Materials that do not fail in a ductile manner will fail in a brittle manner.
Brittle fractures are characterised as having little or no plastic deformation prior to failure.
Materials that usually fracture in a brittle manner are glasses, ceramics, and some polymers and metals. Under some circumstances some metals that are usually ductile will fail in a brittle manner, possibly with catastrophic results.
Like ductile fractures, brittle fractures also have a distinctive fracture surface. The fracture surface of a brittle failure is usually reasonably smooth. The crack propagates through the material by a process called cleavage.
The images below show the fracture surface of a steel that failed in a brittle manner.
Cleavage
Crack propagation (cleavage) in brittle materials occurs through planar sectioning of the atomic bonds between the atoms at the crack tip.