Jawaharlal Nehru Technological University Hyderabad B.Tech III MECHANICAL METALLURGY Previous Question Paper (November/December - 2018 - R13)

Plastic deformation is the permanent, non-recoverable change in the shape or size of a material that occurs when an applied stress exceeds the material's yield strength, resulting from the movement of dislocations through the crystal lattice without breaking the atomic bonds permanently. In contrast, fracture is the separation of a material into two or more pieces under the action of stress, involving the complete rupture of atomic bonds and the creation of new surfaces.

A Frank-Read source is a mechanism for generating multiple dislocation loops from a single pinned dislocation segment. The process occurs as follows:

1. A dislocation segment is pinned at two points, A and B, by obstacles such as precipitates or nodes in the crystal lattice.
2. Under an applied shear stress, the segment bows out into a curved shape, increasing its length and energy.
3. As the stress continues, the dislocation segment bows further until it forms a semi-circular shape.
4. The two segments of the dislocation eventually meet and annihilate each other where they touch, resulting in the formation of a closed dislocation loop that expands outward and a regenerated straight segment between the original pinning points.
5. This process repeats, allowing the source to continuously emit dislocation loops as long as the applied stress is maintained.