Plastic Izod impact testing, also known as the cantilever beam impact test, is a standardized method for evaluating the impact toughness and notch sensitivity of plastic materials. Governed by internationally recognized standards such as 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 consumer goods industries. This article provides a comprehensive technical overview of the test principle, specimen requirements, measurement procedures, and practical applications.
Technical Overview
The Izod impact test, referred to in Chinese standards as the cantilever beam 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 hammer at a defined distance above the clamp. The energy absorbed by the specimen during fracture is measured and used to calculate the impact strength of the material.
The fundamental principle of the test relies on the conservation of energy. A pendulum of known potential energy is released from a fixed height; upon breaking the specimen, the pendulum loses part of its kinetic energy. The difference between the initial energy and the residual energy, corrected for friction and windage losses, represents the energy absorbed by the specimen during fracture. This absorbed energy, normalized by the specimen dimensions, yields the impact strength value.
The principal standards governing plastic Izod impact testing include:
- ISO 180 – Plastics – Determination of Izod impact strength
- ASTM D256 – Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics
- GB/T 1843 – Chinese national standard, technically equivalent to ISO 180
- JIS K 7110 – Japanese industrial standard for Izod impact strength of plastics
Impact performance is a critical design parameter because plastics exhibit pronounced sensitivity to high strain rates, stress concentrations, and temperature. The Izod test provides a rapid, reproducible, and economical means of screening materials for toughness, assessing notch sensitivity, and monitoring process-related changes such as degradation during molding.
Testing Samples
Notched Specimens (Standard Configuration)
Notched specimens are the default configuration for most specifications. The notch acts as a stress concentrator that promotes brittle fracture and provides a more severe and reproducible test condition. Under ISO 180, the standard specimen measures 80 mm × 10 mm × 4 mm, with notch types classified as follows:
- Type A notch – notch tip radius of 0.25 mm, the most commonly specified type
- Type B notch – notch tip radius of 1.0 mm, providing a milder stress concentration
- Type C notch – notch tip radius of 0.1 mm, an intentionally sharp notch for materials with low notch sensitivity
Under ASTM D256, the standard specimen measures 63.5 mm × 12.7 mm (2.5 in × 0.5 in) with a preferred thickness of 3.2 mm, and the notch radius is 0.25 mm with a notch depth of 2.54 mm. Specimens may be machined from molded plaques or directly injection-molded with a molded-in notch, although machined notches generally produce more consistent results.
Unnotched Specimens
Unnotched testing is applied to materials where the notched value is too low to be meaningful or where the end-use application involves no intentional stress concentrators. Highly brittle materials such as general-purpose polystyrene may fracture completely without a notch, whereas ductile materials such as polyethylene may not break at all, in which case the result is reported as "no break."
Specimen Orientation and Designation
ISO 180 defines several specimen orientations, each with a specific designation that must accompany the reported result:
- Edgewise impact (e) – the specimen is struck parallel to its length, with the notch facing the pendulum
- Flatwise impact (f) – the specimen is struck perpendicular to its major face
- Reversed notch impact (r) – the notch faces away from the pendulum, producing a different stress state
The complete designation combines orientation and notch type, for example "notched Izod impact strength, edgewise, Type A notch (a¿iN)." Accurate designation is essential because results from different orientations are not directly comparable.
