ASTM D256 Complete Guide: Izod Impact Specimens, Notching, Hammers, Calculations and Equipment
Updated: 11 hours ago
ASTM D256 is the standard Izod pendulum impact test for plastics, and it is one of the easiest tests in a plastics lab to get wrong without noticing. A notch cut with a worn cutter, a hammer that is too big for the specimen, a bar conditioned for 12 hours instead of 40, or a hinge break averaged in with complete breaks can each move the result by 20 % or more, and the number still looks plausible on a data sheet. This complete guide covers everything you need to run D256 properly: Test Methods A and C, the specimen and its tolerances, notching, conditioning, the pendulum machine and the 85 % rule, the step-by-step procedure, break types, the calculation with worked examples, typical values, precision, low-temperature testing, D256 vs. ISO 180 and the Charpy methods, troubleshooting and a full FAQ.
It also answers a question most equipment pages avoid: what kind of machine D256 actually needs. A plastics Izod test runs on a small pendulum of a few joules, not on a 150 to 750 J metals Charpy machine and not on a tensile tester. We say plainly what Universal Grip does and does not supply for the impact test itself, and where our dies, presses, grips, fixtures, Mark-10 testers and Galdabini universal testing machines fit in a lab that runs D256 alongside D638 and D790.
For the standard reference page with the full dimensional specification and downloads, see our ASTM D256 Izod impact test page. This article focuses on how to set up and run the test, how to avoid the common errors, and how to equip the lab around it.
ASTM D256 at a glance
Full title: Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics.
What it measures: the energy absorbed in breaking a notched bar with one swing of a pendulum, divided by the specimen width. Reported in J/m (or ft·lbf/in; 1 ft·lbf/in = 53.4 J/m).
Methods: the current edition (D256-26) lists Test Method A (notched cantilever beam) and Test Method C (Method A plus a toss-energy correction, required below 27 J/m). Older editions also carried Methods D (notch-radius sensitivity) and E (reversed notch).
Specimen: 63.5 ± 2.0 mm long, 12.70 ± 0.20 mm deep and 3.0 to 12.7 mm wide (3.2 mm and 6.35 mm molded bars are most common).
Notch: 45 ± 1° included angle, 0.25 ± 0.05 mm root radius, leaving 10.16 ± 0.05 mm of material under the notch.
Pendulum: 610 ± 2 mm drop height, about 3.5 m/s at impact, a basic hammer of 2.7 J (2.00 ft·lbf) with heavier hammers that double in steps up to 21.7 J (16 ft·lbf).
85 % rule: use the lightest hammer that breaks the specimen with no more than 85 % of its capacity absorbed.
Conditioning: at least 40 hours at 23 ± 2 °C and 50 % RH after notching (ASTM D618 Procedure A), and test in the same atmosphere, unless the material specification says otherwise.
Specimens: at least five, preferably ten or more, all of the same width; anisotropic sheet is tested in each principal direction.
Result: mean impact resistance of specimens with the same break type (C, H, P), with the number of each break type and of non-breaks (NB) reported.
Typical equipment: a plastics pendulum impact tester with Izod vise, a notching cutter and notch gauge, a micrometer, a conditioning room or cabinet, and usually a universal testing machine for the companion ASTM D638 and ASTM D790 tests.
What ASTM D256 measures and who uses it
D256 measures how much energy a standard notched plastic bar absorbs when it is broken by a single swing of a pendulum. The specimen is clamped vertically as a cantilever with its notch level with the top of the vise and facing the hammer. The striker hits the bar 22 mm above the vise, the bar fractures from the notch root, and the pendulum swings on with less energy than it started with. The difference, corrected for friction and air resistance, is the energy to break. Dividing it by the specimen width (the molded thickness, not the 12.7 mm depth) gives Izod impact resistance in J/m.
The notch is the point of the test. It concentrates stress at a sharp root, raises the local strain rate and forces fracture to start at a known place, so the test ranks how well a material resists a crack running from a sharp flaw at high speed. That is why notched Izod is so sensitive to anything that changes the notch: a 0.25 mm root radius can cut the value of a notch-sensitive polymer such as polycarbonate to a fraction of what the same material gives with a 1.0 mm radius.
