The plastic cantilever beam impact strength test, commonly known as the Izod impact test, is a standardized method for evaluating the impact toughness and notch sensitivity of plastic materials under dynamic loading conditions. Governed primarily by ISO 180, ASTM D256, and GB/T 1843, this test is essential for material qualification, quality control, and product design across the automotive, electronics, packaging, and construction industries. This article provides a comprehensive technical overview of test principles, specimen preparation, procedures, result interpretation, and practical applications.
Technical Overview
The cantilever beam impact test, internationally referred to as the Izod impact test, is one of the most widely used methods for characterizing the impact resistance of rigid plastics. In this test, a rectangular specimen is clamped vertically as a cantilever beam and struck by a swinging pendulum at a defined distance above the clamp. The energy absorbed by the specimen during fracture is measured and converted into impact strength values.
During the test, a pendulum of known potential energy is released from a fixed height. As it strikes and breaks the specimen, part of its kinetic energy is absorbed. The difference between the initial and residual energies, corrected for friction and windage losses, represents the energy consumed in fracturing the specimen. This energy is normalized by the cross-sectional area of the specimen to yield the impact strength, expressed in kJ/m² according to ISO 180 or in J/m according to ASTM D256.
The test can be performed on both notched and unnotched specimens. Notched specimens measure the material's resistance to crack propagation and its notch sensitivity, while unnotched specimens provide a broader assessment of overall toughness. The test is applicable to a wide range of materials including thermoplastics, thermosets, fiber-reinforced composites, and rigid sheets.
- Primary standards: ISO 180, ASTM D256, GB/T 1843, JIS K 7110
- Measured quantity: energy absorbed per unit cross-sectional area (kJ/m²) or per unit width (J/m)
- Instrument: pendulum-type cantilever beam impact tester
- Key capability: evaluation of brittle versus ductile failure behavior under dynamic loading
Testing Samples
Standard Specimen Geometry
According to ISO 180, the preferred specimen is Type 1, measuring 80 mm in length, 10 mm in width, and 4 mm in thickness. The specimen is clamped over a fixed length, leaving a defined free length of 22 mm (for 80 mm long specimens) exposed to the striking pendulum. Under ASTM D256, the standard specimen measures 63.5 mm in length, 12.7 mm in width, and typically 3.2 mm or 6.4 mm in thickness.
Notched Specimens
Notched specimens are machined with a precisely controlled V-shaped notch on one face. The notch serves as a stress concentrator that initiates crack propagation. ISO 180 defines three notch types based on the notch tip radius: Type A (0.25 mm radius), Type B (1.0 mm radius), and Type C (0.1 mm radius). The remaining ligament thickness after notching must be within specified tolerances, typically 8.0 mm ± 0.2 mm for the Type 1 specimen.
Unnotched Specimens
Unnotched specimens are tested in the same configuration but without a machined notch. These results reflect the total energy required to initiate and propagate a fracture and are typically much higher than notched values. Unnotched testing is particularly useful for comparing ductile materials that may not break completely when notched.
Specimen Preparation
Specimens may be produced by injection molding, compression molding, or machining from sheets, plaques, or finished parts. Notches must be machined with a fresh, sharp cutter to avoid introducing compressive residual stresses or heating effects at the notch root, both of which can significantly distort results. Surface defects, voids, and processing-induced orientation can also affect measured values, so representative and consistent preparation is critical.
Conditioning
Prior to testing, specimens shall be conditioned in a controlled atmosphere, typically 23 °C ± 2 °C and 50 % ± 10 % relative humidity for a minimum of 16 hours unless otherwise specified by the relevant material standard.
Testing Items
The cantilever beam impact test program may include the following measurement items, depending on the material specification and customer requirements:
- Notched cantilever beam (Izod) impact strength, Type A, B, or C notch
- Unnotched cantilever beam impact strength
- Impact strength at reduced temperatures (e.g., -40 °C, -30 °C, 0 °C) to assess brittle-ductile transition behavior
- Impact strength at elevated temperatures for heat-resistant grades
- Comparative evaluation of filled, reinforced, or modified material grades
- Failure mode classification: complete break (C), hinge break (H), partial break (P), or non-break (NB)
- Notch sensitivity assessment by comparing notched versus unnotched results
Testing Methods
Apparatus and Calibration
The test employs a pendulum impact machine equipped with interchangeable hammers of different rated energies (for example, 1 J, 2.75 J, 5.5 J, 11 J, and 22 J). The machine must be leveled and calibrated, with friction and air resistance losses verified in accordance with the requirements of ISO 13802 or the calibration section of ASTM D256. The striking edge velocity at impact is nominally 3.5 m/s.
