Tensile testing to GB standards is the core method used in China to evaluate the mechanical performance of plastic materials, covering tensile strength, yield stress, elongation at break, and elastic modulus. The GB/T 1040 standard series defines the specimen geometry, test conditions, and calculation methods that manufacturers, laboratories, and certification bodies rely on for material qualification and quality control. This guide explains the applicable standards, sample requirements, test procedures, typical applications, and frequently asked questions related to GB plastic tensile testing.
Technical Overview
Plastic tensile testing under the Chinese national standard system is governed primarily by the GB/T 1040 standard series, which specifies the general principles, specimen types, and test conditions for determining the tensile properties of plastics. The series is technically aligned with the international ISO 527 standard family, allowing laboratories to conduct tests that are broadly comparable with global practice while remaining compliant with domestic regulatory and contractual requirements.
- GB/T 1040.1 — Plastics: Determination of tensile properties; Part 1: General principles, covering definitions, apparatus, and calculation methods.
- GB/T 1040.2 — Part 2: Test conditions for moulding and extrusion plastics, including rigid and semi-rigid thermoplastics and thermosets.
- GB/T 1040.3 — Part 3: Test conditions for films and sheets.
- GB/T 1040.4 — Part 4: Test conditions for isotropic and orthotropic fibre-reinforced plastic composites.
- GB/T 1040.5 — Part 5: Test conditions for unidirectional fibre-reinforced plastic composites.
Supporting standards complete the test framework: GB/T 2918 defines the standard atmosphere for conditioning and testing, GB/T 17037.1 governs the preparation of injection-moulded test specimens, GB/T 17200 specifies requirements for the testing machine, and GB/T 8804 covers the determination of tensile properties for thermoplastic pipes. When performed by accredited third-party laboratories, GB tensile testing results are widely accepted for product certification, supplier approval, and regulatory submission within China.
Testing Samples
Type 1A and 1B Multipurpose Specimens
Type 1A specimens are the preferred geometry for rigid and semi-rigid moulding and extrusion plastics such as PP, PE, ABS, PC, PA, POM, PET, and PS. Type 1A specimens are directly injection moulded to the standard dumbbell shape, while Type 1B specimens are machined or milled from sheets, plates, compression-moulded slabs, or finished parts. Both types feature a parallel gauge section with a nominal width of 10 mm and thickness of 4 mm, and a gauge length of 50 mm used for strain measurement.
Film and Sheet Specimens
For plastic films and thin sheets, GB/T 1040.3 applies. Specimens are typically die-cut with a sharp, defect-free cutting die to ensure smooth edges free of nicks that could cause premature tearing. Films are usually tested with an initial grip separation of 100 mm and an extensometer gauge length of 50 mm. Because films are frequently anisotropic due to orientation during blown or cast film production, specimens should be cut in both the machine direction and the transverse direction and reported separately.
Small-Scale Specimens
When only a limited quantity of material is available, for example when sampling from finished products, small components, or laboratory-scale compound batches, reduced-size specimens may be used in accordance with the relevant provisions of the standard. It should be noted that results from small specimens are not always directly comparable with those from full-size specimens and must be clearly identified in the test report.
Pipe and Fitting Materials
Tensile properties of thermoplastic pipes, including PVC, PE, and PP piping materials, are determined using strip or dumbbell specimens cut from the pipe wall in accordance with GB/T 8804. Specimen preparation must account for pipe curvature, wall thickness, and orientation effects so that the measured properties represent the extruded pipe material rather than a reprocessed sample.
Specimen Quality Requirements
- Specimens must be free of voids, sink marks, flash, notches, and visible surface defects.
- Machined edges must be smooth and parallel, without burns or machining marks that act as stress concentrators.
- Width and thickness must be measured at multiple points within the gauge section, and the smallest cross-sectional area used for stress calculation.
- Specimens should be conditioned in the standard atmosphere before testing, typically at 23 °C ± 2 °C and 50 % ± 10 % relative humidity per GB/T 2918, commonly for at least 24 hours depending on thickness.
Testing Items
GB tensile testing provides a complete characterization of the tensile stress-strain behaviour of plastics. The principal measured and calculated parameters include:
- Tensile stress at yield — the first stress at which strain increases without a further increase in stress.
- Tensile strength — the maximum tensile stress sustained by the specimen during the test.
- Tensile stress at break — the stress recorded at the moment of specimen rupture.
- Tensile strain at yield — the strain corresponding to the yield point, expressed as a percentage.
- Tensile strain at break — the strain at rupture measured with an extensometer within the gauge length.
- Nominal tensile strain at break — strain calculated from grip separation, used when no extensometer is applied or when the break strain exceeds the extensometer range.
- Tensile modulus of elasticity — the slope of the stress-strain curve determined between the strain limits of 0.0005 and 0.0025 at a test speed of 1 mm/min.
