Impact Test - Charpy & Izod Pendulum Energy

An impact test is a pendulum or drop-tower experiment that measures how much energy a notched material specimen absorbs before it fractures under a single rapid strike. The most common variants are the Charpy test (specimen supported at both ends) and the Izod test (specimen held as a cantilever), both described in ASTM D256, ASTM D6110, and ASTM E23. This calculator turns the swing angles into a joule figure you can record alongside the specimen's break type.

• • Comparing material toughness: Run the same swing geometry on a known specimen and an unknown one, then compare the absorbed joule figure to rank toughness quickly.
• • Verifying a Charpy or Izod reading: Type the swing angles, arm length, and striker mass from your lab notebook into the form to confirm the joule figure reported by the pendulum readout.
• • Translating swing heights into impact velocity: Use the impact velocity output to feed downstream simulations (drop tests, crash models) without re-doing the geometry math by hand.

The math behind a pendulum impact test mirrors a playground swing: the striker trades height for speed on the way down, then trades speed back into height on the way up. The drop in height from before to after the strike is the specimen's absorbed energy, with a small fixed loss (bearing friction, frame vibration, windage) subtracted first per the ASTM D256 calibration run.

If you would rather see the energy from mass and impact velocity directly (without the swing geometry), our impact energy calculator skips the angles and just takes E = 1/2 m v^2 and is the closest peer in this category.

The calculator converts swing angles into heights above the bottom of the swing, subtracts a calibrated energy-loss term, and reports absorbed energy in joules. It also uses V = sqrt(2 g h) to give you the speed the striker had at the moment of contact.

• angle of fall (beta): Pendulum starting angle from vertical before release, entered in degrees. Typical setups use 60 deg to 160 deg.
• angle of rise (alpha): Upswing angle after the specimen has absorbed energy. Zero means the specimen absorbed all the initial potential energy.
• pendulum length (S) and striker mass (m): S is the distance from the pivot to the striker centre in metres. m is the striker mass in kilograms (ASTM D256 pendulums are typically 0.45 to 4 kg).
• energy loss (E_l) and gravity (g): E_l is the calibrated correction for bearing friction, windage, frame vibration, and striker rubbing, obtained by running the pendulum with no specimen. g defaults to the NIST standard-gravity value 9.80665 m/s^2 and can be swapped to the Moon or Mars preset for comparison.

According to ASTM D256, the energy absorbed by a notched Izod specimen equals the difference between the pendulum's initial and final gravitational potential energy, minus a calibrated energy-loss term that covers bearing friction, windage, frame vibration, and striker rubbing.

According to NIST Special Publication 811, the standard acceleration due to gravity adopted for measurement work is 9.80665 m/s^2, and 1 foot-pound equals exactly 1.3558179483314004 joules.

When you already know the striker mass and velocity and want the same E = 1/2 m v^2 expression without the swing geometry, our kinetic energy calculator handles that case in the math-conversion category.

Four small ideas explain every result the calculator shows.

ASTM D256 also lists four break outcomes (complete, hinge, partial, non-break) that lab writeups pair with the joule figure.

If you want to check the swing timing of the same arm before running the test, our pendulum period calculator applies T = 2 pi sqrt(L / g) and is a useful sanity check on the arm length you entered.

Five short steps turn your pendulum readings into a joule figure you can put in a report.

1. 1 Pick the swing angles: Type the angle of fall and angle of rise in degrees. Most readouts report angles to one decimal place.
2. 2 Enter the pendulum geometry: Add the distance from the pivot to the striker centre in metres and the striker mass in kilograms. ASTM D256 pendulums usually print both on the instrument plate.
3. 3 Add the calibrated energy loss: Run the pendulum with no specimen once and record the difference between the two swing heights as E_l. Leave it at 0 J if your instrument does not require a correction.
4. 4 Choose the gravity preset: Keep Earth at 9.80665 m/s^2 for a normal lab. Switch to Moon or Mars only for a different gravitational environment comparison.
5. 5 Read the absorbed energy: Use the absorbed joule figure as the headline result, quote the initial and final heights in your notes, and pull the impact velocity if you are feeding a downstream model.

If your Charpy readout shows beta = 120 deg, alpha = 60 deg, S = 0.8 m, m = 4 kg, E_l = 0.5 J, and g = 9.80665 m/s^2, the calculator returns 30.8813 J (22.7763 ft-lbf), an initial drop height of 1.2 m, a post-impact rise of 0.4 m, and an impact velocity of 4.8514 m/s. That is enough information to update the lab notebook and to feed the same striker velocity into a finite-element model.

If the impact you are modelling is closer to a fast projectile than a pendulum strike, our bullet energy calculator applies the same E = 1/2 m v^2 to a bullet mass and velocity.

A purpose-built calculator removes unit-conversion and sign errors that creep in when the swing math is done by hand.

• • Handles both Charpy and Izod setups: The same form covers both ASTM D256 (Izod) and ASTM D6110 (Charpy) geometries, so you do not need two different spreadsheets.
• • Reports joules and foot-pounds at once: Absorbed energy is shown in both metric and imperial units using the NIST 1.3558179483314004 J/ft-lbf conversion.
• • Surfaces supporting geometry: Initial drop height, post-impact rise, and impact velocity are shown next to the absorbed energy, which makes the calculator useful for downstream models.
• • Flags bad pendulum readings: A negative absorbed-energy value means the post-impact rise is higher than the starting position, which is the easiest way to spot a misread angle or a calibration drift.

For a non-pendulum impact where the striker drops straight down from a known height, our free fall time calculator covers the same m * g * h starting point and is usually a faster fit.

Three variables drive the absorbed-energy number, and two limitations tell you when to double-check the answer.

• • The formula assumes a rigid pendulum arm, no air drag, and a single point-mass striker, so the result is the energy the specimen would absorb in an idealised test rather than a measurement of every dissipation channel.
• • A negative result (post-impact rise higher than the starting height) means the data are inconsistent. Recheck the angles, the calibrated E_l, and whether the right specimen was loaded.

According to Omni Calculator Impact Test page, the worked example with beta = 60 deg, alpha = 30 deg, S = 1 m, m = 0.44 kg, and E_l = 0 J gives 1.5794 J of absorbed energy and an impact velocity of 3.1316 m/s.

For a vehicle-scale impact where the velocity already comes from a different sensor, our car crash force calculator skips the swing geometry and goes straight to mass and velocity.