Plastic tensile strain at break (εB) testing to the Chinese national standard GB/T 1040 is the fundamental procedure for quantifying the ductility and deformation capacity of plastic materials before fracture. This article explains the applicable national standards, specimen requirements, equipment, step-by-step test procedure, calculation of strain at break, and typical industrial applications, serving as a practical reference for manufacturers, quality engineers, and accredited third-party laboratories.
Technical Overview
Tensile strain at break, designated εB in the GB/T 1040 standard series, is defined as the tensile strain of a specimen at the moment of rupture, calculated from the increase in gauge length divided by the original gauge length. It is the primary indicator of plastic ductility: a high strain at break characterizes a tough, deformable material, while a low value signals brittleness or embrittlement caused by aging, excessive filler loading, poor processing, or structural degradation.
The Chinese national standard system for tensile testing of plastics is the GB/T 1040 series,
Plastics — Determination of tensile properties
, which is technically aligned with the ISO 527 international standard series. The system is organized as follows:- GB/T 1040.1 — General principles: definitions, apparatus, calculation methods, and reporting requirements for all tensile property measurements.
- GB/T 1040.2 — Test conditions for moulding and extrusion plastics, including the Type 1A and 1B dumb-bell specimens.
- GB/T 1040.3 — Test conditions for films and sheets less than 1 mm thick.
- GB/T 1040.4 — Test conditions for isotropic and orthotropic fibre-reinforced plastic composites.
- GB/T 1040.5 — Test conditions for unidirectional fibre-reinforced plastic composites.
Specimen conditioning and the standard laboratory atmosphere are governed by GB/T 2918 (23 °C / 50 % RH), ensuring that moisture content and temperature do not distort the measured strain at break.
Testing Samples
Rigid Thermoplastics and Thermosets
Materials such as PP, PE, ABS, PC, PA6, PA66, POM, PBT, PET, PS, PMMA, PVC, PPS, and PEEK are tested using Type 1A or Type 1B dumb-bell specimens per GB/T 1040.2. The Type 1A specimen is directly injection moulded, while the Type 1B specimen is preferred for machining from sheets, plates, laminates, or finished parts. Both types feature a gauge length of 50 mm, a narrow-section width of 10 mm, and a nominal thickness of 4 mm, with an initial grip separation of 115 mm.
Plastic Films and Thin Sheets
For flexible packaging films, BOPP, CPP, PE, and PET films, and thin sheets below 1 mm, GB/T 1040.3 applies. Rectangular strip specimens (typically 10–25 mm wide with a 50 mm gauge length) or small dumb-bell specimens are used. Because film properties are direction-dependent, specimens must be cut and tested separately in both the machine direction (MD) and transverse direction (TD).
Fibre-Reinforced Composites
Glass-fibre or carbon-fibre reinforced plastics are tested under GB/T 1040.4 and GB/T 1040.5 using rectangular or dumb-bell specimens with tabbed ends where necessary to prevent premature grip failure. Strain at break for these materials is characteristically low and must be measured with a high-resolution extensometer.
Small Specimens and Special Formats
When material quantity is limited, Type 2 or the miniature Type 5 specimen may be used, provided this is clearly stated in the report because results are not directly comparable with Type 1A/1B data. Specimens from 3D-printed parts, recycled granulates, compression-moulded plates, or machined components are also accepted, with the preparation route documented.
Testing Items
Although the focus of this testing is tensile strain at break, the GB/T 1040 method simultaneously delivers the full tensile property profile of the material:
- Tensile strain at break εB (%) — the principal measurand of this test.
- Tensile stress at break σB (MPa).
- Tensile strength σM (MPa) — stress at maximum force.
- Tensile stress at yield σY (MPa) and tensile strain at yield εY (%).
- Tensile modulus Et (MPa) — the secant modulus between 0.05 % and 0.25 % strain.
- Nominal strain at break εt (%) — strain based on grip separation when an extensometer is not used.
Testing Methods
Equipment and Calibration
A constant-rate-of-extension universal testing machine is required, with force measurement verified to at least class 1 accuracy per ISO 7500-1 or the corresponding national verification regulation. Extension measurement uses either a class 1 extensometer per ISO 9513 / GB/T 12160 or, for nominal strain, the machine crosshead displacement referenced to the initial grip separation. Wedge, pneumatic, or serrated grips are selected to suit specimen rigidity, and width and thickness are measured with a calibrated micrometer or thickness gauge to 0.02 mm.