Notched Izod is the impact number on almost every North American plastics data sheet. Resin producers and compounders use it to rank grades, impact modifiers and fillers and to release lots; molders use it to check regrind, drying and molding conditions; and automotive, appliance, electrical and medical-device specifications set minimums at room temperature and often at −30 or −40 °C.
D256 is a comparative and quality-control test. The result depends on specimen width, notch quality, molding history and temperature, and it is not a design value for predicting how a part will behave in service.
Test Methods A and C (and the retired D and E)
Test Method A, cantilever beam: the standard notched Izod test. The reading is corrected for windage and friction and divided by width. Most specifications and data sheets quote Method A.
Test Method C, cantilever beam for materials below 27 J/m (0.5 ft·lbf/in): identical to Method A but also subtracts the toss energy, the energy used to throw the broken half of the specimen. It is required for brittle materials below 27 J/m, where the toss energy is a significant share of the reading, and optional above that.
Test Methods D and E: earlier editions included Method D (notch-radius sensitivity, testing at 0.25 and 1.0 mm radii and reporting a slope) and Method E (reversed notch, struck on the face opposite the notch as an estimate of unnotched resistance). The current scope lists only A and C, so if an older specification calls for D or E, check which edition it cites. For unnotched values, ASTM D4812 is the dedicated method.
The Charpy version of the test, once Method B of D256, is now a separate standard, ASTM D6110. It uses the same notched bar supported horizontally as a simple beam.
ASTM D256 specimen dimensions and tolerances
The D256 specimen is a rectangular bar with a V-notch milled across one narrow face. Its length and depth are fixed; the width is the molded or sheet thickness and may vary within a range, and the result is normalized by it.
Length: 63.5 ± 2.0 mm (2.50 ± 0.08 in).
Depth (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 when a thicker, stiffer specimen is wanted, because it is less likely to bend or crush. Keep widths within a test group closely matched; results at different widths are not comparable.
Depth under the notch: 10.16 ± 0.05 mm (0.400 ± 0.002 in), which makes the notch about 2.54 mm deep on a 12.70 mm bar.
Notch angle and radius: 45 ± 1° included angle and 0.25 ± 0.05 mm (0.010 ± 0.002 in) root radius, with the notch plane perpendicular to the specimen face.
Notch position: at mid-length, so that with one end in the vise the notch sits exactly at the top edge of the clamp.
Molded, machined or die-cut?
Izod bars are molded to size or machined from sheet, plate or blocks. They are not die-cut: a 12.7 mm deep rigid bar cannot be cut cleanly with a die, and the ligament must be held to ± 0.05 mm by a milled notch. Record molding conditions, because flow orientation and skin change the result, and test anisotropic sheet in each principal direction.
Die-cutting does belong in the same lab, for the companion tests. Universal Grip makes hardened steel and ribbon steel cutting dies for ASTM D638 Type I to V tensile bars and for ASTM D1822 Type L tensile-impact specimens cut from sheet, and the 26 kN arbor press is sized for D638 and D790 specimens from plastic sheet. We make free test cuts on customer material.
Measuring the specimen
Measure the width of every specimen at the notch with a micrometer reading to 0.025 mm (0.001 in) or better. A 0.1 mm error on a 3.2 mm bar is a 3 % error in the result.
Measure the depth of material remaining under the notch. A ligament outside 10.16 ± 0.05 mm means the notch is too deep or too shallow, and the specimen should be discarded.
Check that the bar is straight and free of sink marks, voids, flash and weld lines near the notch.
Notching: the step that decides the result
Most between-lab disagreements in Izod data trace back to the notch. The notch is machined after molding, not molded in, using a single-tooth milling cutter with zero rake, or a dedicated notching machine, run at a speed and feed that leave a smooth root without heating the plastic. The standard gives example cutter speeds and feeds and allows multi-tooth cutters only if they produce the same profile.
Verify the profile on an optical comparator or with a notch gauge; D256 calls for a check at least every 500 notches, and glass-filled compounds wear cutters faster.
A dull cutter raises the result. As it wears, the root radius grows and the specimen looks tougher than it is; a chipped cutter can do the opposite.
Notch a comparison set together on the same cutter in the same session, so any wear affects all specimens equally.
Keep the notch square across the width, and avoid cutter speeds and feeds that heat the root.
Start the conditioning clock after notching, not after molding.
