Determination of the tensile breaking force of plastics is one of the most fundamental mechanical tests for evaluating how much load a plastic material can withstand before fracture occurs. This article provides a comprehensive technical overview of the test principle, applicable international standards such as ISO 527 and ASTM D638, specimen preparation requirements, step-by-step testing procedures, and the typical applications of tensile breaking force measurement in material qualification and quality control.
Technical Overview
Tensile breaking force determination is a quasi-static mechanical test in which a plastic specimen of standardized geometry is clamped between the grips of a universal testing machine and stretched along its major axis at a constant crosshead speed until rupture. Throughout the test, the applied force and the resulting elongation are continuously recorded, producing a force-extension (or stress-strain) curve that characterizes the tensile behavior of the material.
The tensile breaking force, expressed in newtons (N), is the maximum or ultimate force that the specimen sustains immediately before or at the moment of fracture. When this force is normalized by the original cross-sectional area of the parallel portion of the specimen, it yields the tensile strength at break, expressed in megapascals (MPa). Together with elongation at break and tensile modulus, these parameters form the core dataset used to judge whether a plastic material is suitable for a given load-bearing application.
The test is governed by several internationally recognized standards, primarily ISO 527 (Plastics — Determination of tensile properties), ASTM D638 (Standard Test Method for Tensile Properties of Plastics), ASTM D882 for thin plastic sheeting, GB/T 1040 (the Chinese national standard series, technically aligned with ISO 527), and JIS K 7113. Because plastics are viscoelastic and sensitive to temperature, humidity, and strain rate, strict control of conditioning environment, specimen geometry, and test speed is essential for obtaining reproducible and comparable results.
Testing Samples
Tensile breaking force testing can be performed on a wide range of plastic material forms. Specimen geometry, preparation method, and the applicable standard depend on the product category.
Rigid and Semi-Rigid Moulding and Extrusion Plastics
For thermoplastics, thermosets, and filled compounds supplied as pellets, sheets, or moulded parts, tests are conducted according to ISO 527-2 or ASTM D638. Specimens are typically dog-bone (dumbbell) shaped, with ISO Type 1A (preferred for directly moulded specimens) and Type 1B (preferred for machined specimens) being the most common geometries. ASTM D638 defines specimen Types I through V, among which Type I is the standard geometry for rigid plastics with a thickness of 7 mm or less. Specimens may be injection moulded, compression moulded, or machined from larger stock, and machined surfaces must be free of visible defects, notches, or scratches that could induce premature stress concentration.
Plastic Films and Sheets
Thin flexible materials such as packaging films, barrier laminates, and geomembranes are tested in accordance with ISO 527-3 or ASTM D882. Instead of dumbbell specimens, rectangular strips with a standard width (typically 10 mm to 15 mm) and a gauge length of 50 mm to 100 mm are used. Film thickness is measured with a high-resolution dead-weight micrometer at multiple points along the gauge length, because thickness variation has a strong influence on the calculated breaking stress.
Fibre-Reinforced Plastic Composites
For isotropic and orthotropic fibre-reinforced composites, ISO 527-4 and ISO 527-5 apply. Specimens are usually straight-sided rectangular strips or dumbbell types, and the fibre orientation relative to the loading direction must be clearly documented, since tensile breaking force along the fibre direction can differ by an order of magnitude from the transverse direction.
Finished Products and Components
Wherever feasible, specimens can be cut directly from finished goods such as pipes, profiles, injection-moulded parts, woven or nonwoven plastic textiles, and geosynthetics. Testing specimens taken from actual products allows verification of the effect of processing conditions, welding lines, orientation, and aging on the tensile breaking force.
Testing Items
A standard tensile breaking force determination typically reports the following parameters derived from the force-extension curve:
- Tensile breaking force (N): the maximum force sustained by the specimen up to the point of rupture.
- Tensile strength at break (MPa): the breaking force divided by the original minimum cross-sectional area of the specimen.
- Tensile strength (MPa): the maximum tensile stress recorded during the test, which may occur at yield or at break depending on the material behavior.
- Tensile stress at yield (MPa): the stress at the first point on the curve at which an increase in strain occurs without an increase in stress.
- Elongation at break (%): the increase in gauge length between the initial grip separation and the moment of rupture, expressed as a percentage of the initial gauge length.
- Tensile modulus (Young's modulus, MPa): the slope of the stress-strain curve in the initial linear elastic region, typically measured between strain values of 0.0005 and 0.0025 for ISO 527.
- Secant modulus (MPa): the ratio of stress to strain at a specified strain level, used for materials without a clearly linear region.
- Poisson-related strain data and failure mode descriptions, where instrumented with transverse or longitudinal extensometers.
Testing Methods
The general test procedure, in accordance with ISO 527-1 and ASTM D638, is carried out on a calibrated universal testing machine equipped with suitable grips, a calibrated load cell, and an extensometer or crosshead displacement measurement system. The typical workflow is as follows:
- Specimen preparation: prepare at least five identical specimens for each direction and condition being evaluated. Specimens must be free of flash, voids, notches, and machining marks, and must be conditioned to the standard atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity (ISO 291 or ASTM D618) for a minimum conditioning period, commonly at least 24 hours for many materials.
- Dimensional measurement: measure the width and thickness of the parallel section of each specimen at several points using a calibrated micrometer or caliper, and compute the minimum cross-sectional area used in stress calculations.