Test Procedure
- Condition the specimens at 23 ± 2 °C and 50 ± 10 % relative humidity for at least 88 h, or as specified by the relevant material standard.
- Measure the specimen width and thickness at a minimum of three points within the gauge section and compute the mean cross-sectional area.
- Set the initial grip separation to 115 ± 1 mm for Type 1A/1B specimens and set the extensometer gauge length to 50 ± 0.5 mm.
- Select the test speed from the standard series (1, 2, 5, 10, 20, 50, 100, 200, 250, or 500 mm/min). A speed of 50 mm/min is widely used for unfilled rigid thermoplastics; brittle or highly reinforced materials are tested at slower speeds, while films are frequently tested at 250–500 mm/min.
- Apply a small preload to remove slack, then run the test until the specimen fractures completely.
- Record the force–extension curve and read the extension at break ΔLb.
- Repeat on at least five valid specimens per condition; for films, test at least five specimens in each direction.
Calculation and Validity of Results
The tensile strain at break is calculated as εB = ΔLb / L0 × 100 %, where ΔLb is the extension between gauge marks at break and L0 is the initial gauge length. If the extensometer must be removed before rupture for materials with very high elongation, or if no extensometer is fitted, the nominal strain at break εt is calculated using the initial grip separation instead, and this must be stated in the report. Results from specimens that break at the grips, outside the gauge section, or due to grip-induced damage are treated as invalid and retested.
Reporting
A compliant GB/T 1040 report includes material identification, specimen type and preparation method, measured dimensions, conditioning details, test speed, gauge length, individual and mean strain at break values with standard deviation, break locations, and any deviations from the standard.
Applications
Strain at break data generated under GB/T 1040 supports decisions across the entire plastics value chain:
- Incoming inspection of resins, masterbatches, and compounded pellets against supplier datasheets.
- Quality control of injection-moulded, extruded, blown, and 3D-printed components in automotive, electronics, appliances, packaging, medical device, and pipe-and-fitting production.
- Material selection and structural design, where ductility margins determine allowable deformation in service.
- Formulation development, evaluating the effect of impact modifiers, plasticizers, fillers, and reinforcing fibres on toughness.
- Aging and durability studies, including thermal aging, UV weathering, hydrolysis, and chemical exposure, where retention of εB is a sensitive indicator of embrittlement.
- Failure analysis of cracked or brittle parts and batch-to-batch consistency verification.
- Compliance demonstration for customers, certification bodies, and accredited third-party laboratories in domestic and export markets.
FAQ
What is the difference between tensile strain at break and elongation at break?
They describe the same physical event. Strain at break is a dimensionless ratio (mm/mm) that, when multiplied by 100, is numerically identical to elongation at break expressed in percent. GB/T 1040 formally uses the term tensile strain at break εB.
Which part of GB/T 1040 applies to my sample?
Use GB/T 1040.1 together with GB/T 1040.2 for moulded and extruded plastics thicker than 1 mm, GB/T 1040.3 for films and sheets below 1 mm, and GB/T 1040.4 or 1040.5 for fibre-reinforced composites.
Can strain at break be measured without an extensometer?
Yes. When no extensometer is used, or when it is removed before break, nominal strain at break is calculated from crosshead displacement relative to the initial grip separation. Because grip separation includes machine compliance and specimen deformation outside the gauge section, nominal strain values are less accurate and must be labelled accordingly in the report.
Why do brittle or reinforced materials give scattered strain at break values?
Low-ductility materials fracture at small deformations, so specimen surface defects, machining marks, internal voids, and fibre alignment dominate the result. Testing more specimens, improving specimen preparation quality, and reporting standard deviation are recommended.
How does GB/T 1040 compare with ASTM D638?
Both standards use similar dumb-bell specimens and constant-rate-of-extension testing. GB/T 1040 is aligned with ISO 527, whereas ASTM D638 uses different preferred specimen geometry and speed conventions. Results are broadly comparable but not automatically interchangeable; the standard used must be specified when comparing data.
How does test speed affect strain at break?
Plastics are viscoelastic, so higher testing speeds generally raise strength and lower ductility, while lower speeds increase strain at break. The test speed specified in the relevant product or material standard must therefore be followed exactly for valid comparisons.