Universal Grip's listed manual broaching machine cuts V-notches in 10 × 10 × 55 mm metal Charpy bars. It is not a plastics Izod notcher, which has to hold a 0.25 mm root radius on a 3.2 mm wide, 12.7 mm deep bar. If you need a plastics notcher, contact us and we will help you specify one.
Conditioning and the test atmosphere
Standard conditioning: at least 40 hours at 23 ± 2 °C and 50 % relative humidity after notching, per ASTM D618 Procedure A (use the humidity tolerance in the current D618 edition), unless the material specification gives other conditions.
Test atmosphere: run the test in the same laboratory atmosphere, or within a few minutes of removing specimens from it.
Hygroscopic materials: nylons and other moisture-sensitive polymers can change notched Izod by a large factor between dry-as-molded and 50 % RH equilibrium. Many specifications, such as ASTM D4066 for nylons, call for testing dry as molded; whatever you do, report the moisture state.
Material specifications take precedence. D256 says that conditioning, preparation and test parameters in a material specification override the defaults in the test method.
Which machine runs ASTM D256?
This is the most important equipment question, and the answer is not a tensile tester or a metals Charpy machine. D256 is run on a small pendulum impact tester for plastics with an Izod vise and a set of light hammers. The standard fixes the geometry and the energy range:
Drop height and velocity: the striker falls 610 ± 2 mm, giving about 3.5 m/s (11.4 ft/s) at impact.
Hammer energies: a basic hammer of 2.7 J (2.00 ft·lbf), with heavier hammers that double in steps (about 5.4, 10.8 and 21.7 J) at the same drop height. ISO-series machines carry 2.75, 5.5, 11 and 22 J hammers.
Striker: a hardened edge of 0.80 ± 0.20 mm radius that strikes 22.00 ± 0.05 mm above the top of the vise.
Vise: a clamp that holds the bar vertically with the notch exactly at the top edge of the fixed jaw, usually with a positioning gauge.
Losses: free-swing friction and windage must be small and measured, so the reading can be corrected.
Why a metals Charpy machine cannot run D256
A notched polypropylene or ABS bar absorbs somewhere between about 0.1 J and 3 J. On a 150 J pendulum that is less than 2 % of the scale, far below the range where the reading has useful resolution, and the machine has the wrong striker, the wrong strike height and no Izod vise for a 12.7 mm plastics bar. That is why D256 sets the 85 % rule and why plastics labs use the lightest hammer that will break the specimen.
What Universal Grip supplies, and what it does not
Our listed impact testers are Galdabini STRIKE pendulums of 150 J, 300 J, 450 J, 600 J and 750 J, built for Charpy testing of metals to ASTM E23 and ISO 148: the 150 J, 300 J, 450 J and 600 J machines. The 150 J machine can be fitted with an Izod hammer, but a 150 J Izod configuration is a metals impact setup, not a plastics machine that meets the D256 hammer series. We do not list a plastics Izod pendulum in the few-joule range as a standard product. If you need one, contact us with your materials, expected energy range and specimen widths, and we will quote a suitable plastics pendulum configuration on request rather than sell you a machine that is the wrong size.
Where Universal Grip does fit a D256 lab is everything around the pendulum: specimen cutting dies and presses for the companion tests, tensile grips, bend fixtures and compression platens, environmental chambers, and the Mark-10 and Galdabini machines that run D638, D790 and the other plastics methods on the same molded bars. If your lab also tests metals, our impact tester range and the ASTM E23 page cover Charpy testing of steel.
Choosing the hammer: the 85 % rule in practice
Use the lightest hammer that breaks the specimen with no more than 85 % of its capacity absorbed. To pick one before testing, multiply the expected impact resistance in J/m by the width in metres, divide by 0.85, and choose the smallest hammer at or above the answer.
Polycarbonate, 3.2 mm bar, about 800 J/m expected: 800 × 0.0032 = 2.56 J; 2.56 ÷ 0.85 = 3.0 J, so the 5.4 J hammer.
Impact-modified polypropylene, 3.2 mm bar, about 600 J/m expected: 600 × 0.0032 = 1.92 J; 1.92 ÷ 0.85 = 2.26 J, so the 2.7 J hammer.
