Izod vs Charpy Impact Testing of Plastics: Methods, Notching & Machine Selection

Impact resistance tells you whether a plastic part survives a drop, a knock or a cold-weather shock. Two pendulum methods dominate: Izod (ASTM D 256, ISO 180) and Charpy (ASTM D 6110, ISO 179). Both swing a calibrated hammer into a notched bar and measure absorbed energy — but the specimen mounting and reported units differ.

A combined Izod / Charpy Impact Tester performs both methods on one frame. This article compares them and explains why notch quality decides whether your data can be trusted.

Izod vs Charpy: The Practical Difference

• Izod — specimen clamped vertically as a cantilever, notch facing the hammer; standard for North American datasheets, reported in J/m or ft·lb/in.
• Charpy — specimen supported horizontally at both ends, struck behind the notch; preferred in ISO/European specifications, reported in kJ/m².
• Same physics, different geometry — values from the two methods are not interchangeable and must never be compared directly.

Export manufacturers typically need both: an American buyer will ask for Izod per ASTM D 256, a European buyer for Charpy per ISO 179. A dual-configuration pendulum with interchangeable strikers and vices covers both without buying two machines.

Notching: Where Most Impact Data Goes Wrong

The 'V' notch concentrates stress so failure starts at a defined point. Its depth, angle (45°) and root radius (0.25 mm) are specified tightly — a blunt or torn notch can double the apparent impact strength. That is why standards effectively require machined notches from a Motorized Notch Cutter rather than hand-cut ones.

Verify every batch of notches with a Micro-meter Jig for depth measurement, and prepare clean specimen blanks with a Contour Cutter or hydraulic sample press. For unnotched-specimen work such as pipe fittings, see our falling weight Impact Tester.

Selecting an Impact Tester

Choose pendulum energies that bracket your materials — brittle filled grades need small hammers for resolution, tough copolymers need larger ones so the pendulum retains enough residual energy. Look for hardened knife edges, friction-compensated bearings, digital angle encoders and a rigid base; per the standards, the machine must lose almost no energy to vibration.

Frequently Asked Questions