In ASTM D256, a pendulum hammer is dropped onto a notched plastic sample clamped at one end, and the energy that the sample absorbs until breakage is measured. The notching is the critical factor in this test. It focuses stress at a fixed radius, making it possible to quantify impact resistance to the notch. MatX Lab performs Izod impact testing on molding plastics, filled compounds, and toughened plastics per ASTM D256 and ISO 180 standards.
What Is ASTM D256?
ASTM D256 is the standard test method for determining the Izod pendulum impact resistance of plastics. The specimen stands as a vertical cantilever with the notched face toward the striker, and the pendulum breaks it on the way through.
The loss of energy from the pendulum is the measurement and can be determined by the height to which it swings after impact. The reported values have been normalized either by the notch thickness (joules/meter) or by the residual cross-sectional area (kilojoules/meter squared).
Several approaches can be used within this test method. Approach A is the standard Izod notched impact test. Approach C is used for materials that lose very little energy; in such cases, the energy transferred to the fractured piece must be accounted for. Approach E uses the reverse notch approach.
ASTM D256 Scope and Applications
The method suits rigid plastics that break under the pendulum. Materials too tough to break give a partial break or no break, and those results have to be reported as such rather than as a number.
Areas of application:
- Grade comparison. Comparing impact-modified and unmodified grades of the same polymer, which constitutes the most frequent cause for performing this test.
- Low-temperature testing. Testing specimens in their conditioned state at low temperature, when most polymers become brittle within a narrow temperature range.
- Filler effect quantification. Measuring how much impact resistance a glass or mineral loading costs, since stiffness gains usually come at exactly this expense.
- Regrind and recycled content evaluation. Detecting the impact loss that reprocessing causes before it reaches a production part.
- Lot verification. Confirming a delivered compound performs as qualified.
Charpy impact on the same materials runs under ASTM D6110. Instrumented multiaxial impact, which gives force and displacement rather than a single energy figure, is covered by ASTM D3763. MatX Lab reports measured values and does not certify products or act as a standards body.
Abrasion resistance is an entirely separate property tested to ASTM D1044. Surface energy and wetting properties, which pertain to adhesion rather than friction, are determined by contact angle tests according to ASTM D7490. MatX Lab reports test results and is not a product certification agency or standards organization.
ASTM D256 Test Procedure
The notching process controls this test entirely. A notch cutter with a blunt blade cuts a radius out of specification, and all readings obtained from this batch will be in error.
| Step | What happens |
| Specimen preparation | Bars are molded or machined to 63.5 mm x 12.7 mm, with a thickness commonly 3.2 mm and consistent along the length. |
| Notching | A single-tooth cutter machines the notch to the specified depth and tip radius, leaving the standard remaining width. |
| Notch verification | Notch depth and tip radius are checked optically on samples from the batch, since cutter wear is gradual and invisible without measurement. |
| Conditioning | Specimens are conditioned at standard laboratory conditions, and moisture-sensitive materials are brought to a defined state. |
| Pendulum selection | A pendulum capacity is chosen so that the energy absorbed falls in the useful part of its range, commonly between 10% and 85% of capacity. |
| Machine check | Determine and apply windage and friction losses, and verify the machine before the batch runs. |
| Testing | Clamp each specimen to the specified height with the notch facing the striker, then release the pendulum. |
| Break classification | Classify each break as complete, hinge, partial, or non-break, and report that classification with the energy. |
Limitations: The effect is dependent on notch tip radius in such a way that even a slight error causes a significant difference; hence the poor repeatability between laboratories for this test, even if it does not show up in the numbers obtained. The values obtained are comparative and not design values because an actual component has its own geometry, gates, and stress concentrations. Neither non-failure nor hinge failure can be quantified, and averaging these values with full failures produces a meaningless number. Thickness affects the test outcome; thus, specimens of different gauges cannot be compared. Molding conditions also strongly affect the test result, so specimens with cold melt or poor packing will yield lower values regardless of material.
ASTM D256 Specimen Requirements and Test Conditions
| Parameter | Typical requirement |
| Specimen dimensions | 63.5 mm x 12.7 mm, commonly 3.2 mm thick |
| Notch | Machined, with the standard tip radius and remaining width |
| Replicates | Five per condition minimum, ten for high scatter materials |
| Pendulum capacity | Selected so absorbed energy falls within the usable range |
| Conditioning | Standard laboratory atmosphere, or a defined reduced temperature |
| Test temperature | 23 °C standard, low temperature conditions on request |
| Material required | Molded bars, or enough plaque to machine the specimen count |
| Reporting | Energy plus break classification for every specimen |
Send molded bars where possible rather than machined ones. Machining a bar from a plaque changes the skin structure and usually lowers the result, and the difference can be larger than the difference between two resins.
Tell us the test temperature you need. Many polymers pass through their ductile-to-brittle transition between 0 °C and -30 °C, and a room-temperature result says nothing about a part that will be handled outdoors in winter.
ASTM D256 Test Results and Reporting
In your report, specify impact energy per specimen along with its break category.
The notch geometry of the batch will be verified, as the data on depth and tip radius are used in all the calculations. The dimensions of specimens, conditioning, pendulum capacity, and testing temperature will be included.
The number of specimens for non-breaks and hinge breaks will be mentioned separately; they are not included in the calculation of the average. If a material has mixed break types among replicates, explain it, as it indicates proximity to the transition.
ASTM D256 FAQs
Why do laboratories disagree on Izod values?
Notch tip radius. The result is highly sensitive to it, and small differences in cutter condition between laboratories produce real differences in reported energy on identical material.
Can I use Izod values for design?
No. The data sort materials by performance under a specific notched specimen geometry. The actual component has different dimensions, different stress concentrations, and a different load condition; hence, design requires testing.