Strain at Break Determination Testing: Principles, Standards, Methods, and Applications

Strain at break determination testing, also known as fracture point strain measurement, quantifies the maximum deformation a material sustains at the instant of tensile rupture, providing a critical indicator of ductility and toughness. This experiment is essential for material qualification, product design, and quality control across plastics, metals, rubbers, composites, and films. This guide explains the testing principles, applicable ISO, ASTM, EN, and GB standards, specimen requirements, instrumentation, procedures, and typical industrial applications.

Technical Overview

The fracture point strain determination experiment measures the strain a test specimen accumulates up to the exact moment of fracture under a monotonic tensile load. The reported result, known as strain at break (εB), is calculated as an engineering strain using the formula εB = (Lu − L0) / L0 × 100%, where L0 is the initial gauge length and Lu is the gauge length at rupture. Because engineering strain expressed as a percentage is numerically identical to elongation at break, the two terms are often used interchangeably in material datasheets.

Strain at break is one of the most informative outputs of a tensile test. It distinguishes brittle materials, which fracture at low strain (typically below 5%), from ductile materials, which may sustain strains of several hundred percent before rupture. It also provides direct evidence of a material's capacity for plastic deformation, energy absorption, and resistance to crack propagation. In practice, the fracture point is identified as the point on the stress-strain curve at which the force drops abruptly or reaches zero due to specimen separation.

It is important to distinguish strain at break from other characteristic strain values, including strain at yield, strain at maximum force, and, for metallic materials, total extension at fracture (A) and uniform elongation (Ag). For ductile metals that exhibit necking, the measured fracture strain depends on the gauge length adopted, so the reporting must always state the proportional gauge length (for example, A50mm or A5.65√S0). For polymeric and composite materials, strain measurement is typically performed directly on the specimen with an extensometer rather than derived from crosshead displacement, because machine compliance and grip slip introduce significant errors.

Testing Samples

Rigid and Semi-Rigid Plastic Specimens

Thermoplastics and thermosets are tested using machined or injection-moulded dumbbell specimens. ISO 527-2 specifies the widely used 1A and 1B specimen geometries, while ASTM D638 defines Types I through V for different thickness ranges. The equivalent Chinese standard series is GB/T 1040. Specimens must be free of sink marks, flash, weld lines, and machining damage, as surface defects act as crack initiation sites and can severely underestimate the true strain at break.

Metallic Material Specimens

Metals are tested in accordance with ISO 6892-1, ASTM E8/E8M, EN ISO 6892-1, GB/T 228.1, or JIS Z 2241. Both proportional specimens, where the gauge length is related to the cross-sectional area (L0 = 5.65√S0), and non-proportional specimens with fixed gauge lengths (such as 50 mm) are accepted. Round bars, flat plates, sheets, tubes, and wires can all be evaluated, provided original marks for elongation measurement are applied within the parallel length.

Rubber and Elastomer Specimens

Vulcanized rubbers and thermoplastic elastomers are evaluated under ISO 37, ASTM D412, or GB/T 528 using dumbbell specimens (such as ISO Type 2 or ASTM Type C) or ring specimens. Because these materials elongate by several hundred percent, the strain measurement system must offer a large travel range, which typically requires non-contact video extensometers or long-travel contact extensometers.

Fibre-Reinforced Polymer Composite Specimens

Continuous-fibre composites are tested as straight-sided rectangular coupons per ASTM D3039 or ISO 527-4 and ISO 527-5. Fibre direction must be documented (0°, 90°, or ±45° in-plane shear), and bonded composite tabs are usually required to prevent grip-induced failure. Damage-free specimen cutting with diamond tooling is critical to obtaining valid fracture strain data.

Film and Thin Sheet Specimens

Flexible films below 1 mm thickness are tested as rectangular strips under ASTM D882 or ISO 1184. Specimen preparation, edge quality, and uniform width are decisive, since micro-notches along cut edges dramatically reduce the measured strain at break of thin polymer films.

Testing Items

A fracture point strain determination experiment typically delivers the following results:

• Tensile strain at break (fracture point strain), reported as a percentage to a defined number of significant figures
• Nominal strain at break derived from crosshead displacement, when direct extensometry is not applicable
• Supporting tensile properties measured in the same test, including tensile strength, yield strain, strain at maximum force, and tensile modulus
• Fracture location and failure mode classification (within gauge, at gauge boundary, at grip or tab) with failure codes
• Statistical summary of replicate specimens, including mean value, standard deviation, and coefficient of variation
• Stress-strain curves and raw load-displacement or load-extension data for engineering analysis

Testing Methods

Specimen Conditioning and Preparation

Prior to testing, polymer specimens are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291 or ASTM D618 (typically a minimum of 40 hours) to ensure moisture and temperature equilibrium. Metal specimens are cleaned and deburred, and the cross-sectional dimensions are measured at multiple locations within the gauge length to determine the minimum area. Any anisotropy arising from processing, such as mould flow direction or rolling direction, must be recorded because it strongly influences fracture strain.

