Izod Impact Testing of Plastics: Cantilever Beam Impact Strength Determination, Standards, and Proce

Izod impact testing, also known as cantilever beam impact determination, is the internationally recognized method for evaluating the impact toughness and notch sensitivity of plastic materials. This guide explains the test principle, specimen requirements, applicable standards such as ISO 180, ASTM D256, and GB/T 1843, testing procedures, result calculation, and typical industrial applications. Read on to understand how accredited third-party laboratories perform cantilever beam impact tests and how to interpret the reported data.

Technical Overview

The cantilever beam impact test, internationally known as the Izod impact test, is one of the most widely applied methods for characterizing the impact resistance of plastics. In this test, a rectangular specimen is clamped at one end as a vertical cantilever beam and struck near its free end by a swinging pendulum hammer. The energy absorbed by the specimen during fracture is measured and converted into impact strength, providing a quantitative index of material toughness under high-speed loading conditions.

The cantilever beam impact test is primarily used for quality control, material comparison, and screening of formulations rather than for generating direct design allowables. Results are strongly influenced by specimen geometry, notch quality, temperature, and processing history, so strict adherence to standardized conditions is essential for reproducibility.

• 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 — Japanese industrial standard for Izod impact testing of plastics
• ISO 291 / ASTM D618 — Conditioning atmospheres for specimen state adjustment before testing

Testing Samples

Specimen Geometry

The preferred specimen defined in ISO 180 and GB/T 1843 is the Type 1 bar measuring 80 mm × 10 mm × 4 mm. ASTM D256 uses a similar rectangular bar, typically 63.5 mm × 12.7 mm × 3.2 mm up to 12.7 mm thick. Specimens may be taken directly from injection-molded or compression-molded plates, or machined from finished parts, sheet, pipe, or other product forms. When machined surfaces are present, the original molded surface should be noted because surface condition can affect the measured energy.

Notch Types

Notched specimens are the most common configuration because the notch creates a controlled stress concentration that promotes consistent, reproducible brittle fracture. ISO 180 defines three notch radii:

• Type A notch: 0.25 mm radius, the standard and most frequently used geometry
• Type B notch: 1.00 mm radius, used for more notch-sensitive comparative studies or very brittle materials
• Type C notch: 0.10 mm radius, used for very sharp stress concentration requirements

For a Type 1 specimen, the remaining ligament width under the notch is 8.0 mm. Notches must be machined with a sharp, properly shaped cutter, and the cutter tip radius must be verified periodically, as notch radius is one of the dominant variables affecting the result.

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 (ISO 291) or 40 hours (ASTM D618) prior to testing. Testing at elevated or sub-ambient temperatures may be performed with a thermostatic chamber, with appropriate dwell time allowed for the specimen to reach temperature equilibrium.

Testing Items

Cantilever beam impact testing services typically cover the following determination items:

• Notched Izod impact strength (A-type, B-type, or C-type notch), expressed in kJ/m² per ISO 180/GB/T 1843 or in J/m per ASTM D256
• Unnotched Izod impact strength, reflecting the total energy to initiate and propagate fracture
• Reverse-notched impact strength, where the specimen is struck on the side opposite the machined notch
• Edgewise and flatwise orientations, to evaluate anisotropy in oriented or reinforced materials
• Temperature-dependent impact testing, from low-temperature brittleness screening to elevated-temperature ductility assessment
• Fracture mode classification, including complete break, hinge break, partial break, and non-break outcomes

Testing Methods

Apparatus

The test is performed on a pendulum impact machine equipped with interchangeable hammers of nominal energies typically ranging from 1 J to 25 J. The machine shall be leveled and rigidly mounted, and the center of percussion, friction losses, windage losses, and indicated energy shall be verified through regular calibration and verification. The specimen clamp must hold the bar vertically with the notch plane aligned with the top of the clamp jaws and positioned so the pendulum striker contacts the specimen at the correct distance from the clamping line.

