Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 5M

Year: May 2015

A fracture is the separation of an object or material into two or more pieces under the action of stress.

• The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid.
• If a displacement develops perpendicular to the surface of displacement, it is called a normal tensile crack or simply a crack; if a displacement develops tangentially to the surface of displacement, it is called a shear crack, slip band, or dislocation.
• Fracture strength or breaking strength is the stress when a specimen fails or fractures.

Types

• The theoretical strength of a crystalline material is (roughly)In brittle fracture, no apparent plastic deformation takes place before fracture.
• In brittle crystalline materials, fracture can occur by cleavage as the result of tensile stress acting normal to crystallographic planes with low bonding (cleavage planes).
• In amorphous solids, by contrast, the lack of a crystalline structure results in a conchoidal fracture, with cracks proceeding normal to the applied tension.
• The sinking of RMS Titanic in 1912 from an iceberg collision is widely reported to have been due to brittle fracture of the hull's steel plates.
• The theoretical strength of a crystalline material is (roughly),

$$\sigma_{theoretical} = \sqrt{\frac{E \gamma}{r_0}}$$

where: -

$E$ is the Young's modulus of the material, $\gamma$ is the surface energy, and $r_0$ is the equilibrium distance between atomic centers.

Ductile fracture

Schematic representation of the steps in ductile fractur

In ductile fracture, extensive plastic deformation (necking) takes place before fracture. The terms rupture or ductile rupture describe the ultimate failure of tough ductile materials loaded in tension. Rather than cracking, the material "pulls apart," generally leaving a rough surface. In this case there is slow propagation and an absorption of a large amount energy before fracture.