Determination of Tensile Strain in Plastics by Extensometry: Principles, Methods, and Applications

Determination of tensile strain in plastics using extensometers is a precision measurement technique essential for accurately characterizing elastic modulus, yield strain, and elongation behavior of polymeric materials in accordance with ISO 527 and ASTM D638. This article provides a comprehensive technical overview of extensometer-based strain measurement, covering specimen requirements, testing procedures, calibration classes, and practical applications in material qualification and research. It serves as a valuable reference for engineers, laboratory technicians, and quality professionals involved in plastics testing and characterization.

Technical Overview

Tensile strain measurement using extensometers is a fundamental technique in the mechanical characterization of plastic materials. Unlike crosshead displacement, which includes machine compliance, grip slippage, and load-frame deflection, extensometry directly measures deformation within a defined gauge length on the specimen itself, providing true material strain data with high accuracy and repeatability. This distinction is especially critical when determining the tensile modulus, where strain values in the range of 0.05% to 0.25% must be resolved with confidence.

An extensometer is a strain-measuring device that attaches to (or optically monitors) a specimen and records the change in distance between two reference points, known as the gauge length, as the specimen is loaded in tension. For plastics, a gauge length of 50 mm is most common, consistent with the requirements of ISO 527-1 and ASTM D638. The measured strain is calculated as the change in gauge length divided by the original gauge length, typically expressed as a dimensionless ratio, a percentage, or in millimeters per millimeter (mm/mm).

The principal standards governing this testing area include ISO 527-1 (Plastics — Determination of tensile properties — Part 1: General principles), ISO 527-2 (test conditions for moulding and extrusion plastics), and ASTM D638 (Standard Test Method for Tensile Properties of Plastics). Extensometer performance and calibration are verified against ISO 9513 (Metallic materials — Calibration of extensometer systems used in uniaxial testing) or ASTM E83 (Standard Practice for Verification and Classification of Extensometer Systems). For modulus determination, ISO 527-1 requires an extensometer of accuracy class 1 or better as defined in ISO 9513, corresponding to a maximum relative error of ±1% of the indicated strain.

Extensometers used for plastics fall into two broad categories. Contact extensometers, including clip-on and knife-edge types, physically attach to the specimen via spring-loaded arms and are suitable for rigid and semi-rigid plastics. Non-contact extensometers, including video extensometers and laser-based systems, monitor gauge marks optically and are advantageous for flexible plastics, films, elastomers, and materials that fail at high elongation, as well as for tests conducted inside environmental chambers. Advanced dual-axis configurations with both longitudinal and transverse extensometers allow simultaneous determination of Poisson's ratio.

Testing Samples

Rigid and Semi-Rigid Thermoplastics

Materials such as polypropylene (PP), polyethylene (PE), polyamide (PA), polycarbonate (PC), acrylonitrile butadiene styrene (ABS), polyoxymethylene (POM), and polyethylene terephthalate (PET) are typically tested using ISO 527-1A or 1B dumbbell specimens or ASTM D638 Type I specimens. These specimens have a parallel gauge section of consistent cross-section where the extensometer knife edges or gauge marks are positioned. Specimens are produced by injection molding, compression molding, or machining from sheets and must be free from flash, notches, and visible defects that could induce premature stress concentration.

Thermosetting Plastics and Fiber-Reinforced Composites

Epoxies, phenolics, unsaturated polyesters, and short-fiber or continuous-fiber reinforced plastics are tested with extensometry to capture their characteristically low strain-at-break values and high moduli. For reinforced materials, ISO 527-4 and ISO 527-5 define specific specimen geometries. Because fracture strain may be below 2%, high-resolution extensometers of class 0.5 or class 1 are recommended to resolve the initial stress-strain curve accurately.

Plastic Films and Sheets

Thin polymeric films are tested per ISO 527-3 using strip specimens with extended gauge lengths, often 100 mm. Non-contact video extensometry is strongly preferred for films because the low stiffness of the material cannot support the weight of a clip-on device, and elongations may exceed several hundred percent. Gauge marks are applied as ink dots, adhesive labels, or autographically printed lines.

