As can be demonstrated from the charpy impact test, toughness is not constant for a given material but varies largely with temperature.
At high temperatures many materials require significant levels of energy to cause fracture, however, at low temperatures these same materials fracture with very little impact energy.
This phenomenon is termed the ductile to brittle transition and occurs abruptly at a material specific temperature referred to as the transition temperature.
Not all materials exhibit a ductile to brittle transition temperature, however, those that do must have service temperatures higher than their transition temperatures if they are to be subject to impact.