Pendulum Energy Selection
The pendulum capacity shall be selected such that the energy absorbed by the specimen represents between 10 % and 80 % (per ISO 180) or 10 % and 90 % (per ASTM D256) of the pendulum's available energy. If the absorbed energy falls outside this range, a different hammer must be used to ensure measurement accuracy.
Test Procedure
- Condition the specimens and verify dimensions, including width, thickness, and remaining ligament thickness after notching, using calibrated instruments.
- Select an appropriate pendulum and perform an empty-swing check to determine friction losses.
- Clamp the specimen vertically in the vise with the notch facing the pendulum, ensuring the notch plane is level with the top of the clamp within specified tolerance.
- Release the pendulum from its latched position and allow it to strike the specimen at the specified striking height.
- Record the absorbed energy from the scale, dial, or digital display, and apply corrections for friction and windage.
- Examine the fractured specimen and classify the failure mode as complete break, hinge break, partial break, or non-break.
- Repeat the test on a statistically sufficient number of specimens, typically at least five, and calculate the arithmetic mean and standard deviation.
Calculation of Results
For ISO 180, 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, that is, the product of specimen width and the remaining thickness beneath the notch. The result is reported in kJ/m². For ASTM D256, the absorbed energy is divided by the specimen width and reported in J/m. Results for unnotched specimens are calculated using the full cross-sectional area. Non-breaking specimens shall not be averaged with breaking specimens; instead, results are reported separately with the appropriate failure mode designation.
Factors Influencing Results
- Notch quality, tip radius, and machining condition
- Specimen thickness and molding-induced orientation
- Test temperature and conditioning history
- Pendulum energy range relative to absorbed energy
- Material morphology, filler dispersion, and moisture content
- Clamping torque and specimen positioning accuracy
Applications
Cantilever beam impact strength data are used extensively throughout the plastics value chain. Typical applications include:
- Material data sheets and grade comparison for engineering thermoplastics such as ABS, PC, PA, POM, PP, and their reinforced or impact-modified variants
- Incoming material inspection and lot release testing in manufacturing quality control
- Verification of the effectiveness of impact modifiers, fillers, and elastomer additions
- Assessment of low-temperature embrittlement for outdoor, automotive, and cold-chain applications
- Evaluation of degradation caused by aging, UV exposure, hydrolysis, or thermal processing
- Failure analysis of fractured components in service
- Compliance demonstration for material specifications referenced in product standards and regulatory requirements
Accredited third-party laboratories routinely perform Izod impact testing under ISO/IEC 17025 management systems, providing traceable and mutually recognized test reports for suppliers, manufacturers, and certification bodies.
FAQ
What is the difference between Izod and Charpy impact tests?
The Izod (cantilever beam) test uses a vertically clamped specimen struck above the clamp, while the Charpy (simply supported beam) test uses a horizontally supported specimen struck at midspan opposite the notch. The two methods follow different standards, ISO 180 and ISO 179 respectively, and their results are not directly interchangeable.
Why are notched and unnotched results so different?
The notch acts as a stress concentrator that promotes brittle crack initiation. Notched values reflect notch sensitivity and crack propagation resistance, whereas unnotched values include the energy for crack initiation plus propagation. The ratio of the two provides insight into a material's notch sensitivity.
What do the failure mode designations C, H, P, and NB mean?
C denotes complete break, H denotes hinge break where the two halves remain connected by a thin deformable layer, P denotes partial break, and NB denotes non-break. Specimens that do not break yield only nominal values and must be reported separately, as averaging them with fractured specimens is statistically invalid.
At what temperature is the test performed?
The standard laboratory condition is 23 °C with 50 % relative humidity, but testing can be conducted at subzero or elevated temperatures using environmental chambers to map the brittle-ductile transition of the material.
How many specimens should be tested?
A minimum of five specimens per condition is recommended unless the material standard specifies otherwise. For heterogeneous or oriented materials such as fiber-reinforced composites, larger sample sizes improve statistical reliability.
Can results from ISO 180 and ASTM D256 be compared directly?
Not directly. The two standards use different specimen dimensions, notch geometries, support configurations, and result units (kJ/m² versus J/m). Comparison requires awareness of these differences, and where possible, testing under a single consistent standard is preferred.