The short answer
Izod and Charpy both swing a pendulum into a notched plastic bar and report the energy it took to break it. Izod clamps the bar upright like a cantilever, with the notch facing the striker. Charpy supports the bar at both ends and strikes the side opposite the notch. ASTM D256 Izod reports J/m or ft-lb/in (1 ft-lb/in = 53.38 J/m); ISO 180 Izod and ISO 179 Charpy report kJ/m². J/m and kJ/m² cannot be converted into each other, so compare only values from the same test.
Impact strength is one of the most quoted numbers on a resin data sheet and one of the easiest to misread. This page covers what each test does, what the units mean, and what the numbers can and cannot tell you about a real part. For the rest of the data sheet, see how to read a resin data sheet.
The pendulum tests
ASTM D256 (notched Izod) is the standard U.S. test. A bar 63.5 × 12.7 mm, usually 3.2 mm (1/8 in) thick, has a 45° V-notch 2.54 mm deep with a 0.25 mm tip radius. It is clamped upright with the notch at the top of the vise, facing the striker. The pendulum breaks it and the energy is divided by the bar's thickness, giving J/m or ft-lb/in.
ISO 180 (notched Izod) uses the same cantilever setup on the 80 × 10 × 4 mm bar cut from the ISO multipurpose specimen, with a type A notch (0.25 mm radius) leaving 8 mm of material. The energy is divided by the area under the notch, giving kJ/m².
ISO 179-1 (Charpy) lays the same 80 × 10 × 4 mm bar across two supports 62 mm apart and strikes it in the middle, on the face opposite the notch. The code tells you the setup: 1eA means specimen type 1, struck edgewise (on the narrow side), notch type A. Notch type A (0.25 mm radius) is the usual choice; types B and C are blunter and sharper. A "U" in place of the notch letter means unnotched.
Impact tests at a glance
| Test | Standard | Specimen and setup | Units | Notes |
|---|---|---|---|---|
| Notched Izod (ASTM) | ASTM D256 | 63.5 × 12.7 mm bar, usually 3.2 mm thick; 2.54 mm V-notch, 0.25 mm tip radius; clamped upright, notch facing the striker | J/m or ft-lb/in | Energy divided by bar thickness. 1 ft-lb/in = 53.38 J/m |
| Notched Izod (ISO) | ISO 180/1A | 80 × 10 × 4 mm bar; type A notch, 0.25 mm radius, 8 mm left under the notch; clamped upright | kJ/m² | Energy divided by area under the notch |
| Notched Charpy (ISO) | ISO 179-1/1eA | 80 × 10 × 4 mm bar on supports 62 mm apart; struck edgewise on the face opposite the notch; type A notch | kJ/m² | Common on ISO data sheets; ISO 10350 calls for 23°C / 73°F and −30°C / −22°F |
| Unnotched Izod or Charpy | ISO 180/1U, ISO 179-1/1eU, ASTM D4812 | Same bars with no notch | kJ/m² (ISO), J/m (ASTM) | Tough resins often show no break |
| Charpy (ASTM) | ASTM D6110 | Notched bar, supported at both ends | J/m or ft-lb/in | Less common on resin data sheets |
| Instrumented puncture | ASTM D3763, ISO 6603-2 | Flat plaque or disk, clamped, hit by a round-nosed striker | J (energy), N (peak force) | Closest to how real parts fail; shows ductile or brittle failure |
| Falling weight (Gardner) | ASTM D5420 | Flat plaque struck by a weight dropped from varying heights | J or in-lb | Relative ranking by mean failure energy |
A plain summary, not the full standards. Results depend on specimen preparation, conditioning and test temperature; values on data sheets are typical, and grades vary.
Why J/m and kJ/m² cannot be converted
ASTM D256 divides energy by the bar's thickness (a length), so the result is J/m. ISO 180 and ISO 179 divide by the cross-section left under the notch (an area), so the result is kJ/m². The bars also differ in thickness (3.2 mm vs. 4 mm) and setup, and toughness does not scale in a straight line with thickness. There is no factor that turns one into the other.
The ft-lb/in to J/m conversion, by contrast, is a fixed unit conversion (multiply by 53.38) because it is the same test in different units. Our unit converter handles it.
