Brittleness impact testing determines the temperature and loading conditions under which plastic materials transition from ductile to brittle failure when subjected to rapid impact. This guide explains the technical principles, specimen requirements, and internationally recognized test methods—including ASTM D746, ISO 974, ISO 179-1, and ASTM D256—used by accredited third-party laboratories to evaluate the low-temperature impact performance of polymers.
Technical Overview
Brittleness impact testing evaluates the resistance of plastic materials to sudden, brittle fracture under impact loading, particularly at reduced temperatures. Many polymers that behave in a tough, ductile manner at room temperature can fail catastrophically in a brittle mode when cooled below a critical threshold or subjected to high strain rates, sharp notches, aggressive environments, or long-term aging. Because brittle failure occurs with little or no prior deformation and propagates rapidly, it is one of the most dangerous and least predictable failure modes in plastic components.
The central concept behind this testing family is the ductile-to-brittle transition. As temperature decreases, the mobility of polymer chain segments is restricted, and the yield strength of the material rises while its ability to absorb energy through plastic deformation falls. When the yield stress exceeds the fracture stress of the material, the failure mode switches from ductile tearing to brittle cracking. The temperature at which this transition occurs—often expressed as the impact brittleness temperature or the temperature at which 50% of specimens fail (T50)—is a key design parameter for any plastic part that will see service in cold climates or dynamic loading.
Brittleness impact testing therefore serves two complementary purposes: quantifying the lowest temperature at which a material retains acceptable impact toughness, and characterizing impact energy absorption under standardized impact geometries. Results support material qualification, quality control, comparative material selection, and failure analysis across industries such as automotive, packaging, piping, construction, and consumer electronics.
Testing Samples
Molded Standard Specimens
Most brittleness and impact tests use specimens produced by injection molding or compression molding directly from the compound under evaluation, ensuring that the test piece reflects the morphology and internal stresses of the processed material. Typical specimen geometries include bars of 80 mm × 10 mm × 4 mm for pendulum impact tests, with conditioning performed in a controlled atmosphere (typically 23 °C and 50% relative humidity) prior to testing to ensure reproducible results.
Notched Specimens
Notched specimens are machined with a precisely controlled radius (commonly 0.25 mm, 0.5 mm, or 1.0 mm) to create a stress concentrator. Notching dramatically reduces the apparent impact strength of many plastics because it forces triaxial stress states and suppresses shear yielding. Comparing notched and unnotched performance reveals the notch sensitivity of the material, which is one of the most practical indicators of inherent brittleness.
Film and Sheet Specimens
For flexible packaging films, geomembranes, and thin sheets, brittleness is evaluated using rectangular strips or circular discs subjected to low-temperature impact by a falling dart, pendulum striker, or projectile. These specimens must be free of wrinkles, gels, and edge damage, since flaws act as crack initiators and can bias results toward premature brittle failure.
Finished Products and Components
Molded parts, pipe fittings, containers, and housings can be tested in their end-use configuration, either as whole-component impact tests or by cutting specimens from designated locations. Testing actual products captures the influence of gate locations, weld lines, fiber orientation, wall thickness variation, and assembly features—factors that often dominate real-world brittle failures.
Testing Items
Depending on the material, product form, and application requirements, brittleness impact testing typically covers the following measurable properties:
- Impact brittleness temperature (Tb) and the 50% failure temperature (T50) determined by statistical analysis of pass/fail impact results across a temperature series
- Charpy impact strength, notched and unnotched, expressed in kJ/m²
- Izod impact strength, notched and unnotched, expressed in J/m or kJ/m²
- Tensile impact strength for thin or highly ductile materials
- Instrumented puncture impact behavior, including peak force, total energy, and force–displacement curves from multi-axial impact
- Ductile-to-brittle transition temperature determined from the inflection of the energy-versus-temperature curve
- Notch sensitivity and sharp-notch performance
- Low-temperature impact retention after thermal aging, weathering, or chemical exposure
Testing Methods
Impact Brittleness Temperature Method (ASTM D746 / ISO 974 / GB/T 5470)
This is the classical brittleness test for plastics. Specimens are clamped in a fixture and immersed in a temperature-controlled bath, typically a mixture of methanol or silicone oil cooled with dry ice or a mechanical chiller. After a defined conditioning period at the set temperature, a spring-actuated or motorized striker delivers a single, sharp impact to each specimen at a specified velocity. Each specimen is then examined visually for cracks, breaks, or fissures and recorded as a pass or fail. By testing groups of specimens at incrementally lower temperatures, the 50% failure temperature (T50) is calculated using statistical methods. The lower the T50, the better the material retains flexibility and toughness in cold service.
