Plastic Tensile Testing with Universal Testing Machines: Standards, Methods, and Applications

Plastic tensile testing with a universal testing machine (UTM) is the most widely used method for determining the tensile strength, elastic modulus, and elongation of plastic materials in accordance with ASTM D638 and ISO 527. This technical guide explains the equipment, specimen types, measured properties, step-by-step test procedures, and typical applications involved in tensile testing of plastics. Whether you are setting up a quality control laboratory or validating material performance, understanding these fundamentals will help you achieve accurate, repeatable, and standards-compliant results.

Technical Overview

A universal testing machine, also known as a UTM or tensile tester, is an electromechanical load frame that applies a controlled tensile load to a specimen while precisely measuring force, displacement, and strain. In plastic tensile testing, a dumbbell-shaped or rectangular specimen is clamped between two grips and pulled at a constant crosshead speed until it yields, necks, and ultimately fractures. The load cell records force throughout the test, while an extensometer or crosshead displacement provides strain data, generating a stress-strain curve from which all key tensile properties are calculated.

A typical universal testing machine configured for plastic tensile testing consists of the following components:

• Load frame: single-column design (typically up to 5 to 10 kN) for flexible plastics and films, or dual-column design (10 kN and above) for rigid and reinforced plastics
• Load cell: force transducer with accuracy compliant with ISO 7500-1 or ASTM E4, typically within ±0.5% or ±1% of the indicated force
• Grips: mechanical wedge grips with serrated faces for rigid plastics, or pneumatic grips with smooth or rubber-faced inserts for films and soft materials
• Extensometer: contact or non-contact (video) extensometer meeting ISO 9513 or ASTM E83 requirements, essential for accurate modulus measurement
• Testing software: automates speed control, data acquisition, standards-based calculations, and report generation

Plastic tensile testing is governed primarily by ASTM D638 and ISO 527-1/-2 for rigid and semi-rigid plastics, ASTM D882 and ISO 527-3 for thin films and sheeting, and related methods for specialized specimen sizes. Common materials tested include PE, PP, ABS, PC, PVC, PET, PA (nylon), POM, PEEK, TPU, and glass- or mineral-filled compounds.

Testing Samples

ASTM D638 Specimens for Rigid and Semi-Rigid Plastics

ASTM D638 defines five specimen geometries (Type I through Type V). Type I is the standard specimen for materials up to 7 mm thickness, with an overall length of 165 mm, an overall width of 19 mm, a narrow-section width of 13 mm, a preferred thickness of 3.2 mm, a gauge length of 50 mm, and a grip separation of 115 mm. Type IV is often used for non-rigid plastics or when specimen material is limited, while Type V is used for small-scale evaluation. Specimens may be injection molded, machined from sheet or finished parts, or die cut where permitted by the material specification.

ISO 527 Specimens

ISO 527-2 specifies the 1A specimen for injection-molded plastics, with a 10 mm narrow-section width, 4 mm thickness, 50 mm gauge length, and 115 mm initial grip separation, and the 1B specimen for machined material. Although ASTM and ISO specimens appear similar, their geometries differ enough that results are not directly interchangeable, so the correct standard must be selected based on the specification or customer requirement.

Thin Films and Sheeting

Flexible films and thin sheeting are tested as rectangular strips under ASTM D882 or ISO 527-3 rather than as dumbbells. Strip widths are typically 15 to 25 mm, and grip separations of 50 to 250 mm are selected based on the expected elongation of the material. Because films fail at low forces, a low-capacity load cell and gentle pneumatic grips are used to prevent slippage and premature breaks at the grip faces.

Specimen Preparation and Conditioning

All specimens must be free of voids, notches, sink marks, and machining marks in the narrow section, since any surface defect concentrates stress and reduces measured strength. Unless otherwise specified, specimens are conditioned before testing per ASTM D618 or ISO 291 at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 40 hours to ensure results are comparable between laboratories.

