Izod Impact Test: Procedure, Formula, Standards, and Applications

 Plastics that feel strong in the hand can still shatter when something hits them at speed, and a mould design, a filler, or even a change in cooling rate can quietly turn a tough grade into a brittle one. The Izod Impact Test exists to catch that change early. It measures the energy a notched plastic bar absorbs when a swinging pendulum strikes it, and the result tells you how resistant the material is to sudden shock.

Two standards govern the method. ASTM D256 is the most common in North America and includes several variants, such as Method A for a standard notched cantilever and Method C for a reversed notch. ISO 180 is the international equivalent, and its units differ slightly. In both, the specimen is moulded or machined to about 63.5 mm by 12.7 mm by 3.2 mm, with a 45-degree notch cut 2.54 mm deep and a tip radius of 0.25 mm. The notch must be clean, since a rough one can lower the reading by a surprising margin.

Before testing, specimens are conditioned for at least 40 hours at 23 degrees Celsius and 50 percent relative humidity. The operator checks the pendulum for free swing, selects a hammer energy such as 2.7 or 5.4 joules, and clamps the sample vertically so the notch faces the striker and sits level with the vice top. After release, the pendulum hits at roughly 3.5 metres per second, and the absorbed energy is read from the scale or display. At least five specimens are tested, and a result is accepted only when the break type is recorded, whether complete, hinge, partial or non-break.

The formula under ASTM D256 is impact strength in J/m = absorbed energy (J) / specimen thickness (m) at the notch. For example, a sample that absorbs 2.5 J with a thickness of 3.2 mm gives 2.5 / 0.0032, which equals about 781 J/m. ISO 180 divides by the remaining cross sectional area instead and reports kJ/m².

Polymer producers use the results to compare grades of ABS, nylon, polycarbonate and polypropylene, and automotive suppliers apply it to bumpers and interior trims. Pipe, electrical housing and toy manufacturers rely on it too, and incoming inspection labs use it to confirm that each resin lot matches the data sheet.

Presto Stantest manufactures pendulum impact testers with accurate calibration, rigid frames and easy digital reading. Visit prestogroup.com to explore the range, and speak with the Presto team about the instrument that suits your Izod Impact Test requirements.