Impact Test Calculator (Charpy / Izod pendulum absorbed energy)

Understanding the impact test calculator — concept and principle

This calculator works out the energy a specimen absorbs in a Charpy or Izod pendulum impact test, from the hammer mass, arm length and the release and rise angles. It also gives the impact strength per unit area under the notch. Materials students and test engineers use it to check toughness results and understand how the machine works.

The specimen absorbs the energy the hammer fails to carry up the far side: compare release and rise heights.

How it is calculated

Absorbed energy E = W × R × (cos β − cos α)

Potential energy at release = W × R × (1 − cos α)

W = m × g, with g = 9.80665 m/s²

Impact strength = E ÷ (width × ligament depth), in J/cm² or kJ/m²

α is the release angle and β the rise angle after fracture, both measured from the hanging (lowest) position. R is the distance from the pivot to the hammer's centre of mass.

The principle

The pendulum starts at height R(1 − cos α) and would swing up to the same height on the other side if nothing stopped it. Breaking the specimen takes energy, so the hammer rises only to R(1 − cos β). The difference in potential energy between release and final height is the energy the specimen absorbed. A tough material absorbs a lot and lets the hammer rise only a little; a brittle one barely slows it.

Worked example

Default inputs: hammer 20 kg, arm 0.75 m, α = 140°, β = 70°, specimen width 10 mm, ligament 8 mm.

W = 20 × 9.80665 = 196.1 N.
Energy at release = 196.1 × 0.75 × (1 − cos 140°) = 259.8 J.
Absorbed = 196.1 × 0.75 × (cos 70° − cos 140°) = 147.1 × (0.3420 + 0.7660) = 163 J.
Left after fracture = 96.8 J; share absorbed = 62.7%.
Area = 10 × 8 = 80 mm²; impact strength = 163 ÷ 0.8 cm² = 203.7 J/cm² = 2,037 kJ/m².

Each point is the calculator re-run with only “Pendulum (hammer) mass (kg)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Pendulum (hammer) mass (kg)	Absorbed energy (J)
5	40.7
10	81.5
15	122.2
20	163
25	203.7
30	244.5
40	326

How to read the result

Compare the absorbed energy with the specification at the same temperature and specimen type. Many steels show a ductile-to-brittle transition: the same steel can absorb a lot at room temperature and very little when cold. Results from different specimen sizes or notch types are not directly comparable.

Common mistakes

Measuring the angles from horizontal instead of from the hanging position.
Using the hammer length instead of the distance to its centre of mass.
Ignoring friction and air losses; real machines subtract a free-swing correction.
Comparing J/cm² values with plain joules from a different test standard.

Keep in mind: Formal toughness values must come from a calibrated machine and the applicable test standard. This calculator is for understanding and checking, not for certification.

Frequently asked questions

What is the difference between Charpy and Izod?

Charpy supports the specimen at both ends and strikes the middle; Izod clamps it at one end like a cantilever.

Why is the test done at low temperatures?

To find the temperature at which a steel turns brittle.

Why does the hammer not rise to its starting angle?

The energy used to break the specimen, plus small friction losses, is missing.

Sources: Pendulum energy balance E = W·R·(cos β − cos α), standard potential-energy result (E. Irhowene, MEE 510 project report "Dual purpose impact testing machine")

Checked 9 Nov 2026. Pendulum energy balance E_abs = W·R·(cos β − cos α) as given in the MEE 510 project report “Dual purpose impact testing machine” (E. Irhowene); it is the standard potential-energy result. Specimen size in the hint is the usual Charpy V-notch bar.

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