Plastic Tensile Testing Procedure: Step-by-Step Guide to ISO 527 and ASTM D638

Plastic tensile testing is one of the most widely performed mechanical tests for evaluating the strength, ductility, and elastic behavior of polymeric materials. This article provides a complete, step-by-step overview of the plastic tensile testing procedure in accordance with internationally recognized standards such as ISO 527, ASTM D638, and GB/T 1040, covering specimen preparation, equipment setup, test execution, and data calculation. Whether you work in quality control, product development, or materials research, this guide will help you understand how reliable tensile property data is generated in accredited third-party laboratories.

Technical Overview

Tensile testing of plastics measures how a material responds when it is pulled along its axis until it yields, deforms, or fractures. A standardized specimen is clamped between two grips on a universal testing machine (UTM), and a steadily increasing tensile force is applied at a controlled speed while the applied load and extension are continuously recorded. From the resulting stress-strain curve, key mechanical properties such as tensile strength, yield stress, elongation at break, and tensile modulus are calculated.

The most commonly applied standards include ISO 527 (Parts 1, 2, 3, 4, and 5), ASTM D638 for rigid and semi-rigid plastics, ASTM D882 for thin plastic films, and GB/T 1040, which is the Chinese national standard technically aligned with ISO 527. JIS K 7161 is the corresponding Japanese standard. Because plastics are strain-rate sensitive and sensitive to temperature and humidity, these standards strictly define specimen geometry, conditioning environment, testing speed, and calculation methods. Following the prescribed procedure is essential for obtaining reproducible and comparable results.

Testing Samples

Rigid and Semi-Rigid Plastics (Dumbbell Specimens)

For molded or machined rigid plastics, the standard specimen is a dog-bone (dumbbell) shape. ISO 527-2 specifies Type 1A specimens, typically injection molded with an overall length of 170 mm and a gauge width of 10 mm, while ASTM D638 defines Type I specimens with a similar geometry. The reduced central section ensures that stress is concentrated within the gauge length so that failure occurs at a predictable location.

Films and Thin Sheets

Flexible films below approximately 1 mm in thickness are tested as rectangular strips, typically 10 to 25 mm wide and at least 150 mm long, in accordance with ISO 527-3 or ASTM D882. Specimens are usually cut cleanly with a sharp die or blade to avoid nicks and stress concentrators along the edges that could cause premature tearing.

Fiber-Reinforced and Anisotropic Plastics

Glass-fiber or carbon-fiber reinforced composites are tested using ISO 527-4 and ISO 527-5 specimen geometries, often with bonded end tabs to prevent grip damage. For materials with directional properties, specimens should be cut in both the machine direction and the transverse direction to characterize anisotropy.

Specimen Conditioning

Prior to testing, specimens must be conditioned in a standard laboratory atmosphere. The typical requirement per ISO 291 or ASTM D618 is 23 ± 2 °C and 50 ± 10% relative humidity for a minimum of 40 hours, unless otherwise agreed. Conditioning stabilizes the moisture content and temperature of the polymer, both of which significantly influence tensile results, especially for hygroscopic materials such as polyamide (nylon).

Testing Items

A single tensile test on plastics can generate the following characteristic values, all of which are defined in ISO 527-1 and ASTM D638:

• Tensile strength (maximum tensile stress sustained before break or yield)
• Tensile stress at yield (stress at the first point where strain increases without a stress increase)
• Tensile stress at break (stress at the moment of specimen rupture)
• Tensile strain at yield
• Tensile strain at break and nominal strain at break (elongation)
• Tensile modulus (Young's modulus), calculated as the slope of the stress-strain curve in the initial linear region
• Poisson's ratio (optional, per ISO 527-1 Annex, using transverse and axial strain measurement)

Testing Methods

The complete tensile test procedure can be divided into preparation, setup, execution, and evaluation stages. The following steps reflect the general requirements of ISO 527 and ASTM D638.

Step 1: Verify Specimen Preparation and Conditioning

Confirm that the correct specimen type has been prepared and that all specimens have been conditioned at 23 ± 2 °C and 50 ± 10% relative humidity for the required duration. Inspect each specimen visually for flash, voids, notches, or machining marks that could invalidate the test.

Step 2: Measure Specimen Dimensions

Measure the width and thickness of each specimen within the gauge length at a minimum of three positions using a calibrated micrometer or optical measuring device, and record the minimum cross-sectional area. Dimensional accuracy directly affects the calculated stress values, so the measurement resolution should typically be 0.01 mm or better.

