Plastic Tensile Strain at Break Testing: Methods, Standards, and Technical Guidelines

Tensile strain at break, commonly reported as percent elongation at break, is one of the most important ductility indicators for plastic materials and is measured under internationally recognized standards such as ISO 527 and ASTM D638. This article provides a comprehensive technical overview of plastic tensile strain at break testing, covering specimen preparation, equipment requirements, test procedures, calculation methods, and result validation. It serves as a practical reference for engineers, quality managers, and accredited third-party laboratories involved in material qualification and quality control.

Technical Overview

Tensile strain at break, denoted as εB, is the ratio of the increase in gauge length to the original gauge length of a plastic specimen at the instant of fracture under uniaxial tensile loading. When expressed as a percentage, it is widely known as elongation at break. Because this parameter quantifies the deformation reserve a plastic retains before failing, it is one of the most frequently specified properties on material datasheets, in purchase specifications, and in regulatory submissions.

The measurement of tensile strain at break for plastics is governed by several harmonized and parallel standards systems. The core references include ISO 527-1 (general principles), ISO 527-2 (test conditions for moulding, extrusion and compression-moulding plastics), ISO 527-3 (films and sheets), ASTM D638 (tensile properties of plastics), the GB/T 1040 series (Chinese national standards aligned with ISO 527), and JIS K 7161 and JIS K 7162. Although the test principles are similar across these standards, specimen geometry, test speed, and reporting conventions differ, so results are not directly interchangeable.

Tensile strain at break is used to distinguish ductile from brittle behavior, to verify incoming resin quality, to evaluate the effects of additives, recycled content, processing conditions, and environmental aging, and to confirm that finished products meet mechanical performance requirements. Testing is typically performed in accredited third-party laboratories or in-house quality control facilities using calibrated universal testing machines.

Testing Samples

ISO Multipurpose Dog-Bone Specimens (Types 1A and 1B)

ISO 527-2 defines the multipurpose specimen type 1A for directly injection-moulded material, with an overall length of 170 mm, a parallel section 10 mm wide and 4 mm thick, a gauge length of 50 mm, and a grip separation of 115 mm. Type 1B is intended for specimens machined from sheets, bars, or molded parts and features a shorter parallel length with large fillet radii to reduce stress concentration at the shoulders. Reduced-scale variants such as 1BA and 1BC are available when only limited material can be obtained.

ASTM D638 Specimen Types

ASTM D638 defines five specimen geometries. Type I is the preferred specimen, measuring 165 mm in overall length, 13 mm in narrow-section width, 3.2 mm in thickness, with a 50 mm gauge length and 115 mm grip separation. Types II, III, and IV are designated for particular material classes, thickness ranges, and non-rigid plastics, while the small Type V specimen is used when only a small quantity of material is available. The specimen type used must always be reported, because strain at break values depend strongly on geometry.

Films and Thin Sheets

For plastic films and thin sheets, ISO 527-3 applies and specifies rectangular strip specimens, typically 10 mm to 25 mm wide with a minimum length of 150 mm, cut with a sharp die to avoid edge notches. ASTM D882 is used for sheeting with a thickness of 1 mm or less. Higher test speeds are generally specified for films, and special care is required to prevent slippage and damage at the grips.

Specimens Taken from Finished Products

When products themselves must be evaluated, specimens may be die-cut or machined from finished parts. The cutting method, sample location, and orientation relative to the polymer flow direction must be documented, because molded-in orientation, weld lines, voids, and machining marks can significantly reduce measured strain at break.

Conditioning and Sample Quantity

Specimens shall be conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity according to ISO 291, or per ASTM D618 Procedure A (typically 40 hours) as appropriate. ISO 527-1 requires at least five valid results for each test direction, and additional specimens should be prepared when results scatter widely or when anisotropic materials are evaluated in both the machine and transverse directions.

Testing Items

The primary testing item is the tensile strain at break εB, determined from the extension of the gauge length at the moment of fracture. In routine tensile testing, the following related properties are usually obtained from the same force-extension curve:

• Tensile strain at break (εB) – the gauge-length-based strain at fracture, reported as a percentage.
• Nominal tensile strain at break (εtB) – strain calculated from crosshead extension and initial grip separation, used when an extensometer is not applied or when fracture occurs outside the gauge length.
• Tensile stress at break (σB) – the maximum stress sustained at the point of fracture.
• Tensile stress and strain at yield (σY, εY) – the first point on the curve at which strain increases without an increase in stress, where applicable.
• Tensile modulus (Et) – the slope of the stress-strain curve in the initial linear region, determined at 1 mm/min with an extensometer gauge length of 75 mm for ISO 1A/1B specimens.

Testing Methods

Equipment Requirements

Testing is performed on a screw-driven or servo-hydraulic universal testing machine with force verification to ISO 7500-1 (class 1 or better) or the equivalent ASTM E4 requirement. Essential equipment and accessories include:

• Pneumatic, hydraulic, or wedge-action grips with faces suited to the material, specimen geometry, and expected load level.
• A calibrated extensometer conforming to ISO 9513 class 1 or ASTM E83 class B-2 or better; contact, auto-detaching, or non-contact video extensometers are preferred for large-elongation materials.
• A precision micrometer or thickness gauge for measuring specimen width and thickness at the gauge section.
• An optional environmental chamber when testing at elevated or sub-ambient temperatures is required.

