Plastic pendulum impact strength testing is a core mechanical evaluation method used to quantify how plastic materials resist fracture under high-speed impact loading. This article provides a comprehensive technical overview of Charpy and Izod pendulum impact tests, covering international standards such as ISO 179-1, ISO 180, ASTM D256, and GB/T 1043, along with specimen preparation, test procedures, result calculation, and typical industrial applications.
Technical Overview
Plastic pendulum impact strength testing measures the energy absorbed by a plastic specimen when it is fractured by a single swing of a pendulum hammer. The pendulum is released from a fixed height, strikes a supported specimen at the lowest point of its arc, and swings through to a reduced height after breaking the sample. The difference between the initial and residual potential energies of the pendulum, corrected for friction and windage losses, equals the energy absorbed by the specimen. When this absorbed energy is normalized by the cross-sectional area of the specimen, the result is reported as impact strength in kJ/m² (ISO method) or as energy per unit width in J/m (ASTM method).
Two principal test geometries are used worldwide. In the Charpy configuration, the specimen is supported horizontally on two anvils and struck at mid-span opposite the notch, following ISO 179-1, ASTM D6110, and GB/T 1043. In the Izod configuration, the specimen is clamped vertically as a cantilever beam and struck above the clamping point on the notched face, following ISO 180, ASTM D256, and GB/T 1843. Both tests can be performed on notched or unnotched specimens, and each provides complementary information about a material's ductility, brittleness, and notch sensitivity.
Because plastics are viscoelastic materials whose impact behavior is strongly influenced by temperature, strain rate, molecular orientation, and stress concentration, pendulum impact data are essential for material qualification, quality control, and design decisions involving dynamic loading conditions.
Testing Samples
Notched Specimens
Notched specimens are the most common format for pendulum impact testing. The machined or molded notch acts as a controlled stress concentrator that initiates crack formation, allowing the test to differentiate between materials that resist crack propagation and those that fail in a brittle manner. ISO 179-1 and ISO 180 define three notch types: Type A with a notch root radius of 0.25 mm, Type B with a radius of 1.0 mm, and Type C with a sharp radius of 0.1 mm. Notch quality, surface finish, and machining consistency have a significant influence on test results and must be tightly controlled.
Unnotched Specimens
Unnotched specimens measure the total energy absorption capability of a material without an artificial stress concentrator. This format is useful for comparing overall toughness, particularly for highly filled or fiber-reinforced compounds where the notch effect may dominate the failure mode.
Standard Specimen Dimensions
- Charpy specimen (ISO 179-1): 80 mm × 10 mm × 4 mm, supported on a 62 mm span
- Izod specimen (ISO 180): 80 mm × 10 mm × 4 mm, clamped vertically with the notch aligned to the top of the clamp
- Izod specimen (ASTM D256): 63.5 mm × 12.7 mm × 3.2 mm to 12.7 mm thick, with a standard remaining ligament of 10.16 mm after notching
- Specimens may be taken in the edgewise or flatwise direction to evaluate orientation effects in molded parts
Specimen Conditioning
All specimens must be conditioned before testing, typically at 23 °C ± 2 °C and 50% ± 5% relative humidity in accordance with ISO 291 or ASTM D618, unless a specific conditioning protocol is required by the material specification.
Applicable Materials
The pendulum method is suitable for rigid thermoplastics, thermoplastic elastomers, thermosetting materials, and fiber-reinforced composites. Typical material families include polypropylene, polyethylene, ABS, polycarbonate, polyamide, PVC, PMMA, POM, polyester compounds, and phenolic or epoxy-based thermosets. Flexible films and thin sheets are generally evaluated with other methods such as free-falling dart impact testing.
Testing Items
Pendulum impact programs are typically structured around the following test items, each producing a quantified result supported by fracture classification:
- Charpy impact strength, notched (kJ/m²) — ISO 179-1, GB/T 1043
- Charpy impact strength, unnotched (kJ/m²) — ISO 179-1
- Izod impact strength, notched (J/m or kJ/m²) — ISO 180, ASTM D256, GB/T 1843
- Izod impact strength, unnotched (J/m) — ASTM D256, ASTM D4812
- Charpy impact strength at reduced temperatures (−40 °C, −20 °C, 0 °C) to assess cold-weather brittleness
- Fracture type classification: complete break (C), hinged break (H), partial break (P), non-break (NB), as defined by ISO 179-1
- Instrumented pendulum testing, which records force–displacement curves to separate crack initiation energy from crack propagation energy
Testing Methods
Apparatus and Energy Selection
The test is performed on a calibrated pendulum impact machine equipped with interchangeable hammers of defined energy capacity, for example 1 J, 2.75 J, 5.5 J, 11 J, and 22 J for Charpy testing. The standards require that the absorbed energy fall within an acceptable fraction of the pendulum capacity — typically between 10% and 80% — to ensure measurement accuracy. The machine must be zeroed with an empty swing before each series to quantify friction and air resistance losses.
