Plastic Cantilever Beam Impact Test (Izod): Standards, Procedures, and Applications

The plastic cantilever beam impact test, commonly known as the Izod impact test, is a standardized laboratory method for measuring the impact toughness and notch sensitivity of plastic materials. Governed by ISO 180, ASTM D256, and GB/T 1843, this test determines the energy absorbed by a vertically clamped specimen when fractured by a swinging pendulum. This article provides a comprehensive technical overview of the test principle, specimen requirements, testing procedures, calculation methods, and typical industrial applications.

Technical Overview

The plastic cantilever beam impact test (Izod impact test) is one of the most widely used methods for evaluating the impact resistance and notch sensitivity of rigid and semi-rigid plastics. In this configuration, a rectangular test specimen is clamped vertically as a cantilever beam, and a pendulum hammer is released to strike the specimen near its free end at a fixed velocity of approximately 3.5 m/s. The energy absorbed during fracture is measured and normalized to produce the Izod impact strength, a key indicator of material toughness and brittleness.

The test is primarily governed by the following standards:

• ISO 180 – Plastics – Determination of Izod impact strength, which expresses results in kilojoules per square metre (kJ/m²)
• ASTM D256 – Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics, which expresses results in joules per metre of notch (J/m) or foot-pounds per inch (ft-lb/in)
• GB/T 1843 – Plastics – Determination of Izod impact strength, the Chinese national standard technically aligned with ISO 180
• ISO 291 / ASTM D618, which define the standard conditioning atmosphere (23 ± 2 °C and 50 ± 5% relative humidity) prior to testing

The cantilever beam (Izod) method differs from the simply supported beam (Charpy) method described in ISO 179 and ASTM D6110 in the way the specimen is held: the Izod specimen is gripped in a vise at one end, whereas the Charpy specimen rests horizontally on two supports. Both methods are routinely performed by accredited third-party laboratories and material manufacturers for quality control, material qualification, and research purposes.

Testing Samples

Notched Specimens per ISO 180 / GB/T 1843

The preferred specimen is ISO Type 1, measuring 80 mm × 10 mm × 4 mm. A machined notch is introduced on the narrow long face to serve as a stress concentration point. ISO 180 defines three notch geometries:

• Type A notch: root radius of 0.25 mm, typical notch depth of 2.0 mm (most common configuration)
• Type B notch: root radius of 1.00 mm, providing a milder stress concentration
• Type C notch: root radius of 0.10 mm, providing a sharper stress concentration for notch-sensitive materials

A test is typically designated as "ISO 180/1A", indicating a Type 1 specimen with an A-type notch.

Notched Specimens per ASTM D256

The standard ASTM specimen measures 63.5 mm × 12.7 mm × 3.2 mm (2.5 in × 0.5 in × 1/8 in), with a machined notch depth of 2.03 mm (0.080 in) and a root radius of 0.25 mm (0.010 in) on the 1/8-inch wide face. Thicker specimens (6.4 mm) may be tested when specified, but results from different thicknesses are not directly comparable.

Unnotched Specimens

Unnotched specimens of the same dimensions may be tested to evaluate the inherent toughness of the material without an artificial stress concentrator. Unnotched values are always significantly higher than notched values and highlight the degree of notch sensitivity of the polymer.

Specimen Preparation and Conditioning

• Specimens should be prepared by injection molding or machined from sheet or finished parts, with the processing condition recorded since molding parameters strongly influence results
• Notches must be machined carefully with a calibrated cutting tool, as notch quality directly governs repeatability
• Specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 16 hours (or longer if specified by the material standard)
• A minimum of five (preferably ten) specimens per condition should be tested, and each specimen must be free of visible defects, sink marks, or flash at the notch

Testing Items

Typical test items evaluated under the cantilever beam impact configuration include:

• Notched Izod impact strength (kJ/m² per ISO 180 / GB/T 1843; J/m per ASTM D256) at 23 °C
• Unnotched Izod impact strength, reporting energy per unit cross-sectional area
• Low-temperature impact strength at specified temperatures such as 0 °C, -20 °C, or -40 °C, using a conditioned chamber to assess cold brittleness
• Reverse-notched impact strength, in which the specimen is struck on the face opposite the notch, providing a measure closer to unnotched behavior
• Break type classification per ISO 180: complete break (C), hinge break (H), partial break (P), and non-break (NB)
• Comparative notch sensitivity, determined by testing specimens with different notch radii (A, B, or C) or notched versus unnotched specimens

Testing Methods

Apparatus

The test is performed on a pendulum impact testing machine equipped with interchangeable hammers of defined energies (commonly 1 J, 2.75 J, 5.5 J, 11 J, and 22 J). The machine incorporates a rigid vise that clamps the specimen vertically so that the notch plane is positioned level with the top of the clamp (nominally 22 mm above the clamping point per ISO 180), with the notch facing the striking edge of the pendulum. The machine must be calibrated and verified regularly, with windage and friction corrections applied to the indicated energy.

