Impact Brittleness Performance Testing to Chinese National Standards (GB)

Impact brittleness performance testing to Chinese national standards (GB) determines the low-temperature brittleness temperature of plastics and elastomers, a critical parameter for materials used in cold-climate service. Conducted in accordance with GB/T 5470 and related national standards, this test evaluates how polymer materials transition from ductile to brittle behavior under impact loading. This article explains the technical principles, sample requirements, test procedures, and industrial applications of GB impact brittleness testing.

Impact brittleness performance testing to Chinese national standards (GB) determines the low-temperature brittleness temperature of plastics and elastomers, a critical parameter for materials used in cold-climate service. Conducted in accordance with GB/T 5470 and related national standards, this test evaluates how polymer materials transition from ductile to brittle behavior under impact loading. This article explains the technical principles, sample requirements, test procedures, and industrial applications of GB impact brittleness testing.

Technical Overview

Impact brittleness testing measures the temperature at which a polymeric material loses its ductility and fails in a brittle manner when subjected to a sharp mechanical impact. As ambient temperature decreases, the molecular mobility of polymers diminishes, and materials that are flexible at room temperature can shatter or crack under sudden loading at low temperatures. The governing Chinese national standard for plastics is GB/T 5470 (Determination of the brittleness temperature of plastics by impact), which is technically aligned with ISO 974 and broadly comparable to ASTM D746. For vulcanized rubbers, GB/T 15256, aligned with ISO 812, defines the corresponding low-temperature brittleness method.

The central result of the test is the brittleness temperature, defined statistically as the temperature at which 50% of the specimens tested exhibit brittle failure (cracking, chipping, or complete fracture) when struck by a standardized impactor at a controlled velocity and energy. In many commercial specifications, testing is also performed in a pass/fail mode at a single agreed temperature, such as minus 40 °C, where the material must show no cracks or breaks. Because brittleness is a statistical phenomenon influenced by specimen preparation, notch-free surface condition, and thermal equilibration, strict adherence to the national standard procedure is essential for reproducible results.

Testing Samples

Thermoplastic Resins and Compounds

Polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), ABS, and various engineering plastics can be evaluated. Rigid and semi-rigid thermoplastics intended for outdoor or low-temperature service are the most common submissions to accredited third-party laboratories.

Elastomers and Vulcanized Rubber

Natural rubber, SBR, NBR, EPDM, neoprene, and thermoplastic elastomers used in seals, gaskets, hoses, and shoe soles are tested to GB/T 15256. These materials are expected to remain flexible at low temperatures, and brittleness data define their minimum service temperature.

Films, Sheets, and Coatings

Thin polymeric films, geomembranes, insulation tapes, and coating layers may be assessed using adapted specimen geometries, provided the material can be cut into the standard strip dimensions without cracking or excessive orientation effects.

Finished Product Cut Sections

Sections cut from finished goods such as cable jackets, pipe fittings, automotive trim, and protective components can be tested when the product thickness permits machining to standard specimen sizes. Testing of actual product material captures the combined effect of processing, additives, and aging on low-temperature performance.

Testing Items

• Brittleness temperature T50: the statistically derived temperature at which 50% of specimens fail under impact, obtained from a series of tests at incremental temperatures.
• Single-temperature pass/fail verification: confirmation that all specimens remain uncracked at a contractually specified temperature, such as minus 30 °C or minus 40 °C.
• Comparative brittleness assessment: side-by-side evaluation of competing materials, formulations, or supplier batches at identical test conditions.
• Low-temperature performance after conditioning: evaluation of brittleness following heat aging, UV exposure, or chemical immersion to simulate degraded service states.
• Verification testing for incoming inspection and lot acceptance against national standard or purchase specification limits.

