Impact absorbed energy testing measures the energy a plastic specimen dissipates when fractured by a swinging pendulum, providing an essential index of material toughness and notch sensitivity. This guide explains the Charpy and Izod test principles, the governing ISO, ASTM, and GB standards, specimen preparation requirements, and the typical industrial applications of impact energy determination for plastic materials.
Technical Overview
Impact absorbed energy refers to the quantity of mechanical energy that a plastic specimen dissipates when it is struck and fractured by a moving striker, typically the hammer of a pendulum impact machine. The pendulum is released from a defined height; part of its stored potential energy is consumed to initiate and propagate a crack through the specimen, and the difference between the initial energy and the residual energy of the pendulum is recorded as the absorbed energy. When this energy is normalized by the cross-sectional area of the specimen, or by the area remaining beneath a notch, the result is the impact strength, generally expressed in kilojoules per square meter (kJ/m²).
Because plastics exhibit strong strain-rate dependence and marked temperature sensitivity, impact behavior cannot be reliably inferred from slow-speed tensile data. High-speed loading exposes brittleness, notch sensitivity, and internal defects such as voids, poor weld lines, or molecular degradation. Impact energy data therefore serve as a key acceptance criterion in material specifications, component qualification, quality control, and failure analysis across the plastics industry.
Testing Samples
Standard Injection-Molded Bars
The reference specimen for Charpy and Izod testing is the ISO multi-purpose bar, measuring 80 mm × 10 mm × 4 mm. Specimens are typically injection molded directly to final dimensions or machined from the central portion of molded plates. Fiber orientation, gate location, and molding conditions strongly influence results, so the sampling position and direction must always be documented.
Notched Specimens
Notches are machined into the narrow face of the bar to create a controlled stress concentration. ISO 179 and ISO 180 define notch geometries by cutting-tool radius: Type A (0.25 mm radius), Type B (1.00 mm radius), and Type C (0.10 mm radius), leaving a residual ligament width of 8.0 mm beneath the notch. Notch cutting quality, including tool sharpness and surface finish, directly affects the measured energy, so notches must be freshly machined with calibrated cutters in accordance with ISO 2818.
Specimens from Finished Parts
When testing must reflect an actual component, specimens can be machined from molded parts, extruded profiles, pipes, sheets, or films. In these cases the resulting values are comparative rather than absolute material data, and deviations from the standard 4 mm thickness must be clearly noted in the report.
Conditioning
Specimens are conditioned at 23 °C ± 2 °C and 50 % ± 10 % relative humidity in line with ISO 291, normally for at least 16 hours, and for extended periods with moisture-sensitive materials such as polyamides. Test temperature has a pronounced effect on the outcome, particularly near the glass transition or ductile–brittle transition of the material.
Testing Items
- Charpy impact strength, unnotched (edgewise and flatwise)
- Charpy impact strength, notched (Type A, Type B, and Type C notches)
- Izod impact strength, notched and unnotched
- Instrumented Charpy and Izod impact testing with force–deflection recording
- Tensile impact strength (ISO 8256) for thin or highly ductile materials
- Temperature-dependent impact testing, typically from −40 °C to 23 °C, for cold-weather performance evaluation
- Multiaxial puncture and falling-mass impact testing for films, sheets, and finished components (ISO 6603, ASTM D5628)
Testing Methods
Charpy Impact Test (ISO 179-1, GB/T 1043.1, ASTM D6110)
In the Charpy configuration, the specimen rests horizontally on two anvils separated by a 62 mm span and is struck at mid-span by the pendulum striker. The absorbed energy is read from the encoder and corrected for friction and air-resistance losses. Tests are performed edgewise or flatwise, with the notch oriented toward or away from the striker as specified. Break behavior is classified as complete break, hinge break, partial break, or non-break, and this classification is always reported together with the numerical value.
Izod Impact Test (ISO 180, GB/T 1843, ASTM D256)
For the Izod test, one end of the vertical specimen is clamped as a cantilever, and the free notched end is struck by the pendulum at a fixed distance above the clamp. Izod results are commonly cited in North American material datasheets, while Charpy data dominate in Europe and Asia. The two values are not interchangeable, even for identical materials, because the stress states and clamping conditions differ fundamentally.
Instrumented Impact Testing (ISO 179-2)
Instrumented machines incorporate force transducers in the striker and high-speed displacement measurement, producing a complete force–deflection curve. This allows the energy consumed in crack initiation to be separated from the energy consumed in crack propagation, which is critical for understanding toughening mechanisms, identifying ductile–brittle transitions, and validating finite-element simulation models.
Equipment and Calibration Requirements
Pendulum machines must be verified in accordance with ISO 13802, covering potential energy, striking velocity (commonly 2.9 m/s ± 10 % for plastics), friction losses, and frame stiffness. A hammer capacity is selected so that the absorbed energy falls within the recommended working range of the pendulum; if the specimen does not break, a lower-energy hammer is used. At least five, and preferably ten, valid specimens are tested for each condition, and the mean value with standard deviation is calculated and reported.
Applications
- Automotive: bumper systems, interior trim, and under-hood components, verifying low-temperature toughness required by OEM material specifications
- Electrical and electronic equipment: enclosures, connectors, and switch housings, supporting compliance with UL and IEC safety requirements for impact resistance
- Packaging: films, bottles, crates, and pallets, assessing drop resistance and transport durability
- Pipes and fittings: evaluating resistance to impact damage during handling and installation in accordance with EN and ISO product standards
- Construction: window profiles, roofing sheets, glazing, and safety products exposed to mechanical abuse
- Consumer goods and appliances: housings, handles, and sports equipment requiring durable, shatter-resistant performance
- Material development: ranking impact modifiers, recycled-content compounds, and formulation changes; incoming inspection and lot-release testing
- Failure analysis: comparing the impact energy of failed parts against reference material to detect degradation, contamination, or processing faults
FAQ
What is the difference between Charpy and Izod impact tests?
The Charpy test strikes a horizontally supported bar at mid-span, while the Izod test strikes a vertically clamped cantilever specimen. The two methods impose different stress states at failure, so results differ and must never be compared directly; the test method must always be stated alongside the reported value.
Why are notches introduced into impact specimens?
A machined notch creates a triaxial stress concentration that promotes brittle fracture and reduces data scatter, making the measurement highly sensitive to differences in notch sensitivity between materials. Notched impact strength reflects resistance to crack propagation, whereas unnotched values characterize overall toughness.
What does a "no break" result mean?
When a specimen bends or deforms without fracturing, which is common with tough engineering plastics, the result is reported as "no break". The operator then selects a lower-energy hammer or a lower test temperature; alternatively, tensile impact or instrumented puncture methods may be more appropriate for such materials.
Which factors most influence the measured impact energy?
Notch geometry and machining quality, specimen thickness and fiber orientation, molding defects, test temperature and humidity, hammer capacity selection, and machine calibration all significantly affect results. Consistent specimen preparation and conditioning are essential for reproducible data.
How is impact strength calculated and expressed?
Impact strength equals the corrected absorbed energy divided by the original cross-sectional area for unnotched specimens, or by the area beneath the notch for notched specimens, and is reported in kJ/m². For instrumented tests, characteristic forces, deflections, and partitioned energy values may also be reported.
Can impact tests be performed at low temperatures?
Yes. Temperature-controlled chambers permit testing from below −70 °C up to elevated temperatures, allowing ductile–brittle transition behavior to be mapped. This capability is particularly relevant for outdoor products, automotive cold-climate requirements, and refrigeration appliances. Accredited third-party laboratories routinely offer these conditioned-temperature programs.