Charpy Pendulum Impact (ISO 179-1 / ASTM D6110)
In the Charpy configuration, a horizontally supported specimen is struck at mid-span by a pendulum hammer. The energy absorbed in breaking the specimen is calculated from the difference between the initial and residual heights of the pendulum. The test can be performed with edgewise, flatwise, or unnotched geometries and at temperatures from −70 °C to +150 °C using environmental chambers. Charpy testing is the dominant method in Europe and is particularly effective for materials that exhibit complete brittle fracture, providing a direct measure of fracture energy per unit cross-sectional area.
Izod Pendulum Impact (ASTM D256 / ISO 180)
The Izod test clamps the specimen vertically as a cantilever beam and strikes it above the clamp with the notch facing the striker. Widely used in North America and for quality control of engineering resins, the Izod method yields results in joules per meter of notch depth (ft·lb/in in US units). Notched Izod values are highly sensitive to notch radius and processing conditions, making them excellent for detecting embrittlement caused by moisture, degradation, poor dispersion of fillers, or excessive regrind content.
Instrumented Falling Dart and Drop Weight Impact (ISO 6603-2 / ASTM D3763)
Instrumented multi-axial impact testing drives a hemispherical striker through a clamped disc or plate at a controlled velocity while a force sensor records the entire force–displacement history. This method distinguishes between ductile failure, which shows a gradual force decline after yield, and brittle failure, which shows a sharp force drop with radial cracking and low absorbed energy. Because it captures the full energy signature rather than a single value, it is the preferred technique for mapping the ductile-to-brittle transition of automotive exterior panels, pipes, and packaging structures.
All methods require strict control of striker geometry, impact velocity, support span, specimen temperature at the moment of impact, and the time interval between removal from the cold environment and striking. Temperature overshoot of even a few degrees can shift results significantly, which is why accredited third-party laboratories maintain calibrated cryogenic equipment and documented cooling medium procedures in accordance with the referenced standards.
Applications
Brittleness impact testing is indispensable wherever plastics encounter cold temperatures, dynamic loads, or impact events during manufacture, transport, or service. Representative application areas include:
- Automotive: bumpers, body panels, battery housings, and interior trim that must withstand low-speed collisions and stone impact at sub-zero temperatures
- Packaging: films, containers, and closures requiring drop and cold-chain impact resistance during filling, distribution, and freezer storage
- Pipes and fittings: water, gas, and drainage systems verified for toughness in cold-climate installation and trenching conditions
- Construction: window profiles, roofing membranes, and insulation materials exposed to seasonal temperature cycling
- Consumer electronics and appliances: housings and enclosures subjected to drop impact in end-use environments
- Material development: screening impact modifiers, elastomer blends, and recycled content for low-temperature toughness retention
- Failure analysis: determining whether a service failure was caused by material embrittlement, notch effects, or environmental stress
FAQ
What is the impact brittleness temperature of a plastic?
It is the temperature at which a plastic, when struck under specified impact conditions, transitions from ductile to brittle behavior. In standardized testing it is commonly reported as the temperature at which 50% of the specimens fail (T50), determined statistically from pass/fail results across a temperature series according to ASTM D746, ISO 974, or GB/T 5470.
How is a brittle failure different from a ductile failure in impact testing?
A ductile failure involves significant plastic deformation, yielding, drawing, or tearing, and absorbs a large amount of energy before separation. A brittle failure occurs with little deformation, produces clean, sharp fracture surfaces, sometimes with mirror and mist zones, and absorbs very little energy. Instrumented impact testing distinguishes the two modes clearly from the shape of the force–displacement curve.
Why are both notched and unnotched impact results reported?
Unnotched values reflect the intrinsic toughness of the material, while notched values reveal how it behaves in the presence of sharp stress concentrators such as scratches, weld lines, or sharp corners. A large difference between the two indicates high notch sensitivity, which is a warning sign for brittle fracture in real components. Standards such as ISO 179-1 and ASTM D256 specify several notch radii so that this sensitivity can be quantified.
What factors influence brittleness test results?
Key factors include test temperature, impact velocity, notch radius and quality, specimen conditioning (temperature and humidity), thermal history and internal mold stresses, material degradation during processing, filler and impact modifier content, crystallinity, specimen thickness, and the accuracy of the cooling medium temperature. Consistent control of these parameters is essential for reproducible, comparable data.
Which standard should be selected for a given project?
The choice depends on the material form, market region, and specification requirements: ASTM D746 or ISO 974 for brittleness temperature of molded bars, ASTM D256 or ISO 180 for Izod impact, ISO 179-1 or ASTM D6110 for Charpy impact, and ISO 6603-2 or ASTM D3763 for instrumented puncture impact on plates and finished parts. Accredited third-party laboratories can advise on the most appropriate method and specimen preparation for each application and can combine several methods to construct a complete low-temperature toughness profile of a material.