Testing Methods

Apparatus and Test Medium

The standard apparatus consists of a thermally insulated bath vessel, a specimen holder that clamps each strip as a cantilever, and a motor-driven striking arm that sweeps through the specimen at a controlled linear velocity. A low-temperature heat transfer medium, typically silicone oil, ethanol, or methanol cooled with solid carbon dioxide (dry ice) or liquid nitrogen, maintains the bath at the target temperature with a tolerance of plus or minus 0.5 °C. A calibrated low-temperature thermometer or thermocouple immersed at the specimen position verifies bath uniformity.

Specimen Preparation

For GB/T 5470, the standard plastic strip specimen is typically 20.0 mm long, 2.5 mm wide, and 1.6 mm thick, die-cut or machined from compression-molded plaques or product sections with smooth, defect-free edges. Specimens must be conditioned at standard laboratory atmosphere, usually 23 °C and 50% relative humidity, for a minimum period before testing. Edge nicks, machining marks, and embedded voids act as stress concentrators and must be excluded, as they bias the measured brittleness temperature upward.

Test Procedure

Specimens are mounted in the holder and immersed in the bath at the selected temperature for a defined equilibration period, commonly 3 minutes for standard thickness specimens, ensuring the entire specimen reaches bath temperature. After equilibration, the striker is released and impacts each specimen within a short, standardized interval after leaving the bath. Each specimen is immediately examined for cracks, chips, or complete fracture. The procedure is repeated at several temperatures bracketing the expected transition region, typically in increments of 2 °C to 5 °C, until both all-pass and all-fail temperatures are established.

Calculation of Brittleness Temperature

The failure data at each temperature are treated statistically using the geometric-mean probability method prescribed by the standard, which weights the nearest all-pass and all-fail temperature groups to interpolate the temperature corresponding to 50% failure probability. This calculated value is reported as the brittleness temperature T50. The test report must state the standard designation, specimen dimensions, bath medium, test temperatures, number of failures at each temperature, and the calculated brittleness temperature.

Applications

• Automotive engineering: qualification of exterior seals, gaskets, bellows, hose compounds, battery components, and body trim that must survive cold-region winters without cracking.
• Wire and cable: verification that insulation and sheathing compounds for power, communication, and automotive cable remain impact-resistant at rated low service temperatures.
• Pipeline and infrastructure: assessment of PVC and polyolefin pipes, fittings, valves, and geomembranes intended for buried or exposed cold-environment installations.
• Footwear and consumer goods: confirmation that rubber outsoles, elastic components, and flexible plastic parts retain toughness in winter conditions.
• Packaging and films: evaluation of freezer-grade and cold-chain packaging films that must withstand handling shocks at refrigeration temperatures.
• Quality control and certification: lot release testing, supplier qualification, and support for product declarations referencing GB, ISO, or ASTM low-temperature requirements.

FAQ

What is the difference between GB/T 5470 and ASTM D746? Both standards determine the impact brittleness temperature of plastics using a cantilever specimen struck at low temperature. GB/T 5470 is aligned with ISO 974, while ASTM D746 uses similar apparatus with slightly different specimen geometry and calculation conventions; results are usually comparable but not automatically interchangeable for specification purposes.

How many specimens are required? A typical determination uses 15 to 30 specimens distributed across several test temperatures, with at least five specimens per temperature, to allow statistical interpolation of the 50% failure temperature. Single-temperature pass/fail verification commonly requires a smaller defined set of specimens per lot.

Is brittleness temperature the same as glass transition temperature (Tg)? No. Tg is a physical transition measured by thermal analysis, whereas the impact brittleness temperature is an engineering performance limit measured mechanically. The brittleness temperature typically lies below Tg for many plastics and reflects both material behavior and test geometry.

Which cooling media are used in the bath? Silicone oil, ethanol, and methanol are the most common media, cooled by dry ice or liquid nitrogen. The medium must remain fluid and thermally stable at the test temperature and must not chemically affect the specimen during the short immersion period.

How should results be interpreted for product design? The brittleness temperature defines a lower bound for impact-loaded service, and designers generally apply a safety margin above it. For pass/fail specifications, the agreed test temperature should reflect the lowest service temperature plus an appropriate margin for impact severity and material aging.