Plastic Tensile Strain Testing: Principles, Standards, Methods, and Applications

Plastic tensile strain testing is a fundamental mechanical evaluation method used to measure how plastic materials deform and ultimately fail under uniaxial tensile loading. Governed by internationally recognized standards such as ISO 527, ASTM D638, and GB/T 1040, this test determines critical properties including tensile modulus, yield strain, tensile strength, and elongation at break. This article provides a comprehensive technical guide covering test principles, specimen requirements, measurement procedures, and practical applications of plastic tensile strain testing.

Technical Overview

Plastic tensile strain testing is a quasi-static mechanical test in which a standardized plastic specimen is loaded in tension at a constant crosshead speed until yielding or fracture occurs. Throughout the test, the applied force and the resulting deformation are continuously recorded, producing a stress-strain curve that characterizes the material's stiffness, ductility, and strength.

Tensile strain (ε) is defined as the relative change in length of the specimen gauge section and is expressed as a percentage:

• ε = ΔL / L₀ × 100%, where L₀ is the initial gauge length and ΔL is the change in length measured by an extensometer or video measurement system.
• Engineering stress (σ) is calculated as σ = F / A, where F is the applied force and A is the original cross-sectional area of the narrow section.
• The tensile modulus (Young's modulus, Eₜ) is determined as the slope of the stress-strain curve within a defined strain interval, typically between strains of 0.0005 and 0.0025 for rigid plastics.

Strain measurement is a critical aspect of the test because crosshead displacement alone includes machine compliance, grip slippage, and deformation outside the gauge section. Therefore, direct strain measurement using contact extensometers or non-contact video extensometers is required for accurate modulus and yield strain determination.

The principal standards governing plastic tensile strain testing include:

• ISO 527-1: Determination of tensile properties — General principles.
• ISO 527-2: Test conditions for moulding and extrusion plastics, including the multipurpose specimen.
• ISO 527-3: Test conditions for films and sheets.
• ASTM D638: Standard Test Method for Tensile Properties of Plastics.
• GB/T 1040.1 and GB/T 1040.2: Chinese national standards technically aligned with ISO 527.
• JIS K 7161 and JIS K 7162: Japanese industrial standards corresponding to ISO 527-1 and ISO 527-2.

Accredited third-party laboratories routinely perform these tests to support product development, incoming material inspection, quality control, and compliance verification for regulatory and certification purposes.

Testing Samples

Moulding and Extrusion Plastics (Rigid and Semi-Rigid)

For rigid and semi-rigid thermoplastics, thermosets, and thermoplastic elastomers, ISO 527-2 defines the preferred dumbbell-shaped multipurpose specimens:

• Specimen 1A: preferred multipurpose specimen with a total length of 170 mm, a parallel width of 10 mm, and a thickness of 4 mm, typically injection moulded directly.
• Specimen 1B: machined from sheets, plates, or finished parts, with the same nominal dimensions as 1A but adapted for machining from thicker stock.
• ASTM D638 Type I specimen: 165 mm total length, 13 mm width in the narrow section, and a nominal thickness of 3.2 mm, widely used in North America.
• ASTM D638 Type IV and Type V specimens: smaller geometries used when limited material is available or for testing thin sections.

Films and Sheets

Thin plastic films and sheets with a thickness of 1 mm or less are tested according to ISO 527-3 or ASTM D882. These specimens are typically rectangular strips, commonly 10 mm to 25 mm in width, cut with sharp dies to avoid edge defects that could initiate premature tearing.

Anisotropic and Reinforced Materials

For fibre-reinforced plastics, oriented films, and injection-moulded parts containing weld lines or flow-induced orientation, specimens are prepared in multiple directions relative to the processing direction to characterize anisotropy in tensile strain behaviour.

Specimen Conditioning

All specimens must be conditioned prior to testing to ensure reproducible results:

• Standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity per ISO 291 or ASTM D618, unless otherwise specified.
• Minimum conditioning duration of at least 40 hours for most plastics, extended for hygroscopic materials such as polyamides (nylon).
• Specimens must be free of surface defects, notches, bubbles, and machining marks that could act as stress concentrators.

Testing Items

Plastic tensile strain testing yields a complete set of mechanical properties derived from the stress-strain curve:

• Tensile modulus (Eₜ): the slope of the stress-strain curve in the initial linear region, indicating material stiffness.
• Tensile stress at yield (σᵧ): the first stress at which strain increases without an increase in stress, applicable to ductile materials exhibiting a yield point.
• Tensile strain at yield (εᵧ): the strain corresponding to the yield stress, expressed as a percentage.
• Tensile strength (σₘ): the maximum tensile stress sustained by the specimen before break or yield plateau.
• Tensile stress at break (σ_B) and tensile strain at break (ε_B): values recorded at the moment of specimen rupture.
• Nominal tensile strain at break (εₜB): strain calculated from crosshead displacement relative to the initial distance between grips, used when the specimen breaks outside the gauge section or necks beyond the extensometer range.
• Poisson's ratio (μ): the ratio of transverse strain to axial strain, measured with a biaxial extensometer per ISO 527-1.