Break types and "no break"
Each specimen is reported by how it broke: complete break (C), hinge break (H, a thin skin holds the halves together), partial break (P) or non-break (NB). A value labeled NB is not an energy figure, and it cannot be averaged with values that did break. Tough resins such as many polycarbonates can show partial or hinge breaks on 3.2 mm notched bars, and many ductile resins do not break at all unnotched.
Notch sensitivity and thickness
The notch is there to concentrate stress, like a sharp inside corner in a part. Some resins are notch-sensitive: tough without a notch, much weaker with one. Polycarbonate is the textbook case. It reads about 600–900 J/m (11–17 ft-lb/in) on a 3.2 mm notched bar, but thicker bars or sharper notches can switch it to a brittle break at a fraction of that energy. Generous radii at inside corners help any resin, and they matter most for notch-sensitive ones.
Temperature
Plastics generally get less tough as they get colder, and some have a fairly sharp ductile-to-brittle transition. A room-temperature value can hide that. ISO data sheets commonly add a value at −30°C (−22°F), and many ASTM sheets add −40°C (−40°F). If your part works outdoors or in a freezer, compare grades at the cold temperature. See the best plastic for cold temperatures.
Dry vs. conditioned nylon
Nylon absorbs moisture, which makes it tougher and less stiff. ISO data sheets typically give two columns: dry (as molded) and conditioned to equilibrium at 23°C (73°F) and 50% relative humidity. ASTM sheets may say "DAM" (dry as molded) and "50% RH". The conditioned impact value is usually noticeably higher. Make sure you compare dry with dry. See nylon and moisture.
Real parts: multiaxial and falling-weight tests
A notched bar is struck in one direction at one point. A dropped housing or a stone hitting a bumper loads the part in many directions at once. Instrumented puncture tests (ASTM D3763 and ISO 6603-2) drive a round-nosed striker through a clamped plaque and record force against displacement, showing total energy and whether the failure was ductile or brittle. The Gardner falling-weight test (ASTM D5420) ranks plaques by the energy that cracks half of them. For a final answer, nothing beats dropping or striking real molded parts at the coldest service temperature.
How to compare grades on impact
- Match the test: ASTM D256 with ASTM D256, ISO 179-1/1eA with ISO 179-1/1eA.
- Match the conditions: temperature, and dry or conditioned for nylon.
- Check the break type, and treat NB as "very tough", not as a number.
- Check cold values if the part works below room temperature.
- Test parts when impact is the main failure risk. Our resin comparison tool and the best plastic for impact guide help narrow the field first.
What to send us for a quote
- The impact value and test method your print calls for, and the temperature
- The grade you run now, or the part and resin family
- Quantity and ship-to city
We aim to send your quote within one to three business days.
Common questions
Can I convert notched Izod J/m to kJ/m²?
No. ASTM D256 divides the energy by the bar's thickness and reports J/m; ISO 180 and ISO 179 divide by the area under the notch on a different bar and report kJ/m². Compare grades only with values from the same test.
How do I convert ft-lb/in to J/m?
Multiply by 53.38. For example, 12 ft-lb/in is about 641 J/m. To go the other way, divide J/m by 53.38.
Is Izod or Charpy better?
Neither is better; they are different setups. U.S. data sheets mostly report notched Izod (ASTM D256), while many ISO data sheets report notched Charpy (ISO 179-1/1eA). Use whichever your specification calls for and compare like with like.
What does NB mean on an impact test?
NB means non-break: the specimen did not break, or the crack ran less than 90% of the way through, so there is no meaningful energy value. It is common in unnotched tests and for very tough resins such as some polycarbonates and impact-modified grades.
Why is my nylon's impact strength different dry and conditioned?
Moisture absorbed from the air plasticizes nylon, making it tougher and less stiff. ISO data sheets list dry and conditioned values; ASTM sheets may list dry-as-molded and 50% relative humidity values. Check which one you are comparing.
Does a high notched Izod mean my part won't break?
Not necessarily. Notched Izod ranks materials on a small standard bar. Real parts fail at their own corners, thicknesses, temperatures and speeds, so drop or puncture tests on parts, or instrumented multiaxial impact tests (ASTM D3763, ISO 6603-2), are better predictors.
Why do data sheets list impact at −30°C (−22°F) or −40°C (−40°F)?
Many plastics turn brittle as they get colder, and room-temperature values hide that. ISO data sheets commonly add −30°C (−22°F); many ASTM sheets add −40°C (−40°F).
Tell us the impact you need
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
- Ship-to city and state or province