Plastic cantilever beam impact testing, commonly known as the Izod impact test, is a standardized laboratory method for evaluating the impact toughness and notch sensitivity of plastic materials under dynamic loading conditions. Governed by internationally recognized standards such as ISO 180, ASTM D256, and GB/T 1843, this test provides critical data for material qualification, quality control, and product design across industries including automotive, electronics, construction, and packaging. This article explains the testing principle, sample requirements, test procedures, and typical applications of cantilever beam impact testing for plastics.
Technical Overview
The cantilever beam impact test (Izod impact test) measures the energy absorbed by a plastic specimen when it is fractured by a single blow from a swinging pendulum. In this configuration, the specimen is clamped vertically as a cantilever beam, with the notch facing the direction of the strike, and the pendulum impacts the free end of the specimen at a defined distance from the clamping point. The absorbed impact energy is calculated from the difference between the initial and residual height of the pendulum, corrected for friction and windage losses.
The result is typically expressed as impact strength in kJ/m², obtained by dividing the absorbed energy by the cross-sectional area of the specimen (or by the residual area beneath the notch for notched specimens). Because thermoplastics often exhibit brittle behavior in the presence of stress concentrators, notched testing is particularly valuable for assessing notch sensitivity, while unnotched testing reflects the overall ductility and toughness of the material.
The most widely applied standards for this test 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 – Determination of Izod impact strength of plastics (Chinese national standard, technically aligned with ISO 180)
JIS K 7110 – Testing method for Izod impact strength of plastics (Japanese industrial standard)
Testing Samples
Standard Injection-Molded Specimens
Preferred test pieces are injection-molded bars produced under controlled molding conditions, as molding parameters significantly influence the measured impact behavior. Typical specimen dimensions are 80 mm × 10 mm × 4 mm for ISO 180, and 63.5 mm × 12.7 mm × 3.2 mm (2.5 in × 0.5 in × 1/8 in) for ASTM D256.
Notched Specimens
Notches are machined into the narrow face of the specimen to create a controlled stress concentration. ISO 180 defines three notch geometries: type A (notch tip radius 0.25 mm), type B (radius 1.0 mm), and type C (radius 0.1 mm). The notch must be cut with a sharp, properly shaped milling cutter, and its geometry is verified using a profile projector or optical measuring system. The notch radius has a strong influence on the result and must always be reported.
Specimens Machined from Finished Products
When evaluating pipes, sheets, profiles, or molded parts, specimens may be machined directly from the product. In such cases, the sampling position and orientation relative to the processing direction (e.g., extrusion or flow direction) must be documented, as anisotropy can significantly affect the measured impact strength.
Specimen Conditioning
Unless otherwise specified, specimens shall be conditioned at 23 °C ± 2 °C and 50 % ± 10 % relative humidity for a minimum of 16 hours in accordance with ISO 291 or ASTM D618. For low-temperature evaluations, specimens are additionally conditioned at the target temperature (for example −20 °C or −40 °C) immediately before testing.
Testing Items
Common testing items and reported parameters include:
Notched cantilever beam (Izod) impact strength, expressed in kJ/m²
Unnotched cantilever beam impact strength, expressed in kJ/m²
Impact strength at reduced or elevated temperatures to assess ductile-to-brittle transition behavior
Comparative notch sensitivity evaluation using different notch radii (type A, B, or C)
Fracture mode classification: complete break (C), hinged break (H), partial break (P), or non-break (NB)
Testing Methods
Testing is performed on a pendulum impact tester equipped with an appropriate cantilever beam (Izod) support fixture and striking edge. The general procedure is as follows:
Select a pendulum with an energy capacity such that the absorbed energy falls within the valid working range of the instrument (typically between 10 % and 80 % of the pendulum capacity under ISO 180).
Verify the zero point and perform an empty-swing calibration to determine friction and windage corrections.
Measure and record the width and thickness of each specimen at the notch, and calculate the residual cross-sectional area.
Clamp the specimen vertically in the Izod fixture with the notch aligned with the top of the clamping jaws and facing the striker.
Release the pendulum (standard impact velocity approximately 3.5 m/s) and record the absorbed energy.
Classify the fracture mode of each specimen according to the definitions in the applicable standard.
Test a statistically sufficient number of specimens, typically a minimum of 10, and report individual values, the arithmetic mean, and the standard deviation.
The test report should identify the standard used, material designation, specimen preparation method, notch type, conditioning conditions, pendulum energy, test temperature, and all calculated impact strength values together with their fracture classifications. Results obtained under different standards (for example ISO 180 versus ASTM D256) are not directly comparable due to differences in specimen geometry, notch dimensions, and clamping configuration.
Applications
Cantilever beam impact testing is widely used throughout the plastics industry value chain, including:
Material research and development, comparing the toughening efficiency of impact modifiers, fillers, and new polymer formulations
Incoming material inspection and batch-to-batch quality control in accredited third-party laboratories and in-house facilities
Automotive engineering, evaluating interior trim, bumpers, battery housings, and cold-temperature impact performance of exterior components
Electrical and electronics, assessing housings, connectors, and structural parts made from ABS, PC, PC/ABS, and PVC
Pipes, fittings, and construction products, where low-temperature impact resistance is a key acceptance criterion
Packaging and consumer goods, verifying the drop and impact durability of rigid plastic containers and appliance components
Failure analysis, where unexpectedly low impact strength can reveal degradation, contamination, or improper processing
FAQ
What is the difference between the Izod test and the Charpy test?
In the Izod (cantilever beam) test the specimen is clamped vertically at one end and struck above the clamp, while in the Charpy (simply supported beam) test the specimen rests horizontally on two supports and is struck at the midpoint. The specimen orientations, notch directions, and energy calculations differ, so the two methods yield different numerical results.
Why are both notched and unnotched results useful?
Unnotched impact strength reflects the inherent toughness of the material, whereas notched impact strength reveals its sensitivity to stress concentrations such as sharp corners, weld lines, or scratches. A material with high unnotched strength but low notched strength is notch-sensitive and requires careful design of part geometry.
How many specimens are required for a valid test?
Most standards recommend testing at least 10 specimens per condition, because plastic impact data are inherently scattered. Additional specimens may be required if several fracture modes occur, since only results from comparable fracture types should be averaged.
What happens if the specimen does not completely break?
The result is reported as a partial break, hinged break, or non-break according to the standard. The pendulum energy may be insufficient, in which case a higher-capacity pendulum should be used. Non-break values cannot be averaged with complete-break values and should be reported separately.
Why is low-temperature impact testing often required?
Many thermoplastics, especially polypropylene and other polyolefins, undergo a ductile-to-brittle transition as temperature decreases. Testing at subzero temperatures verifies that components such as automotive exterior parts and buried piping will retain adequate toughness in cold service environments.
Are ISO 180 and ASTM D256 results interchangeable?
No. The two standards use different specimen dimensions, notch geometries, and support configurations, so the numerical values differ systematically. Comparisons should only be made between results generated under the same standard and specimen conditions.