The cantilever beam impact test, universally known as the Izod impact test, is one of the most widely applied methods for evaluating the impact toughness and notch sensitivity of plastic materials. This technical guide explains the complete determination method in accordance with ISO 180, ASTM D256, and GB/T 1843, covering test principles, specimen preparation, procedural steps, calculation of impact strength, and typical industrial applications. Whether you are involved in material development, quality control, or third-party verification, this article provides the essential technical knowledge needed to understand and correctly interpret Izod impact test results.
Technical Overview
The cantilever beam impact test, commonly referred to as the Izod impact test, is a standardized pendulum-based method used to determine the resistance of plastics to failure under impact loading. A specimen is clamped vertically as a cantilever beam, and a swinging pendulum hammer strikes the free end of the specimen at a fixed height above the clamping point. The energy absorbed by the specimen during fracture is measured and normalized to a characteristic cross-section of the specimen, yielding the impact strength.
The principal international and national standards governing this method 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 (the Chinese national standard, technically aligned with ISO 180), and JIS K 7110. Although the fundamental principle is identical, ISO and ASTM systems differ in specimen geometry, notch details, and result expression, so data quoted under different standards must not be compared directly without conversion.
Test Principle
A pendulum of known potential energy is released from a defined angle. As it strikes and breaks the clamped specimen, part of its kinetic energy is absorbed by the fracture process. The residual energy carries the pendulum up to a lower final angle. The difference between the initial and residual energies, after correction for friction and windage losses, represents the energy absorbed by the specimen. Dividing this absorbed energy by the cross-sectional area of the specimen at the fracture plane gives the impact strength in kJ/m² (ISO method) or, alternatively, by the specimen width to give J/m (ASTM method).
Failure Mode Classification
ISO 180 requires the mode of failure to be recorded because it fundamentally affects how results are interpreted:
- C — Complete break: the specimen separates into two or more pieces.
- H — Hinge break: an incomplete break in which the two halves are held together only by a thin peripheral layer of moderate flexibility.
- P — Partial break: an incomplete break that does not meet the hinge-break criterion.
- NB — Non-break: the specimen bends but does not break, or breaks in an irregular manner not meeting other categories.
Impact strength values are strictly comparable only when the failure mode is the same. Non-break results (NB) must not be averaged with break values; they are reported as exceeding the capacity of the applied impact energy.
Testing Samples
Specimen Geometry
ISO 180 and GB/T 1843 specify a preferred specimen of 80 mm × 10 mm × 4 mm, injection molded or machined from sheet, plate, or molded parts. ASTM D256 specifies specimens of 63.5 mm × 12.7 mm (2.5 in × 0.5 in) with thicknesses of 3.2 mm (1/8 in) or 6.4 mm (1/4 in). Specimens may be taken directly from finished components for verification testing, provided orientation and machining effects are documented.
Notch Types
Notched specimens are the standard configuration because the notch creates a controlled stress concentration and improves the reproducibility of fracture. ISO 180 defines three notch geometries cut into the 10 mm wide face:
- Type A notch: tip radius 0.25 mm, remaining ligament thickness 8.0 mm — the default and most widely reported configuration.
- Type B notch: tip radius 1.0 mm, remaining ligament thickness 8.0 mm — used to evaluate materials with high notch sensitivity.
- Type C notch: tip radius 0.10 mm, remaining ligament thickness 8.0 mm — the sharpest notch, used to accentuate brittle behavior.
ASTM D256 uses a Type A notch with a 0.25 mm tip radius and a remaining ligament of 10.16 mm as the reference condition. Notch quality is critical: cutters must be sharp, and the notch profile should be verified periodically with a profile projector or optical comparator, since a dull or incorrectly radiused cutter can alter results by a significant margin.
Unnotched and Reverse-Notched Specimens
Unnotched specimens are tested when evaluating the inherent toughness of ductile materials. For very tough polymers that do not break in the unnotched state, a reverse-notch configuration may be used, in which the specimen is impacted on the side opposite the notch.
Conditioning
Specimens shall be conditioned prior to testing, typically at 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 16 hours, in accordance with ISO 291 or ASTM D618, unless the material standard specifies otherwise. Conditioning is essential because moisture content and temperature strongly influence the impact behavior of many plastics, particularly polyamides and other hygroscopic polymers.
Testing Items
The following quantities are typically determined by the cantilever beam impact test:
- Notched Izod impact strength (a_iN): the core test result, expressed in kJ/m², obtained on Type A, B, or C notched specimens.
- Unnotched Izod impact strength (a_iU): the impact strength of the full cross-section without a stress concentrator.
- Reverse-notch impact strength: applicable to tough materials that resist fracture in the standard configuration.
- Temperature-dependent impact strength: testing at sub-ambient or elevated temperatures, for example at −40 °C or −30 °C, to assess the ductile-to-brittle transition relevant to cold-service applications.
- Notch sensitivity comparison: comparing results across notch radii (A, B, C) to quantify notch sensitivity of a material.
