Toughness, as far as tool materials are concerned, is a measure of the resistance of a tool to breakage, chipping, or cracking while under impact loading or constant stress. You could view toughness as the opposite of brittleness. There are a number of different ways in which to characterize material toughness including various impact tests and bend fracture tests.
In impact testing, such as a Charpy impact toughness test, a small sample is held in a fixture and is fractured by a calibrated weight swung on a pendulum. The toughness is equal to the amount of energy the sample absorbs before it breaks, and this is usually expressed in terms of ft-lbs or Joules. Brittle materials absorb relatively little energy before breaking, so their impact toughness values tend to be quite low. Tough materials will absorb much more energy before fracturing, so their impact toughness values tend to be higher. One inherent chaacteristic of the toughness test is that most materials are notch sensitive, which means that any small notch or defect present on the surface of the sample will allow it to fracture at a much lower energy. So sample preparation is extremely important in toughness testing. One variation on the Charpy impact toughness test is to notch the sample with either a V shaped notch or a C shaped notch. Since these materials tend to be notch sensitive, the sample will fracture at a known location, namely the C or V notch. This makes testing somewhat more predictable in terms of where the sample will break. By standardizing these type of tests, a valid comparison may be drawn between two different materials that are under consideration for a given application.
