Determination of Elongation at Break of Plastics: Standards, Methods, and Technical Guide

Elongation at break is one of the most important mechanical properties for evaluating the ductility and deformation capacity of plastic materials before fracture. This technical article explains the determination of tensile elongation at break of plastics in accordance with ISO 527, ASTM D638, and GB/T 1040, covering specimen preparation, testing equipment, procedures, calculation methods, and practical applications.

Technical Overview

Elongation at break, also known as tensile elongation at rupture, is defined as the increase in gauge length of a test specimen measured at the moment of fracture, expressed as a percentage of the original gauge length. It quantifies the ability of a plastic material to undergo deformation under tensile stress before failing, making it a key indicator of ductility, toughness, and process-induced molecular orientation.

The determination of elongation at break is typically performed using a universal testing machine (tensile tester) equipped with suitable grips and an extensometer or crosshead displacement measurement system. During the test, a dumbbell-shaped or strip specimen is pulled at a constant speed until rupture, and the change in distance between the gauge marks is recorded continuously as a function of tensile force.

This test is widely applied in material qualification, incoming material inspection, quality control of finished products, and research and development of polymer formulations. Commonly referenced standards include ISO 527-1 and ISO 527-2, ASTM D638, GB/T 1040.1 and GB/T 1040.2, as well as ISO 527-3 and ASTM D882 for plastic films and thin sheets.

Testing Samples

Rigid and Semi-Rigid Thermoplastics

Materials such as polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), and polyamide (PA) are commonly tested using injection-molded or machined dumbbell specimens. These materials typically exhibit moderate to high elongation at break, depending on molecular weight, crystallinity, and the presence of impact modifiers.

Engineering Plastics and Thermosets

High-performance polymers such as polyetheretherketone (PEEK), polyoxymethylene (POM), polybutylene terephthalate (PBT), and glass-fiber or carbon-fiber reinforced compounds usually show lower elongation at break values due to their rigid molecular structure or reinforcement content. Thermosetting materials, such as epoxy and phenolic molding compounds, are also evaluated using this method.

Plastic Films and Sheets

Flexible packaging films, including polyethylene, polypropylene, polyester (PET), and multilayer laminated films, are tested in strip form according to ISO 527-3 or ASTM D882. These materials often exhibit very high elongation at break values, sometimes exceeding several hundred percent, requiring instruments with large travel capacity.

Elastomeric and Flexible Materials

Thermoplastic elastomers (TPE), thermoplastic polyurethanes (TPU), and flexible PVC compounds demonstrate extremely high elongation at break, frequently above 300 to 1000 percent, which reflects their rubber-like deformability.

Testing Items

A tensile test for the determination of elongation at break is typically conducted together with a full tensile property profile, which may include the following parameters:

• Elongation at break: the percentage increase in gauge length at the moment of rupture
• Tensile strength: the maximum tensile stress sustained by the specimen
• Tensile stress at break: the stress recorded at the point of rupture
• Tensile stress at yield: the stress at which the stress-strain curve shows a first maximum
• Elongation at yield: the strain corresponding to the yield point
• Tensile modulus (Young's modulus): the slope of the initial linear portion of the stress-strain curve
• Secant modulus, where specified by product standards

Testing Methods

Standards and Test Conditions

The determination is primarily carried out in accordance with ISO 527-1 and ISO 527-2, ASTM D638, or GB/T 1040. Unless otherwise specified, specimens are conditioned at a standard atmosphere of 23°C ± 2°C and 50% ± 10% relative humidity for at least 24 hours prior to testing, and the test is performed under the same conditions. The applicable test speed depends on the specimen type and material, typically ranging from 1 mm/min for modulus determination to 50 mm/min, 100 mm/min, 200 mm/min, or 500 mm/min for ductile materials and films.

Specimen Preparation

Specimens may be prepared by injection molding, compression molding, machining, die-cutting from sheets, or punching from films, depending on the material form. ISO 527-2 specifies the commonly used Type 1A and 1B dumbbell specimens with a preferred gauge length of 50 mm, while ASTM D638 defines Type I through Type V specimens, with Type I being the most widely used for rigid plastics with a thickness of 7 mm or less. Specimen dimensions, surface quality, and freedom from notches, voids, and stress concentrations are critical, as defects can cause premature rupture and artificially low elongation values.

