Standardized Determination of Tensile Strain at Break for Plastics: Test Principles, Methods, and Ap

Tensile strain at break is a core ductility indicator that quantifies how far a plastic material can be stretched before rupture, making its standardized determination essential for material qualification, quality control, and product design. This article explains the measurement principles of ISO 527, ASTM D638, and GB/T 1040, covering specimen preparation, instrumentation, test procedures, data evaluation, and typical industrial applications. It serves as a practical technical reference for engineers and laboratory personnel performing strain-at-break testing in accredited third-party laboratories or in-house facilities.

Technical Overview

Tensile strain at break, denoted as εB, is defined by ISO 527-1 as the engineering strain of a plastic specimen at the moment of rupture, referenced to the initial gauge length L0 and expressed as a percentage. It is calculated using the relationship εB = (ΔLB / L0) × 100%, where ΔLB is the increase in gauge length between the initial state and the breaking point. Together with tensile stress at break and stress at yield, strain at break characterizes whether a material behaves in a brittle, semi-ductile, or highly ductile manner under tensile loading.

The magnitude of strain at break varies dramatically across polymer families. Brittle materials such as general-purpose polystyrene or unfilled thermosets may rupture at strains of only a few percent, while ductile polyolefins such as HDPE can sustain strains of several hundred percent before failure. Because the result is highly sensitive to molecular weight, crystallinity, processing-induced orientation, specimen geometry, test speed, temperature, and humidity, standardized determination is indispensable for obtaining comparable data.

The governing framework consists of ISO 527-1 (general principles) together with ISO 527-2 (test conditions for moulding and extrusion plastics) and ISO 527-3 (films and sheets). The American counterpart is ASTM D638, while GB/T 1040.1 and GB/T 1040.2 are the Chinese national adoptions harmonized with ISO 527. JIS K 7161 and JIS K 7162 align with the ISO methodology for Japanese users. Machine verification is performed per ISO 7500-1 or ASTM E4, and extensometers are classified according to ISO 9513 or ASTM E83.

Testing Samples

Rigid and Semi-Rigid Moulding and Extrusion Plastics

Thermoplastics such as PP, PE, ABS, PC, PA6, PA66, PET, PVC, POM, and PMMA, as well as filled or short-fiber-reinforced compounds, are typically tested using machined or moulded dumbbell specimens. The preferred ISO specimen 1A has an overall length of 170 mm, a width of 10 mm in the narrow parallel section, a thickness of 4 mm, and a gauge length of 50 mm. Specimen 1B is similar with a slightly shorter shoulder design. Multi-purpose specimens produced according to ISO 20753 or ISO 3167 ensure consistent, comparable results across laboratories.

Plastic Films and Thin Sheets

For flexible films and sheets up to approximately 1 mm in thickness, ISO 527-3 specifies small dumbbell specimens (such as type 5A or 5B with a 20 mm gauge length) or parallel strip specimens. These are widely used for PE, PP, PET, PA, and PVC packaging and converting films, where high ductility and seal performance must be verified.

Machined Specimens from Finished Products

When testing sheets, profiles, molded parts, or aged components, specimens may be machined and polished to remove stress concentrations and surface defects at the edges, since scratches and machining marks can substantially reduce the measured strain at break of ductile materials.

ASTM Specimen Types

ASTM D638 defines specimen Types I through V, with Type I (165 mm overall length, 13 mm narrow-section width, 3.2 mm thickness, 50 mm gauge length) being the most common for rigid plastics. Type V small specimens are used when material quantity is limited, and Types II, III, and IV cover alternative thicknesses and configurations.

Testing Items

A tensile strain at break determination is normally performed as part of a complete tensile characterization, which typically includes the following reported parameters:

• Tensile strain at break (εB), expressed in percent, referenced to the gauge length
• Tensile stress at break (σB), calculated from the force at rupture and the initial cross-sectional area
• Tensile stress at yield (σY) and tensile strain at yield (εY), for materials exhibiting a yield point
• Tensile strength (σM), the maximum stress sustained during the test
• Tensile modulus (ET), determined as a secant modulus between strains of 0.05% and 0.25% per ISO 527-1
• Complete force-extension curves for ductile materials exhibiting necking and cold drawing

Testing Methods

Apparatus and Calibration

Testing is performed on a calibrated universal testing machine equipped with constant-crosshead-speed control, suitable wedge or pneumatic grips, and an extensometer attached directly to the gauge length. Force measuring systems should meet accuracy class 1 of ISO 7500-1, and extensometers should meet class 1 of ISO 9513 or class B-1/B-2 of ASTM E83. Non-contact video extensometers are strongly recommended for very ductile materials, since they allow strain measurement through to rupture without specimen contact and without the risk of the extensometer arms being damaged during break.

