Plastic Tensile Strain at Break Sample Testing: Standards, Methods and Applications

Tensile strain at break testing quantifies the deformation a plastic specimen sustains before rupture and is one of the most frequently performed mechanical tests on polymeric materials. Accredited third-party laboratories carry out this testing in accordance with ISO 527, ASTM D638, GB/T 1040 and JIS K 7161 to support quality control, material certification, failure analysis and product development. This article explains the specimen requirements, measurable parameters, test procedures and typical applications of tensile strain at break testing for plastic samples.

Technical Overview

Tensile strain at break, designated εtB for the nominal value and εB when measured with an extensometer, is the strain of a plastic test specimen at the instant of rupture. It is calculated as the change in gauge length divided by the initial gauge length and is expressed as a percentage. Together with tensile strength and tensile modulus, it forms the core set of tensile properties used to characterize rigid and semi-rigid plastics, and it serves as the primary indicator of ductility versus brittleness.

The governing international standard is the ISO 527 series, in particular ISO 527-1 (general principles) and ISO 527-2 (test conditions for moulding, extrusion and cast plastics). Equivalent or related methods include ASTM D638, the GB/T 1040 series, JIS K 7161 and EN ISO 527 as adopted in Europe. For films and sheets thinner than 1 mm, ISO 527-3 and ASTM D882 apply instead. It should be noted that the 2019 revision of ISO 527-1 introduced a clear distinction between strain at break measured with an extensometer over the gauge length and nominal strain at break calculated from the grip separation, and these two values are not directly interchangeable.

Strain at break is highly sensitive to material state and processing history. Factors such as molecular weight, filler or fiber content, residual molding stress, moisture conditioning, thermal or UV aging, and specimen surface defects all influence the result. For this reason, the test is widely used for incoming resin inspection, batch release, reprocessed-material evaluation, and before-and-after comparisons in aging and weathering studies.

Testing Samples

Applicable Materials

Tensile strain at break testing is applicable to a broad range of rigid and semi-rigid polymers, including:

• Commodity thermoplastics: PE, PP, PS, HIPS, ABS, PVC, PMMA
• Engineering thermoplastics: PC, PA6, PA66, POM, PBT, PET, PPO blends, modified PPS
• High-temperature and specialty resins: PEEK, PEI, PSU, PPSU, fluoropolymers
• Reinforced compounds: glass-fiber or mineral-filled grades, long-fiber thermoplastics
• Thermosets: phenolics, epoxies, unsaturated polyesters, sheet and bulk molding compounds
• Biodegradable polymers such as PLA and PHA, and 3D-printed specimens

Soft elastomeric materials with very high elongation are normally tested under ISO 37 or ASTM D412 rather than ISO 527-2, and thin films below 1 mm are tested under ISO 527-3 or ASTM D882.

Specimen Geometry

The most widely used specimens are dumbbell-shaped bars with a reduced central gauge section:

• ISO 3167 Type A multipurpose specimen, or the reduced ISO 527-2 Type 1A (overall length 170 mm, gauge length 50 mm, preferred thickness 4 mm) and Type 1B variants
• ASTM D638 Type I specimen (overall length 165 mm, gauge length 50 mm, width of narrow section 13 mm, preferred thickness 3.2 mm)
• ASTM D638 Type II, III and IV geometries for specific thicknesses and forms
• ASTM D638 Type V miniature specimen (overall length 63.5 mm, gauge length 7.62 mm) for limited material volumes
• Film strips or dumbbells per ISO 527-3 for sheets and membranes

Specimen Preparation

Specimens should be prepared by injection molding in accordance with ISO 294-1, or machined from plaques, sheets or finished parts in accordance with ISO 2818. Cut edges must be free of notches, scratches and machining marks, because stress concentrations in the gauge section artificially reduce strain at break. When parts are tested directly, the sampling location and direction relative to flow must be recorded, as anisotropy significantly affects ductility.

Conditioning

Before testing, specimens are conditioned in a standard atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 or GB/T 2918, or per ASTM D618 Procedure A. Hygroscopic materials such as polyamides may require extended equilibration or accelerated conditioning, for example under ISO 1110, and dried or as-received states must be clearly documented since moisture content can change strain at break by an order of magnitude.

Sample Quantity

ISO 527-1 requires a minimum of five valid specimens per condition and test direction. When scatter is large, when results are near a specification limit, or when certification data are needed, testing ten or more specimens is recommended so that mean values and standard deviations are statistically meaningful.

Testing Items

A standard tensile test on plastics yields a complete engineering stress-strain curve from which the following items are reported:

• Nominal tensile strain at break εtB (based on grip separation or crosshead travel)
• Tensile strain at break εB (measured with a contact or video extensometer over the gauge length)
• Tensile strain at yield εY
• Stress at yield σY and stress at break σB
• Tensile strength σM (stress at the maximum force encountered)
• Tensile modulus Et, determined in ISO 527 as the secant between strains of 0.05 % and 0.25 %
• Optional: Poisson's ratio per the annex of ISO 527-1, full force-extension raw data, and statistics such as mean, median, standard deviation and coefficient of variation

Testing Methods

Instrumentation

Testing is performed on a calibrated universal testing machine meeting ISO 5893 class 1 or better, equipped with suitable load cells so that the measured force lies within the calibrated range. Extensometers, whether contact or non-contact video types, must comply with ISO 9513 class 1 or better. Pneumatic or mechanical wedge grips with serrated or profiled faces prevent slippage on ductile materials, while film specimens are held with pneumatic side-action grips and low clamping pressure. For environmental testing, temperature chambers typically covering approximately -70 °C to +300 °C are fitted to the frame.