Test Speed Selection

ISO 527 specifies nominal test speeds of 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min. The tensile modulus is determined at 1 mm/min, while strain at break is measured at the speed designated by the relevant product or material standard, commonly 50 mm/min for ductile thermoplastics, 5 mm/min for stiff or brittle materials, and up to 500 mm/min for films. ASTM D638 similarly prescribes standard crosshead speeds, most commonly 5, 50, or 500 mm/min, depending on the specimen type and material specification. Because plastics are viscoelastic, the selected speed must always be reported and held constant for valid comparisons.

Test Procedure

• Condition the specimens in the specified atmosphere and measure the width and thickness of each specimen at the gauge section, recording the mean of at least three measurements.
• Mark the gauge length on the specimen or attach the extensometer, verifying proper seating and zero balance.
• Mount the specimen in the grips with the specified initial grip separation, typically 115 mm for ISO 1A/1B and ASTM Type I specimens, applying a small preload to remove slack.
• Select the test speed specified by the applicable material standard and start the test, running without interruption until the specimen fractures.
• Record the complete force-extension curve, the extension at break, and the exact location of fracture.
• Calculate the tensile strain at break and repeat the test until at least five valid results are obtained.

Calculation and Reporting

The tensile strain at break is calculated as εB = ΔLb / L0, where ΔLb is the increase in gauge length between the gauge marks at break and L0 is the initial distance between the gauge marks, typically 50 mm. The result is usually reported as a percentage. The test report shall include the material identification, specimen type and preparation method, conditioning details, test speed, gauge length, temperature and humidity, individual and mean results with standard deviation, fracture locations, and a statement of whether the extensometer or crosshead displacement was used for strain determination.

Result Validity and Common Error Sources

• Fracture at the grips or fillets – results from fractures outside the gauge section shall be flagged and, in most protocols, retested, since stress concentrations artificially lower strain at break.
• Extensometer slippage – knife-edge slippage causes non-physical strain readings; auto-detaching or video extensometers are recommended for highly extensible materials.
• Crosshead-based strain – machine compliance and grip settling make crosshead displacement unsuitable for accurate strain at break; it may only be reported as nominal strain.
• Edge defects from die cutting – notches and burrs act as crack initiation sites and must be removed by careful die maintenance.
• Moisture history – hygroscopic resins such as polyamides show markedly different strain at break in the dry-as-molded and conditioned states; the moisture state must be documented.
• Orientation and anisotropy – injection-moulded and calendered materials must be tested in defined directions, as transverse strain at break can be substantially lower.

Applications

Tensile strain at break data support a wide range of engineering and regulatory activities across industries:

• Resin and compound qualification – verifying that incoming materials meet datasheet ductility limits before processing.
• Automotive engineering – confirming that interior trims, bumpers, and under-hood components retain sufficient ductility across service temperatures.
• Packaging development – assessing the formability, toughness, and seal-area performance of polyolefin, polyester, and multilayer films.
• Pipes, fittings, and geomembranes – evaluating long-term ductility and resistance to slow crack growth for infrastructure materials.
• Medical devices and pharmaceutical packaging – supplying mechanical evidence for design validation and regulatory submissions.
• Aging and weathering studies – detecting embrittlement caused by UV exposure, thermal oxidation, or hydrolysis, since strain at break is one of the most sensitive degradation indicators.
• Recycling and sustainability programs – quantifying the effect of regrind and recycled content on mechanical performance.
• Additive manufacturing – characterizing anisotropy between print orientations and optimizing build parameters.

FAQ

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

They describe the same physical quantity. Strain at break is a dimensionless ratio of extension to original gauge length, while elongation at break is the same value expressed as a percentage. ISO 527 terminology uses tensile strain at break (εB), whereas ASTM D638 reports percent elongation at break.

Are ISO 527 and ASTM D638 results directly comparable?

No. The two standards use different specimen geometries, gauge lengths, and preferred test speeds, which lead to systematically different values for the same material. The standard to be applied should be defined by the product specification, contractual agreement, or target market regulation.

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

A fracture occurring in the grip region or fillet usually indicates stress concentration, grip damage, or a machining defect. Such results should be marked invalid, and the test should be repeated with adjusted grip pressure or a new specimen. If the material consistently fails in this manner, the nominal strain at break based on grip separation may be reported with a clear notation.

How many specimens are required for a valid test?

ISO 527-1 requires a minimum of five valid results for each test direction, and ASTM D638 similarly recommends testing at least five specimens. For highly anisotropic materials, five specimens parallel and five perpendicular to the principal axis are typically required.

Why does the test speed change the measured strain at break?

Plastics are viscoelastic, so their stiffness and ductility depend on the strain rate. Higher speeds generally reduce elongation at break for ductile materials by suppressing molecular relaxation. The speed specified in the applicable material standard must therefore be used without modification for comparable results.