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

GRIFFIN'S THEORY OF FRACTURE

METHODS OF PROTECTION AGAINST FRACTURE

               As per Griffin's theory surface cracks are more effective than internal cracks and causes brittle fracture. The surface crack of length C has the same effect as internal crack of length 2C. Fatigue cracks often propagate from the surface inwards. Protection against fracture or failure by crack propagation, in many cases, involves the following precautions and prevention steps:

1. Use fine grained polycrystalline materials which offer good protection against brittle fracture.
2. Avoid sharp corners and notches in the parts to eliminate points of stress concentration (one may take care of it at designing stage itself).
3. Provide better finish to the surface, by polishing, it offers better protection as it removes some of the cracks from the surface.
4. Avoid impurities in the base metal as well as during welding process.
5. Introduce compressive stresses on the surface to counteract the tensile stress causing crack to propagate.
6. Stress applied should be parallel to the direction of elongation.
7. Use composite/reinforced materials to provide protection against brittle fracture.
8. Ensure proper distribution of solute atoms in the metals to prevent the formation of brittle phase at grain boundaries.