Once a crack is present in any material, it will tend to grow under the influence of cyclic loading.

The crack may be initiated by fatigue, or may be pre-existing from manufacture, or may be caused by impact, or similar event (i.e. thermal shock).

The crack will grow to a critical length then fracture of the component will occur.

....brittle failure generally occurs because a flaw or crack propagates throughout the material. The start of a fracture at low stresses is determined by the cracking tendencies at the tip of the crack. If a plastic flaw exists at the tip, the structure is not endangered because the metal mass surrounding the crack will support the stress. When brittle fracture occurs (under the conditions for brittle fracture stated above), the crack will initiate and propagate through the material at great speeds (speed of sound).