Specimen Conditioning and Quantity

Unless otherwise specified, specimens are conditioned at 23°C ± 2°C and 50% ± 10% relative humidity in accordance with ISO 291 or ASTM D618 for a minimum of 88 hours for most thermoplastics. Hygroscopic materials such as polyamides may require extended conditioning or testing in the as-received moisture state per material specification. A minimum of five valid specimens per sample is standard per ISO 527-1, and typically ten specimens are recommended when testing films or materials exhibiting scatter in failure behavior. The thickness and width of each specimen within the gauge section are measured to the nearest 0.02 mm at multiple locations before testing.

Testing Items

Extensometer-based strain measurement enables determination of the following tensile properties:

• Tensile modulus (Young's modulus, E_t) — the slope of the stress-strain curve between two specified strains, typically 0.05% and 0.25% for ISO 527-1, or the chord modulus between 0.05% and 0.25% strain per ASTM D638; alternatively, the secant modulus at 1% strain (secant modulus) may be reported
• Tensile stress at yield and tensile strain at yield — identifying the first point on the stress-strain curve at which stress ceases to increase with increasing strain
• Tensile strength and tensile strain at tensile strength — the maximum stress sustained and the strain at which it occurs
• Tensile strain at break — the strain at specimen rupture, measured within the extensometer gauge length
• Proportional limit stress and strain — where relevant for linear-elastic brittle materials
• Poisson's ratio — determined with a biaxial (longitudinal-transverse) extensometer configuration
• Secant moduli at specified strains — such as the 1% secant modulus for design calculations
• Nominal strain and strain-rate-dependent behavior — for comparative material evaluation at different crosshead speeds

Testing Methods

Apparatus and Calibration Requirements

The test system comprises a calibrated universal testing machine (UTM) with force accuracy of class 0.5 or better per ISO 7500-1, appropriate grips (pneumatic, wedge, or manual depending on specimen rigidity), and a calibrated extensometer. Prior to testing, the extensometer must be verified per ISO 9513 or ASTM E83, and its calibration certificate must be current. Class 1 extensometers (relative error within ±1%) are the minimum requirement for modulus determination under ISO 527-1, while class 0.5 (±0.5%) provides improved confidence for high-modulus engineering plastics and composites.

Specimen Preparation and Gauge Marking

Specimen dimensions are measured at the gauge section, and the cross-sectional area is computed. For contact extensometry, the knife edges are attached symmetrically about the specimen centerline with the gauge length set precisely, commonly 50 mm. For non-contact video extensometry, two high-contrast gauge marks are applied to the parallel section, and the system is calibrated to define the initial gauge length. The specimen is then mounted in the grips with the gauge section centered and the extensometer zeroed at negligible preload, typically not exceeding 5% of the force at which modulus measurement begins.

Test Speed and Modulus Determination Procedure

The preferred test speed is specified by the relevant material standard, with 1 mm/min, 5 mm/min, and 50 mm/min being common for rigid plastics. For modulus measurement, ISO 527-1 prescribes a two-part or single-part procedure: the extensometer records strain continuously while a constant crosshead speed is applied, and the modulus is calculated as the secant slope between the stress points corresponding to strains ε₁ = 0.0005 (0.05%) and ε₂ = 0.0025 (0.25%). A straight line is fitted to the stress-strain data within this interval by least-squares regression. The modulus is reported in megapascals (MPa) to three significant figures.

Determination of Yield, Strength, and Strain at Break

Testing continues beyond the modulus interval at the specified speed until yield and, where required, fracture. The extensometer records strain throughout; for tests requiring strain at break, the extensometer must remain attached until rupture, or a non-contact system must track the gauge marks to failure. If the fracture occurs outside the gauge length, ISO 527-1 permits the use of nominal strain as a fallback, though the result must be clearly identified. For ductile materials with elongation exceeding the extensometer range, the extensometer may be removed at a predefined strain and the test continued using crosshead displacement, with the reported strain at break calculated from the combined data set.

Poisson's Ratio Measurement

When a transverse extensometer is fitted, width contraction is recorded simultaneously with axial extension within the elastic region. Poisson's ratio is computed as the negative ratio of transverse strain to axial strain over the modulus interval, typically between 0.05% and 0.25% axial strain. For thin films, optical full-field measurement techniques may be used to capture transverse contraction without mechanical contact.

