ASTM D256
Plastics Notched Izod Impact Test

Downloads: Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics - PDF

Written by Chris Thomas, Universal Grip Company · Reviewed by Frank Bertini · Updated October 17, 2026

ASTM D256 is the standard test method for determining the Izod pendulum impact resistance of plastics. A notched bar is clamped vertically as a cantilever, a swinging pendulum hammer strikes it 22 mm above the clamp on the notched face, and the energy absorbed in breaking it, divided by the specimen width, is reported as Izod impact resistance in joules per metre. It is the impact number on nearly every plastics data sheet (together with its Charpy sibling ASTM D6110), and it is used to rank resins, compounds, colorants and regrind levels, to qualify molded parts, and to check toughness after aging, moisture or cold exposure.

Most of what is published about D256 online stops at the specimen size. This guide gives the full dimensional specification with tolerances, the pendulum series and the 85 % capacity rule, how notches must be cut and checked, the 40-hour conditioning requirement that several pages claim does not exist, the difference between Methods A and C, the break-type codes that decide which results can be averaged, a worked calculation in J/m, ft·lbf/in and kJ/m², the precision you can expect between labs, and why ISO 180 results cannot be converted to D256 results. It also says plainly what kind of pendulum machine the test needs, because it is not the same machine that runs Charpy on metals.

In this guide: What ASTM D256 Measures · Test Methods A and C · Specimen Dimensions and Tolerances · Notching: The Step That Decides the Result · The Pendulum Machine: Energies, Striker and the 85 % Rule · Conditioning and Test Temperature · Step-by-Step Procedure (Method A) · Break Types: C, H, P and NB · Calculation and Worked Example · Precision: What Lab-to-Lab Spread to Expect · ASTM D256 vs ISO 180, D6110, D4812 and D1822 · Common Errors · Equipment for ASTM D256 · FAQ

What ASTM D256 Measures

D256 measures the energy required to break a standard notched specimen under a single swing of a pendulum, expressed per unit of specimen width. The notch is a deliberate stress concentrator: it forces a brittle, high-strain-rate fracture to start at a defined point, so the test measures resistance to crack propagation under impact rather than the energy to deform the whole bar. That is also why the result is extremely sensitive to the notch radius: polycarbonate, for example, can lose most of its Izod value when the notch radius drops from 1.0 mm to the standard 0.25 mm, and a dull or chipped cutter changes the number for every material.

The result is called Izod impact resistance (older documents say impact strength) and is reported in J/m of width, with ft·lbf/in as the inch-pound unit (1 ft·lbf/in = 53.4 J/m). It is a comparative, quality-control property rather than a design value: it depends on specimen width, notch geometry, molding conditions and temperature, and it is not a material constant that can be used to predict part performance. Related methods are ASTM D6110 for the Charpy (simple-beam) version on the same specimen, D4812 for unnotched Izod, D1822 for tensile impact of films and thin sheet, and D3763 for instrumented puncture impact of flat sheet. The current edition is D256-26.

Test Methods A and C

The current scope of D256 lists two methods. Older editions also carried Method D (notch-radius sensitivity, run at 0.25 and 1.0 mm radii and reported as a slope) and Method E (reversed notch, struck on the unnotched face); check the edition a specification cites before offering those, and treat a request for Method E as a proxy for unnotched testing per D4812.

• Method A, notched cantilever beam: the standard Izod test. The specimen is clamped with the notch flush with the top of the vise and facing the striker, and the energy to break is read from the machine and corrected for windage and friction.

• Method C: identical to Method A but adds a correction for the energy used to throw the broken half of the specimen (the toss energy). It is required for materials with impact resistance below 27 J/m (0.5 ft·lbf/in), where the toss energy is a significant fraction of the reading, and is optional above that.

Method C measures the toss energy by placing the broken free end back on the vise and striking it again with the pendulum; the reading from that second strike is subtracted from the first. Reported values from the two methods are not interchangeable at low impact levels, so the method is always part of the report.

Specimen Dimensions and Tolerances

The D256 specimen is a rectangular bar with a V-notch milled across one face. The depth of the bar and the depth of material under the notch are fixed; the width (which is the molded or sheet thickness) may vary within a range, and the result is normalized by it.

• Length: 63.5 ± 2.0 mm (2.50 ± 0.08 in).

• Depth (the 12.7 mm dimension, in the direction of the notch): 12.70 ± 0.20 mm (0.500 ± 0.008 in).