Testing Methods

Equipment Configuration

The test is performed on a calibrated universal testing machine (UTM) equipped with:

• A calibrated load cell with an accuracy class of 1 or better, selected so that the measured forces fall within 10 % to 90 % of its capacity range where possible.
• Appropriate grips — mechanical wedge grips, pneumatic grips, or capstan grips for films — that securely hold the specimen without slippage or premature failure at the jaw faces.
• A contact extensometer or non-contact video extensometer with accuracy class 1 or better, directly measuring displacement within the gauge length (typically 50 mm for ISO 1A/1B specimens and 50 mm for ASTM D638 Type I).
• A data acquisition system recording force and extension at a sufficient sampling rate to resolve yield phenomena and fracture events.

Test Procedure

The general procedure for plastic tensile strain testing is as follows:

• Measure the width and thickness of each specimen at several points within the gauge section using calibrated instruments, and calculate the mean cross-sectional area.
• Condition the specimens in the standard atmosphere and mark the gauge length, typically 50 mm, with reference marks that can be tracked by the video extensometer.
• Mount the specimen in the grips, ensuring axial alignment to avoid bending stresses that distort strain readings.
• Attach or focus the extensometer on the gauge marks and set the preload to a small, standardized value.
• Apply the tensile load at the specified constant crosshead speed. Common speeds include 1 mm/min for modulus determination, and 5, 50, or 500 mm/min for strength and elongation measurements, depending on the material and standard.
• Record force and extension continuously until the specimen yields, necks, and/or breaks.
• Calculate stress, strain, modulus, and the other reported properties; repeat on the required number of specimens (typically at least five valid results per ISO 527 and ASTM D638).

Strain Measurement Considerations

Accurate strain measurement requires attention to several factors:

• Gauge length verification: the modulus and strain values depend directly on the initial gauge length, so L₀ must be precisely established before loading.
• Extensometer removal: for ductile materials that neck beyond the extensometer range, the device is removed at a predefined strain level and the remaining deformation is tracked as nominal strain from crosshead separation.
• Valid failure location: specimens breaking within the gauge section are considered valid; breaks at the grips require investigation and retesting.
• Speed sensitivity: plastics are viscoelastic, so test speed significantly influences modulus, yield stress, and strain at break; the selected speed must always be reported with the results.

Key Differences Between ISO 527 and ASTM D638

Although conceptually similar, the two standard systems differ in ways that affect comparability of results:

• Specimen geometry: ISO 527-2 uses the 1A/1B dumbbell with a 10 mm wide parallel section, while ASTM D638 Type I has a 13 mm wide narrow section.
• Radius and fillet dimensions differ, producing different stress distributions near the shoulder transitions.
• Modulus determination strain ranges and rounding conventions differ slightly between the standards.
• Results obtained under ISO 527 and ASTM D638 are generally not directly interchangeable and should never be compared without noting the specific test conditions.

Applications

Plastic tensile strain testing serves a wide range of engineering and commercial purposes across many industries:

• Material development and comparison: evaluating new polymer formulations, nucleating agents, plasticizers, fillers, and recycled content on stiffness and ductility.
• Quality control in manufacturing: verifying incoming resin lots, monitoring injection moulding processes, and detecting degradation from excessive processing temperatures or moisture exposure.
• Automotive engineering: validating interior trim, bumpers, ducts, clips, and structural components against OEM specifications and long-term durability requirements.
• Packaging industry: characterizing films for elongation, puncture-related stiffness, and formability in thermoforming and stretch applications.
• Construction products: assessing pipes, fittings, geosynthetics, and profiles against product standards referencing tensile property limits.
• Electronics and consumer goods: confirming the mechanical reliability of enclosures, connectors, and cable insulation materials.
• Medical devices: supporting material qualification for catheters, packaging, and housings in accordance with regulatory submissions.
• Certification and compliance: generating test reports accepted in product certification programs, where testing is conducted by accredited third-party laboratories under ISO/IEC 17025 management systems.
• Failure analysis: comparing tensile strain properties of failed parts against baseline material data to identify embrittlement, UV or chemical degradation, and processing defects.

FAQ

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

Tensile strain is the general measure of relative deformation at any point on the stress-strain curve, while elongation at break specifically refers to the strain at the moment of specimen rupture, expressed as a percentage of the initial gauge length.

Why is an extensometer needed instead of using crosshead displacement?

Crosshead displacement includes machine deflection, grip movement, and deformation outside the gauge section, which systematically overstates strain. Direct extensometer measurement within the gauge length provides accurate strain data, which is essential for correct modulus and yield strain values.

How many specimens should be tested?

Both ISO 527 and ASTM D638 recommend testing at least five specimens per condition. For highly anisotropic materials or parts, additional specimens in different orientations are required.

Which test speed should be selected?

A speed of 1 mm/min is standard for modulus determination, while 5 mm/min, 50 mm/min, or 500 mm/min are commonly used for strength and elongation, depending on the material type specified by the product standard. Because plastics are strain-rate sensitive, the speed must always be reported with results.

Can results from ISO 527 and ASTM D638 be compared directly?

No. Differences in specimen geometry, gauge length conventions, and calculation methods mean results from the two standards are not directly interchangeable. Consistency of method is essential for valid material comparisons.

What types of plastics are suitable for this test?

Virtually all plastic categories can be tested, including rigid thermoplastics, thermosets, fibre-reinforced compounds, thermoplastic elastomers, and flexible films, with the appropriate specimen type and speed selected according to the relevant part of the standard.

How is Poisson's ratio measured in tensile strain testing?

Poisson's ratio is measured with a biaxial extensometer that simultaneously records axial strain and transverse (width) strain within the initial elastic region, typically at low strain levels below the proportional limit, as described in ISO 527-1.