Test Procedure

• Select a hammer capacity such that the specimen absorbs an appropriate fraction of the available energy (generally between about 10% and 80% of the nominal capacity), ensuring the pendulum still carries sufficient energy to complete the swing after breaking the specimen.
• Clamp the specimen as a vertical cantilever with the notch facing the pendulum striker, in accordance with ISO 180 and ASTM D256 requirements.
• Release the pendulum from its raised position and allow it to strike and fracture the specimen.
• Read the absorbed energy directly from the scale or digital display, applying corrections for friction and air resistance when required.
• Repeat the test on a statistically sufficient number of specimens, typically at least 10, and record the fracture appearance for each.
• Discard and replace results from specimens that do not break, break at the clamp line, or deform without fracture, and note such occurrences in the report.

Calculation and Expression of Results

Under ISO 180 and GB/T 1843, the Izod impact strength is calculated by dividing the corrected absorbed energy by the cross-sectional area of the specimen at the fracture line (remaining width under the notch multiplied by thickness), and the result is expressed in kJ/m². Under ASTM D256, the notched Izod impact resistance is expressed as energy per unit notch width in J/m. Because the notched ISO result is normalized by the full fractured area while the notched ASTM result is normalized by specimen width alone, numerical values from the two standards cannot be compared directly without unit and basis conversion. Test reports shall identify the standard used, notch type, specimen type, hammer energy, mean value, standard deviation, and the number of specimens tested.

Applications

Cantilever beam impact data are used across the full plastics value chain, from resin producers and compounders to converters and end-product manufacturers. Typical applications include:

• Quality control of incoming resins, masterbatches, and compounded materials against material datasheet limits
• Material selection and comparison of candidate polymers, blends, and filled or reinforced grades for toughness-critical parts
• Formulation development, such as evaluating the effect of impact modifiers, plasticizers, fillers, fibers, and recycled content on toughness
• Processing optimization, including melt temperature, mold temperature, gate design, and weld-line influence on part performance
• Temperature and aging studies, such as cold-temperature brittleness of outdoor parts, thermal aging, and UV weathering effects on impact retention
• Industry sectors including automotive interior and exterior components, electrical enclosures and housings, household appliances, pipes and fittings, packaging, sporting goods, and consumer electronics
• Regulatory and specification compliance for products referenced in UL, IEC, EN, and GB product standards that cite Izod impact requirements

Accredited third-party laboratories routinely perform cantilever beam impact determinations as part of material qualification programs, failure analysis investigations, and supplier audit packages, providing traceable results under ISO 180, ASTM D256, or GB/T 1843.

FAQ

What is the difference between the Izod and Charpy impact tests?

Both tests use a pendulum to break a notched specimen, but in the Izod (cantilever beam) test the specimen stands vertically and is clamped at one end with the notch facing the striker, while in the Charpy (simply supported beam) test the specimen lies horizontally on two supports and is struck opposite the notch. The applicable standards also differ: ISO 180 and ASTM D256 govern Izod testing, whereas ISO 179 and ASTM D6110 govern Charpy testing. Some rigid materials that fail in Izod testing may be testable only by the Charpy method.

Why are specimens notched before testing?

The notch acts as a stress concentrator that initiates fracture at a defined location, which improves reproducibility and accentuates differences in notch sensitivity between materials. Notched and unnotched results serve different purposes: notched values reveal resistance to crack propagation from sharp defects, while unnotched values reflect overall energy absorption capability.

Which factors most strongly affect the results?

Test temperature, specimen conditioning, notch radius and machining quality, specimen thickness, material orientation and anisotropy, processing-induced residual stress, clamping torque, and hammer capacity selection all have measurable effects. This is why standards mandate tightly controlled notch geometry, conditioning atmosphere, and specimen alignment.

How should ISO and ASTM Izod results be compared?

Direct numerical comparison is not valid without conversion because the two standards use different specimen geometries and different normalizing bases. ISO 180 reports notched impact strength in kJ/m² based on the fractured area, while ASTM D256 reports notched impact resistance in J/m based on specimen width. Any comparison should state the standard, notch type, and specimen thickness used.

What does it mean when the specimen does not break?

A non-break result indicates the material is too tough for the selected hammer capacity or that the material behaves in a highly ductile manner at the test temperature. Standards specify that such results be reported separately and that a higher-energy hammer, a sharper notch, or an alternative method such as instrumented impact or tensile impact testing be considered for full characterization.