Specimen Conditioning

Specimens are conditioned per ISO 291 at 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 88 hours, unless the relevant material or product standard specifies otherwise. ASTM D618 procedure A is commonly applied for ASTM D638 testing. Special attention is required for moisture-sensitive polymers such as polyamides, for which the conditioning state (dry-as-moulded, 50% RH equilibrium, or saturated) must be documented because it strongly affects ductility.

Test Procedure

Gauge marks are applied accurately to define L0, the specimen is mounted in the grips with careful axial alignment, and a small preload removes slack. When the tensile modulus is required, testing begins at 1 mm/min over the initial strain range, after which the speed is changed. Per ISO 527-2, the preferred test speed for specimen 1A and 1B for strength and strain determination is 50 mm/min, while ASTM D638 commonly uses 5 mm/min for Type I specimens unless the material specification dictates otherwise. Testing continues until the specimen ruptures, and the extension at break ΔLB is recorded. At least five valid results are required for isotropic materials by both ISO 527-1 and ASTM D638.

Calculation and Validity Criteria

Strain at break is calculated as εB = (ΔLB / L0) × 100%. When a contact extensometer has a limited travel range on highly ductile materials, it may be removed before rupture, or strain may be recorded as nominal strain based on crosshead separation with documented system compliance correction. Tests in which the specimen breaks outside the gauge length, at a visible flaw, or in the grips are considered invalid and must be discarded and repeated, as required by ISO 527-1. Results are reported as the arithmetic mean together with the observed range or standard deviation.

Reporting

A complete report includes full material identification, specimen type and source, dimensions and gauge length, conditioning and test atmosphere, test speed, number of specimens, individual and mean values of strain at break and associated tensile properties, and a description of the break type (brittle or ductile). This traceability is essential when data are submitted for customer specification, regulatory documentation, or certification by accredited third-party laboratories.

Applications

Standardized strain at break determination supports a wide range of engineering and commercial activities:

• Automotive: ductility verification of PP bumper compounds, interior trim materials, and PA66 under-hood components, where cold-temperature embrittlement resistance is safety-relevant
• Packaging: elongation performance of PE, PP, and PET films for form-fill-seal processes, lamination, and pouch integrity
• Pipes and fittings: tensile property verification of PE and PVC piping materials referenced to product standards built on ISO tensile methods
• Electrical and electronics: incoming inspection and UL material recognition support for housings, connectors, and insulation parts
• Medical devices: catheter tubing, film packaging validation, and biocompatibility test programs requiring mechanical baseline data
• Research and development: comparison of formulations, additives, recycled content levels, and molecular weight grades
• Aging and durability studies: quantification of embrittlement after heat aging, hydrolysis, UV or weathering exposure, and chemical contact in failure analysis

FAQ

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

Both describe deformation at rupture as a percentage, but ISO 527 defines strain at break strictly with reference to the initial gauge length L0 measured by an extensometer. Elongation values based on grip separation or crosshead travel include machine and grip compliance as well as deformation outside the gauge section, so they should be reported as nominal strain and are not directly interchangeable with extensometer-based values.

Are ISO 527 and ASTM D638 results interchangeable?

No. The two standards use different specimen geometries (for example ISO 1A versus ASTM Type I), different thicknesses, and different preferred test speeds, all of which influence the measured strain at break. For specification purposes, the exact standard used must be stated, and any correlation between the two methods must be established experimentally for the specific material.

Why does test speed affect the strain at break result?

Plastics are viscoelastic, so a higher strain rate generally raises stiffness and strength and lowers ductility. Comparisons between laboratories or data sets are only valid when the test speed, specimen type, and conditioning history are identical.

Why did my specimen break outside the gauge length, and what should be done?

Premature breaks outside the gauge section usually indicate grip damage, misalignment, edge defects from machining, or internal flaws. Such results are invalid under ISO 527-1 and must be discarded, the cause investigated, and additional specimens tested until the required number of valid results is obtained.

How many specimens should be tested?

ISO 527-1 and ASTM D638 require a minimum of five valid specimens for isotropic materials. For anisotropic materials such as fiber-reinforced compounds or oriented films, testing in both principal directions with additional specimens is recommended to account for direction-dependent behavior and scatter.

What factors most strongly influence strain at break reproducibility?

The dominant factors are specimen preparation quality, accurate gauge length definition, extensometer class and attachment, temperature and humidity control, conditioning of moisture-sensitive polymers, and consistency of test speed. Careful control of these variables, supported by routine calibration per ISO 7500-1 and ISO 9513, ensures reproducible and defensible results.