ASTM D256 Impact Resistance Test: What the Numbers Really Tell You

Why ASTM D256 Matters

If you specify plastics for housings, enclosures, or structural parts, you will almost certainly encounter an ASTM D256 impact resistance test on a datasheet. It is the most common pendulum impact method in North America, and it gives you a single number — usually in J/m or ft·lb/in — that claims to describe how well a material handles a sudden blow.

That number is useful, but only if you understand what the test actually does and where it falls short. This explainer walks through the method, the key clauses, and the practical traps that catch engineers who treat the Izod value as a universal toughness score.

The Test Setup: Pendulum, Specimen, and Notch

ASTM D256, formally Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics, describes a swinging pendulum that strikes a clamped cantilever beam specimen. The specimen is held vertically in a vice, and the pendulum hits it just above the notch. The energy absorbed in breaking the specimen is calculated from the difference between the pendulum's initial and final heights.

The standard actually covers several procedures, but the one you will see on datasheets is Test Method A — the notched Izod impact test. Clause 4.1 of ASTM D256 specifies the standard specimen: a rectangular bar 63.5 mm (2.5 in) long, 12.7 mm (0.5 in) wide, and 3.2 mm (0.125 in) thick. The notch is machined into the middle of the bar, 2.54 mm (0.100 in) deep, with a 0.25 mm (0.010 in) radius at the root.

Specimen preparation matters enormously. The standard requires that the notch be cut with a single pass of a milling cutter or similar tool — not filed, not molded, not stamped. A poorly machined notch with a ragged root can absorb energy inconsistently)Skip and make results scatter. Clause 6.2 spells out that specimens must be conditioned at 23°C ± 2°C and 50% ± 5% relative humidity for at least 40 hours before testing, unless your material specification says otherwise.

Notched vs. Unnotched: Know Which One You're Reading

Datasheets often list both notched Izod and unnotched Izod values. They are not interchangeable. The notched test (Test Method A) concentrates stress at the notch root, which mimics a sharp crack or a molded-in stress raiser. The unnotched variant (Test Method E, added later) measures the energy to break a plain bar, which is more relevant for parts with no sharp corners.

For ductile materials, the unnotched value can be several times higher than the notched value. That gap itself tells you something: a large difference means the material is notch-sensitive. For example, a polycarbonate might show 800 J/m notched and 3200 J/m unnotched, while a brittle polystyrene might show 20 J/m notched and only 60 J/m unnotched. If you only quote the notched number, you are hiding how the material behaves when a crack starts.

ASTM D256 also includes Test Method B (a reversed notch geometry) and Test Method C (a cantilever beam with a different support), but they are rarely used in commercial datasheets. Stick with Method A unless you have a specific reason.

Units and Reporting: J/m vs. ft·lb/in

The standard test result is the energy absorbed per unit width of the specimen, reported as J/m (joules per meter) in SI units or ft·lb/in (foot-pounds per inch) in inch-pound units. To convert, multiply ft·lb/in by 53.38 to get J/m. For example, 2.0 ft·lb/in is roughly 107 J/m.

Here is the trap: the width used in the calculation is the specimen width behind the notch, which is 10.16 mm (0.400 in) after the 2.54 mm notch is cut. The standard is explicit in Clause 8.3 that you divide the absorbed energy (in joules) by the remaining width (in meters) to get J/m. If you accidentally use the full 12.7 mm width, you will understate the result by about 20%.

Also, note that the result is not normalized by thickness. A 3.2 mm thick specimen gives a different J/m value than a 6.4 mm thick specimen of the same material. ASTM D256 does not correct for thickness, so you cannot compare Izod numbers from specimens of different thicknesses. If you need a thickness-independent measure, look at the Charpy test (ASTM D6110) or a falling dart test (ASTM D3763).

Practical Limits and Common Misconceptions

Many engineers assume that a higher Izod number always means a tougher part. That is not true. The Izod test measures energy absorption in a single high-speed strike, but real-world impact events are often slower, or involve repeated loading, or occur at different temperatures. A material with a high Izod value can still crack under a low-velocity impact if it has a sharp internal stress concentration.

Temperature matters, too. ASTM D256 specifies testing at room temperature (23°C) by default, but you can request testing at other temperatures. Many plastics show a ductile-to-brittle transition — polycarbonate and nylon, for example, lose a large fraction of their Izod energy when tested at −20°C. Always check the test temperature on a datasheet; a value measured at 23°C does not apply to a part used outdoors in winter.

Another misconception is that the Izod value is a direct measure of "impact strength" that can be used in finite element analysis or design calculations. It cannot. The test is comparative, not predictive. It is useful for screening materials, for quality control, and for comparing batches of the same material, but it does not give you a stress or a strain limit.

How to Use ASTM D256 in Material Selection

When you compare materials, follow these practical rules:

1. Compare only same-thickness specimens. A 3.2 mm specimen is standard; if a datasheet lists a different thickness, note it and avoid cross-comparison.
2. Check the notch type. Make sure both materials were tested with the same notch radius (0.25 mm is standard). A larger radius will give a higher value.
3. Look at the standard deviation. ASTM D256 allows for a wide scatter in results, especially for brittle materials. If the datasheet shows a ± value that is more than 10–15% of the mean, treat the number with caution.
4. Consider the temperature. If your application sees cold temperatures, ask for Izod data at −20°C or −30°C, not just room temperature.
5. Pair Izod with another test. Use a tensile test (ASTM D638) for stiffness and a falling dart test (ASTM D3763) for multi-axial impact behavior. No single number tells the whole story.

Takeaway

ASTM D256 is a fast, inexpensive, and widely accepted way to rank plastics under a specific set of conditions. But it is not a universal toughness meter. Read the fine print on the datasheet — specimen thickness, notch radius, test temperature, and conditioning — before you make a decision.

For critical parts, never rely on Izod alone. Combine it with a ductility measure (like elongation at break) and a real-world prototype drop test. That is how you avoid the classic mistake of choosing a material that looks tough on paper but shatters in the field.