Elongation at Break Test: Operating Procedure, Standards, and Technical Guide

Elongation at break is a fundamental mechanical property that quantifies how far a material can stretch before fracturing, making it a critical ductility indicator for plastics, rubbers, films, textiles, and cable materials. This technical guide explains the complete elongation at break test operation, covering specimen preparation, universal testing machine setup, standard test methods such as ISO 527 and ASTM D638, calculation formulas, and practical tips for achieving accurate, repeatable results in accredited third-party laboratories and in-house quality control environments.

Technical Overview

Elongation at break, also known as tensile strain at break, is defined as the increase in gauge length of a test specimen at the moment of rupture, expressed as a percentage of the original gauge length. It is one of the most widely measured ductility parameters in materials testing and is typically determined as part of a full tensile test performed on a universal testing machine (UTM). During the test, a prepared specimen is clamped between two grips and pulled at a controlled constant speed until it fractures, while the force and extension are continuously recorded.

The elongation at break is calculated using the following formula:

Elongation at Break (%) = [(Lb − L0) / L0] × 100

Where L0 is the initial gauge length (the distance between the gauge marks or the extensometer measurement length before loading) and Lb is the gauge length at the moment of break. High elongation values generally indicate a ductile, flexible material, while low values indicate brittleness. Commonly applied standards include ISO 527-1 and ISO 527-2, ASTM D638, ASTM D412, ISO 37, ASTM D882, ISO 13934-1, and the corresponding Chinese national standards GB/T 1040 and GB/T 528.

Testing Samples

Rigid and Semi-Rigid Plastics

Thermoplastics and thermosets such as PP, PE, ABS, PC, PA, PVC, and epoxy compounds are typically tested using machined or injection-molded dumbbell (dog-bone) specimens. The standard ISO 527-2 Type 1A specimen has an overall length of 170 mm and a preferred thickness of 4 mm, while the ASTM D638 Type I specimen has an overall length of 165 mm, a thickness of 3.2 mm, and a 50 mm narrow-section gauge length.

Rubber and Elastomeric Materials

Vulcanized rubbers, thermoplastic elastomers, and silicone compounds are tested with smaller dumbbell specimens according to ISO 37 or ASTM D412. The ISO 37 Type 2 specimen, for example, uses a 20 mm gauge length and a 2 mm preferred thickness. Due to the very high elongation of elastomers (often exceeding 300% to over 1000%), the test machine must offer sufficient crosshead travel and grip separation range.

Plastic Films and Thin Sheets

Flexible packaging films, laminates, and thin membranes below approximately 1 mm in thickness are tested as rectangular strips per ASTM D882 or ISO 527-3. Strip widths are commonly 10 mm to 15 mm with gauge lengths of 50 mm to 100 mm, and specimen cutting quality directly affects the measured elongation values.

Textiles, Fabrics, and Yarns

Woven and nonwoven fabrics are tested using the strip method described in ISO 13934-1 or ASTM D5035, typically with 50 mm wide strips and 200 mm gauge lengths. Yarns and threads are tested with specialized capstan or pneumatic yarn grips, and elongation is usually measured directly from the grip separation or with contact extensometers.

Wire and Cable Insulation and Sheathing Materials

Polymeric insulation and sheath compounds for wires and cables are tested on dumbbell specimens or tubular sections in accordance with IEC 60811-501 and related product standards. Elongation at break is a mandatory acceptance parameter, including testing in the aged condition to evaluate thermal degradation resistance.

Testing Items

A typical tensile test conducted to determine elongation at break also yields the following related mechanical properties:

• Elongation at break (tensile strain at break)
• Elongation at yield
• Tensile strength (stress at break)
• Tensile stress at yield
• Tensile modulus (Young's modulus, generally measured between 0.05% and 0.25% strain for plastics)
• Force at break and nominal strain at break
• Tensile energy absorption or work to break, where required

Testing Methods

Specimen Preparation and Conditioning

Specimens must be prepared with smooth, defect-free edges, since notch marks or die-cut burrs act as stress concentrators and cause premature, invalid breaks. Injection-molded plaques, compression-molded sheets, or products are die-cut or machined to the dimensions specified by the applicable standard. Before testing, specimens shall be conditioned in a standard atmosphere, typically 23°C ± 2°C and 50% ± 10% relative humidity for at least 24 hours according to ISO 291, or 23°C ± 2°C and 50% ± 5% relative humidity for 40 hours according to ASTM D618, unless otherwise specified.