Test Procedure

• Condition the specimens at 23 ± 2 °C and 50 ± 5% relative humidity for the specified period, and test within a defined time window after removal from conditioning
• Measure the width and thickness of each specimen (and the remaining ligament width under the notch) with a calibrated micrometer to an accuracy of 0.02 mm
• Select a pendulum capacity such that the absorbed energy lies between 10% and 80% of the available energy; if breakage does not occur, switch to a smaller hammer
• Clamp the specimen vertically in the vise with the notch facing the pendulum and set the zero position of the energy scale
• Release the pendulum from its raised position, allow it to strike and fracture the specimen, and record the corrected absorbed energy from the indicator
• Examine and classify the break type (C, H, P, or NB); specimens that do not break (NB) are recorded as such and are not averaged with broken values
• Repeat for at least five valid specimens and calculate the arithmetic mean and standard deviation

Calculation and Expression of Results

For notched specimens per ISO 180, the Izod impact strength is calculated as:

aiN = (Ec / (h × bN)) × 10³, where Ec is the corrected absorbed energy in joules, h is the thickness of the specimen in millimetres, and bN is the remaining width under the notch in millimetres. The result is expressed in kJ/m². For unnotched specimens, the full original width b is used in place of bN. Under ASTM D256, the absorbed energy is divided by the specimen width to yield J/m, which may be converted to ft-lb/in. Results must always be reported together with the specimen type, notch type, test temperature, and break classification, because these factors materially affect the reported value.

Applications

Cantilever beam impact test data are used extensively across the plastics industry:

• Material selection and datasheet generation: Izod impact strength is a standard entry on resin supplier datasheets, enabling engineers to compare the toughness of ABS, PC, PP, PA, POM, PVC, and their reinforced or filled grades
• Incoming quality control: Manufacturers verify that incoming resin lots meet the minimum notched impact values specified in purchase agreements
• Automotive engineering: qualification of interior trim, bumpers, clips, and connectors that must withstand impact loads across a wide temperature range, including -40 °C cold impact requirements
• Electrical and electronics: evaluation of housings, switch components, and appliance parts where drop and impact resistance is a safety consideration
• Packaging and consumer goods: assessment of containers, closures, and household products for transport and handling durability
• Pipe, fitting, and construction products: verification of toughness requirements in related product standards that reference notched impact performance
• Research and development: studies of brittle–ductile transitions, the toughening effect of elastomer modifiers, the influence of recycled content, and the effect of processing conditions or annealing on impact behavior
• Failure analysis: comparing the impact strength of failed parts against virgin material to identify degradation, contamination, or improper molding

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 with the notch facing the striker. In the Charpy (simply supported beam) test per ISO 179 / ASTM D6110, the specimen lies horizontally on two anvils and is struck at midspan opposite the notch. The two methods use different specimen orientations, support conditions, and strike geometries, so their numerical results are not directly interchangeable.

Why are specimens notched before testing?

The machined notch acts as a controlled stress concentrator that promotes brittle fracture and intensifies the local strain rate at the notch root. Notched results reveal a material's sensitivity to surface defects, sharp corners, and scratches in real parts, which is critical for design. Notch radius significantly affects the result: sharper notches (Type C) generally yield lower impact strength than milder notches (Type A or B).

Can ISO 180 results be converted to ASTM D256 results?

No direct conversion exists. The two standards use different specimen dimensions, notch depths, striking edge radii, and result units (kJ/m² versus J/m). Data comparisons should only be made within the same test standard, specimen type, and notch geometry.

What does it mean when a specimen is reported as "non-break" (NB)?

Non-break means the specimen did not fracture completely and the pendulum could not meaningfully fracture it with the selected energy; the specimen may bend or suffer only partial damage. Per ISO 180, NB results are recorded qualitatively and cannot be averaged with broken specimen values. When NB occurs, a lower-energy hammer is normally used to obtain a measurable fracture.

Which factors most influence the repeatability of the test?

Key factors include notch machining quality (tool sharpness and root radius accuracy), specimen conditioning and test temperature, molding-induced internal stresses and orientation, clamping torque consistency, machine calibration, and correct pendulum energy selection. Accredited third-party laboratories control these variables through documented procedures to ensure reproducible results.

At what temperatures can the test be performed?

The reference test temperature is 23 °C, but the test is frequently performed at elevated or reduced temperatures (for example, -40 °C for automotive cold impact or elevated temperatures near heat-deflection limits) using a temperature-controlled chamber. Specimens must be conditioned at the test temperature for a defined period and broken within a short time (typically seconds) after removal from the chamber.