Tensile strength at break is one of the most fundamental mechanical properties used to evaluate the performance, durability, and quality of plastic materials. This article provides a comprehensive technical guide to the determination of tensile strength at break for plastics, covering relevant international standards such as ISO 527 and ASTM D638, specimen preparation, test procedures, calculation methods, and typical industrial applications.
Technical Overview
Tensile strength at break, also known as ultimate tensile strength at rupture, is the maximum tensile stress a plastic specimen can sustain before fracture occurs during a uniaxial tensile test. It is expressed in megapascals (MPa) and is calculated by dividing the maximum force recorded at rupture by the original cross-sectional area of the narrow parallel section of the specimen.
For ductile plastics such as polyethylene (PE) and polypropylene (PP), the stress-strain curve typically exhibits a yield point followed by cold drawing before break, meaning the tensile stress at break may be lower than the tensile stress at yield. For brittle materials such as polystyrene (PS) or thermosetting resins, fracture often occurs before yielding, and the stress at break corresponds to the maximum stress of the curve. Understanding this distinction is essential for correct material selection and product design.
The determination of tensile strength at break is primarily governed by ISO 527 (Parts 1, 2, and 3), ASTM D638, GB/T 1040, and JIS K 7113. These standards define specimen geometry, conditioning requirements, test speeds, and data processing rules to ensure reproducible and comparable results across accredited third-party laboratories worldwide.
Testing Samples
Rigid and Semi-Rigid Thermoplastics
Common materials include polypropylene (PP), polyethylene (HDPE, LDPE), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polyamide (PA6, PA66), polyoxymethylene (POM), polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polymethyl methacrylate (PMMA). These are typically tested using ISO 527-2 Type 1A or 1B dumbbell specimens, or ASTM D638 Type I specimens.
Thermosetting Plastics
Materials such as epoxy resins, phenolic molding compounds, unsaturated polyester, and melamine-formaldehyde are tested as molded or machined specimens. Since thermosets are generally brittle, accurate measurement of specimen dimensions and careful specimen preparation are critical to avoiding premature stress concentration failures.
Fiber-Reinforced Plastic Composites
Glass-fiber or carbon-fiber reinforced compounds, sheet molding compounds (SMC), and bulk molding compounds (BMC) require dedicated specimen types and end-tab reinforcement in some cases. ISO 527-4 and ISO 527-5 specify test conditions for isotropic and anisotropic fiber-reinforced plastics respectively.
Plastic Films and Sheets
Thin films for packaging applications are tested according to ISO 527-3, which defines rectangular strip specimens (typically Type 2, 15 mm wide) and specialized grip arrangements to prevent slippage and premature tearing at the grips.
Testing Items
A complete tensile test program for plastics typically includes the following measured and calculated parameters:
- Tensile strength at break (ultimate stress at rupture)
- Tensile stress at yield
- Tensile strength (maximum stress recorded during the test)
- Elongation at break (percentage strain at rupture)
- Elongation at yield
- Tensile modulus (Young's modulus, determined between 0.05% and 0.25% strain per ISO 527-1)
- Secant modulus (e.g., 1% secant modulus per ASTM D638)
- Nominal strain at break (for specimens without extensometer measurement beyond grip separation limits)
Testing Methods
Instrumentation
Testing is performed on a calibrated universal testing machine (UTM) equipped with a suitable load cell matched to the expected force range, typically ensuring that the breaking force falls between 20% and 80% of the load cell capacity. Pneumatic or wedge-action grips with serrated or smooth faces are used depending on the material hardness. Strain measurement for modulus determination requires a contact extensometer or non-contact video extensometer attached directly to the gauge length of the specimen.
Specimen Preparation
Specimens are produced by injection molding, compression molding, machining from sheets or finished parts, or die-cutting (for films). Edges must be smooth and free of notches, scratches, and voids, as surface defects significantly reduce measured strength. ISO 527-2 Type 1A specimens have an overall length of 170 mm, a width of 20 mm in the grip section, 10 mm in the parallel section, and a thickness of 4 mm, with a preferred gauge length of 50 mm (Type 1B: overall length 150 mm, gauge length 50 mm).
Conditioning
Unless otherwise specified, specimens are conditioned at 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 88 hours in accordance with ISO 291 or ASTM D618 prior to testing. Moisture-sensitive materials such as polyamides may require special drying or humidity conditioning as agreed between parties.
Test Procedure and Speeds
The specimen is mounted in the grips so that its longitudinal axis aligns with the direction of force application. The preferred test speed for ISO 527-2 Type 1A and 1B specimens is 50 mm/min for strength determination, while a speed of 1 mm/min is used for modulus measurement. Under ASTM D638, Type I specimens are tested at 50 mm/min (5 mm/min for modulus). At least five valid specimens are tested for each direction and condition, and results are reported as mean values with standard deviation.
Calculation
Tensile strength at break is calculated using the equation sigma = F / A, where F is the maximum force at rupture in newtons and A is the original minimum cross-sectional area (width multiplied by thickness) in square millimeters. Elongation at break is calculated as the increase in gauge length at rupture divided by the original gauge length, expressed as a percentage. The modulus is determined from the slope of the stress-strain curve within the specified strain interval using a regression analysis.
Applications
Determination of tensile strength at break supports a wide range of industrial and regulatory applications:
- Automotive engineering: validation of interior trim, bumper systems, under-hood components, and fuel system parts against OEM material specifications
- Packaging: quality control of blown and cast films, bottles, closures, and laminates for food and consumer packaging
- Construction: verification of plastic pipes, fittings, profiles, cable ducts, and geomembranes for civil engineering projects
- Electrical and electronics: qualification of connector housings, insulators, and structural enclosures
- Medical devices: evaluation of disposable device components, sterile barrier packaging, and tubing
- Research and development: comparison of new resin formulations, additive effects, recycling content impact, and processing parameter optimization
- Failure analysis: determination of whether material degradation, aging, or environmental stress cracking contributed to in-service part failures
- Regulatory compliance: demonstration of conformity to product standards, material datasheets, and customer specifications through accredited third-party laboratories
FAQ
What is the difference between tensile strength at break and tensile strength at yield?
Yield strength is the stress at which the material begins to deform plastically, marked by an increase in strain without an increase in stress. Strength at break is the stress at the moment of fracture. For ductile plastics, break stress is often lower than yield stress; for brittle plastics, they may be nearly identical.
What are the key differences between ISO 527 and ASTM D638?
Both standards are conceptually similar, but they differ in specimen dimensions, preferred test speeds, conditioning atmospheres, and modulus calculation intervals. ISO 527-2 Type 1A specimens are 170 mm long with a 4 mm thickness, whereas ASTM D638 Type I specimens are 165 mm long with a preferred thickness between 3.0 and 7.0 mm. Results from the two standards are not directly interchangeable.
How many specimens are required for a valid test?
Both ISO 527-1 and ASTM D638 recommend testing at least five specimens per sample condition. Additional specimens should be tested if the scatter is high or if results fall outside specification limits. Specimens that fail outside the gauge section are typically discarded and retested.
Which factors most strongly influence test results?
Key influencing factors include specimen preparation quality, notch-free edge conditions, conditioning temperature and humidity, test speed, grip alignment and slippage, load cell accuracy, and measurement precision of specimen dimensions. Any variation in these parameters can shift the measured strength by several percent.
Can finished products be tested directly?
Yes. When specimens are cut from finished parts, the test is classified as a product test rather than a material test, because processing conditions, orientation, and geometry may affect the results. It is good practice to record the specimen origin, orientation relative to flow direction, and thickness, and to compare results only among specimens of identical configuration.