Impact brittleness performance testing determines the lowest temperature at which plastics, rubbers, and elastomers can withstand impact loading without cracking or shattering. Governed by internationally recognized standards such as ASTM D746, ISO 974, ISO 812, and GB/T 5470, this method is essential for evaluating the cold-temperature reliability of materials used in automotive, cable, construction, and outdoor applications. This article explains the test principles, specimen requirements, operating procedures, data analysis, and practical applications of impact brittleness testing.
Technical Overview
Impact brittleness testing, commonly known as brittleness temperature testing or low-temperature impact testing, is a standardized method used to determine the temperature at which polymeric materials lose ductility and fracture in a brittle manner under impact. As temperature decreases, molecular mobility in plastics and rubbers diminishes, reducing the material's ability to absorb impact energy through ductile deformation. Below a critical temperature, even a modest impact can initiate cracks or cause complete fracture.
In the test, a cantilever-held specimen is immersed in a temperature-controlled bath and struck by a precision-machined striker moving at a controlled velocity. Each specimen is inspected for cracks or breaks, and the results are analyzed statistically to determine the brittleness temperature, defined as the temperature at which 50 percent of the specimens fail (commonly designated BT50). The method provides a comparative, single-point index of low-temperature toughness and is widely referenced in material datasheets, procurement specifications, and quality control programs.
Governing Standards
- ASTM D746 — Standard Test Method for Brittleness Temperature of Plastics and Elastomers by Impact
- ISO 974 — Plastics: Determination of the brittleness temperature by impact
- GB/T 5470 — Chinese national standard for determination of brittleness temperature of plastics by impact
- ISO 812 — Rubber, vulcanized or thermoplastic: Determination of low-temperature brittleness
- ASTM D2137 — Rubber Property: Brittleness Point of Flexible Polymers and Coated Fabrics
- JIS K 7216 — Japanese standard for the impact brittleness temperature of plastics
- IEC 60811-504 — Cold impact test for insulation and sheathing materials of electric cables
Testing Samples
Rigid and Flexible Plastics
Thermoplastics such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), and polyamides (PA). Specimens are typically cut from compression-molded or injection-molded plaques, or taken directly from film, sheet, pipe, or finished components.
Rubbers and Elastomers
Vulcanized thermoset rubbers, including natural rubber (NR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), ethylene propylene diene monomer (EPDM), silicone, and fluorocarbon rubbers, as well as thermoplastic elastomers such as TPE, TPU, and TPV. These materials are commonly evaluated for seals, gaskets, hoses, diaphragms, and vibration isolators.
Films, Sheets, and Coated Fabrics
Packaging films, geomembranes, roofing membranes, agricultural films, and polymer-coated fabrics that must remain flexible during cold-weather transportation, handling, and installation.
Cable Insulation and Sheathing Compounds
Cross-linked or thermoplastic insulation and jacketing compounds, along with short finished-cable sections, evaluated for cold impact performance as part of cable material qualification programs.
Testing Items
Typical test items offered by accredited third-party laboratories include:
- Brittleness temperature (BT50) — the temperature at which 50 percent of specimens fail under the specified impact conditions
- Pass/fail verification at a contract-specified temperature, such as −40 °C or −55 °C, as required by product specifications
- Low-temperature impact resistance of cable insulation and sheaths in accordance with cable testing standards
- Comparative brittleness ranking of different formulations, production batches, or material suppliers
- Verification of low-temperature performance after heat aging, weathering, ozone exposure, or fluid immersion
Testing Methods
Specimen Preparation
For plastics tested to ASTM D746 or ISO 974, specimens are typically rectangular strips approximately 20 mm in length and 2.5 mm in width, with a preferred thickness of about 2 mm. For rubbers tested to ISO 812 or ASTM D2137, specimens are strips cut from cured sheets, commonly on the order of 40 mm long and a few millimeters wide. All specimens must be free of voids, nicks, flash, and surface defects, since stress concentrators can bias the result. Before testing, specimens are conditioned in a standard laboratory atmosphere, typically 23 ± 2 °C and 50 ± 10 percent relative humidity, until moisture and temperature equilibrium is reached.