General-purpose polystyrene, 3.2 mm bar, about 25 J/m expected: 0.08 J, so the 2.7 J hammer, and because the value is below 27 J/m, Method C.
If the specimen does not break, move up to the next hammer. If it still does not break with the largest hammer, it is a non-break (NB), not a value.
Testing machines for the companion tests: Mark-10 or Galdabini
A plastics lab rarely runs Izod alone. The same molded bars and plaques feed tensile (ASTM D638, ISO 527-2), flexural (ASTM D790, ISO 178) and compression tests, and those run on a tensile tester or universal testing machine. That is where the Mark-10 vs. Galdabini choice applies.
Mark-10 tensile testers: basic and QC plastics testing
A Mark-10 motorized test frame suits incoming inspection and production QC where the forces stay within about 6.7 kN:
100 lb (F105), 300 lb (F305) and 500 lb (F505): 18 in (457 mm) of travel, speeds to 330 mm/min (1,100 mm/min with the extended option). Good for D790 flexure, film and low-force tensile.
750 lb (F755) and 1,500 lb (F1505): 32 in (813 mm) of travel and 10 to 600 mm/min as standard; D638's 5 mm/min for rigid bars is below that range, so check the speed option.
All F-series frames hold speed to ±0.2 % and sample load at 5,000 Hz.
Check the force: a 3.2 mm D638 Type I bar has about 41.6 mm² of cross-section (13 mm × 3.2 mm). At 40 MPa (polypropylene) that is about 1.7 kN; at 180 MPa (30 % glass-filled nylon) about 7.5 kN, beyond a 1,500 lbF Mark-10. See Mark-10 test frame vs. universal testing machine.
Galdabini universal testing machines: full plastics labs
For resin and compound development, certification, glass-filled grades, modulus measurement and testing at temperature, a Galdabini benchtop universal testing machine is the better fit:
2.5 kN Quasar: single column, up to 1,000 mm/min with 1 m of stroke, for film, thin sheet, flexure and high-elongation polyolefins.
5 kN Quasar: dual column with 175 mm of column clearance, room for an environmental chamber.
10 kN and 25 kN: for unfilled and glass-filled D638 bars, D790, D695 compression and ISO 527 on one frame.
UTM software stores a method per standard and calculates D638 modulus and D790 flexural properties. See the universal testing machine buyer's guide.
Grips and fixtures: where they apply in a D256 lab
The Izod vise is part of the pendulum machine, so D256 needs no tensile grips or test fixtures of its own. The grips and fixtures below are for the companion tests on the same materials.
Wedge grips: the standard choice for rigid and glass-filled D638 bars; clamping force rises with load.
Pneumatic grips: constant clamping for flexible plastics and film, with a foot pedal.
Vise grips: for occasional testing of rigid bars at moderate force.
Bend testing fixtures and the small bend fixture: 3-point fixtures for ASTM D790 and 4-point fixtures for ASTM D6272. The small bend fixture comes in aluminum (2.5 kN) or steel (10 kN), with spans of 4 to 150 mm and interchangeable rollers.
Compression platens: for ASTM D695 compression of molded blocks and prisms.
Adapters: to fit our grips and fixtures to Instron, MTS, ZwickRoell, Shimadzu, Tinius Olsen, Galdabini and Mark-10 frames.
Do you need an extensometer for ASTM D256?
No. Izod impact resistance is an energy divided by a width; there is no strain measurement, and no extensometer can follow a fracture that lasts milliseconds. (Instrumented pendulums record force through a gauged striker, which is part of the machine.)
Extensometers matter elsewhere in the lab: tensile modulus to ASTM D638 and ISO 527 must be measured on the specimen, not from crosshead travel. The usual options:
Clip-on extensometers for modulus on rigid bars, removed before break.
Video extensometers for modulus and elongation to break without contact, including through an environmental chamber window. The UGC-VE-2109 gives a 220 mm field of view at ISO 9513 Class 0.5 and 440 mm at Class 1.
XL long-travel extensometers for high-elongation polyolefins and TPEs, including the Plast long-stroke model up to 700 mm.
ASTM D256 test procedure, step by step (Method A)
Step 1, verify the machine: check that the pendulum hangs at zero, swings freely with low friction, and that the windage and friction correction is known for the hammer you will use.
Step 2, measure each specimen: width at the notch to 0.025 mm and depth under the notch; record both and discard specimens outside tolerance.
