Plastic tensile extensometer measurement is a precision testing technique used to determine the strain-dependent mechanical properties of plastic materials, including tensile modulus, yield strain, and strain at break, in accordance with ISO 527 and ASTM D638 standards. By measuring deformation directly on the specimen gauge length rather than relying on crosshead displacement, extensometers deliver significantly higher accuracy for modulus determination and material characterization. This article explains the principles, sample requirements, test methods, and practical applications of extensometer-based tensile testing for plastics.
Technical Overview
An extensometer is a strain-measuring instrument attached directly to a tensile specimen to record elongation within a defined gauge length with high resolution. Unlike crosshead displacement, which includes machine compliance, grip slippage, and deformation outside the gauge section, extensometer-based measurement captures the true strain of the material itself. This distinction is critical for plastics, where accurate determination of the tensile modulus requires strain resolution typically in the range of 0.01% or better.
In plastic tensile testing, extensometers are primarily used to measure properties that are highly sensitive to strain accuracy, such as the tensile modulus (Young's modulus), yield strain, and Poisson's ratio. Modern extensometer systems fall into two main categories: contact extensometers, which use knife edges or clip-on arms attached to the specimen, and non-contact systems, such as video extensometers and laser extensometers, which track gauge marks optically. Both types must be calibrated and classified according to ISO 9513 or ASTM E83 to ensure traceable measurement accuracy.
For modulus determination, ISO 527-1 and ASTM D638 require an extensometer of at least Class 1 accuracy (Class 0.5 is recommended for stiffer materials), because even small strain errors can produce large deviations in the calculated modulus. The use of a properly classified extensometer is therefore a mandatory element of any accredited tensile testing program for plastics.
Testing Samples
Rigid and Semi-Rigid Thermoplastics (Injection-Moulded Specimens)
Standard multipurpose dumbbell specimens according to ISO 527-2, Type 1A (gauge length 75 mm, preferred for injection moulding) or Type 1B (gauge length 50 mm, preferred for machined sheets), are the most common samples. These specimens provide a parallel gauge section suitable for attaching clip-on or video extensometer gauge marks.
Flat Specimens per ASTM D638
ASTM D638 specifies rectangular dog-bone specimens such as Type I (wide section for rigid plastics with thickness of 7 mm or less), Type II, and Type V (small specimens for limited material quantities). The nominal gauge length is 50 mm for Types I and II and 7.6 mm for Type V, and the extensometer class and gauge length must be selected accordingly.
Films and Sheets
Thin plastic films tested under ISO 527-3 are typically measured with non-contact video extensometers or long-travel contact extensometers, because the low specimen stiffness can be disturbed by the weight of a conventional clip-on device. Gauge marks are applied with ink dots, adhesive markers, or pattern speckles.
Machined Specimens from Finished Products
Specimens cut from pipes, profiles, molded parts, or components may be tested with small-gauge-length extensometers, provided the gauge section remains uniform and free of stress concentrators or surface defects introduced during preparation.
Testing Items
Extensometer-based tensile measurement enables accurate determination of the following strain-dependent properties:
- Tensile modulus (Young's modulus, Et) — determined as the slope of the stress-strain curve between two specified strain limits, typically 0.0005 and 0.0025 (0.05% to 0.25%) according to ISO 527-1, or as the tangent modulus per ASTM D638.
- Yield strain — the strain at which the stress-strain curve reaches its first local maximum (yield point).
- Strain at break — the strain at specimen rupture, measured within the gauge length; requires either a long-travel extensometer or removal of the device before break to prevent damage.
- Strain at nominal strain limits — strain values used for comparative purposes such as nominal strain at yield.
- Poisson's ratio — determined using a biaxial (longitudinal plus transverse) extensometer configuration that measures lateral contraction simultaneously with axial extension.
- Complete stress-strain curve — used to characterize material behavior from the initial linear region through yield, strain softening or hardening, and failure.
Testing Methods
Applicable Standards
- ISO 527-1 — Plastics: Determination of tensile properties, Part 1: General principles.
- ISO 527-2 — Test conditions for moulding and extrusion plastics.
- ISO 527-3 — Test conditions for films and sheets.
- ASTM D638 — Standard Test Method for Tensile Properties of Plastics.
- ISO 9513 / ASTM E83 — Calibration and classification of extensometer systems (accuracy classes 0.2, 0.5, 1, and 2).
Typical Test Procedure
The general workflow for extensometer-based tensile testing of plastics is as follows:
- Condition the specimens as specified (typically 23 °C and 50% relative humidity for at least 24 hours) and measure the width and thickness of the gauge section at multiple points to calculate the cross-sectional area.
- Select an extensometer of appropriate class, travel range, and gauge length; verify that its calibration certificate is valid and traceable.