Testing Items

The universal testing machine generates a complete stress-strain curve, from which the following properties are determined:

• Tensile strength (ultimate): the maximum force sustained by the specimen divided by its original cross-sectional area
• Tensile strength at yield: the stress at the yield point, where the material continues to elongate without an increase in stress
• Tensile stress at break: the stress recorded at the moment of rupture
• Tensile modulus (elastic modulus): the slope of the initial linear region of the curve, calculated in ASTM D638 and ISO 527 as the chord modulus between strains of 0.05% and 0.25%
• Secant modulus: the slope of the line drawn from zero strain to a specified strain, such as the 1% secant modulus
• Elongation at yield: the strain corresponding to the yield point
• Elongation at break: the strain at rupture, an important indicator of ductility
• Nominal strain and energy to break: optional values derived from crosshead separation and the area under the curve

Testing Methods

A standard tensile test on plastics follows this general procedure in accordance with ASTM D638 or ISO 527:

• Measure the width and thickness of the narrow section at several points along each specimen and calculate the minimum or average cross-sectional area
• Set the grip separation to 115 mm for ASTM Type I and ISO 1A specimens, ensuring the full shoulder length is clamped without contacting the gauge section
• Select the crosshead speed: 5 mm/min for modulus determination under ASTM D638 and 1 mm/min under ISO 527, with 50 mm/min commonly used for strength determination unless the material specification requires otherwise
• Attach the extensometer to the gauge section and verify proper zeroing of both load and strain channels
• Start the test and run until the specimen fractures completely, recording the full stress-strain response
• Calculate the required properties using the software template for the applicable standard
• Repeat the test on a minimum of five specimens and report the arithmetic mean and, where required, the standard deviation

Specimens that fracture inside the grips, outside the narrow section, or at an obvious flaw should be discarded and replaced, as such breaks do not represent the true strength of the material. Testing should be performed in a controlled laboratory atmosphere of 23 ± 2°C, and the load cell should be selected so that the anticipated breaking force falls within its calibrated working range, ideally between 10% and 90% of capacity.

Applications

Plastic tensile testing on universal testing machines supports a wide range of industrial and research activities:

• Material development: comparing resin grades, additives, fillers, and reinforcement levels during formulation and R&D
• Quality control: incoming resin inspection, production lot release testing, and verification of molding process stability
• Automotive: validating interior trim, bumper components, and under-hood plastic parts against OEM specifications
• Packaging: evaluating the machine-direction and transverse-direction strength, ductility, and orientation of films
• Medical devices: characterizing catheter tubing, instrument housings, and sterilized polymer components
• Electronics and appliances: qualifying connector housings, enclosures, and insulating materials
• Construction: assessing plastic pipes, sheets, and profiles for compliance with product standards
• Certification: generating data for datasheets, regulatory submissions, and third-party laboratory comparisons

FAQ

What is the difference between ASTM D638 and ISO 527?

Both standards measure the same fundamental properties, but they use different specimen geometries, default test speeds, and certain calculation details. For example, the ASTM Type I specimen has a 13 mm narrow section and 3.2 mm preferred thickness, while the ISO 1A specimen has a 10 mm narrow section and 4 mm thickness. Modulus is measured at 5 mm/min in ASTM D638 versus 1 mm/min in ISO 527. Because of these differences, results from the two standards should not be compared directly without a documented correlation.

What machine capacity is needed for plastic tensile testing?

Most unfilled plastics tested with standard dumbbell specimens break at forces well below 5 kN, so a 5 kN single-column or 10 kN dual-column universal testing machine covers the majority of applications. Glass-filled and highly reinforced compounds may require higher capacities. In practice, laboratories often equip one frame with interchangeable load cells so that both very ductile films and stiff engineering plastics can be tested accurately.

How many specimens should be tested?

Both ASTM D638 and ISO 527 require a minimum of five specimens per material sample. Testing a larger number improves statistical confidence, and results from specimens that break at flaws or in the grips should be replaced with additional valid tests.

Is an extensometer required?

An extensometer is strongly recommended whenever modulus or precise strain values are required. Strain calculated from crosshead displacement includes machine compliance, grip seating, and specimen slip, which can significantly distort modulus results. Contact extensometers meeting ISO 9513 Class 1 or ASTM E83 Class B-1, or non-contact video extensometers, provide the accuracy needed for chord modulus measurements between 0.05% and 0.25% strain.

Can the same machine test plastic films and thin sheeting?

Yes. By changing to a low-capacity load cell and pneumatic or rubber-faced grips, the same universal testing machine can perform film tensile tests under ASTM D882 or ISO 527-3, using rectangular strip specimens instead of dumbbells and lower testing speeds appropriate to flexible materials.

Why do results vary between laboratories?

Common sources of variation include differences in specimen preparation and molding conditions, insufficient conditioning, incorrect crosshead speed, strain measured from crosshead travel instead of an extensometer, and grip-induced premature failure. Controlling these factors in line with the applicable standard is essential for reproducible, comparable results.