July 10, 2012

Brittle Fracture:

Brittle fracture takes place without any appreciable deformation, and by rapid crack propagation.

The direction of crack motion is very nearly perpendicular to

the direction of the applied tensile stress and yields a relatively

flat fracture surface

Dislocation theories of Brittle Fracture:The process of brittle fracture consists of three stages.

1)Plastic deformation which involves the pile-up of dislocation

along their slip planes at an obstacle.

2)The buildup of shear stress at the head of the pile-up to

nucleate a micro-crack.

3) In some cases the stored elastic strain energy drives the

micro-crack to complete fracture, a distinct growth stage is

observed in which an increased stress is required to

propagate the micro-crack.

July 10, 2012

Brittle fracture is not possible unless the crack(s) which is/are

nucleated can propagate at a high velocity throughout the

metal.

Fracture surfaces of materials that failed in a brittle manner will

have their own distinctive patterns; no signs of gross plastic deformation.

For example, in some steel, a series of V-shaped ‘chevron’ markings may form near the center of the fracture cross

section.

Other brittle fracture surfaces contain lines or ridges that

radiate from the origin of the crack in a fanlike pattern.

July 10, 2012

For very hard and fine-grained metals, there will be no visible

fracture pattern.

Brittle fracture in amorphous materials, such as ceramic

glasses, yields a relatively shiny and smooth surface.

For most brittle crystalline materials, crack propagation

corresponds to the successive and repeated breaking of atomic

bonds along specific crystallographic planes; such a process is

termed cleavage. Or breaking of atomic bonds along specific crystallographic planes.

July 10, 2012

The cleavage mode of fracture is controlled by tensile stresses

acting normal to a crystallographic cleavage plane.

Based on metallographic examination, fracture in

polycrystalline sample are classified as transgranular (the

crack propagate through the grains) or

transcrystalline/intergranular (the crack propagate along the

grain boundary)

Figure: Intergranular fracture surface

Figure: Crossover from transgranular to intergranular fracture

July 10, 2012

Figure: Scanning Electron Fractograph of ductile cast iron showing a transgranular fracture furface.

Figure: Scanning Electron Fractograph showing interganular fracture surface

July 10, 2012

Behavior described Term usedStrain to Fracture Ductile Brittle

Crystallographic Mode Shear CleavageAppearance of Fracture Fibrous Granular(Shine)

July 10, 2012