Equipment Setup and Calibration

The universal testing machine shall be calibrated and verified against ISO 7500-1 or ASTM E4 for force accuracy, and extensometers shall comply with ISO 9513 or ASTM E83. Select a load cell capacity appropriate for the expected breaking force so that the measured force lies within the verified calibration range, ideally between 20% and 80% of capacity. Suitable grip types include wedge-action grips for rigid plastics, pneumatic or roller grips for films and elastomers, and capstan grips for yarns. Verify grip pressure to prevent specimen slippage without causing grip-zone fracture.

Gauge Length Marking and Specimen Mounting

Mark the gauge length on the narrow parallel section of each specimen using a soft marking tool or a template, taking care not to notch or scratch the surface. Measure and record the width and thickness of the narrow section at three points and use the mean cross-sectional area for stress calculation. Insert the specimen symmetrically into the grips so that its longitudinal axis aligns with the direction of the applied force; misalignment introduces bending stresses and distorts the elongation result. Apply a small straightening preload if permitted by the standard, then attach the extensometer or zero the crosshead displacement.

Test Execution and Speed Selection

Set the crosshead speed according to the material standard and specimen type. Typical speeds include 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min; for example, ISO 527-2 recommends 1 mm/min for modulus determination and commonly 50 mm/min for tensile strength and elongation of Type 1A specimens, while elastomers under ASTM D412 are usually tested at 500 mm/min. Start the test and record the force-extension curve continuously until complete fracture. Do not touch or adjust the specimen during the test, and note any anomalies such as slippage, partial breaks, or failure at the grips.

Data Recording and Calculation

At the moment of break, capture the extension between the gauge marks. If an extensometer is used, it should remain attached until break unless the standard permits its removal at a specified strain, in which case continue the measurement by crosshead separation and combine the two readings. Calculate elongation at break as the extension at break divided by the initial gauge length, multiplied by 100. Alternatively, when permitted, nominal elongation at break may be calculated from grip separation, though this overestimates true elongation due to grip-region deformation and must be clearly identified in the report.

Result Validation and Reporting

Test at least five valid specimens per material condition, and report the arithmetic mean, standard deviation, and coefficient of variation. A break is considered valid when fracture occurs within the gauge length; specimens that fail at the grips or outside the gauge marks shall be discarded and replaced if they would affect the median or mean beyond the acceptance criteria of the standard. The test report shall include the material identification, specimen type and dimensions, gauge length, conditioning details, testing speed and temperature, individual and mean elongation at break values, the number of specimens tested, and the standard method applied.

Applications

Elongation at break testing supports material qualification, product development, and quality control across a wide range of industries:

• Plastics and composite part manufacturing for incoming resin inspection and lot release
• Flexible and rigid packaging for film formability, sealing performance, and toughness evaluation
• Automotive components such as hoses, seals, interior trim, and under-hood parts
• Wire and cable production for insulation and sheath acceptance and aging studies
• Medical devices, tubing, and single-use plastics requiring documented ductility evidence
• Textile and geotextile products for strength and extensibility verification
• Construction materials, waterproofing membranes, and sealants
• Research and development for comparing formulations, processing conditions, and recycling effects on ductility

FAQ

What is a typical range of elongation at break values?

Brittle plastics such as general-purpose PS or thermoset epoxies may show elongation below 5%, engineering plastics typically range from 10% to 100%, polyethylene films can reach 300% to 600%, and elastomers may exceed 800%. Values always depend on grade, processing, specimen geometry, and test speed.

Why is crosshead speed important in elongation testing?

Most polymers are strain-rate sensitive: a higher test speed generally raises tensile strength and lowers elongation at break, while a slower speed produces greater ductility. Using the speed specified in the standard is therefore essential for comparability between laboratories.

Can elongation at break exceed 100%?

Yes. Elongation at break is expressed as a percentage of the original gauge length, so a specimen that stretches to twice its initial length before breaking has an elongation at break of 100%. Ductile films and rubbers routinely exceed this value.

What causes specimens to break inside the grips, and how is it prevented?

Grip-zone breaks are usually caused by excessive clamping pressure, worn or contaminated grip faces, sharp grip edges, or specimen misalignment. They can be prevented by using pneumatic grips with regulated pressure, protective tabbing or reinforced ends where allowed, correct alignment, and replacing worn grip inserts. Invalid breaks must be discarded and retested.

Should elongation be measured with an extensometer or from crosshead displacement?

An extensometer provides higher accuracy because it measures deformation only within the gauge length, excluding grip slip and machine compliance. Crosshead-based measurement is acceptable for screening or when the standard permits nominal strain, but the method used must be stated in the test report.

How many specimens are required for a valid test?

ISO 527 and ASTM D638 require a minimum of five valid specimens, while ASTM D412 recommends at least three. Additional specimens should be tested when scatter is high or when invalid breaks occur, and all anomalies must be documented.