Elongation at Break Test Procedure: Standards, Methodology, and Technical Guidelines

Elongation at break is one of the most important mechanical properties for evaluating the ductility and deformability of plastics, rubbers, films, and textile materials. This article provides a comprehensive guide to the elongation at break test procedure, covering applicable standards such as ISO 527, ASTM D638, ASTM D412, and GB/T 1040, specimen preparation, equipment requirements, step-by-step testing methodology, and result calculation. Whether you are a laboratory engineer, quality control specialist, or materials researcher, this guide will help you perform tensile elongation tests accurately and consistently.

Technical Overview

Elongation at break, also known as tensile elongation at rupture, is defined as the increase in gauge length of a test specimen measured at the moment of break, expressed as a percentage of the original gauge length. It quantifies the ability of a material to undergo plastic deformation before failure and is a fundamental ductility indicator alongside tensile strength and tensile modulus.

The test is performed on a universal testing machine (UTM), in which a specially shaped specimen is clamped between two grips and stretched at a constant crosshead speed until rupture occurs. The elongation at break is calculated using the following formula:

• εb (%) = (Lb − L0) / L0 × 100, where L0 is the initial gauge length and Lb is the gauge length at break.

Depending on the material category, the test is governed by different international and national standards, including:

• ISO 527-1 / ISO 527-2 and GB/T 1040 series — tensile properties of rigid and semi-rigid plastics.
• ASTM D638 — tensile properties of plastics.
• ISO 37 and GB/T 528 — tensile stress-strain properties of vulcanized and thermoplastic rubbers.
• ASTM D412 — tensile properties of vulcanized rubber and thermoplastic elastomers.
• ASTM D882 — tensile properties of thin plastic sheeting and films.
• ISO 13934-1 and ASTM D5035 — tensile properties of textile fabrics.
• ASTM D3039 — tensile properties of polymer matrix composite materials.
• IEC 60811-501 — tensile strength and elongation tests on cables and wire insulation.

Testing Samples

Rigid and Semi-Rigid Plastics

Plastic materials are typically tested using dumbbell-shaped (dog-bone) specimens. Under ISO 527-2 and GB/T 1040.2, the preferred specimens are Type 1A (preferred thickness 4 mm) and Type 1B. Under ASTM D638, Type I specimens (3.2 mm nominal thickness) are most common, while Type V miniature specimens are used when material is limited. Specimens may be injection molded, compression molded, or machined from sheets, plates, or finished parts, and must be free of flash, notches, and visible defects.

Rubber and Thermoplastic Elastomers

Elastomers are tested using die-cut dumbbell specimens, such as Type 1 or Type 2 dumbbells under ISO 37 and GB/T 528, or Die C dumbbells under ASTM D412. Standard thickness is typically 2 mm. Sharp cutting dies and a supportive backing plate are essential to produce smooth, defect-free edges, since nicks and burrs cause premature failure and artificially low elongation values.

Plastic Films and Thin Sheets

For films with thickness below approximately 0.25 mm, ASTM D882 and equivalent ISO methods use rectangular strip specimens rather than dumbbells. Typical strip dimensions are 10 mm to 25.4 mm in width with gauge lengths of 50 mm to 250 mm, depending on the material extensibility. Specimens must be cut with sharp blades to avoid stress-concentrating edge defects.

Textiles, Composites, and Cable Materials

Textile fabrics are tested as strips or raveled specimens per ISO 13934-1 and ASTM D5035. Polymer matrix composites use straight-sided rectangular coupons per ASTM D3039 with bonded tabs to prevent grip damage. Insulation and sheathing compounds from wires and cables are tested using tubular or dumbbell specimens per IEC 60811-501 after removal from the conductor.

Testing Items

An elongation at break test is typically conducted as part of a full tensile characterization, and the following parameters can be determined from a single stress-strain curve:

• Elongation at break (εb) — strain at the moment of rupture.
• Tensile strength at break — maximum stress sustained at rupture.
• Tensile strength — maximum stress during the test.
• Tensile stress at yield and elongation at yield.
• Tensile modulus (Young's modulus) — slope of the initial linear region.
• Nominal elongation at break — calculated from crosshead displacement when an extensometer is not used.

Testing Methods

Specimen Conditioning

Unless otherwise specified, specimens shall be conditioned at 23 ± 2 °C and 50 ± 10 % relative humidity for a minimum of 40 hours prior to testing, in accordance with ISO 291 or ASTM D618. Moisture-sensitive materials such as polyamides may require special drying or humidity conditioning as defined in the relevant material specification. Testing shall be carried out in the same conditioned atmosphere whenever possible.

Equipment Requirements

The test requires a calibrated universal testing machine with:

• A load cell selected so that the breaking force falls within an appropriate range (typically 15 % to 85 %) of the cell capacity, verified under ISO 7500-1 or ASTM E4.
• Suitable grips — pneumatic, mechanical wedge, or elastomeric-capped grips — that secure the specimen without slippage or premature crushing.
• A contact or video extensometer with accuracy class meeting ISO 9513 or ASTM E83, required for precise strain measurement, particularly for modulus determination.
• Crosshead speed capability from 1 mm/min to 500 mm/min with calibrated displacement accuracy.
• Dimension-measuring instruments: micrometers or dead-weight thickness gauges with appropriate resolution for width and thickness measurement.

