cleavagefracture

Cleavage fracture is a brittle fracture mode in which crack propagation occurs primarily along crystallographic cleavage planes, producing a relatively flat, facet-like fracture surface. It involves little plastic deformation and proceeds rapidly once a crack is initiated. The phenomenon is most common in materials with limited ductility, where the stress cannot be redistributed by plastic flow.

In metals, cleavage can be transgranular, occurring through individual grains along specific lattice planes, or intergranular, along grain boundaries. It is favored at low temperatures or high strain rates, when the material’s fracture toughness is reduced. The microstructure—grain size, phase distribution, and presence of impurities or brittle inclusions—greatly influences susceptibility to cleavage. Ceramics and some cast irons are particularly prone to cleavage due to their inherently limited plasticity.

Fracture surfaces produced by cleavage are characterized by distinct facets that can reveal the crystallographic planes involved. In technical analysis, surface morphology, cleavage facet angles, and patterns such as river marks help identify the propagation direction and conditions of fracture. Cleavage is associated with low energy absorption compared with ductile fracture, and it often results in lower impact toughness. Mechanical design aims to avoid cleavage by improving toughness through grain refinement, alloying to promote ductility, reducing stress concentrations, and controlling operating temperatures to stay above ductile-to-brittle transition thresholds. Fracture mechanics concepts, including critical stress intensity factors, are used to assess and mitigate cleavage risk in engineering materials.