• Width (molded thickness): 3.0 to 12.7 mm (0.118 to 0.500 in); 3.2 mm (1/8 in) is the most common molded bar, 6.35 mm (1/4 in) is used where a thicker specimen is wanted. Widths within one test group must agree within ± 0.13 mm, and results from specimens of different widths are not comparable, because thinner bars absorb extra energy in crushing, bending and twisting.

• Depth of material under the notch: 10.16 ± 0.05 mm (0.400 ± 0.002 in), which fixes the notch depth at 2.54 mm on a 12.70 mm bar.

• Notch: included angle 45 ± 1°, root radius 0.25 ± 0.05 mm (0.010 ± 0.002 in), with the plane bisecting the notch perpendicular to the specimen face within 2°.

• Notch position: centred along the length, 31.8 ± 1.0 mm from the clamped end, so that the notch sits exactly at the top edge of the vise.

Specimens are injection molded to size or machined from sheet, plate or molded blocks; they are not die-cut, so a cutting die is only relevant to the companion tensile-impact or D638 specimens. Molded bars are the norm for data sheets, and molding conditions must be recorded because flow orientation, skin and gate position change the result. Sheet materials that are anisotropic are tested in each principal direction and reported separately. Measure each specimen's width to 0.025 mm at the notch and the depth under the notch; a 0.1 mm error in width is a 3 % error in a 3.2 mm bar.

Notching: The Step That Decides the Result

The notch is cut after molding, with a milling cutter or a dedicated notching machine, never molded in. D256 specifies a single-tooth cutter with zero rake and 15 to 20° relief, run at a surface speed and feed that produce a clean, smooth notch without heating the plastic above its glass-transition temperature; the standard gives example cutter speeds of roughly 53 to 150 m/min and feeds of 36 to 160 mm/min. Multi-tooth cutters are allowed only if they give the same profile. The notch profile (angle, radius and depth) must be verified on an optical comparator or similar at least every 500 notches, and more often when cutting abrasive glass-filled compounds, because a worn cutter rounds the root radius and gives falsely high values.

• Notch all specimens of a comparison on the same cutter in the same session, so that any cutter wear affects them equally.

• Check the notch radius with a 0.25 mm radius gauge or on a profile projector; a chipped cutter leaves a sharper root and a lower result, a dull one a larger radius and a higher result.

• Keep the cutting plane square to the specimen face; a tilted notch loads one edge first and scatters results.

• Condition the specimens after notching, not before; the 40-hour clock starts when the notch is cut.

• Do not notch below the material's embrittlement temperature or above its Tg; both change the notch-root microstructure.

The Pendulum Machine: Energies, Striker and the 85 % Rule

An Izod machine for plastics is a small pendulum impact tester with interchangeable hammers. The requirements in D256 are specific, and they are not met by the metals Charpy machines that most testing-equipment catalogs show.

• Drop height and velocity: the striker falls through a height of 610 ± 2 mm, which gives a striking velocity of approximately 3.5 m/s (11.4 ft/s). The effective pendulum length is 0.33 to 0.40 m.

• Hammer energies: the basic hammer has a capacity of 2.76 ± 0.14 J (2.00 ± 0.10 ft·lbf); larger hammers double in steps up to 21.7 J (16 ft·lbf), so a typical set is 2.7, 5.4, 10.8 and 21.7 J (ISO-series machines use 2.75, 5.5, 11 and 22 J), each within 0.5 % of nominal. Hammers are changed by swapping or adding weights, with the same drop height.

• Striker: a hardened edge of radius 0.80 ± 0.20 mm that contacts the specimen along a line at the centre of percussion, 22.00 ± 0.05 mm (0.876 ± 0.002 in) above the top surface of the vise.

• Vise: the top of the fixed jaw is a plane that must bisect the notch within 0.12 mm; the jaw edges have a radius of 0.25 ± 0.12 mm; the specimen is clamped with the notch facing the hammer.

• Verification: the pendulum must swing freely with a first-swing indicated energy of less than 2.5 % of capacity and read within 0.2 % of full scale when hanging at rest; the machine's windage and friction losses are measured and applied as a correction (Annex A2 of the standard). ISO 13802 describes the full verification procedure that most machine makers certify to.

• Choosing the hammer: use the lightest hammer that breaks the specimen with no more than 85 % of its capacity absorbed. The intent is to keep the reading in the usable part of the scale; readings below about 10 to 15 % of capacity have poor resolution, which is also why a metals machine of 150 J or more cannot read a 2 J plastics break with any accuracy.

