Izod Impact Test Specimen Dimensions: A Complete Reference

What Is the Izod Impact Test?

ASTM D256 is the Standard Test Method for Determining the Izod Pendulum Impact Resistance of Plastics. It measures a plastic’s resistance to a sudden, high-speed impact by swinging a pendulum hammer into a notched specimen held in a cantilevered clamp. The notch concentrates stress and standardizes where fracture begins, making results reproducible for material comparison, quality control, and product design across the automotive, packaging, electronics, and construction industries.

Specimen Dimensions

• Standard ASTM specimen: 63.5 x 12.7 x 3.2 mm (2.5 x 0.5 x 0.125 in)
• Most common thickness: 3.2 mm, though 6.4 mm is preferred where available since thinner bars are more prone to bending or crushing on impact
• Notch: 45-degree V-notch machined to a depth of 2.54 mm, with a tip radius of 0.25 mm, leaving 10.2 mm of material depth behind the notch
• ISO 180 equivalent specimen (Type 1A, end tabs removed): 80 x 10 x 4 mm, with an 8 mm notch depth

Test Procedure

The notched specimen is clamped vertically in the Izod fixture with the notch facing the pendulum’s striking edge. The pendulum is released from a fixed height, striking through the specimen in a single swing. The energy absorbed during fracture is calculated from the difference between the pendulum’s release height and its height of rise after impact. ASTM D256 includes Method A (standard notched Izod), Method C (reversed notch, used when impact resistance falls below 27 J/m), and Method D (unnotched). Results are averaged across five specimens and reported in J/m or ft-lb/in (energy per unit thickness), while ISO 180 reports energy per unit cross-sectional area (kJ/m2) — the two are not directly interchangeable.

Factors Affecting Results

Impact values shift with polymer type and molecular weight, notch sharpness and geometry, specimen conditioning (per ASTM D618), test temperature (impact resistance typically drops at lower temperatures), filler content, and processing history that affects crystallinity or molecular orientation. Even minor deviations in notch depth or surface quality can produce misleading impact numbers, which is why specimen preparation is often the biggest source of test-to-test variability.

Industry Specifications Referencing ASTM D256

• Automotive & consumer plastics: nylon, polycarbonate, ABS, and impact-modified polypropylene qualification
• ISO 180: the international equivalent, with differing specimen geometry
• ASTM D4812: companion method for unnotched Izod testing

Also Read – Bakelite vs Plastics: Key Differences, Properties & Testing

Conclusion

The Izod impact test remains one of the most widely specified toughness checks for engineering plastics, valued for its speed and repeatability as a quality control screen. Because results are geometry-dependent, comparisons should only be made between specimens of matching width, notch depth, and test method.

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