- Poisson's ratio — optionally determined with transverse and longitudinal strain measurement devices.
Testing Methods
Apparatus
Testing is performed on a calibrated universal testing machine equipped with a force measurement system meeting accuracy class 1 or better per GB/T 17200. Self-tightening wedge or pneumatic grips are used for rigid specimens, while capstan or smooth-face grips with extended contact length are preferred for films to prevent slippage and premature break at the jaw faces. An extensometer, either contact-type or non-contact video type, with accuracy class 1 or better is required for modulus determination and for accurate strain-at-break measurement.
Test Speed Selection
The standard specifies a range of preferred test speeds, including 1, 2, 5, 10, 20, 50, 100, 200, 250, and 500 mm/min. For rigid thermoplastics tested with Type 1A and 1B specimens, 50 mm/min is the most widely applied speed, whereas 1 mm/min is mandatory for modulus determination. Films are commonly tested at higher speeds such as 100, 200, 250, or 500 mm/min as specified in the material standard or agreed between parties. The selected speed must always be reported because tensile properties of viscoelastic plastics are strongly rate-dependent.
Test Procedure
- Condition the specimens in the standard atmosphere and verify freedom from defects.
- Measure the width and thickness of each specimen at several points within the gauge section and calculate the original cross-sectional area.
- Set the specified gauge length, mount the specimen symmetrically in the grips, and attach the extensometer where required.
- Select the specified test speed and pre-load if applicable, then start the test and record the complete force-extension curve until rupture.
- Repeat the test on at least five valid specimens per direction; specimens that break in the grips or outside the gauge length are discarded for strain-at-break evaluation and replaced.
- Calculate tensile stress as force divided by the original cross-sectional area, determine strain from extensometer displacement, and derive the modulus from the regression slope between the specified strain limits.
Data Reporting
The test report should identify the material and specimen preparation method, specimen type and dimensions, conditioning atmosphere, test speed, individual and mean results for each property, standard deviation, and the standard designation. When comparing results with ISO 527 or ASTM D638 data, differences in specimen geometry, speed, and modulus calculation conventions must be considered, and results from different test systems should not be interchanged without technical justification.
Applications
GB tensile testing is applied across the entire plastics value chain, from raw material qualification to finished product verification. Typical use cases include:
- Incoming inspection of resins and compounds against supplier data sheets and purchase specifications.
- Generation of material property data for product design, simulation, and material substitution studies.
- Evaluation of formulation changes, additive packages, filler or reinforcement content, and recycled material incorporation.
- Validation of injection moulding, extrusion, and film-processing parameters and their effect on mechanical performance.
- Verification of performance retention after heat ageing, weathering, or chemical exposure by comparing tensile properties before and after exposure.
- Compliance evidence for product certification, tender documents, and regulatory submissions in the Chinese market, typically issued by accredited third-party laboratories.
- Failure analysis of cracked or degraded plastic components in automotive, electronics, packaging, construction, and consumer goods applications.
FAQ
What is the difference between GB/T 1040 and ASTM D638?
Both standards determine tensile properties, but they use different specimen geometries: GB/T 1040 follows the ISO-style Type 1A/1B dumbbell with a 10 mm × 4 mm gauge cross-section, while ASTM D638 typically uses the Type I specimen with a 13 mm × 3.2 mm cross-section. Preferred speeds and reporting conventions also differ, so tensile results obtained under the two systems are not directly interchangeable and should be treated as separate datasets.
Are GB/T 1040 and ISO 527 results comparable?
The GB/T 1040 series is developed in alignment with the corresponding parts of ISO 527, and in most cases the technical requirements are consistent. For commercial or regulatory purposes where both standards are cited, laboratories can often issue combined-standard reports after verifying that all requirements of both documents are satisfied, but the specific clauses adopted must be clearly stated.
How many specimens are required for a valid test?
A minimum of five valid specimens is required for each direction tested. For anisotropic materials such as films, fibre-reinforced composites, and injection-moulded parts with flow-induced orientation, separate specimen sets must be prepared for each principal direction, and additional specimens may be needed if scatter is high.
Why is the modulus measured at 1 mm/min?
Plastics are viscoelastic, and their measured stiffness depends on deformation rate. The standard fixes the modulus test speed at 1 mm/min and calculates the slope between strain limits of 0.0005 and 0.0025 to ensure that modulus values are reproducible and comparable between laboratories.
What happens if a specimen breaks outside the gauge length?
When a specimen breaks at or in the grips or outside the gauge length, the strain-at-break result is invalid and the specimen must be replaced. Strength values may be reported with annotation depending on the applicable procedure, but the anomaly must always be documented in the test report.
Can finished products be tested directly instead of moulded specimens?
Yes. Specimens can be machined from finished parts or small specimens can be used when limited material is available. However, processing history, orientation, and anisotropy of the actual part will influence the results, so such data characterize the specific product rather than the base resin and should be reported accordingly.