Apparatus and Temperature Control
The apparatus consists of a specimen holder, a striker with a defined striking-edge geometry, a drive mechanism delivering a controlled impact velocity (commonly in the range of 1.8 to 2.1 m/s for plastic tests), and a low-temperature bath. The bath uses a heat-transfer medium that does not attack the material, such as dry ice in ethanol or methanol, silicone oil or alcohol cooled by liquid nitrogen, or a mechanically refrigerated bath. The medium must maintain the set temperature within ±0.5 °C, verified by a calibrated thermocouple or thermometer positioned adjacent to the specimens.
Test Procedure
- Establish the bath at the target temperature and allow it to stabilize.
- Mount a group of specimens in the holder and immerse them in the medium, conditioning for approximately three minutes to ensure thermal equilibrium.
- Release the striker so that each specimen is struck once at the specified velocity while at temperature.
- Remove the specimens, allow them to return to room temperature, and inspect each one visually, with magnification if necessary, for cracks, partial breaks, or complete fracture.
- Record each specimen as failed or passed, then repeat the sequence at additional temperatures chosen to bracket the transition from fully ductile to fully brittle behavior.
Determination of Brittleness Temperature
For full characterization, testing is performed at a series of temperatures, and the percentage of failed specimens at each temperature is plotted against temperature. Using the graphical or statistical method prescribed by the governing standard, the brittleness temperature is interpolated as the temperature corresponding to 50 percent failures. For pass/fail verification, a defined group of specimens is tested at a single specified temperature, and the material is accepted if no failures occur or if failures remain within the allowable fraction stated in the product specification.
Applications
Impact brittleness testing supports material selection, product qualification, and quality control across many industries:
- Automotive — validating elastomeric seals, gaskets, hoses, boots, and exterior components for service in cold climates
- Wire and cable — confirming that insulation and sheathing compounds remain impact-resistant at the rated low-temperature service limit
- Construction — assessing window seals, roofing membranes, geomembranes, and pipeline coatings installed in freezing environments
- Aerospace and defense — screening non-metallic materials for low-temperature and high-altitude exposure
- Consumer goods and footwear — ensuring that outsoles, grips, and housings retain toughness in winter conditions
- Chemical processing — qualifying hoses, diaphragms, and linings used in refrigerated or cryogenic service
Specification limits vary widely depending on the polymer family, ranging from around −10 °C for some commodity polyolefins to −70 °C or lower for specialty elastomers and fluoropolymers.
FAQ
What exactly does brittleness temperature mean?
Brittleness temperature is the empirically determined temperature at which 50 percent of specimens fail when struck under standardized impact conditions. It is a performance index that reflects the combined effects of polymer chemistry, formulation, morphology, and test geometry rather than an intrinsic material constant.
How is brittleness temperature different from glass transition temperature?
Glass transition temperature (Tg) is a fundamental viscoelastic transition measured by techniques such as DSC or DMA, whereas brittleness temperature is a dynamic, pass/fail mechanical test result obtained under impact at a high deformation rate. The two values are related but not interchangeable, and the brittleness temperature typically provides a more direct indication of impact performance in cold service.
How many specimens are required?
For a full BT50 determination, testing is conducted at several temperatures, commonly with ten or more specimens per temperature to obtain a statistically valid failure distribution. For single-temperature pass/fail screening, groups of five to ten specimens are typically tested as specified by the applicable standard or purchase specification.
Which standard should be selected?
ASTM D746, ISO 974, and GB/T 5470 follow the same basic principle for plastics and differ mainly in specimen details and statistical treatment, so the choice usually follows the region or specification governing the product. Rubber materials are normally tested to ISO 812 or ASTM D2137, while cable insulation and sheaths are evaluated using the cold impact method defined in IEC 60811-504.
Can the brittleness temperature be used directly as a minimum service temperature?
Not by itself. The brittleness temperature is a comparative screening value, and real-world serviceability also depends on deformation rate, stress state, part geometry, and long-term aging. It is best used in combination with other low-temperature evaluations, such as cold bend, low-temperature elongation, or temperature retraction tests, when defining safe service limits.
What media are used to reach very low temperatures?
Common heat-transfer media include dry ice in ethanol or methanol for temperatures near −70 °C, and silicone oil or alcohol baths cooled by liquid nitrogen for temperatures down to approximately −100 °C or lower. The selected medium must remain fluid and chemically inert toward the specimens at the test temperature.