Step 3, choose the hammer: estimate the energy to break and select the lightest hammer that keeps the reading at or below 85 % of capacity.
Step 4, clamp the specimen: vertically in the vise, notch facing the striker, notch level with the top of the fixed jaw (use the positioning gauge), clamped firmly and the same way every time.
Step 5, release the pendulum: let it strike, then record the indicated energy and the break type.
Step 6, correct the reading: subtract windage and friction; for Method C also subtract the toss energy measured by re-striking the broken free end placed back on the vise.
Step 7, repeat: at least five specimens, preferably ten or more, and for anisotropic sheet a set in each principal direction.
Step 8, classify and calculate: compute impact resistance for each specimen, group by break type and width, and calculate the mean and standard deviation of each group.
Break types: C, H, P and NB
Every specimen is classified after the test, and the classification decides what can be averaged.
C, complete break: the specimen separates into two or more pieces. The normal result for brittle and semi-ductile materials.
H, hinge break: an incomplete break in which one part cannot support itself above the horizontal when the other part is held vertically (less than a 90° included angle).
P, partial break: an incomplete break that does not meet the hinge definition but has fractured at least 90 % of the distance between the notch vertex and the opposite side.
NB, non-break: the fracture extends less than 90 % of that distance. An NB is not an impact resistance value; report only the number and percentage of non-breaks.
Average only specimens with the same break type, and report the number of each type. A set of ten bars with seven complete breaks at 620 J/m and three hinge breaks at 780 J/m is reported as two groups, not as one mean of 668 J/m. If a material will not break at room temperature (many polycarbonates, TPUs and impact-modified nylons), test at a lower temperature if the specification allows, use a sharper or different method, or report it as NB.
ASTM D256 calculation and worked examples
Izod impact resistance (J/m) = corrected energy to break (J) ÷ specimen width (m).
Inch-pound units: ft·lbf ÷ width in inches. 1 ft·lbf/in = 53.4 J/m.
Optional area basis: D256 also allows energy ÷ (width × depth under the notch), in kJ/m². This is not an ISO 180 value and cannot be converted to J/m by a fixed factor between materials.
Width is the specimen width at the notch (the molded thickness), not the 12.7 mm depth.
Worked example (Method A, impact-modified polypropylene): a 3.20 mm wide bar is expected to give about 600 J/m, so the 2.7 J hammer is chosen (600 × 0.0032 ÷ 0.85 = 2.26 J). The machine indicates 1.88 J and the windage and friction correction for that hammer is 0.03 J, so the energy to break is 1.85 J. Capacity check: 1.85 ÷ 2.7 = 69 %, within the 85 % limit. Impact resistance = 1.85 ÷ 0.00320 = 578 J/m, or 578 ÷ 53.4 = 10.8 ft·lbf/in. On the area basis, the ligament is 3.20 × 10.16 = 32.5 mm², so 1.85 J ÷ 32.5 mm² = 0.0569 J/mm² = 56.9 kJ/m².
Reporting the set: five complete breaks give 578, 561, 590, 552 and 584 J/m. The mean is 2,865 ÷ 5 = 573 J/m and the sample standard deviation is 16 J/m. Report: 573 J/m (SD 16), 5 C, 0 H, 0 P, 0 NB, Method A, 2.7 J hammer, 3.20 mm width, 40 h at 23 °C / 50 % RH.
Worked example (Method C, general-purpose polystyrene): a 3.20 mm bar broken with the 2.7 J hammer gives 0.090 J after the windage and friction correction. The broken free end is placed back on the vise and struck again, reading 0.012 J of toss energy. Net energy = 0.090 − 0.012 = 0.078 J, so impact resistance = 0.078 ÷ 0.00320 = 24.4 J/m. Without the toss correction the result would be 0.090 ÷ 0.00320 = 28.1 J/m, about 15 % high and on the wrong side of the 27 J/m threshold, which is exactly why Method C is mandatory for brittle materials.
Typical ASTM D256 notched Izod values
Values depend heavily on grade, molding and moisture, so use these ranges only for orientation, and use the hammer-selection step with your own expected value:
Brittle amorphous polymers (general-purpose polystyrene, acrylic, many phenolics and SAN): roughly 15 to 30 J/m; Method C territory.