- Mount the specimen in the grips with proper alignment to minimize bending stresses, which are a major source of modulus error.
- Attach the contact extensometer knife edges precisely at the gauge marks, or focus the video extensometer on the gauge marks, ensuring the measured gauge length equals the specified value.
- Apply a small preload and zero both the force and strain channels.
- Run the test at the specified speed. For modulus determination per ISO 527, a low crosshead speed (typically 1 mm/min) is used within the modulus strain range; for yield and break properties, the speed is selected from the standard based on specimen type and material ductility.
- For brittle materials, run the extensometer through failure; for ductile materials, remove the contact extensometer (or pause video tracking region limits) once the strain exceeds the range needed, to protect the instrument.
- Calculate the tensile modulus using chord slope regression between the specified strain limits, and record yield strain, break strain, and the full stress-strain curve.
Equipment and Accuracy Requirements
- Universal testing machine with force accuracy of at least Class 0.5 or Class 1 per ISO 7500-1.
- Extensometer of Class 1 or better for modulus; long-travel or non-contact types for strains exceeding 50%.
- Climatic chamber capability when properties must be evaluated at elevated or sub-ambient temperatures.
- Data acquisition rate high enough to resolve the initial linear region, typically at least 10 Hz for standard-speed tests.
Key Sources of Error and Their Control
- Specimen bending due to misalignment — controlled through matched grips, alignment verification, and averaging strain from two opposing sides when feasible.
- Slippage or knife-edge cutting into soft materials — mitigated by using non-contact video extensometers or reduced spring-force attachments.
- Incorrect gauge length setup — verified before each test series by measuring the actual knife-edge separation or marker spacing.
- Machine compliance contamination of strain data — eliminated because strain is measured on the specimen rather than derived from crosshead travel.
Applications
Extensometer-based tensile measurement of plastics serves a wide range of engineering and regulatory purposes:
- Material data sheets and design allowables — accurate modulus and strain values used in finite element analysis and structural design of plastic components.
- Research and development — comparison of polymer formulations, filler and fiber loading effects, and processing conditions on stiffness and ductility.
- Quality control — incoming resin verification, batch-to-batch consistency monitoring, and release testing in automotive, electronics, packaging, and medical device manufacturing.
- Material substitution studies — validating bio-based, recycled, or high-strength engineering plastics against incumbent materials.
- Temperature and environmental characterization — measuring modulus retention under thermal aging, humidity exposure, or chemical conditioning.
- Failure analysis — establishing whether material strain capacity met design expectations in failed components.
- Certification support — generating test reports from accredited third-party laboratories for regulatory submissions and customer qualification.
FAQ
Why is an extensometer required for modulus testing instead of crosshead displacement?
Crosshead displacement includes machine deflection, grip movement, and strain outside the gauge section, which typically overstates specimen strain by a large margin in the low-strain region. Because the modulus is calculated from strain values as small as 0.05% to 0.25%, these errors can shift the modulus result by 10% to 50%. An extensometer measures strain directly on the gauge length, providing the accuracy required by ISO 527 and ASTM D638.
Which extensometer accuracy class is needed for plastic tensile testing?
For tensile modulus determination, ISO 527-1 requires at least a Class 1 extensometer per ISO 9513, and Class 0.5 is recommended when high precision is needed. ASTM D638 similarly requires a Class B-1 or better extensometer per ASTM E83 for modulus measurement. For yield strain and strain at break, Class 1 or Class 2 is generally sufficient.
Can an extensometer measure strain up to specimen failure in ductile plastics?
Standard clip-on extensometers have limited travel and must be removed before break in ductile materials such as polyethylene. Long-travel extensometers or non-contact video extensometers can follow strain beyond 100% and remain in place through rupture, allowing direct measurement of strain at break within the gauge length.
How is Poisson's ratio of a plastic measured?
Poisson's ratio is determined by simultaneously measuring axial strain with a longitudinal extensometer and transverse strain with a width-sensing (transverse) extensometer. The ratio of transverse contraction strain to axial extension strain in the elastic region yields Poisson's ratio, typically falling between 0.3 and 0.45 for most plastics.
What is the difference between strain measured by an extensometer and nominal strain?
Extensometer strain is the change in gauge length divided by the original gauge length, measured locally on the specimen. Nominal strain is calculated from grip separation or crosshead travel and includes deformation outside the gauge section. The two values diverge significantly after yield, which is why ISO 527 distinguishes between strain at break and nominal strain at break.
How does temperature affect extensometer-based tensile results?
Plastics are viscoelastic and highly temperature sensitive; modulus can drop substantially with only modest temperature increases. When testing at elevated or low temperatures, the extensometer must be rated for the chamber environment, and results should always be reported together with the test temperature and conditioning state.