Fracture

Typically as the yield strength of a material is increased, its ductility and toughness decrease. Toughness is the energy absorbed in fracturing. If a material has a high yield strength, it can be subjected to stresses high enough to cause fracture before there has been much plastic deformation to absorb energy. Factors that inhibit plastic flow lower ductility as schematically indicated in Figure 5.1. These factors include decreased temperatures, increased strain rates, and the presence of notches.

Engineers should be interested in ductility and fracture for two reasons. Ductility is required to form metals into useful parts by forging, rolling, extrusion, or other plastic working processes. Also, some plastic deformation is necessary to absorb energy so as to prevent failure in service.

Fractures can be classified several ways. A fracture may be described as being ductile or brittle, depending on the amount of deformation that precedes it. Failures may also be described as intergranular or transgranular, depending on the fracture path. The terms cleavage, shear, void coalescence, etc., are used to identify failure mechanisms. These descriptions are not mutually exclusive. A brittle fracture may be intergranular, or it may occur by cleavage.

The failure in a tensile test of a ductile material occurs well after the maximum load is reached and a neck has formed. In this case, fracture usually starts by nucleation of voids in the center of the neck where the hydrostatic tension is the greatest.