How this instrument works
Absorbed impact energy measures how much work a material soaks up while fracturing under a sudden blow, rather than under the slow, steady pull of a tensile test. A Charpy pendulum is released from a fixed height, swings through an arc, strikes a notched specimen clamped at the bottom, breaks it, and continues upward on the far side. Whatever height it fails to regain represents the energy the specimen took out of the swing.
The formula comes straight from conservation of energy, not from any property of the material itself. Gravitational potential energy at release, mgh₁, converts to kinetic energy at the bottom of the swing; after the strike, whatever kinetic energy remains carries the hammer back up to mgh₂. Subtract one from the other and the mass and gravity terms factor out cleanly, leaving E = mg(h₁ − h₂) — the height difference alone, scaled by weight, is the specimen's toll on the pendulum.
The idealization ignores small losses that real machines calibrate away: bearing friction, air drag on the hammer, and the momentum the broken fragments carry off after the strike. Laboratory Charpy testers correct for these with a friction-and-windage calibration before every batch of tests, which is why a certified machine's readout can be trusted to the joule even though the raw physics here is just two heights and a mass. The method also only reports energy at one temperature; steel in particular loses most of its toughness over a narrow band called the ductile-to-brittle transition, so a single result never tells the whole story.
- Enter the Pendulum hammer mass — the weight of the striking hammer, typically 20 kg or another standardized Charpy mass.
- Enter the Release height — how far above the strike point the hammer starts before it is let go.
- Enter the Swing-through height after impact — how far above the strike point the hammer rises on the far side once it has broken the specimen.
- Read the Absorbed impact energy — the joules the specimen took out of the swing, switchable to kilojoules for larger tests.
Worked example — a 20 kg Charpy hammer through a steel coupon
A 20 kg Charpy pendulum is released from 0.6 m and swings down to strike a notched steel coupon clamped at the bottom of the arc. After breaking it, the hammer continues upward but only reaches 0.4 m on the far side. Plugging m = 20 kg, h₁ = 0.6 m, and h₂ = 0.4 m into E = mg(h₁ − h₂) gives E = 20 × 9.80665 × (0.6 − 0.4) = 39.2266 J, reported as 39.23 J.
That 39.23 J is the coupon's Charpy V-notch toughness at the test temperature, the figure a mill certificate needs to show a pipeline or pressure-vessel steel will absorb enough energy before it snaps rather than bends — a minimum commonly set around 27 J for structural steel under ASME and API specifications, below which the material is rejected for cold-weather service regardless of how strong it tested in tension.
Questions
What does the absorbed energy actually measure?
It measures toughness — the material's capacity to absorb energy while fracturing under a sudden blow, not its strength under a slow pull. A brittle material can test strong in tension yet absorb almost no energy here, snapping cleanly with h₂ nearly equal to h₁; a tough one drags the hammer down hard, leaving h₂ far short of h₁.
Why measure heights instead of testing the specimen directly?
Because energy lost from a swing is far easier to capture than force applied during a fracture that lasts milliseconds. Conservation of energy lets the test skip measuring force and time entirely: whatever gravitational potential energy the pendulum fails to recover on the far side of its swing is exactly what the specimen absorbed, with no instrumentation on the specimen itself required.
What if the swing-through height equals or exceeds the release height?
Equal heights mean the specimen absorbed nothing — the hammer swung clear through without breaking it, or the specimen was missed entirely. A swing-through height above the release height is physically impossible for an unpowered pendulum and signals a measurement or setup error, so the calculator rejects h₂ greater than h₁ rather than return a negative energy.
Why is impact toughness tested at low temperatures?
Because many metals, steel especially, lose toughness sharply below a threshold called the ductile-to-brittle transition temperature, absorbing far less energy in the cold than at room temperature. Running the same Charpy test across a range of temperatures traces that drop-off — the same effect behind the brittle hull fractures that sank several WWII Liberty ships in cold Atlantic water, and why Arctic pipeline steel is qualified at its coldest service temperature, not at room temperature.
Is a higher absorbed energy always the better result?
Only relative to what the application needs, not as a universal rule. Structural and pressure-vessel steel wants high absorbed energy so it bends and warns before it breaks; a file or a hardened cutting tool is deliberately made harder and more brittle, trading impact toughness for wear resistance, because the failure mode it must resist is abrasion, not a sudden blow.
How does the Charpy setup differ from the Izod test?
Charpy specimens sit as a simply supported beam, notch facing away from the hammer, and are struck directly behind the notch; Izod specimens are clamped upright as a cantilever, notch facing the hammer, and struck near the free end. Charpy dominates steel and structural qualification in the US and Europe; Izod remains more common for plastics testing.