Test Procedure
- Condition the specimens to equilibrium at the specified temperature and humidity
- Measure the width and thickness (or remaining ligament thickness for notched specimens) at the fracture plane to an accuracy of 0.02 mm
- Select a hammer capacity appropriate for the expected energy absorption
- Perform an empty swing and verify the zero reading
- Position the specimen with the notch aligned toward the striker and correctly seated against the supports or clamp
- Release the pendulum from its latched height and record the absorbed energy from the indicating scale or digital readout
- Inspect the broken halves and classify the fracture type
- Test a statistically valid number of specimens, with ISO methods typically requiring at least ten valid results, and calculate the mean and standard deviation
Calculation of Results
For ISO-based Charpy and Izod tests, the notched impact strength is calculated by dividing the corrected absorbed energy, in joules, by the cross-sectional area of the specimen at the notch, in square centimeters or square millimeters, yielding kJ/m². For ASTM D256 Izod tests, the impact resistance is calculated by dividing the absorbed energy by the specimen thickness, yielding J/m. Because the two calculation bases differ, ISO and ASTM values must never be compared directly, and the reporting standard must always be stated alongside the result.
Factors Affecting Results
Key variables include test temperature, hammer energy and impact velocity, notch geometry and machining quality, specimen orientation relative to processing direction, material moisture content (notably for polyamides), and thermal history of the molded specimen. Laboratories must document these parameters for results to be reproducible and comparable between batches and suppliers.
Applications
Pendulum impact strength data support a wide range of engineering and commercial decisions across the plastics industry:
- Material datasheet generation and comparative selection of engineering resins
- Incoming resin and compound quality control, including verification of impact modifier content
- Injection molding process validation, detecting degradation or excessive residual stress in molded parts
- Automotive component qualification for interior trim, exterior body panels, bumper systems, and under-hood parts requiring ductile behavior at low temperature
- Pipes, fittings, and construction products where impact durability is a regulatory and safety requirement
- Packaging materials evaluation for crates, containers, and closures subjected to drop and handling loads
- Research and development of toughened, filled, or recycled compounds, where impact retention is a key performance indicator
- Certification support for accredited third-party laboratories conducting conformity assessment against ISO, ASTM, EN, JIS, and GB standards
FAQ
Q: What is the difference between the Charpy and Izod test? A: In the Charpy test the specimen lies horizontally on two supports and is struck at mid-span, while in the Izod test the specimen is clamped vertically as a cantilever and struck above the clamp. The two geometries apply stress differently and produce numerically different results, so the test method must always accompany the reported value.
Q: Why are notched specimens used? A: The notch creates a reproducible stress concentration that promotes crack initiation, sharpens the distinction between brittle and ductile materials, and measures resistance to crack propagation, which reflects real-world service conditions where scratches, weld lines, and sharp corners act as stress raisers.
Q: Which units are reported? A: ISO-based Charpy and Izod results are expressed in kJ/m² based on the fracture cross-section, whereas ASTM D256 Izod results are expressed in J/m based on specimen width.
Q: How does temperature affect impact strength? A: Most plastics become significantly more brittle as temperature decreases. Testing at subambient temperatures, such as −40 °C, is common for outdoor, automotive, and cold-chain applications to confirm that the material does not undergo a ductile-to-brittle transition within the service range.
Q: What does a partial or non-break result mean? A: Per ISO 179-1, a hinged or partial break indicates substantial ductility, while a complete break indicates brittle behavior. Non-break results exceed the practical energy range of the machine configuration and may require a higher-capacity hammer or unnotched testing to obtain valid comparative data.
Q: Can fiber-reinforced plastics be tested? A: Yes. Reinforced compounds are commonly tested with Charpy unnotched or notched methods, and results are reported alongside the observed fracture classification, since aligned fibers introduce strong anisotropy that must be considered when interpreting the data.