Short note: fracture mode of materials

Fracture is the separation of a body into two or more parts under stress. There are two main modes.

Ductile fracture

• Occurs after extensive plastic deformation and necking.
• Absorbs a lot of energy and gives warning.
• The surface is dull and fibrous, with a cup-and-cone shape in tension.
• Mechanism: voids form in the neck and join up, and the remaining rim shears at 45 degrees. Examples: mild steel, aluminium.

Brittle fracture

• Occurs suddenly with very little or no plastic deformation.
• The crack grows extremely rapidly, absorbing little energy.
• The surface is flat, bright and granular, perpendicular to the tensile stress.
• It occurs by cleavage (breaking of atomic bonds along crystal planes) or along grain boundaries. Examples: cast iron, glass, concrete, steel at low temperature.

Other modes

Fatigue fracture (cyclic loading) and creep fracture (long-term loading at high temperature).

Factors that promote brittle fracture: low temperature, high strain rate, and notches and sharp corners (stress concentration).