Izod Notched Impact Testing of Plastics: Technical Guide to ISO 180 and ASTM D256

Izod notched impact testing is the most widely used pendulum-impact method for evaluating the impact toughness, brittleness, and notch sensitivity of plastic materials. Governed by standards such as ISO 180, ASTM D256, and GB/T 1843, this test measures the energy a notched cantilever-beam specimen absorbs when fractured by a single pendulum strike. This guide explains the test principle, specimen and notch requirements, calculation methods, and typical industrial applications.

Technical Overview

Izod notched impact testing, also known as cantilever beam impact testing, is a standardized method for determining the resistance of plastic materials to breakage under impact bending loads. In this test, a rectangular specimen with a machined notch is clamped vertically as a cantilever beam, with the notch positioned level with the top of the clamp and facing the striking edge. A swinging pendulum of defined energy strikes the notched face of the specimen at a fixed distance above the notch, and the energy absorbed in breaking the specimen is measured by the difference between the initial and residual pendulum energy.

The notch acts as a stress concentrator, imposing a severe triaxial stress state at the notch root. This accelerates brittle crack initiation and allows materials that might otherwise bend without breaking to be ranked by their notch sensitivity. The test is therefore one of the most demanding and informative assessments of plastic toughness.

The principal standards governing this test include ISO 180 (Plastics — Determination of Izod impact strength), ASTM D256 (Determining the Izod Pendulum Impact Resistance of Plastics), GB/T 1843 (the Chinese national standard technically equivalent to ISO 180), and JIS K 7110. Results are expressed in kJ/m² under the ISO system and in J/m or ft·lb/in under the ASTM system.

Testing Samples

Specimen Geometry per ISO 180

• Type 1 specimen: 80 mm × 10 mm × 4 mm, the preferred geometry for most thermoplastics.
• Type 2 specimen: 80 mm × 10 mm × 3 mm, used where the product wall thickness dictates a thinner sample.
• Type 3 specimen: 120 mm × 15 mm × 10 mm, used for cast or thick-section materials.
• Type 4 specimen: 125 mm × 13 mm × 13 mm, corresponding to ASTM-style geometry.

Specimen Geometry per ASTM D256

The standard ASTM specimen measures 63.5 mm × 12.7 mm × 3.2 mm, with thicknesses up to 12.7 mm permitted for certain applications. The notch depth is 2.54 mm, leaving a remaining ligament of 10.16 mm beneath the notch.

Notch Types

• Type A notch: tip radius 0.25 mm, the preferred notch for routine testing.
• Type B notch: tip radius 1.00 mm, a less severe notch used for very brittle materials.
• Type C notch: tip radius 0.10 mm, a sharper notch for highly notch-resistant materials.

Specimen Preparation and Conditioning

Specimens may be directly injection molded, compression molded, or machined from sheets, rods, or finished parts in accordance with ISO 2818. Notches must be freshly cut with a sharp single-point milling cutter, since notch quality directly governs the measured result. Prior to testing, samples are conditioned at 23 °C ± 2 °C and 50 % ± 5 % relative humidity per ISO 291 or ASTM D618. A minimum of five (ASTM) or ten (ISO) specimens is recommended for each condition to account for the inherent scatter of impact data.

Testing Items

Typical test items offered by accredited third-party laboratories and in-house quality laboratories include:

• Notched Izod impact strength at room temperature (23 °C).
• Notched Izod impact strength at reduced temperatures (for example −20 °C, −30 °C, −40 °C) to characterize low-temperature embrittlement.
• Unnotched Izod impact strength for comparison with notched values.
• Notch sensitivity evaluation, expressed as the ratio of unnotched to notched impact strength.
• Fracture mode classification: complete break, hinged break, partial break, or non-break.
• Anisotropy assessment by testing specimens cut parallel and perpendicular to the molding direction.