Step 3: Mark the Gauge Length and Select the Extensometer

Apply gauge marks on the specimen to define the original gauge length, typically 50 mm for standard dumbbell specimens. For modulus determination, attach a contact extensometer or use a non-contact video extensometer directly to the gauge section, because crosshead displacement alone is not accurate enough for modulus measurement.

Step 4: Configure the Universal Testing Machine

Select a load cell appropriate for the expected maximum force so that the measured loads fall within the calibrated range, ideally between 20% and 80% of capacity. Verify that the machine has a valid calibration certificate, choose suitable grips (wedge, pneumatic, or capstan grips depending on the material), and check grip alignment to minimize bending stresses.

Step 5: Select the Test Speed

Set the crosshead speed according to the material and the standard:

• ISO 527-2: 1 mm/min for tensile modulus determination; 50 mm/min for standard Type 1A/1B strength testing
• ASTM D638: 50 mm/min for Type I specimens; 5 mm/min or lower for rigid materials and modulus measurement; 500 mm/min for thin, extensible materials
• ISO 527-3 and ASTM D882 for films: typically 100 to 500 mm/min depending on film type

Step 6: Clamp the Specimen

Insert the specimen symmetrically into the upper and lower grips so that its long axis coincides with the axis of the applied force. Apply sufficient clamping pressure to prevent slippage without crushing the specimen, and apply a small preload (as permitted by the standard) to remove slack.

Step 7: Run the Test

Start the test and let the machine pull the specimen at the constant selected speed. The software records force and extension continuously until the specimen breaks. For modulus measurement, the extensometer data between 0.05% and 0.25% strain (per ISO 527) is used; the extensometer may be removed after this interval for materials with very large elongation.

Step 8: Inspect the Fracture and Validate the Result

After break, examine the fracture location. A break occurring outside the gauge length or a break at a grip or flaw generally requires the result to be discarded and the test repeated. Record the failure mode for the report.

Step 9: Calculate the Results

Calculate engineering stress by dividing force by the original minimum cross-sectional area, and strain by dividing extension by the original gauge length. Compute the tensile modulus from the secant between the prescribed strain limits, and derive strength and elongation values from the recorded curve.

Step 10: Repeat and Report

Test a minimum of five valid specimens per condition, as required by ISO 527 and ASTM D638, and report the arithmetic mean, standard deviation, and individual values. The final report should include material identification, specimen type and preparation method, conditioning details, test speed, gauge length, machine and extensometer details, and all calculated properties.

Applications

Tensile testing data for plastics supports a wide range of industrial and regulatory activities:

• Quality control of incoming resins and outgoing molded parts in injection molding and extrusion production
• Material qualification and comparison during new product development and material substitution studies
• Verification of mechanical performance for automotive interior and exterior components, packaging films, pipes and fittings, medical devices, cables, and consumer electronics housings
• Compliance demonstration against product standards and buyer specifications in accredited third-party laboratories
• Aging and durability studies, including evaluation of degradation after heat, UV, or chemical exposure by comparing tensile properties before and after treatment
• Input data for finite element analysis (FEA) and structural simulation of plastic parts

FAQ

What is the difference between ISO 527 and ASTM D638? Both standards measure the same fundamental properties, but they differ in specimen geometry, gauge length definitions, testing speed conventions, and some calculation details. Results from the two standards are similar but not always directly interchangeable, so the required standard should be confirmed with the customer or end specification before testing.

How many specimens are required for a valid test? Both ISO 527 and ASTM D638 require a minimum of five specimens per sample. Additional specimens may be needed if breaks occur outside the gauge length or if high scatter is observed.

Why is test speed so important? Plastics are viscoelastic and strain-rate sensitive: a higher test speed generally produces a higher apparent tensile strength and lower elongation. Fixed speeds defined by the standards ensure that results from different laboratories are comparable.

Is an extensometer always necessary? For tensile modulus determination, yes, because grip and machine compliance errors make crosshead displacement unsuitable for accurate strain measurement in the initial linear region. For simple strength and elongation results on ductile films, crosshead extension or gauge mark separation may be acceptable within the limits of the standard.

What causes specimens to break at the grips? Common causes include excessive clamping pressure, grip serrations biting into soft materials, poor alignment, edge defects from specimen preparation, or stress concentrators. Such breaks are normally treated as invalid and the test is repeated with adjusted grip pressure or tabbed specimens.