ميكانيك الكسر

The other common mode of fracture is known as brittle fracture that takes place with little or no preceding plastic deformation. It occurs, often at unpredictable levels of stress, by rapid crack propagation. The direction of crack propagation is very nearly perpendicular to the direction of applied tensile stress. This crack propagation corresponds to successive and repeated breaking to atomic bonds along specific crystallographic planes, and hence called cleavage fracture. This fracture is also said to be transgranular because crack propagates through grains. Thus it has a grainy or faceted texture. Most brittle fractures occur in a transgranular manner. However, brittle fracture can occur in intergranular manner i.e. crack propagates along grain boundaries. This happens only if grain boundaries contain a brittle film or if the grain-boundary region has been embrittled by the segregation of detrimental elements.
In analogy to ductile fracture, as supported by number of detailed experiments, the brittle fracture in metals is believed to take place in three stages - (1) plastic deformation that causes dislocation pile-ups at obstacles, (2) micro-crack nucleation as a result of build-up of shear stresses, (3) eventual crack propagation under applied stress aided by stored elastic energy.
As mentioned earlier, brittle fracture occurs without any warning sign, thus it needs to be avoided. Hence brittle fracture and its mechanism have been analyzed to a great extent compared to ductile fracture. Brittle fracture usually occurs at stress levels well below those predicted theoretically from the inherent strength due to atomic or molecular bonds. This situation in some respects is analogous to the discrepancy between the theoretical strength shear strength of perfect crystals and their observed lower yield strength values.