Acetal, unfilled polyamides dry as molded, PBT, homopolymer polypropylene: roughly 30 to 100 J/m.
Glass-filled engineering compounds: often 50 to 150 J/m; the fibres raise strength more than notched toughness.
Impact copolymer polypropylene and ABS: roughly 100 to 600 J/m depending on rubber content.
Polycarbonate and impact-modified nylons: commonly 600 to 900 J/m at room temperature with hinge breaks or non-breaks, falling sharply at low temperature.
Precision: how much scatter to expect
D256's interlaboratory study shows large between-lab differences even when every lab follows the method. Reproducibility limits in the study ranged from about 11 J/m on a 30 J/m phenolic to about 159 J/m on an 876 J/m polycarbonate, and on an acetal of about 77 J/m the reproducibility limit was larger than the mean itself. In practice, two competent labs testing the same lot can differ by 10 to 40 % on most materials, while repeatability within one lab is much tighter.
So a specification minimum needs a margin that reflects the method, a 15 % supplier-customer disagreement is often not a material difference, and the place to reduce scatter is the notch, hammer selection and conditioning.
Testing Izod impact at low and high temperature
Why it matters: most thermoplastics have a ductile-to-brittle transition. A polycarbonate that hinges at 23 °C can break completely at −30 °C, and many specifications set a minimum at −30 or −40 °C.
Conditioning and transfer: hold specimens at temperature in a freezer, cabinet or bath until they reach equilibrium, then move each bar to the room-temperature pendulum and break it within a few seconds; 5 seconds is the limit commonly used.
Companion tests at temperature: for D638 and D790 at −80 to +280 °C, our environmental chambers mount on Galdabini frames, with a video extensometer viewing through the window. See the temperature chamber buyer's guide.
What to report
The standard, edition and test method (A or C).
Material, specimen source (molded or machined), molding conditions and orientation.
Specimen width and depth under the notch.
Conditioning, moisture state and test temperature.
Hammer capacity.
Mean and standard deviation for each break-type group in J/m (or ft·lbf/in), with the number and percentage of C, H, P and NB.
Any deviations from the standard or material specification.
Common ASTM D256 problems and how to fix them
Results drift upward over weeks: the notch cutter is wearing and the root radius is growing. Verify the profile and resharpen or replace the cutter.
High scatter within a set: inconsistent notch depth, tilted notches, voids or weld lines at the notch, or inconsistent clamping.
Reading at the bottom of the scale or above 85 %: wrong hammer. Use the lightest hammer that breaks the bar within 85 %.
Low-value materials reading high: Method A used below 27 J/m. Apply Method C.
Nylon results that disagree with the data sheet: different moisture state. Test as the specification requires and report it.
Specimens tested too soon: conditioning counted from molding instead of notching, or shorter than 40 hours.
One mean for mixed breaks: C, H and P results averaged together, or NB values included.
Width confusion: dividing by the 12.7 mm depth, or mixing 3.2 mm and 6.35 mm bars in one set.
Wrong machine or wrong comparison: a metals pendulum used for plastics, or D256 J/m compared with ISO 180 kJ/m² or Charpy values.
ASTM D256 vs. ISO 180, ASTM D6110, D4812 and D1822
ISO 180 (Izod for plastics): different specimen (80 × 10 × 4 mm, cut from a multipurpose bar), a 2 mm notch leaving 8.0 mm, notch types A (0.25 mm), B (1.0 mm) and C (0.10 mm), and a result in kJ/m² (energy ÷ width × remaining depth), typically on ten specimens. There is no conversion factor between ISO 180 and D256 results.
ASTM D6110 (Charpy for plastics): the same notched bar supported horizontally on two anvils and struck behind the notch. Formerly D256 Method B. Izod and Charpy values for the same material differ.
ISO 179 (Charpy for plastics): the ISO counterpart of D6110 and the impact method most European data sheets quote.
ASTM D4812 (unnotched Izod): the cantilever test without a notch, for materials where the notched value is too low or too sensitive to be useful.
ASTM D1822 (tensile impact): for films, thin sheet and materials too flexible or too thin for a 12.7 mm bar. We make cutting dies for D1822 Type L specimens.
