D4 Impact Tests (Charpy)

Topic

The Charpy pendulum—the experimental method described by this model—involves dropping a heavy hammer from a known height to strike a notched specimen of the metal in a single impact; it then measures the energy absorbed during this dynamic fracture based on the height to which the hammer rebounds after breaking the specimen. The less height the hammer regains after impact, the more energy the specimen absorbed upon fracturing; conversely, higher energy absorption by the specimen causes the Charpy pendulum hammer to rebound to a lower height.

Measuring this absorbed energy across several identical specimens—each tested at a different temperature—allows for the plotting of the metal's ductile-to-brittle transition curve using a sigmoidal fit. At sufficiently high temperatures, the metal absorbs a high, roughly constant amount of energy (the ductile regime); at sufficiently low temperatures, it absorbs a low, also roughly constant amount (the brittle regime); and between these extremes, the absorbed energy drops sharply over a relatively narrow temperature range. As the testing temperature drops through this transition range, the energy absorbed during fracture falls—following that same sigmoidal curve—from the high ductile level to the low brittle level.

Thus, this model—based on the Charpy pendulum and the associated absorbed energy—allows for an approximate estimate of the metal's fracture toughness (a property also defined by another model in this set) using the Barsom-Rolfe empirical correlation. This correlation converts Charpy energy—which is far simpler and less costly to measure than a standard fracture toughness test—into an approximate estimate of that fracture toughness. The fact that the Charpy energy measured for this metal is higher means that, according to this Barsom-Rolfe correlation, the fracture toughness estimated from it is also higher.

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