Causes of brittle fracture

Brittle fractures that occur in service are invariably initiated by defects that are initially present in the manufactured product or fabricated structure or by defects that develop during service. The defects are essentially stress concentrators and may take any one of the following five forms.

Notches, which are discontinuities caused by abrupt changes in the direction of a free surface, are often fracture initiators. Among the common intentional notches are sharp fillets and corners, holes, threads, splines, and keyways. Notches can also be produced accidentally by mechanical damage, such as from dents, gouges, or scratches. When fracture occurs it always initiates at the notche, regardless of whether the presence of a notche is intentional or accidental.
Laps, folds, flakes, large inclusions, forging bursts, laminations in sheet and plates and undesirable grain, flow introduced during working operations also contribute to failure.
Segregation, inclusions, undesirable microstructures, porosity, tears, cracks or surface discontinuities introduced during melting, deoxidation, grain refining, and casting operations may or may not lead to failure.
Cracks resulting from machining, quenching, fatigue, hydrogen embrittlement, liquid-metal embrittlement, or stress corrosion also lead, to brittle fracture. In fact, the single most prevalent initiator of brittle fràcture is the fatiguie crack, which conservatively accounts for at least 50% of all brittle fractures in manufactured products.
Residual stresses, although not defects in geometric sense can be an important factor in the initiation of brittle fractures.