In practice: estimate the expected J/m, multiply by the width in metres, divide by 0.85, and pick the smallest hammer at or above that energy. A 3.2 mm polycarbonate bar at 800 J/m absorbs about 2.6 J, so it needs the 5.4 J hammer; a 3.2 mm unfilled polystyrene bar at 25 J/m absorbs 0.08 J and needs the 2.7 J hammer plus Method C. Universal Grip's pendulum impact testers are 150 to 750 J machines for ASTM E23 and ISO 148 Charpy testing of metals; for D256 you need a 2.7 to 21.7 J plastics pendulum, which we can quote on request through our contact page.

Conditioning and Test Temperature

D256 conditions specimens per ASTM D618 Procedure A: at least 40 hours at 23 ± 2 °C and 50 ± 5 % relative humidity after notching, with the test run in the same atmosphere (referee tests hold ± 1 °C and ± 2 % RH). Pages that say the standard specifies no conditioning time are wrong, and the time matters: moisture-sensitive polyamides can change notched Izod by a factor of several between dry-as-molded and 50 % RH equilibrium, which is why the standard allows hygroscopic materials such as nylons to be tested dry-as-molded under ASTM D4066 when the specification calls for it, and why the report must state which.

For sub-ambient testing, specimens are conditioned in a cold chamber or bath at the test temperature and transferred to the vise and broken within a few seconds; the commonly used limit is 5 seconds, because a 3 mm bar warms several degrees in that time. An environmental chamber beside the machine, or a machine with an integrated conditioning cabinet, is the usual arrangement; the pendulum itself stays at room temperature. Impact resistance of most thermoplastics falls sharply through the ductile-brittle transition, so the temperature is as much part of the result as the material.

Step-by-Step Procedure (Method A)

• 1. Verify the machine: check free-swing and rest readings, confirm the hammer capacity and the windage/friction correction for that hammer.

• 2. Measure the width of each specimen at the notch to 0.025 mm and the depth under the notch; record both.

• 3. Select the hammer from the expected energy so that the reading will be at or below 85 % of capacity.

• 4. Clamp the specimen vertically in the vise with the notch facing the striker and the notch bisector flush with the top of the vise (use the machine's positioning gauge); clamp firmly and consistently.

• 5. Release the pendulum from its latched height and let it strike; record the indicated energy and the break type.

• 6. Correct the reading for windage and friction, and in Method C also for toss energy.

• 7. Repeat on at least 5 specimens, preferably 10 or more, and for anisotropic sheet on each principal direction.

• 8. Calculate impact resistance per specimen, then the mean and standard deviation of specimens with the same width and the same break type.

Break Types: C, H, P and NB

Every specimen is classified after the test, and the classification decides what can be averaged and reported.

• C, complete break: the specimen separates into two pieces. The normal result for brittle and semi-ductile materials, and the value used on data sheets.

• H, hinge break: the halves remain attached by a thin hinge that cannot support itself, with an included angle of less than 90°. Reported as a valid result but kept separate from complete breaks.

• P, partial break: at least 90 % of the ligament has fractured but the specimen is not a hinge. Reported separately.

• NB, non-break: less than 90 % of the ligament has fractured; the specimen bent or was simply pushed through the vise. NB values are not Izod impact resistance; they are reported only as the number and percentage of non-breaks.

Only specimens of the same break type and width are averaged together, and the report gives the number and percentage of each type. A material that gives three C and two NB results is reported that way, not as a five-specimen mean; averaging in the NB values would understate the energy of the breaks and overstate the material. Tough polymers that never break at room temperature (polycarbonate, many TPUs, impact-modified nylons) are often tested at reduced temperature, with a sharper notch under a product specification, or by a different method such as instrumented impact.

Calculation and Worked Example

Izod impact resistance (J/m) = corrected energy to break (J) ÷ specimen width (m). The width is the molded thickness measured at the notch, not the 12.7 mm depth. For the inch-pound form, divide ft·lbf by width in inches; 1 ft·lbf/in = 53.4 J/m. The standard's optional alternative divides the energy by the ligament area (width × depth under the notch) to give kJ/m², but its appendix is explicit that the two units cannot be converted into each other and that a kJ/m² value from D256 is still not an ISO 180 value.