Instrumentation and Strain Measurement

Testing is performed on a calibrated universal testing machine meeting the force accuracy requirements of ISO 7500-1, ISO 5893, or ASTM E4. Strain measurement on the specimen gauge section employs one of the following devices, each verified against ISO 9513 or ASTM E83 accuracy classes:

• Clip-on or bolt-on contact extensometers with knife edges attached to the gauge length, suitable for rigid plastics, metals, and low-elongation composites
• Long-travel contact extensometers for elastomers and highly ductile plastics
• Video or laser non-contact extensometers that track gauge marks, avoiding knife-edge influence on soft or thin specimens
• Digital image correlation (DIC) systems that deliver full-field strain maps, useful for locating necking, strain localisation, and the true fracture zone

Test Procedure

The specimen is mounted in self-tightening or wedge-action grips, carefully aligned to eliminate bending stresses. A small preload is applied to remove slack, the extensometer gauge length is set or verified, and the crosshead is driven at the speed prescribed by the governing standard. For plastics, ISO 527 commonly uses 1 mm/min for modulus determination followed by 50 mm/min through fracture, while ASTM D638 typically specifies 5 mm/min for modulus and 50 mm/min for rupture of Type I specimens. Metals are tested under stress-rate or displacement-rate control as defined in ISO 6892-1 Method A. Rubbers are tested at 500 mm/min per ASTM D412. The test proceeds until complete separation of the specimen, and the strain value at the fracture point is captured from the extensometer or video system at the instant of load drop.

Data Processing and Validity Criteria

Strain at break is computed from the extension at fracture divided by the initial gauge length, multiplied by 100. Results from replicate specimens (a minimum of five valid specimens is standard for plastics and composites) are averaged, and outliers are handled according to the statistical rules of the applicable standard. A specimen is generally invalidated and retested if fracture occurs within the grips, at a tab, or outside the gauge length region, because such failures do not represent the true material response. All reports must state the standard used, specimen type and orientation, conditioning, test speed, gauge length, temperature and humidity, and the type of strain measurement device employed.

Applications

Fracture point strain data underpin safety, design, and regulatory decisions across a broad range of industries:

• Automotive engineering: verifying ductility of crash-relevant components, battery housing materials, and interior polymers under impact and cold-temperature service conditions
• Aerospace and composites: qualification of carbon-fibre laminates, adhesive bonds, and primary structure materials where allowable strain limits define design envelopes
• Medical devices: evaluating catheter tubing, packaging films, implants, and disposable polymer components for flexibility and reliability
• Packaging industry: optimising film formability, seal integrity, and stretch behaviour of mono-layer and multi-layer films
• Wire and cable: assessing insulation and sheathing compounds for elongation performance before and after ageing
• Additive manufacturing: comparing print orientations, infill strategies, and post-processing effects on the ductility of 3D-printed parts
• Research and failure analysis: benchmarking new formulations, detecting embrittlement from UV, chemical, or thermal exposure, and diagnosing in-service fracture incidents

FAQ

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

There is no numerical difference when engineering strain is expressed as a percentage: elongation at break is simply the strain at break multiplied by 100. The distinction matters mainly for true strain, which accounts for progressive reduction of the gauge cross-section and is used in advanced forming and fracture analyses.

Can strain at break be measured from crosshead displacement alone?

Crosshead-derived (nominal) strain includes machine compliance, grip slip, and grip-region deformation, so it systematically overestimates strain at break, especially for stiff, low-elongation materials. Accredited third-party laboratories therefore measure strain directly on the gauge section with a calibrated extensometer whenever the governing standard requires it.

How many specimens are required for a valid result?

ISO 527 and ASTM D638 require a minimum of five valid specimens per material condition, and ASTM D3039 requires at least five valid coupons per fibre direction and environmental condition. Additional specimens are tested if failures occur in the grips or outside the gauge length.

What happens if the specimen breaks outside the gauge length?

Fracture occurring in the grip region, at a tab, or beyond the gauge marks is considered a non-representative failure. The data point is discarded, the failure mode is documented with a failure code, and a replacement specimen is tested until the required number of valid results is obtained.

How do temperature and strain rate affect fracture strain?

Most polymers and metals become less ductile at lower temperatures and higher strain rates, shifting the ductile-to-brittle transition. When service conditions involve cold, heat, or impact loading, fracture point strain determination should be repeated under conditioned-temperature chambers and at representative loading rates to capture realistic behaviour.

Which extensometer accuracy class is required?

For modulus-critical and low-strain measurements, ISO 9513 Class 0.5 or Class 1 extensometers, or the ASTM E83 equivalent Class B-1, are typically specified. For high-elongation elastomers, long-travel or non-contact video extensometers are preferred, provided their calibration range covers the expected fracture strain.