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Wednesday, 14 October 2015

FRACTURE IN METALS

INTRODUCTION:

             Fracture is the braking of a metal to yield an irregular surface. The topography of the fracture surface is the important characteristics intimately connected with the crystalline form and in some cases it may give an indication of purity of metal.
         In poly-crystalline material there can be three distinct modes of fracture as given below:
  • Inter granular fracture by separation of crystals at the grain boundaries.
  • Fracture due to shear between the crystallographic planes.
  • Cleavage fracture through pulling a part of crystallographic plane.

DUCTILE AND BRITTLE FRACTURE:

Depending upon the fracture mechanism it is divided into two distinct types:
                          1. Ductile fracture                    2. Brittle fracture

              In the case of Ductile fracture there occurs an appreciable plastic deformation prior to failure and the fractured surface gives cup and cone appearance (shown in Fig.) The fracture is found to start only after a necked portion shown on the test piece. The first formed micro-cracks and cavities grow large and finally join together to form a crack in the centre of the necked portion. The cavity then spread in direction inclined at 45° to the tensile axis. The size of the cup depends on the relative shear and cleavage strength values. Metals with high yield strength give a smaller cup. The fracture faces are dull, irregular fibrous in appearance.

Different progressive stages of ductile fracture are shown in Fig.


                In case of brittle fracture failure of the metal occurs when the fracture crack propagates through the cross-section without an appreciable plastic deformation. The fracture crack may start from any location where there are stress raisers. The surface condition of the metal can be critical and markings on it can initiative cracks. Such a fracture is more likely to occur in metal with poor plasticity and low temperature.







COMPARISON BETWEEN DUCTILE  AND BRITTLE FRACTURE:

             DUCTILE FRACTURE
              BRITTLE FRACTURE
Material fractures after plastic 
deformation and slow propagation
of crack
Materials fractures with very little
or no plastic deformation, e.g. in china clay,
glass etc.
Fractured surfaces are dull or fibrous
in appearance
Fractured surfaces are bright and crystalline 
in appearance
Percentage elongation is about 
30% prior to fracture occurs
Percentage elongation is about
0.5% or almost nil prior to fracture occurs
There is reduction in
cross-sectional area of the specimen
There is virtually no change in the 
cross-sectional area
Fracture takes place after necking 
with little sound
Fracture occurs rapidly often accompanied
by a loud noise
Ductile fracture occurs in most 
metals (not too cold)
Brittle fracture occurs in Ceramics, 
Glasses, Ice

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