Fracture Mechanics & Failure Analysis Lecture Brittle Fracture

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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.

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 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.

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 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.

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 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

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

Figure: Scanning Electron Fractograph showing interganular
fracture surface

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Behavior described Term used
Strain to Fracture Ductile Brittle
Crystallographic Mode Shear Cleavage
Appearance of Fracture Fibrous Granular(Shine)

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