Step-by-Step Test Procedure

• Step 1: Verify specimen geometry and reject specimens with visible defects, nicks, or voids.
• Step 2: Measure the width and thickness at several points within the gauge length and record the median values; calculate the initial cross-sectional area A0.
• Step 3: Mark the original gauge length L0 (e.g., 50 mm for ISO 527 1A/1B and ASTM D638 Type I specimens; 25 mm for common ASTM D412 dumbbells) using a benchmark gauge or the extensometer reference points.
• Step 4: Select the test speed according to the governing standard and material type — for example, 1 to 50 mm/min for rigid plastics per ISO 527, 5 mm/min (0.2 in/min) typical for ASTM D638 Type I, 500 mm/min for rubbers per ASTM D412 and ISO 37, and 25 to 500 mm/min for films per ASTM D882.
• Step 5: Clamp the specimen symmetrically in the grips, ensuring axial alignment to avoid bending stresses; apply a small preload to remove slack.
• Step 6: Attach the extensometer (or activate the video extensometer) and zero the load and extension readings.
• Step 7: Start the test and stretch the specimen at constant speed, recording force and extension continuously until the specimen ruptures.
• Step 8: Record the extension at break Lb, remove the broken specimen, and inspect the fracture surfaces.
• Step 9: Repeat the test on at least five valid specimens per standard requirements.

Calculation of Results

Elongation at break is calculated as εb = (Lb − L0) / L0 × 100 %. For elastomer standards such as ASTM D412, elongation may alternatively be calculated using the initial distance between the grips when preferred. The reported result is typically the median (ISO 527, ISO 37) or the mean (ASTM D638) of the individual values, together with the standard deviation or coefficient of variation.

Result Validity and Reporting

A specimen shall be discarded and retested when failure occurs within the grips, when slipping is detected, or when the specimen breaks outside the gauge length due to obvious defects. The test report shall include: the standard applied, specimen type and preparation method, conditioning details, test speed, specimen dimensions, individual and statistical results, and any deviations from the standard procedure.

Factors Affecting Test Results

• Test speed — higher strain rates generally increase strength and reduce elongation for ductile polymers due to viscoelastic effects.
• Temperature and humidity — elevated temperature increases elongation; humidity strongly affects hygroscopic materials such as polyamide.
• Specimen preparation — machining marks, die-cut nicks, and molding defects act as stress concentrators and lower measured elongation.
• Grip alignment and slippage — misalignment introduces bending stresses and produces non-conservative results.
• Measurement method — crosshead displacement includes machine and grip compliance, so extensometer-based strain is more accurate, especially for rigid materials.

Applications

Elongation at break testing is widely applied across industries for material qualification, quality control, and research and development:

• Plastics and compounds — incoming resin inspection, batch release, and datasheet verification for polyethylene, polypropylene, ABS, PET, and engineering polymers.
• Rubber and elastomers — quality control of seals, gaskets, O-rings, footwear components, and tires.
• Packaging — evaluation of stretch films, pouches, and laminated films for formability and puncture resistance.
• Wire and cable — verification of insulation and sheathing ductility before and after aging per IEC 60811-501.
• Medical devices — mechanical qualification of catheter tubing, gloves, and flexible packaging per relevant ISO and ASTM material standards.
• Automotive and construction — durability assessment of interior trims, hoses, geomembranes, and waterproofing sheets.
• Failure analysis and material comparison — determination of embrittlement after thermal, UV, or chemical exposure.

Manufacturers, research institutes, and accredited third-party laboratories routinely perform these tests to demonstrate compliance with product specifications, regulatory requirements, and customer-defined acceptance criteria.

FAQ

What is the difference between elongation at break and nominal elongation at break?

Elongation at break is measured with an extensometer over a defined gauge length on the parallel section of the specimen, whereas nominal elongation at break is calculated from crosshead displacement relative to the initial grip separation. Nominal values include machine and grip compliance and are therefore less accurate, particularly for rigid, low-elongation materials.

How many specimens should be tested?

ISO 527, ISO 37, and ASTM D638 generally require a minimum of five valid specimens per condition. Additional specimens should be tested if results show excessive scatter or if invalid failures (grip breaks or slippage) occur.

Which crosshead speed should be selected?

The speed is prescribed by the governing standard and depends on the material class: for example, 50 mm/min is common for ductile plastics under ISO 527-2 with 1A specimens, 5 mm/min for ASTM D638 Type I rigid plastics, and 500 mm/min for rubbers per ASTM D412 and ISO 37. Changing the speed significantly changes the measured elongation, so the specified speed must always be reported.

What are typical elongation at break values for common materials?

Brittle materials such as polystyrene and thermosets typically show elongation below 5 %; general-purpose polyolefins such as polypropylene and polyethylene often exceed 300 %; and elastomers such as silicone rubber and TPU can reach 500 % to over 1000 %. Exact values depend on grade, processing, and test conditions.

Why do my test results show large scatter?

Common causes include die-cut or machining defects on specimen edges, inconsistent specimen thickness, grip slippage, misalignment, inadequate conditioning, and testing at speeds or temperatures different from the standard. Systematic review of specimen preparation and machine setup usually identifies and eliminates the source of variability.