Impact Test Calculator

The impact test calculator determines the energy absorbed in Izod and Charpy impact tests.

Energy absorbed106.522 JmgR(cos β − cos α) — the drop in the pendulum's potential energy, which is what the specimen took to break

Energy at release277.1037 Jraised 1,412.8 mm above the lowest point

Energy remaining after the break170.5817 Jthe pendulum carries on to 869.7 mm

Fraction absorbed38.441%

Impact speed at the lowest point5.2641 m/sindependent of the pendulum mass, as every free-fall speed is

Impact toughness133.1525 J/cm²energy per unit fracture area, which is the comparable figure between specimens of different size

Fracture area80 mm²a standard Charpy V-notch specimen has 80 mm² behind the notch

What this test does NOT givea design propertyCharpy energy is a comparative index, not a material constant. It depends on specimen size, notch geometry and strain rate, so it cannot be fed into a stress calculation — its real use is ranking materials and finding the transition temperature

The transition temperaturewhy the test existsbody-centred cubic metals like carbon steel switch from ductile to brittle over a narrow temperature range, losing most of their impact energy. Running this test across temperatures locates that transition — the failure mode behind the Liberty ships and the Titanic's hull plate

The formula

E = mgR(cos β − cos α); toughness = E/A

A pendulum that measures what it loses

The Charpy test releases a pendulum from a known height, lets it break a notched specimen at the bottom of its swing, and measures how high it rises afterwards. The difference in potential energy is what the specimen absorbed. Nothing more elaborate than mgh is involved, which is why the test has survived essentially unchanged for over a century.

It is an index, not a property

Charpy energy cannot be used in a stress calculation. It depends on the specimen size, the notch geometry and the strain rate, so it is comparative rather than fundamental — two labs testing the same steel to different standards get different numbers.

Its real value is finding the ductile-to-brittle transition. Body-centred cubic metals such as carbon steel lose most of their impact toughness over a narrow temperature range, becoming brittle while every other property looks unchanged. Running the test across temperatures locates that transition — a failure mode that broke Liberty ships in cold water and is implicated in the Titanic's hull plate. Face-centred cubic metals like austenitic stainless and aluminium have no such transition and stay tough all the way down.