We don’t want engineering materials to fail because they can cause loss of life, economic loss or a loss of products or services.
The usual causes of material failure are incorrect materials selection, incorrect processing, incorrect manufacturing procedures, inadequate design or incorrect use.
Fracture is the separation of a body into two or more pieces as a result of an imposed stress.
Fast fracture occurs when a pre-existing crack in a material suddenly becomes unstable and grows rapidly through the material. This form of fracture is highly undesirable. It is a catastrophic failure that occurs without warning.
The two most common failures to occur without warning are fast fracture and fatigue. Both fast fracture and fatigue cause failure below the nominal yield stress of a material. Fast fracture occurs through rapid propagation of a pre-existing crack leading to failure. Fatigue occurs through slow stepwise crack propagation under cyclic stresses with fast fracture occurring as a final step to failure.
In both fast fracture and fatigue, crack growth occurs from the presence of stress concentrations where there is a localised build up of stress around the crack tip.
Ultimate failure occurs when the crack has a length greater than the critical crack length for which the material can support its load. At this point the crack advances rapidly through the material resulting in fracture.