Plastic tensile strength testing is one of the most fundamental mechanical evaluations for determining how plastic materials behave under uniaxial pulling forces, and it is governed by internationally recognized standards such as ISO 527, ASTM D638, and GB/T 1040. This guide provides a professional overview of the applicable standards, specimen requirements, tested parameters, test procedures, and typical industrial applications, helping manufacturers select the correct tensile testing standard for product development, quality control, and regulatory compliance.
Technical Overview
Tensile strength is the maximum engineering stress a plastic material can withstand while being stretched before it breaks or yields. It is one of the most widely specified mechanical properties in material datasheets, product drawings, and purchase agreements, because it directly reflects the load-bearing capability of plastic components in service.
Tensile testing of plastics is performed on a universal testing machine (UTM), in which a standardized dumbbell-shaped or strip specimen is clamped between grips and pulled at a constant crosshead speed until failure. During the test, the force and elongation are continuously recorded, and the stress-strain curve is generated from which key parameters such as tensile strength, yield stress, elongation at break, and tensile modulus are derived.
The principal standards governing plastic tensile strength testing include:
- ISO 527-1 / ISO 527-2 — General principles and test conditions for rigid and semi-rigid moulding and extrusion plastics (internationally, including European markets).
- ASTM D638 — Standard test method for tensile properties of plastics (widely used in North America).
- GB/T 1040.1 / GB/T 1040.2 — Chinese national standards, technically aligned with ISO 527 series, commonly required for the Chinese domestic market and CCC-related submissions.
- ISO 527-3 / ASTM D882 — Tensile testing of plastic films and thin sheets below 1 mm thickness.
- JIS K 7161 / JIS K 7162 — Japanese industrial standards harmonized with the ISO 527 series.
Although these standards are conceptually similar, they differ in specimen geometry, gauge length, testing speed, and modulus calculation, so results obtained under different standards are generally not directly interchangeable. Manufacturers should therefore select the standard that matches the requirements of their target market, product specification, or certification scheme, and accredited third-party laboratories can confirm the appropriate method before testing begins.
Testing Samples
Rigid and Semi-Rigid Moulding and Extrusion Plastics
For materials such as ABS, PP, PE, PC, PA (nylon), POM, PET, and reinforced or filled compounds, the preferred specimens are injection-moulded dumbbels: Type 1A (overall length 170 mm, thickness 4 mm) under ISO 527-2 and GB/T 1040.2, or Type I under ASTM D638 (overall length 165 mm, thickness 3.2 mm). Machined specimens cut from compression-moulded plates, sheets, or finished parts are also acceptable when moulding is not feasible, provided machining marks and overheating are avoided.
Sheets, Plates, Pipes, and Finished Products
When specimens must be taken from real products — for example pipe segments, profiles, or large parts — smaller specimen types such as ISO Type 1B or ASTM Types II, III, or V are machined from the article. The sampling location and direction (parallel or perpendicular to processing direction, e.g., extrusion or rolling direction) must be recorded, because orientation can significantly influence tensile results.
Plastic Films and Thin Sheets
For flexible films and sheets with thickness below 1 mm, strip specimens specified in ISO 527-3 or ASTM D882 are used instead of rigid dumbbels. Typical strip widths are 10–25 mm with long gauge lengths, and testing may be performed in both machine direction (MD) and transverse direction (TD) to evaluate anisotropy.
Specimen Conditioning
Unless otherwise agreed, specimens shall be conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 or ASTM D618, typically for at least 24 hours prior to testing. For hygroscopic materials such as PA, additional drying or humidity conditioning may be required by the material specification, because moisture content has a pronounced effect on tensile properties.
Testing Items
A full tensile test according to ISO 527 or ASTM D638 typically reports the following parameters:
- Tensile strength (σM) — the maximum stress sustained by the specimen before rupture or yield.
- Tensile stress at yield (σY) — the stress at the first point where strain increases without a further increase in stress, relevant for ductile materials.
- Tensile stress at break (σB) — the stress at the moment of specimen rupture.
- Tensile strain at yield (εY) — the strain corresponding to the yield point.
- Tensile strain at break / elongation at break (εB) — the strain at rupture, expressed as a percentage of the initial gauge length.
- Tensile modulus (Et) — the slope of the stress-strain curve in the initial linear region; ISO 527 defines it by the secant between 0.05 % and 0.25 % strain.
- Secant modulus and offset yield strength — optional parameters for materials without a distinct linear region or clear yield point.
Results are reported as the arithmetic mean of valid specimens together with the standard deviation, specimen type, conditioning, and testing speed, ensuring full traceability of the measurement.
Testing Methods
The general test procedure under ISO 527, ASTM D638, and GB/T 1040 can be summarized as follows:
- Equipment verification: A universal testing machine calibrated in accordance with ISO 7500-1 or ASTM E4, fitted with a load cell appropriate to the expected force range and grips (wedge, pneumatic, or capstan types) that prevent slippage without initiating premature failure.