Testing Items
Typical measurement items within a plastic Izod impact testing program include:
- Notched Izod impact strength – the primary reported value, expressed in kJ/m² under ISO or J/m (ft-lb/in) under ASTM
- Unnotched Izod impact strength – used to quantify total toughness for ductile or notch-insensitive materials
- Impact strength at elevated or reduced temperatures – typically at -40°C, -20°C, 0°C, 23°C, and elevated temperatures using environmental chambers, to map ductile-to-brittle transition behavior
- Fracture mode classification – complete break (C), hinge break (H), partial break (P), or non-break (NB) as defined by ISO 180
- Comparative and lot-release testing – verification of conformance to material datasheet values or purchase specifications
- Notch sensitivity evaluation – comparison of results across different notch radii to characterize susceptibility to stress concentrations
Testing Methods
Testing is performed on a calibrated pendulum impact tester equipped with interchangeable hammers of varying nominal energies. ISO 180 specifies pendulum capacities of 1 J, 2.75 J, 5.5 J, 11 J, and 22 J, while ASTM D256 permits energies up to approximately 25 ft-lb. Key elements of the procedure are as follows:
- Conditioning – specimens are conditioned at 23°C ± 2°C and 50% ± 10% relative humidity for a minimum of 16 hours (longer for hygroscopic materials such as nylon), unless otherwise specified
- Dimensional verification – the width and thickness of each specimen at the notch plane are measured to an accuracy of 0.02 mm, as these dimensions directly enter the calculation
- Calibration check – friction and windage losses of the apparatus are determined by performing free swings without a specimen and applying appropriate corrections
- Energy selection – a hammer capacity is chosen so that the absorbed energy falls between approximately 10% and 80% of the pendulum capacity to ensure measurement accuracy
- Clamping and striking – the specimen is clamped as a vertical cantilever with the notch plane aligned with the top of the clamp; the pendulum strikes the specimen on the notched face at a defined point above the clamp
- Data collection – the absorbed energy is recorded for each specimen, and the fracture appearance is classified
The calculations differ between the two principal standard systems:
- ISO 180 (kJ/m²) – the absorbed energy is divided by the cross-sectional area of the specimen at the notch, yielding an energy per unit area
- ASTM D256 (J/m or ft-lb/in) – the absorbed energy is divided by the specimen width beneath the notch, yielding an energy per unit length
A minimum of five specimens is normally tested for isotropic materials, and at least ten for reinforced or directionally oriented materials, with the mean value reported. Individual values that differ significantly from the mean should be investigated rather than discarded. It is important to note that ISO and ASTM results are not numerically interchangeable due to differences in specimen geometry, notch dimensions, and units; correlation factors must be established empirically when comparing data across the two systems. Testing should be conducted by accredited third-party laboratories or qualified in-house facilities with calibrated equipment and validated procedures to ensure data reliability and comparability.
Applications
Plastic Izod impact testing supports a broad range of engineering and commercial activities:
- Material selection and comparison – ranking candidate thermoplastics, including ABS, PC, PC/ABS blends, PP, PA, POM, and PBT, for impact-critical applications
- Automotive engineering – qualification of interior trim, instrument panels, bumpers, body panels, and under-hood components that must withstand impact across a wide temperature range
- Electrical and electronics – evaluation of housings, enclosures, connectors, and appliance components subject to drop, knock, and assembly loads
- Pipes, fittings, and construction products – impact verification for PVC, PE, and PP piping systems, profiles, and sheets
- Packaging – assessment of containers, caps, and transport packaging for drop and shock resistance
- Quality control and process monitoring – detection of material contamination, moisture-related degradation, improper molding conditions, or inadequate annealing that reduce toughness
- Regulatory compliance and certification – demonstration of conformance to material specifications, UL requirements for polymeric materials, and customer-specific engineering standards
- Research and development – study of toughening mechanisms, rubber modification, filler effects, recycled content, and ductile-to-brittle transitions
FAQ
What is the difference between Izod and Charpy impact testing?
In the Izod (cantilever beam) test, the specimen is clamped vertically at one end and struck above the clamp, whereas in the Charpy (simply supported beam) test, the specimen rests horizontally on two supports and is struck at midspan. Izod testing per ASTM D256 is dominant in North America, while Charpy testing per ISO 179 is more common in Europe. The two methods produce different values for the same material and the results are not directly interchangeable.
Why are notches machined into the specimens?
The notch creates a controlled, triaxial stress concentration at its tip that promotes brittle, crack-initiation-controlled fracture. This makes the test more discriminating for materials with low notch sensitivity and improves repeatability. Notched impact strength also correlates better with real-world performance where sharp corners, weld lines, or scratches act as stress raisers.
How are ISO and ASTM results converted between kJ/m² and J/m?
There is no universal conversion factor because the standards use different specimen geometries, notch depths, and normalization bases. For a given material family, an empirical correlation can be established, but for formal specification purposes, testing should be performed according to the exact standard invoked by the applicable specification.
What does "non-break" (NB) mean in a test report?
Non-break indicates that the specimen did not fracture completely, typically because the material is highly ductile at the test temperature. Under ISO 180, non-break results are reported as such and are not used in the arithmetic mean, since the absorbed energy only represents a lower bound of the material's toughness.
How does temperature affect Izod impact strength?
Impact strength generally decreases as temperature decreases. Many plastics, particularly polypropylene and nylon, exhibit a ductile-to-brittle transition below room temperature. Testing at low temperatures such as -40°C is therefore common for outdoor, automotive, and cold-chain applications to confirm adequate toughness in service conditions.
How many specimens should be tested?
ISO 180 recommends a minimum of five specimens for isotropic materials and ten for fiber-reinforced or anisotropic materials. Additional specimens may be required if the scatter is high or if statistical characterization, such as a mean with confidence interval, is needed for specification compliance.