Worked example. A 3.18 mm wide ABS bar is broken with the 5.4 J hammer. The machine indicates 2.31 J; the windage and friction correction for that hammer at that swing angle is 0.04 J, so the net energy is 2.27 J. Capacity check: 2.27 ÷ 5.4 = 42 % of capacity, within the 85 % limit and well up the scale. Izod impact resistance = 2.27 J ÷ 0.00318 m = 714 J/m, which is 714 ÷ 53.4 = 13.4 ft·lbf/in. The break is a complete break, type C. On the ligament-area basis the same test gives 2.27 J ÷ (3.18 mm × 10.16 mm) = 70.3 kJ/m², a number that must not be compared with an ISO 180 result. Ten specimens with complete breaks averaging 705 J/m and a standard deviation of 28 J/m are reported as 705 J/m, SD 28, 10 C, 0 H, 0 P, 0 NB, Method A, 5.4 J hammer, 3.18 mm width.

Method C addition. For a 3.2 mm polystyrene bar the 2.7 J hammer reads 0.095 J; the re-strike of the broken half reads 0.012 J, so the net energy is 0.083 J and the impact resistance is 0.083 ÷ 0.0032 = 26 J/m. Without the toss correction the result would be 30 J/m, a 15 % overstatement, which is why the correction is mandatory below 27 J/m.

Precision: What Lab-to-Lab Spread to Expect

The interlaboratory study in D256 (Method A, five-specimen averages) gives reproducibility limits IR of roughly 11 J/m on a 30 J/m phenolic, 91 J/m on a 77 J/m acetal, 37 J/m on a 106 J/m reinforced nylon, 87 J/m on a 142 J/m polypropylene, 88 J/m on a 577 J/m ABS and 159 J/m on an 876 J/m polycarbonate. In plain terms, two labs testing the same lot can differ by 10 to 40 % on most materials and by more than 100 % on acetal, even when both are correct. Within one lab the repeatability is far better, typically 5 to 10 %. The consequences: a specification minimum should sit well below the typical value, a supplier-customer dispute over 15 % is usually not a material difference, and notch quality, hammer selection and conditioning are where the between-lab spread comes from.

ASTM D256 vs ISO 180, D6110, D4812 and D1822

• ISO 180 (Izod, plastics): the international Izod method, which D256 itself says resembles it in title only. ISO uses an 80 × 10 × 4 mm bar (a Type 1 tensile bar with the tabs removed) with a 2 mm deep notch leaving 8.0 mm under it, three notch types (A, radius 0.25 mm; B, 1.00 mm; C, 0.10 mm), results in kJ/m² (energy divided by width × remaining width), ten specimens, and a usable window of 10 to 80 % of hammer capacity. Different specimen, different ligament, different strike geometry, different units: a PA66 reported at 5.5 kJ/m² under ISO 180/1A and at about 1.0 ft·lbf/in (53 J/m) under D256 are the same material, but there is no conversion factor between the two methods.

• ASTM D6110 (Charpy): the same notched bar supported horizontally as a simple beam and struck behind the notch; formerly Method B of D256. Charpy and Izod values for one material differ and are not interchangeable.

• ASTM D4812 (unnotched Izod): the same cantilever test without a notch, for materials too tough or too brittle to give a useful notched value.

• ASTM D1822 (tensile impact): for films, thin sheet and materials too flexible or too thin for a 12.7 mm bar.

• ISO 179 (Charpy): the ISO counterpart of D6110, and the method most European data sheets quote; ISO 179-1eA is the unnotched/notched value usually listed beside ISO 180.

Metals impact testing is a different world: ASTM E23 and ISO 148 use 10 × 10 × 55 mm Charpy specimens on 150 to 750 J machines, with the energy reported in joules, not per unit width.

Common Errors

• Worn, chipped or multi-tooth notch cutter; notch radius not verified.

• Notch bisector not flush with the vise top (specimen set too high or low), which changes the strike-to-notch distance.

• Hammer too large, so the reading sits in the bottom 10 % of the scale; or too small, exceeding 85 % of capacity.

• Windage and friction correction not applied, or applied from a different hammer.

• Method A used below 27 J/m without the Method C toss correction.

• Conditioning counted from molding instead of from notching, or shorter than 40 hours.

• Nylons tested at an unstated moisture state.

• Averaging complete breaks with hinge, partial or non-break results.

• Mixing specimen widths in one data set, or reporting J/m without the width.

• Comparing D256 J/m with ISO 180 kJ/m² or with Charpy values.

• Using a metals Charpy pendulum (150 J and up) for a plastics Izod test.