- Dimension measurement: Width and thickness of the narrow parallel section are measured at several points with a precision of 0.01 mm or better, and the minimum cross-sectional area is used for stress calculation.
- Gauge length setting: For ISO Type 1A/1B specimens, the gauge length L0 is 50 mm with an initial distance between grips of 115 mm; ASTM D638 Type I uses a 50 mm gauge length with 115 mm grip separation.
- Testing speed selection: Crosshead speeds typically range from 1 mm/min to 500 mm/min depending on specimen type and material. For modulus determination under ISO 527-2, a speed of 1 mm/min is prescribed, whereas common speeds for strength measurement are 5–50 mm/min. The selected speed must be stated in the report.
- Deformation measurement: An extensometer (contact or video/optical, non-contact) is attached to or focused on the gauge section for accurate strain and modulus measurement, especially for rigid materials with small elongations.
- Data acquisition and calculation: Stress is calculated as σ = F / A0 (force divided by original cross-sectional area) in MPa. Tensile strength is the maximum stress; modulus is derived from the slope between the specified strain limits; elongation values are computed from extensometer or crosshead displacement.
- Validity and repetition: At least five valid specimens are tested for each condition per ISO and ASTM requirements; specimens that fail outside the gauge section or slip in the grips are discarded and replaced.
Testing is normally performed at 23 °C and 50 % relative humidity. When materials are evaluated for low-temperature or elevated-temperature service, tests can be conducted inside an environmental chamber at the specified temperature, following the same standard procedure.
Applications
Tensile strength testing in accordance with the above standards supports a wide range of industrial and regulatory purposes:
- Raw material acceptance and incoming inspection: Verifying that resin batches, compounds, and masterbatches meet datasheet values before production.
- Product and quality control: Periodic batch release testing of injection-moulded parts, extruded profiles, geomembranes, and pipe fittings against internal or client specifications.
- Automotive industry: Qualification of interior and exterior plastic components, with tensile data supporting material cards for CAE simulation and PPAP documentation.
- Packaging industry: Evaluation of PE, PP, PET, and multi-layer films for tensile strength and elongation to guarantee machineability and seal performance.
- Pipes and construction products: Demonstrating conformity of PE, PVC, and PP-R piping materials with product standards referenced in building regulations and certification schemes.
- Electronics and consumer goods: Confirming the mechanical reliability of housings, connectors, and insulating parts in accordance with UL- or IEC-referenced material requirements.
- Medical devices: Supporting biocompatibility and design-control dossiers with mechanical property data for polymers used in device components.
- Research and failure analysis: Comparing formulations, evaluating the effects of aging, UV exposure, or recycling content, and diagnosing in-service failures by testing material retrieved from failed parts.
Test reports issued by accredited third-party laboratories are widely accepted by regulators, certification bodies, and international buyers as objective evidence of conformity.
FAQ
Q1: Which standard should I choose — ISO 527, ASTM D638, or GB/T 1040?
The choice depends mainly on the target market and the product specification. ISO 527 and GB/T 1040 are technically aligned and preferred for European and Chinese markets, while ASTM D638 dominates in North America. If a buyer or certification scheme does not specify a standard, ISO 527 is generally the most internationally neutral choice.
Q2: Are ISO 527 and ASTM D638 results comparable?
Not directly. The specimen geometries, thicknesses, and test speeds differ, and measured values — particularly tensile strength and modulus — can deviate by several percent between the two methods. For contractual purposes, the exact standard should be agreed upon in advance rather than converting results between methods.
Q3: How many specimens are required for one test?
Both ISO 527 and ASTM D638 require a minimum of five valid specimens per material condition. Additional specimens are recommended for highly variable materials, reinforced compounds, or when testing in multiple directions.
Q4: What specimen is used for plastic film?
Films and thin sheets below 1 mm are tested as strips under ISO 527-3 or ASTM D882 rather than as rigid dumbbels, and tests are usually performed in both the machine direction and transverse direction.
Q5: What is the tensile modulus and how is it determined?
The tensile modulus (Young's modulus) characterizes the stiffness of the material in the initial elastic region. Under ISO 527 it is calculated as the secant slope between 0.05 % and 0.25 % strain using an extensometer, while ASTM D638 typically uses a tangent to the initial linear portion of the curve.
Q6: What factors can affect tensile test results?
Key influences include specimen preparation quality, conditioning temperature and humidity (especially for hygroscopic nylons), testing speed, grip alignment, notch-free specimen edges, and material orientation. Strict adherence to the standard minimizes these variables and ensures reproducible results.
Q7: Can tensile testing be performed at high or low temperatures?
Yes. Equipped with an environmental chamber, accredited third-party laboratories can conduct tensile tests at temperatures from approximately -70 °C to +300 °C to evaluate material performance under realistic service conditions.