Equipment and Procedure

The test is performed on a calibrated universal testing machine with a force capacity appropriate for the expected load range. The typical procedure is as follows:

• Measure and record the specimen dimensions, including thickness and width within the narrow section, and mark the initial gauge length
• Mount the specimen in the grips, ensuring axial alignment to avoid bending stresses
• Attach a contact extensometer or configure a non-contact video extensometer for accurate strain measurement, particularly for materials where grip separation cannot represent true strain
• Set the crosshead speed according to the relevant standard and material specification
• Start the test and continuously record the force-displacement or force-elongation curve until the specimen ruptures
• Note the elongation at the moment of rupture, defined as the point where the force drops significantly or the specimen separates completely

Calculation

Elongation at break is calculated using the following formula: ε_b expressed as a percentage equals (L − L₀) / L₀ × 100, where L₀ is the initial gauge length and L is the gauge length at break. The results of at least five valid specimens are usually reported as the arithmetic mean, along with the standard deviation. Ruptures occurring outside the gauge length or at the grips are typically discarded and retested, as they do not represent the true material behavior.

Factors Affecting Results

• Test speed: higher strain rates generally reduce elongation at break for ductile materials
• Temperature and humidity: elevated temperature increases ductility, while low temperature embrittles many plastics
• Specimen preparation: injection molding parameters, melt orientation, and machined surface quality significantly influence results
• Specimen alignment: misalignment introduces bending stresses and causes premature failure
• Material conditioning: moisture-sensitive polymers such as polyamide require drying or humidity equilibration before testing

Applications

Determination of elongation at break serves a broad range of industries and purposes:

• Material qualification and comparison of resin grades, compounds, and recycled materials
• Incoming inspection of plastic raw materials and semi-finished products such as sheets, pipes, and profiles
• Quality control in injection molding, extrusion, blow molding, blown film, and thermoforming production lines
• Verification of the effects of additives, plasticizers, fillers, reinforcements, and impact modifiers
• Assessment of aging, weathering, UV exposure, thermal degradation, and chemical resistance through before-and-after comparison
• Compliance verification for automotive components, electrical and electronic enclosures, packaging materials, medical devices, wire and cable insulation, and consumer goods
• Support for research and development of new polymer formulations and processing parameter optimization

Accredited third-party laboratories routinely perform elongation at break testing to support regulatory compliance, supplier approvals, failure analysis, and certification programs, ensuring that reported data are traceable to internationally recognized standards.

FAQ

What is elongation at break and why is it important?

Elongation at break is the percentage increase in gauge length of a plastic specimen at the moment it ruptures during a tensile test. It is a direct measure of ductility: materials with high elongation at break can deform substantially before failure, which is essential for applications involving bending, stretching, or impact loading, while low values indicate brittleness.

Which standards govern the determination of elongation at break for plastics?

The principal standards are ISO 527-1 and ISO 527-2 for molding and extrusion plastics, ISO 527-3 for films and sheets, ASTM D638 for rigid plastics, ASTM D882 for thin plastic sheeting and films, and GB/T 1040.1 and GB/T 1040.2 as the corresponding Chinese national standards. The choice depends on the material form, specimen geometry, and market or regulatory requirements.

How many specimens are required for a valid test?

ISO 527 and ASTM D638 generally require at least five valid specimens for isotropic materials and ten for anisotropic materials such as oriented films or fiber-reinforced compounds. Specimens that fail outside the gauge section or at the grips are normally rejected and replaced.

What is the difference between elongation at break and elongation at yield?

Elongation at yield is the strain at which the material begins to deform plastically, marked by a maximum or plateau in the stress-strain curve, while elongation at break is the strain at complete rupture. For ductile plastics, elongation at break is considerably higher than elongation at yield; for brittle materials, the two values may be close or fracture may occur before yield.

Why do elongation at break results vary between laboratories?

Variations commonly arise from differences in specimen preparation, conditioning history, test speed, grip alignment, strain measurement technique, and environmental conditions. Strict adherence to the specified standard, calibrated equipment, and consistent specimen production are essential for reproducible results.

Can elongation at break be tested on finished products?

Yes. Specimens can be die-cut or machined from finished parts, films, or components, provided the sampling location and cutting direction are documented. Testing on actual products reflects the influence of processing, orientation, and service aging, which is particularly valuable for failure analysis and product durability assessment.