Fracture of metallic components during service is of great importance and consequence. The proper selection of a material for a component is a critical step in avoiding unwanted failures. Fracture of metals may generally be classified as either ductile or brittle. This is easily observed by performing simple static tension tests. Ductile fracture is accompanied by sever plastic deformation prior to failure. On the contrary, brittle shows little or no deformation prior to fracture and is therefore more problematic. In some cases, under high loading rates or lowered temperatures, originally ductile materials behave in a brittle fashion called ductile to brittle transition. Therefore selection of materials for components that operate in cold temperatures should be done with care.
Since defects such as microcracks weaken a material, engineers use the concept of fracture toughness based on the assumption of pre-existing flaws (fracture mechanics) to design components that are safer. The concept of stress intensity factor, K, at a crack tip is used to represent the combined effect of stress at the crack tip and the crack length.
The failure of metallic components under cyclic or repeated loading called fatigue failure is of tremendous importance to engineers. Its importance is due to the low level of stresses at which such failures occur, the hidden nature of the damage (inside the material), and its sudden and abrupt failure. Another form of failure that occurs at high temperatures and constant loading is called creep, which is defined as progressive plastic deformation over a period of time. Engineers are very conscious of such failures and use high factors of safety to guard against them.
Engineers and scientists always search for new materials that offer higher strength, ductility, fatigue resistance, and are in general resistant to failure. Nanocrystalline materials promise to be the materials of choice for the future offering a combination of properties that will greatly enhance a material’s resistance to fracture. However, more research is needed by materials scientists to achieve this goal.