Data Processing and Reporting

The test report, per ISO 527-1 and ASTM D638, shall include: complete identification of the material tested, specimen type and dimensions, conditioning history, test temperature and humidity, test speed, extensometer type, gauge length, and accuracy class, individual and mean values of all reported properties with standard deviations, and the modulus calculation interval used. Stress-strain curves should be retained as documentary evidence of test validity, including the identification of yield behavior, fracture location, and any anomalies such as grip slippage or premature fracture outside the gauge section.

Applications

Extensometer-based tensile strain measurement supports a wide range of industrial and scientific applications:

• Material data sheet generation — producing certified values of tensile modulus, yield strain, and strain at break used in product literature and material selection databases
• Finite element analysis (FEA) input — supplying true stress-strain and modulus data for structural simulation of plastic components in automotive, aerospace, electronics, and consumer goods
• Quality control and incoming inspection — verifying that incoming resin lots, compounds, or molded parts conform to purchase specifications and material standards
• Material development and formulation studies — comparing the effects of fillers, plasticizers, nucleating agents, impact modifiers, and recycling content on stress-strain behavior
• Regulatory compliance and certification — supporting conformity assessment for products governed by UL, IEC, EN, GB, and JIS standards where mechanical property verification is required
• Long-term performance studies — establishing baseline tensile properties before and after heat aging, weathering, chemical exposure, or UV conditioning to quantify degradation
• Process validation — evaluating the influence of injection molding parameters, weld lines, orientation, and annealing on mechanical anisotropy
• Academic and contract research — providing high-precision strain data for publications, standards development, and round-robin interlaboratory comparisons at accredited third-party laboratories

FAQ

Why is an extensometer required for modulus measurement instead of crosshead displacement?

Crosshead displacement includes machine frame deflection, load cell deformation, grip movement, and specimen deformation outside the gauge section. At the small strain values used for modulus determination (0.05% to 0.25%), these systematic errors can exceed the material strain itself, leading to significant underestimation of modulus. An extensometer measures strain directly on the specimen gauge section, eliminating machine compliance effects and satisfying the accuracy requirements of ISO 527-1.

What extensometer accuracy class is needed for plastics testing?

ISO 527-1 requires a minimum of class 1 per ISO 9513 (relative error within ±1%) for modulus determination. For high-modulus materials, fiber-reinforced plastics, and applications demanding enhanced precision, class 0.5 (±0.5%) or class 0.2 is recommended. ASTM D638 similarly requires the extensometer to meet class B-1 or better per ASTM E83 for modulus measurement.

Contact or non-contact extensometer — which should be used?

Contact (clip-on) extensometers are economical and precise for rigid plastics with moderate elongation. Non-contact video or laser extensometers are preferred for films, flexible plastics, elastomers, high-elongation materials, tests in environmental chambers, and brittle specimens whose gauge sections cannot bear the weight or knife-edge pressure of a contact device. Both types can achieve class 1 accuracy when properly calibrated.

What gauge length is used, and does it matter?

The standard gauge length for ISO 527-1A/1B and ASTM D638 Type I specimens is 50 mm, located within the parallel section of the dumbbell. Using the specified gauge length ensures strain is measured in a region of uniform stress. A shorter gauge length increases strain resolution for brittle materials but reduces absolute displacement range; the selected length must be documented in the test report.

How is strain at break handled when the extensometer range is exceeded?

For ductile plastics with elongation beyond the extensometer travel, the common practice is to record strain with the extensometer up to its limit, remove it before damage occurs, and complete the test using crosshead displacement. The reported strain at break is then calculated by combining the extensometer-measured strain with the additional nominal strain, and this methodology is noted in the report. Alternatively, a long-travel or non-contact extensometer can measure strain continuously to rupture.

What test conditions and how many specimens are required?

The standard atmosphere is 23°C ± 2°C and 50% ± 10% relative humidity per ISO 291, with specimens conditioned for at least 88 hours unless the material standard dictates otherwise. A minimum of five valid specimens is required by ISO 527-1, and values are reported as means with standard deviations; specimens breaking outside the gauge length during strain-at-break determination are retested or flagged according to the applicable standard.

Can this method be applied to anisotropic and reinforced plastics?

Yes. For fiber-reinforced plastics tested per ISO 527-4 and ISO 527-5, extensometry provides accurate modulus and strain-at-break data for both longitudinal and transverse directions relative to fiber orientation. Biaxial extensometry additionally yields Poisson's ratio, which is essential input for orthotropic material models used in composite structural analysis.