Handbook-Mechanical Properties

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9 The difference between the distance which the hammer travels after breaking the specimen, and the distance it would have travelled had there been no specimen to hit, is a direct measure of the energy absorbed by the specimen before it fractured. This energy, expressed as units of force, is correctly described as impact energy, rather than impact strength. Two steels which have equal tensile strengths at room temperatures may vary widely in their impact energies (strengths) especially when tested at low temperatures. Fracture toughness refers specifically to the resistance of a material to rapid crack propagation (the brittle fracture mentioned earlier) when a slight crack already exists and a massive load is applied suddenly. For a homely example, think of a loaded grocery bag sitting on the floor. If the bottom of the bag has no holes or tears, you can usually yank it up suddenly without mishap. If there is a small break in the bottom of the bag, you can often lift the bag slowly and get your arm under it, but if you try to yank it up, you’re likely to wind up with groceries scattered on the floor. Similarly, a very small crack in a metal member, which might reduce the strength of that member only slightly were high tensile stress to be created gradually, can sometimes spread with great speed, and destroy the member completely, when an equal load is applied suddenly. Fracture toughness depends on several factors which are not directly involved in the standard Charpy V-notch test, so the results of such tests are often considered only a rough measure of fracture toughness.