Testing Methods

Apparatus

The test uses a calibrated pendulum impact machine with standard energies of 1 J, 2.75 J, 5.5 J, 11 J, and 22 J, delivering a striking velocity of approximately 3.5 m/s. The machine is equipped with a rigid vise that clamps the specimen as a vertical cantilever, and an encoder or pointer system that records the absorbed energy.

Test Procedure

• Select a pendulum capacity appropriate to the expected energy absorption.
• Measure the specimen width and the remaining thickness beneath the notch to 0.02 mm accuracy.
• Clamp the specimen vertically with the notch level with the top of the vise and facing the striker.
• Release the pendulum from its latched height and allow it to strike the specimen above the notch.
• Record the absorbed energy, classify the fracture mode, and repeat for all specimens.

Calculation and Validity Criteria

Under ISO 180 and GB/T 1843, the notched Izod impact strength is calculated by dividing the corrected absorbed energy by the cross-sectional area of the specimen at the notch, and is reported in kJ/m². Under ASTM D256, the result is expressed as energy per unit notch width in J/m. To be valid, the absorbed energy should fall between 10 % and 80 % of the pendulum capacity; otherwise a different hammer must be used. Only complete breaks and hinged breaks yield quantitative values, while partial breaks and non-breaks are reported qualitatively.

Factors Influencing Results

Test results are sensitive to temperature, notch radius and machining quality, specimen thickness, molecular orientation from molding, moisture content (particularly for polyamides), and material aging. Strict control of these variables is essential for reproducible, comparable data.

Applications

Izod notched impact testing is applied throughout the plastics value chain, from resin producers to end-product manufacturers:

• Material data sheets and grade-to-grade comparison of engineering thermoplastics.
• Incoming inspection and quality control of resin lots and molded components.
• Automotive interior and exterior parts requiring validated impact performance.
• Electrical appliance housings, power tool enclosures, and consumer electronics cases.
• Pipes, fittings, and profiles subject to impact during installation and service.
• Packaging materials and containers evaluated for drop and impact resistance.
• Verification of recycled content effects, heat aging, UV weathering, and chemical exposure on toughness.
• Failure analysis and root-cause investigation of brittle in-service fractures.

FAQ

What is the difference between Izod and Charpy impact tests?

In the Izod test the specimen is clamped vertically as a cantilever beam and struck on the notched face, whereas in the Charpy test the specimen lies horizontally on two supports and is struck on the face opposite the notch. The two methods follow different standards (ISO 180 / ASTM D256 for Izod versus ISO 179 / ASTM D6110 for Charpy) and produce results that are not directly interchangeable.

Why is the specimen notched before testing?

The notch concentrates stress and promotes brittle, reproducible fracture. Without a notch, many ductile plastics would simply bend without breaking, making it impossible to rank materials by notch sensitivity or to detect embrittlement caused by processing, aging, or low temperature.

Are ISO 180 and ASTM D256 results comparable?

No. The specimen dimensions, notch depth, and result units differ between the two standards. ISO 180 reports kJ/m² based on the notched cross-sectional area, while ASTM D256 reports J/m based on specimen width. Data sheets should always state which standard was used, and conversion between the two is not valid.

What does a low notched impact strength indicate?

A low value indicates brittle behavior in the presence of stress concentrations, which may lead to cracking around sharp corners, gate marks, or scratches in service. Designers often respond by increasing radii, selecting a tougher grade, or modifying the polymer with impact modifiers.

Can the test be performed at low temperatures?

Yes. Specimens can be conditioned in a cooled chamber or liquid bath and tested rapidly after removal. Low-temperature Izod testing is commonly used to identify the ductile-to-brittle transition of materials intended for outdoor, automotive, or refrigeration applications.

How should samples be submitted for testing?

Specimens should ideally be molded or machined to the standard geometry with notches cut immediately before testing, or finished parts can be submitted to an accredited third-party laboratory for specimen extraction and preparation under ISO 2818.