Impact Embrittlement Strength Testing – Brittleness Temperature Determination for Plastics and Elast

Impact embrittlement strength testing, also known as brittleness temperature testing by impact, is a standardized laboratory method used to determine the lowest temperature at which plastics, rubbers, and elastomeric materials can withstand impact loading without brittle fracture. This critical low-temperature performance test supports material qualification and quality control in industries such as automotive, wire and cable, construction, and aerospace. Accredited third-party laboratories conduct this testing in accordance with recognized standards including ASTM D746, ISO 974, ISO 812, and GB/T 5470.

Technical Overview

Impact embrittlement strength testing measures the temperature at which a polymeric material transitions from ductile, energy-absorbing behavior to brittle, fracture-prone behavior when subjected to a sudden impact. As the ambient temperature decreases, molecular mobility in plastics and elastomers is progressively restricted, and the material loses its ability to deform plastically under load. Below a critical temperature, even a modest impact can initiate cracking or complete fracture.

The test is performed by clamping strip-shaped specimens as cantilevers in a temperature-controlled bath and striking them with a calibrated impactor moving at a specified velocity. A specimen is considered to have failed if it breaks, cracks, or shows visible cracking when subsequently bent. The brittleness temperature is statistically defined as the temperature at which 50% of the specimens fail, providing a reproducible index of low-temperature toughness.

This test method is widely specified in material datasheets, purchasing specifications, and regulatory requirements for components that must remain flexible and impact-resistant in cold service environments.

• ASTM D746 – Standard Test Method for Brittleness Temperature of Plastics and Elastomers by Impact
• ISO 974 – Plastics: Determination of the brittleness temperature by impact
• ISO 812 – Rubber, vulcanized or thermoplastic: Determination of low-temperature brittleness
• GB/T 5470 – Determination of brittleness temperature of plastics by impact
• GB/T 15256 – Determination of low-temperature brittleness of vulcanized or thermoplastic rubber
• JIS K 7216 – Testing method for brittleness temperature of plastics by impact

Testing Samples

Thermoplastic Resins and Molded Articles

Compression-molded or injection-molded sheets of polypropylene, polyethylene, PVC, ABS, polystyrene, polyamide, and other thermoplastics can be cut into standard strip specimens. Testing of molded plaques allows direct evaluation of the compound as it will be processed in production.

Rubber and Elastomeric Compounds

Vulcanized compounds such as natural rubber, SBR, NBR, EPDM, silicone rubber, and fluoroelastomers are frequently tested to verify low-temperature flexibility for seals, gaskets, and hoses. Both cured sheets and specimens taken from finished parts may be evaluated.

Films, Sheets, and Coated Fabrics

Flexible packaging films, geomembranes, roofing membranes, and polymer-coated textiles used in outdoor or cold-climate applications can be assessed for impact resistance at low temperature using adapted specimen geometries.

Cable Insulation and Sheathing Compounds

Insulation and jacketing materials intended for outdoor, underground, or arctic cable installations are routinely screened for brittleness temperature to confirm compliance with product safety standards referenced by IEC and UL specifications.

Testing Items

Depending on the engineering requirement, testing can be performed as a full statistical determination or as a simple go/no-go verification at a specified temperature:

• Determination of the brittleness temperature at which 50% of specimens fail (T50)
• Impact embrittlement verification at a specified low temperature, reported as pass or fail
• Low-temperature impact screening for batch release and incoming material inspection
• Comparative evaluation of formulations, plasticizer levels, or resin grades
• Verification of low-temperature performance after heat aging, fluid exposure, or weathering
• Failure analysis support for components that cracked or shattered in cold service

Testing Methods

Apparatus and Test Principle

The test uses a dedicated brittleness temperature apparatus consisting of a specimen holder with a clamping device, a motor-driven or spring-actuated impactor with a defined striking edge, and a temperature-controlled bath filled with a heat transfer medium such as silicone oil, methanol, or ethanol cooled with dry ice or liquid nitrogen. The impactor strikes the cantilevered specimen at a velocity of approximately 1.8 to 2.2 m/s, as required by the applicable standard, while the specimen is immersed at the test temperature.

Specimen Preparation

Typical specimens for plastics are strips measuring approximately 31.75 mm in length and 6.35 mm in width, cut from sheets of uniform thickness near 2 mm. Edges must be smooth and free of nicks or voids, which could act as crack initiation sites. Rubber standards specify similar strip geometries appropriate to the compound and product form. Specimens are conditioned at standard laboratory atmosphere before immersion in the cooled medium for the specified equilibration period, typically around three minutes.

Test Procedure

After thermal equilibration, each specimen is struck once by the impactor while still immersed in the medium. The specimen is then removed and inspected: any complete break, visible crack, or crack revealed by gently bending the specimen constitutes a failure. Testing is repeated at a series of temperatures using groups of specimens, with temperature steps chosen to bracket the region where the failure rate transitions from 0% to 100%.

Data Evaluation

The failure and survival data collected at each temperature are analyzed statistically, commonly using a bracketing and interpolation approach, to calculate the temperature at which the failure probability equals 50%. This value is reported as the brittleness temperature, together with the test standard, specimen details, impact velocity, and medium used. For go/no-go verification, all specimens must survive at the specified temperature for the material to pass.

Applications

Impact embrittlement strength testing is applied across many sectors where reliable low-temperature performance is essential:

• Automotive: weatherstrips, door seals, hoses, grommets, battery components, and underbody parts exposed to winter conditions
• Wire and cable: insulation and jacketing compounds for outdoor, underground, and arctic installations
• Pipes and fittings: plumbing and pressure pipe systems installed in cold climates
• Aerospace and defense: seals, O-rings, dampers, and elastomeric components exposed to high-altitude or extreme-cold environments
• Construction: roofing membranes, geomembranes, window gaskets, and expansion joints
• Consumer products: footwear, appliances, sporting goods, and packaging used in freezing conditions
• Research and quality control: formulation development, supplier qualification, batch release, and root-cause investigation of cold-weather failures

FAQ

What is the difference between brittleness temperature and glass transition temperature?

The glass transition temperature (Tg) is a fundamental thermal property measured by techniques such as DSC, describing the temperature range over which the amorphous phase changes from glassy to rubbery behavior. The brittleness temperature is an engineering performance value determined directly by impact testing and reflects the combined influence of molecular structure, crystallinity, defects, and specimen geometry. Brittleness temperature is therefore the more practical criterion for specifying impact resistance in service.

How many specimens are required for the test?

For a full brittleness temperature determination, groups of specimens are tested at several temperatures to bracket the 50% failure point, commonly requiring a total of several dozen specimens. For a simple pass/fail verification at a specified temperature, a smaller defined group of specimens is tested and all must pass.

What temperature range can be achieved?

Using dry-ice-cooled media, temperatures down to approximately minus 70 degrees Celsius are routinely achievable, while liquid nitrogen systems can extend the range to below minus 100 degrees Celsius, covering the requirements of virtually all commercial polymer materials.

Can finished products be tested directly?

Yes, where geometry permits, specimens can be cut from finished parts such as seals, hoses, or cable jackets. Where direct sampling is not possible, test plaques molded from the same compound under controlled conditions provide an acceptable alternative.

How are the results applied in material selection?

Designers typically require the brittleness temperature of a material to be comfortably below the lowest expected service temperature, with a safety margin accounting for impact severity, aging, and processing variations. A lower brittleness temperature indicates better retention of toughness in cold environments.