ASTM E23 and ISO 148 (metals): impact testing of metals on 150 J and larger machines, reported in joules. See our ASTM E23 page and Charpy testing guide.
Related tests on the same materials: ASTM D638 tensile, ASTM D790 and ASTM D6272 flexure (see 3-point vs. 4-point bend testing), ASTM D882 film tensile (see the plastic film testing guide), and our plastics testing page for every fixture and die.
Equipment packages for a plastics lab that runs ASTM D256
These packages cover what Universal Grip supplies. The Izod pendulum itself is quoted separately on request.
Basic QC station
A plastics Izod pendulum with 2.7 and 5.4 J hammers, a notching cutter and a notch gauge (ask us for an Izod configuration)
Mark-10 F305 or F505 for D790 flexure and low-force tensile checks
Small bend fixture for D790 and vise grips or wedge grips for D638 bars
ASTM D638 cutting dies and a 26 kN arbor press for sheet specimens
A micrometer reading to 0.01 mm and a 23 °C / 50 % RH conditioning area
Advanced development and certification lab
A plastics pendulum with the full 2.7 to 21.7 J hammer series, a motorized notcher and low-temperature conditioning (quoted on request)
Galdabini 10 kN or 5 kN universal testing machine with plastics test software
Wedge grips and pneumatic grips, a bend testing fixture for D790 and D6272, and compression platens for D695
UGC-VE-2109 video extensometer or a clip-on extensometer for D638 modulus
Environmental chamber for tensile and flexural testing from −80 to +280 °C
ASTM D256 FAQ
What is ASTM D256?
The ASTM standard test methods for determining the Izod pendulum impact resistance of plastics. A notched bar is clamped vertically as a cantilever and broken by one swing of a pendulum; the corrected 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), with a 45° notch of 0.25 mm root radius that leaves 10.16 ± 0.05 mm of material under the notch.
What test methods are in the current ASTM D256?
The current edition lists Test Method A (notched cantilever beam) and Test Method C (Method A with a toss-energy correction, required below 27 J/m). Methods D and E appeared in earlier editions; the Charpy test is now ASTM D6110.
What pendulum energy does ASTM D256 use?
A basic 2.7 J (2.00 ft·lbf) hammer, with heavier hammers that double in steps up to 21.7 J (16 ft·lbf), all falling 610 mm to strike 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 metals Charpy impact tester run ASTM D256?
No. A 150 J or larger metals pendulum cannot resolve the 0.1 to 3 J a plastics bar absorbs, and it lacks the Izod vise, striker and strike height that D256 specifies. D256 needs a small plastics pendulum.
Does Universal Grip sell an ASTM D256 Izod tester?
Our listed impact testers are 150 to 750 J Galdabini pendulums for Charpy testing of metals, which are the wrong size for D256. We do not list a plastics Izod pendulum as a standard product, but we can quote a suitable plastics configuration on request, and we supply the dies, presses, grips, fixtures and testing machines for the rest of a plastics lab.
How many specimens does ASTM D256 require?
At least five, preferably ten or more, all of the same width. Anisotropic sheet is tested in each principal direction and reported separately.
How are ASTM D256 specimens conditioned?
For at least 40 hours at 23 ± 2 °C and 50 % relative humidity after notching, per ASTM D618 Procedure A, and tested in the same atmosphere, unless the material specification sets other conditions. Hygroscopic materials such as nylon are often tested dry as molded; report the moisture state.
How is Izod impact resistance calculated?
Divide the corrected energy to break in joules by the specimen width in metres. For example, 1.85 J on a 3.20 mm wide bar is 578 J/m, or 10.8 ft·lbf/in (1 ft·lbf/in = 53.4 J/m).
When is Method C required?
For materials with an Izod impact resistance below 27 J/m (0.5 ft·lbf/in). Method C subtracts the energy used to toss the broken half, measured by re-striking it on the vise; without it, brittle materials read high.
What do break types C, H, P and NB mean?
C is a complete break, H a hinge break, 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 results are reported only as a count and percentage.
Is ASTM D256 the same as ISO 180?
No. ISO 180 uses an 80 × 10 × 4 mm bar with 8 mm under the notch and reports kJ/m² of ligament area; D256 uses a 63.5 × 12.7 mm bar with 10.16 mm under the notch and reports J/m of width. There is no conversion factor between them.
Related reading
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