- Failure mode classification: complete, hinge, partial, or non-break, reported alongside each numeric value.
Testing Methods
Apparatus
The test is performed on a pendulum impact testing machine equipped with a cantilever (Izod) support fixture. The machine shall be level, rigidly mounted, and periodically verified in accordance with ISO 13802 or equivalent calibration practice. The pendulum capacity shall be selected so that the energy absorbed by the specimen falls typically within 10% to 80% of the nominal impact energy, ensuring adequate measurement resolution and accuracy.
Test Procedure
- Condition the specimens and allow them to reach thermal equilibrium at the test temperature, normally 23 °C.
- Measure the width and the thickness (or remaining ligament thickness at the notch) of each specimen to an accuracy of 0.02 mm using a calibrated micrometer.
- Select a pendulum of suitable energy capacity and verify the friction/windage correction by performing free-swing checks.
- Clamp the specimen vertically in the fixture with the notch facing the striker, positioning the notch plane at the specified distance of 22 mm above the clamping edge (ISO configuration).
- Release the pendulum from the defined starting angle and allow it to strike the specimen once; prevent any secondary impacts.
- Read the absorbed energy from the indicating device and apply the correction for friction and air resistance.
- Record the failure mode of each specimen.
- Repeat the test on the required number of specimens — at least 10 specimens per ISO 180, and a minimum of 5 (preferably more) per ASTM D256 for statistical reliability.
Calculation
For notched specimens, the impact strength is calculated by dividing the corrected absorbed energy by the cross-sectional area of the remaining ligament at the notch: energy (J) divided by the product of specimen width and remaining thickness, expressed in kJ/m². For unnotched specimens, the full specimen thickness is used in the denominator. Under ASTM D256, results are commonly expressed in joules per meter of notch width (J/m) or ft·lb/in. Because the ISO result is normalized to area and the ASTM result to width, values from the two systems are not directly interchangeable without knowledge of specimen dimensions.
Test Report
A complete report shall include: the standard followed, identification of the material and its conditioning, specimen type and dimensions, notch type and method of cutting, the number of specimens tested, pendulum capacity, test temperature and humidity, individual and arithmetic mean impact strength values, and the observed failure mode for each specimen.
Applications
The cantilever beam impact test is one of the most frequently requested mechanical tests in plastics laboratories and serves a broad range of engineering and commercial purposes:
- Material selection and ranking: comparing the impact toughness of candidate materials such as ABS, PC, PP, PE, PA, POM, PVC, PBT, and their modified or filled grades.
- Quality control: verifying incoming resin batches, compounded materials, and molded parts against material datasheet limits.
- Product compliance: supporting specification compliance for automotive interior and exterior components, electrical appliance housings, pipes and fittings, consumer products, and packaging.
- Low-temperature service evaluation: assessing brittleness at sub-zero temperatures for outdoor, refrigeration, and cold-climate applications.
- Formulation studies: quantifying the effects of impact modifiers, plasticizers, fillers, glass fiber reinforcement, recycled content, and processing conditions.
- Failure analysis: comparing impact performance before and after aging, weathering, hydrolysis, or thermal exposure in durability investigations.
- Regulatory and certification testing: generating data for submissions to accredited third-party laboratories, certification bodies, and end-user specifications.
FAQ
What is the difference between the Izod (cantilever beam) test and the Charpy test?
In the Izod test the specimen is clamped vertically as a cantilever and struck above the clamp, whereas in the Charpy test the specimen lies horizontally on two supports and is struck at midspan as a simply supported beam. The specimen geometries, support conditions, and reported reference dimensions differ, so the two methods produce different numerical results and are governed by separate standards (ISO 180 versus ISO 179).
Why is the specimen notched?
The notch acts as a controlled stress concentrator that promotes a reproducible, rapid crack initiation. It reduces scatter, enables brittle fracture in otherwise ductile materials, and allows the notch sensitivity of a polymer to be quantified — a property of major engineering importance, since real components frequently contain sharp corners, weld lines, and scratches that behave like notches.
Why must ISO and ASTM Izod results not be compared directly?
The two standards use different specimen dimensions, notch ligament depths, striker geometries, and result units (kJ/m² versus J/m). A material may therefore show substantially different nominal values under each system even though its intrinsic toughness is unchanged. Comparisons should always be made within a single test standard.
What should be done if the specimen does not break?
A non-break (NB) result is recorded and the value is reported as greater than the applied energy. Such results must not be averaged with break values. If meaningful data are required, a higher-energy pendulum may be used, or the test configuration changed — for example, using a sharper Type C notch, testing at a lower temperature, or employing the reverse-notch method.
Which factors most strongly influence the test result?
Key influences include the notch tip radius and cutting quality, specimen thickness, test temperature, moisture conditioning, material orientation from molding, and the capacity of the pendulum relative to the absorbed energy. Strict adherence to the conditioning, notching, and dimensional requirements of the governing standard is essential for reproducible results.