- Machine setup: select a load cell whose capacity is appropriate for the expected breaking force, generally ensuring the measured force falls within the calibrated range of the load cell. Set the initial gauge length, typically 50 mm for ISO 527-2 Type 1A/1B and ASTM D638 Type I specimens.
- Grip mounting: clamp the specimen in the grips so that its longitudinal axis coincides with the direction of pull, apply a small preload (for example, up to 1 % of the expected maximum force) to remove slack, and align the extensometer on the gauge section if direct strain measurement is required.
- Speed selection: apply the crosshead speed specified by the material standard. ISO 527 defines preferred speeds of 1, 2, 5, 10, 20, 50, 100, 200, and 500 mm/min, with 50 mm/min being typical for many unreinforced thermoplastics tested with Type 1A specimens; lower speeds are generally used for modulus determination.
- Test execution: start the machine and extend the specimen at constant speed until rupture. Continuously record force and elongation, and note the failure location and mode (e.g., brittle fracture, ductile tearing, failure within or outside the gauge length).
- Validity check: results are considered invalid if the specimen fails in the grips or outside the gauge section, or if slip occurs, unless such failure is representative of the product. Invalid specimens are discarded and replaced so that at least five valid results are obtained.
- Calculation and reporting: compute stress as force divided by original cross-sectional area, elongation from the extension relative to the initial gauge length, and modulus from the slope of the linear region. Report the arithmetic mean, standard deviation, and coefficient of variation for each parameter, together with specimen type, conditioning, test speed, temperature, humidity, and machine calibration information.
For materials that exhibit necking, a long gauge-length extensometer or video extensometer is recommended to capture post-yield strain accurately. For materials that fail in a brittle manner, grip alignment and lubricated or padded grip faces help prevent premature failure at the grip interface. In all cases, measurement traceability to national standards and verification of the force measuring system are prerequisites for reliable data.
Applications
Determination of the tensile breaking force of plastics supports decision-making across the entire product lifecycle, from material development to in-service performance verification. Typical applications include:
- Quality control in plastics manufacturing: routine verification of incoming resins, compounds, and finished parts against internal specifications and lot-to-lot consistency requirements.
- Material selection and comparison: ranking candidate materials for stiffness, strength, and ductility during design and engineering development.
- Compliance and certification: demonstrating conformity with product standards and regulatory requirements in automotive, construction, electrical, medical, and packaging sectors, often through accredited third-party laboratories.
- Processing optimization: evaluating how moulding parameters, weld lines, orientation, regrind content, or extrusion conditions influence mechanical performance.
- Aging and durability studies: comparing tensile breaking force before and after heat aging, UV weathering, chemical exposure, hydrolysis, or sterilization cycles to assess service life.
- Failure analysis: testing material taken from failed components to determine whether degradation, contamination, or processing defects contributed to the failure.
- Research and development: characterizing new polymer formulations, blends, biobased and biodegradable plastics, nanocomposites, and recycled materials.
Sectors that rely heavily on this test include flexible and rigid packaging, automotive interior and exterior components, pressure pipes and fittings, wire and cable insulation, medical devices and consumables, consumer electronics housings, and geosynthetic and agricultural films.
FAQ
What is the difference between tensile breaking force and tensile strength?
Tensile breaking force is the absolute load, in newtons, at which the specimen fractures, whereas tensile strength is the corresponding stress, in megapascals, obtained by dividing the force by the original cross-sectional area of the specimen. Tensile strength allows fair comparison between specimens of different dimensions, while breaking force is the raw load capacity value.
Which standards govern plastic tensile testing?
The primary standards are ISO 527 (Parts 1 through 5), ASTM D638, ASTM D882 for thin plastic sheeting, GB/T 1040 series, and JIS K 7113. The choice depends on the material form, the market region, and the specification invoked by the customer or product standard.
How many specimens are required for a valid test?
Both ISO 527 and ASTM D638 require a minimum of five valid specimens per material condition and test direction. For materials with high variability, or for statistically robust quality control, a larger sample size is recommended, and results should always be reported with mean, standard deviation, and coefficient of variation.
Why does test speed affect the results?
Plastics are viscoelastic, meaning their mechanical response depends on the rate of deformation. Higher crosshead speeds generally increase the measured tensile strength and modulus while reducing elongation at break. For this reason, standards prescribe specific test speeds, and comparisons between datasets are only valid when the speeds and specimen geometries match.
What does a low elongation at break indicate?
A low elongation at break typically indicates brittle behavior, which may result from the intrinsic nature of the polymer, excessive filler content, degradation during processing, low temperature, or aging. Ductile materials with high elongation at break are generally preferred for applications requiring impact resistance and energy absorption, while brittle materials may be acceptable in rigid structural roles.
Can thin films be tested with the same method as rigid plastics?
No. Thin films and sheets require dedicated methods such as ISO 527-3 or ASTM D882, which use rectangular strip specimens, lighter grips, lower preloads, and higher sensitivity load cells. Using rigid-plastic dog-bone methods on films can lead to grip-induced failures and unreliable breaking force values.
How should results be reported and interpreted?
Reports should state the standard used, specimen type and preparation method, conditioning atmosphere, test speed, temperature, and humidity, individual and mean values of breaking force, tensile strength, elongation at break, and modulus, plus the failure mode of each specimen. Interpretation should always consider statistical scatter and compare the data against the acceptance criteria defined in the relevant product specification.