Equipment for ASTM D256

• Pendulum impact tester for plastics: a 2.7 to 21.7 J (or 2.75 to 22 J ISO-series) machine with Izod vise, 0.80 mm striker, interchangeable hammers and windage/friction correction. Universal Grip's impact testers cover the 150 to 750 J metals range for Charpy per E23; for a plastics Izod/Charpy pendulum, contact us with your expected energy range and we will quote a suitable machine.

• Notching cutter: a single-tooth or certified multi-tooth notcher with 45°/0.25 mm profile, plus a profile projector or notch gauge for the 500-notch check.

• Conditioning: a 23 °C / 50 % RH room or cabinet, and an environmental chamber or cold bath for sub-ambient tests.

• Measurement: a micrometer reading to 0.01 mm for width and depth under the notch.

• Companion tests on a universal testing machine: ASTM D638 tensile, D790 flexure and D695 compression on the same molded bars, run on our benchtop universal testing machines with cutting dies for sheet specimens.

For the full plastics property set, see our plastics testing page.

ASTM D256 FAQ

What is ASTM D256?

The ASTM standard test method for Izod pendulum impact resistance of plastics. A notched bar is clamped as a cantilever and struck by a pendulum hammer 22 mm above the clamp; the energy to break divided by the specimen width is the Izod impact resistance in J/m or ft·lbf/in.

What are the ASTM D256 specimen dimensions?

63.5 ± 2.0 mm long, 12.70 ± 0.20 mm deep and 3.0 to 12.7 mm wide (usually 3.2 or 6.35 mm molded thickness), with a 45 ± 1° notch of 0.25 ± 0.05 mm root radius leaving 10.16 ± 0.05 mm of material under the notch.

How many specimens does ASTM D256 require?

At least five per sample, preferably ten or more, all of the same width. Anisotropic sheet is tested in each principal direction and reported separately.

What is the conditioning for ASTM D256?

At least 40 hours at 23 ± 2 °C and 50 ± 5 % relative humidity after notching, per ASTM D618 Procedure A, with the test in the same atmosphere. Hygroscopic materials such as nylon may be tested dry-as-molded if the specification requires it.

What pendulum energy is used for ASTM D256?

The basic hammer is 2.76 J (2 ft·lbf) and larger hammers double in steps to 21.7 J (16 ft·lbf), all falling from 610 mm at about 3.5 m/s. Use the lightest hammer that breaks the specimen with no more than 85 % of its capacity absorbed.

Can a Charpy impact tester for metals run ASTM D256?

No. Metals machines of 150 J and above cannot resolve the 0.1 to 3 J absorbed by a plastics specimen, and they lack the Izod vise, 0.80 mm striker and 22 mm strike height. D256 needs a 2.7 to 21.7 J plastics pendulum.

What is the difference between Method A and Method C in ASTM D256?

Both are notched cantilever tests. Method C adds a correction for the energy used to toss the broken half of the specimen, measured by re-striking it, and is required for materials below 27 J/m (0.5 ft·lbf/in).

How is Izod impact resistance calculated?

Corrected energy to break in joules divided by the specimen width in metres, giving J/m. For example 2.27 J on a 3.18 mm wide bar is 714 J/m, or 13.4 ft·lbf/in (1 ft·lbf/in = 53.4 J/m).

What do the break types C, H, P and NB mean?

C is a complete break, H a hinge break with the halves attached by a thin hinge, P a partial break with at least 90 % of the ligament fractured, and NB a non-break with less than 90 % fractured. Only specimens of the same type are averaged, and NB values are not reported as impact resistance.

What is the difference between ASTM D256 and ISO 180?

ISO 180 uses an 80 × 10 × 4 mm bar with 8 mm under the notch, three notch types, a 10 to 80 % capacity window and reports kJ/m² (energy per ligament area); D256 uses a 63.5 × 12.7 mm bar with 10.16 mm under the notch and reports J/m (energy per width). The results cannot be converted between the standards.

What is the difference between Izod and Charpy impact?

In Izod the specimen is clamped vertically as a cantilever and struck on the notched face above the clamp; in Charpy it rests horizontally on two supports and is struck behind the notch at mid-span. Charpy on plastics is ASTM D6110 and ISO 179; the two values for one material differ.

How repeatable is ASTM D256?

Between laboratories the reproducibility is typically 10 to 40 % of the mean, and over 100 % for some materials such as acetal; within one lab it is usually 5 to 10 %. Notch quality, hammer selection and conditioning cause most of the spread.

Need a fixture, grips or a complete system for ASTM D256? Contact Universal Grip with your specimen details, capacity and machine connection and we will quote the right setup.