Test Procedure

The general procedure is as follows:

• Measure the width and thickness of the narrow section at several points to 0.01 mm accuracy and record the minimum cross-sectional area
• Mark the initial gauge length L0, typically 50 mm for Type 1A and Type I specimens
• Mount the specimen with the gauge marks aligned to the extensometer, and apply a small preload to remove slack
• Select the test speed specified by the material standard or the client agreement
• Run the test to complete rupture while recording force and extension continuously
• Check the break location: only breaks within the gauge section are valid; breaks at grip faces, at visible flaws, or outside the gauge require retesting with a replacement specimen

Test Speeds and Parameters

Tensile modulus is determined at 1 mm/min under ISO 527 or 5 mm/min under ASTM D638, using an extensometer. For strength and strain at break, the commonly selected speed is 50 mm/min for ISO Type 1A and ASTM Type I specimens, with 5, 10, 20, 100, 200 and 500 mm/min used where material standards or specifications dictate. Because plastics are viscoelastic, strain at break decreases with increasing speed and decreasing temperature, so the speed used must always accompany the reported value.

Calculation and Expression of Results

Tensile stress is calculated as the force divided by the original cross-sectional area. Tensile strain at break is calculated as the change in gauge length at rupture divided by the initial gauge length, multiplied by 100 to obtain a percentage. The tensile modulus is the slope of the secant between the two defined strain points on the initial linear region. Results for each specimen, together with mean values and standard deviations, are reported, and the specimen type, preparation method, conditioning state, test speed and temperature are always stated because they materially affect comparability.

Quality Assurance

Accredited third-party laboratories operating under ISO/IEC 17025 quality management systems perform these tests with calibrated equipment, traceable reference materials and documented procedures, ensuring that reported strain-at-break data are defensible for regulatory submissions, customer specifications and international trade documents.

Applications

Tensile strain at break data are used across the full plastics value chain, from resin producers to end-product manufacturers. Typical applications include:

• Incoming inspection and lot release of resins, compounds and masterbatches against datasheet limits
• Automotive components such as interior trim, clips, bumpers and under-hood parts, where low-temperature ductility retention is critical
• Packaging films, sheets and laminates requiring defined elongation for form-fill-seal processes
• Pressure pipes and fittings in PVC-U, PE and PP-R, where elongation indicates fusion quality
• Electrical and electronic housings, connectors and cable insulation materials
• Medical device components requiring validated mechanical performance
• Validation of 3D-printed parts and evaluation of print orientation effects
• Assessment of regrind content, recyclate quality and batch-to-batch consistency
• Aging, heat and UV weathering studies, where retained strain at break is the key degradation indicator
• Failure analysis, comparing failed parts against virgin or reference material

FAQ

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

Both describe the deformation at rupture expressed as a percentage. ISO 527-1:2019 formally distinguishes strain at break measured with an extensometer over the gauge length from nominal strain at break calculated from the grip separation. Because the grip separation includes the shoulder regions of the specimen, nominal values are lower for brittle materials and cannot be directly compared with extensometer-based values.

Should I test to ISO 527 or ASTM D638?

The choice depends on the market, customer specification and applicable regulation. The two standards use different specimen geometries, particularly in the shoulder radii and parallel section, and slightly different modulus procedures, so results are similar in trend but not numerically identical. GB/T 1040 is technically aligned with ISO 527, and JIS K 7161 likewise mirrors the ISO method.

Can strain at break be determined from crosshead displacement?

Only as a nominal value. Crosshead displacement includes machine and grip compliance plus deformation outside the gauge section, so an extensometer or video extensometer is strongly recommended whenever strain at break is a specified acceptance criterion.

Why did my specimen fail with a very low strain at break?

Common causes include surface notches or machining marks, poor fusion or voids from molding, insufficient or excessive drying of hygroscopic resins, material degradation from processing or regrind, testing at an incorrect speed or temperature, or inherent brittleness of the formulation. Comparing fracture surfaces and testing reference material helps isolate the cause.

How many specimens are required and what sample size must I submit?

A minimum of five valid breaks per condition is required by ISO 527-1, so laboratories typically request six to ten specimens per condition to allow for invalid breaks. For ISO Type 1A bars this means submitting molded or machined bars about 170 mm long, while ASTM Type V mini-specimens only require material for 63.5 mm bars.

Do temperature and humidity affect the results?

Yes. Plastics are viscoelastic, so strain at break generally increases with temperature and decreases with test speed. Moisture has a strong plasticizing effect on polyamides and some polyesters, which is why conditioning state must be controlled and reported alongside the numerical result.