Brittle fracture takes place without any appreciable deformation, and by rapid crack propagation. The direction of crack motion is very nearly perpendicular to the direction of the applied tensile stress and yields a relatively flat fracture surface, as indicated in Figure 9. 1c. Fracture surfaces of materials that failed in a brittle manner will have their own distinctive patterns; any signs of gross plastic deformation will be absent.
For example, in some steel pieces, a series of V-shaped ‘‘chevron’’ markings may form near the center of the fracture cross section that point back toward the crack initiation site (Figure 9. 5a). Other brittle fracture surfaces contain lines or ridges that radiate from the origin of the crack in a fanlike pattern (Figure 9. 5b). Often, both of these marking patterns will be sufficiently coarse to be discerned with the naked eye. For very hard and fine-grained metals, there will be no discernible fracture pattern.
Brittle fracture in amorphous materials, such as ceramic glasses, yields a relatively shiny and smooth surface. For most brittle crystalline materials, crack propagation corresponds to the successive and repeated breaking of atomic bonds along specific crystallographic planes; such a process is termed cleavage. This type of fracture is said to be transgranular (or transcrystalline), because the fracture cracks pass through the grains. Macroscopically, the fracture surface may have a grainy or faceted texture (Figure 9. b), as a result of changes in orientation of the cleavage planes from grain to grain. This feature is more evident in the scanning electron micrograph shown in Figure 9. 6a. In some alloys, crack propagation is along grain boundaries; this fracture is termed intergranular. Figure 9. 6b is a scanning electron micrograph showing a typical intergranular fracture, in which the three-dimensional nature of the grains may be seen. This type of fracture normally results subsequent to the occurrence of processes that weaken or embrittle grain boundary regions.