIS Z 2241. For metals, the parameter is usually reported as percentage elongation after fracture, using a gauge length of 5.65 × √S0 (short proportional gauge) unless otherwise specified.

Testing Items

A tensile test performed for elongation at break determination normally yields the following reported items:

• Elongation at break (fracture strain)
• Elongation at yield, where applicable
• Tensile strength and tensile stress at break
• Yield stress
• Tensile modulus (Young's modulus), determined with an extensometer
• Percentage elongation after fracture for metallic materials
• Complete force-extension (stress-strain) curves and failure mode description

Testing Methods

The general test method uses a driven universal testing machine that extends the specimen at a constant crosshead speed while continuously recording force and extension until fracture. The detailed procedure below applies to polymer materials tested per ISO 527 and ASTM D638/D412; variations for other material classes are noted.

Equipment and Apparatus

• Universal testing machine verified per ISO 7500-1 or ASTM E4, with a calibrated load cell appropriate to the expected breaking force
• Grips suited to the material: wedge-action grips for rigid plastics, pneumatic or roller grips for films and elastomers, capstan or cord grips for yarns
• Contact or video extensometer classified per ISO 9513 or ASTM E83 when accurate strain measurement or modulus is required
• Micrometer, thickness gauge, or calibrated calipers for dimensional measurement
• Standard atmosphere facility for conditioning, typically 23 °C ± 2 °C and 50% ± 10% relative humidity per ISO 291 or ASTM D618
• Die cutter or machining equipment, gauge-marking tools, and a timer

Specimen Preparation and Conditioning

• Prepare specimens in the machine direction and, where relevant, the transverse direction; label each specimen clearly.
• Inspect all specimens and reject any with voids, notches, die-cut defects, or visible damage.
• Measure width and thickness at several points along the gauge section and record the minimum cross-sectional area.
• Condition specimens in the standard atmosphere, for example a minimum of 40 hours at 23 °C and 50% relative humidity for plastics (ISO 291, ASTM D618), or 23 °C per ASTM D1349 for rubber.
• Mark the initial gauge length L0 on the parallel section using a gauge-marking template, ink, or the extensometer reference points.

Step-by-Step Test Procedure

• Step 1: Verify that the machine, load cell, and extensometer hold valid calibration certificates and complete a brief warm-up.
• Step 2: Select a load cell so that the anticipated breaking force falls within the calibrated working range, ideally between 20% and 80% of capacity.
• Step 3: Set the grip separation or gauge length specified by the standard, for example 115 mm between grips for ISO 527-2 type 1A specimens, 50 mm initial grip separation for many film tests, and the specified gauge length for rubber dumbbells.
• Step 4: Set the crosshead speed as required by the standard and material, for example 50 mm/min for common ISO 527-2 and ASTM D638 plastic specimens, 500 mm/min typical for rubber per ISO 37 and ASTM D412, and 1 mm/min for modulus determination.
• Step 5: Mount the specimen centrally in the grips, ensuring axial alignment to avoid bending stresses; apply a small preload or pretension to remove slack where the standard permits.
• Step 6: Attach the extensometer to the gauge section, or confirm that gauge marks are visible for travel measurement or video analysis.
• Step 7: Start the test and record the force-extension curve continuously at the set constant speed.
• Step 8: Continue until the specimen fractures; capture the extension at break ΔLb from the extensometer or crosshead displacement as permitted by the standard.
• Step 9: For metallic materials, remove the fractured halves, carefully rejoin them, and measure the final gauge length Lu for calculating percentage elongation after fracture.
• Step 10: Record the failure location and mode; discard results for specimens that break within the grips or outside the gauge section, as these are invalid in most standards.
• Step 11: Repeat the test on the required number of specimens, normally at least five for plastics and rubber, and ten per direction for films tested per ASTM D882.
• Step 12: Calculate elongation at break for each valid specimen and compile the statistical summary.

Calculation and Reporting

For each valid specimen, calculate elongation at break as: εb (%) = (Lb − L0) / L0 × 100, or equivalently εb (%) = ΔLb / L0 × 100, where ΔLb is the extension at break. For metals, compute percentage elongation after fracture as A (%) = (Lu − L0) / L0 × 100. Report the individual values together with the median (common practice for rubber per ISO 37 and ASTM D412) or the arithmetic mean and standard deviation (common practice for plastics), and state the specimen type, dimensions, gauge length, crosshead speed, conditioning atmosphere, and test temperature. Any deviation from the standard method must be documented.

Applications

Elongation at break determination is applied across nearly every industry that uses materials subject to mechanical loading:

• Incoming inspection and supplier qualification for resins, compounds, films, textiles, and metal stock.
• Production quality control for plastic parts, packaging films, rubber seals, conveyor belts, and cable insulation.
• Research and development of polymer formulations, blends, and composites, where ductility is a primary optimization target.
• Aging and durability evaluation, comparing elongation retention after heat aging, UV weathering, ozone exposure, or chemical immersion.
• Compliance and certification testing for electrical cables per IEC and UL standards, construction products per EN standards, and general products per GB, ISO, and JIS requirements.
• Failure analysis, where abnormally low elongation at break often reveals embrittlement, contamination, or processing defects.

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, marked by the peak or first plateau on the stress-strain curve, while elongation at break is the strain at final fracture. For ductile materials, elongation at break is always equal to or greater than elongation at yield; for brittle materials, fracture may occur before any yield point is reached.

How many specimens should be tested?

ISO 527 and ASTM D638 require a minimum of five valid specimens for isotropic materials, with additional specimens for anisotropic materials tested in multiple directions. ISO 37 and ASTM D412 likewise use at least three to five dumbbells, and ASTM D882 specifies ten strips in each principal direction. Retests are performed whenever breaks occur in the grips or results show excessive scatter.

What should be done if the specimen breaks outside the gauge length?

When a specimen fails within the grips or outside the marked gauge section, the stress and strain distribution is disturbed and the result is not representative. Most standards require discarding the result and testing an additional specimen, while recording the number of such failures in the report, since a high frequency may indicate grip slippage, improper specimen preparation, or unsuitable grip type.

Which test speed should be selected?

The crosshead speed is prescribed by the governing standard and often depends on specimen type and material rigidity. Typical values include 50 mm/min for ISO 527-2 and ASTM D638 plastic dumbbells, 500 mm/min for rubber per ISO 37 and ASTM D412, and low speeds around 1 mm/min for modulus measurement. Polymers are strain-rate sensitive, so the speed must always be reported together with the results and kept constant when comparing materials.

Why do elongation results sometimes differ between laboratories?

Common causes include differences in specimen preparation quality, conditioning history, actual crosshead speed, grip alignment, and whether extension is measured with an extensometer or inferred from crosshead travel. Machine compliance can inflate crosshead-based elongation for stiff specimens, which is why extensometer measurement is preferred for precision work. Strict adherence to the specified standard and interlaboratory comparison programs minimize such variation.

How is elongation at break determined for metals?

Per ISO 6892-1 and ASTM E8/E8M, the fractured specimen halves are fitted back together and the final gauge length Lu is measured, from which percentage elongation after fracture is calculated. Because the result depends on the gauge length, the gauge length used must always be stated, and proportional gauge lengths such as 5.65 × √S0 are preferred for comparability across specimen sizes.