The deformation of solid materials has two elements: change in shape (distortion) and change in volume (dilatation). However, dilation is considered negligible in solid materials. Thus, deformation is used as synonymous with distribution.
Types of Deformation:
Deformation can be permanent or temporary.
- Permanent deformation is irreversible; the deformation stays even after removal of the applied forces, while the temporary deformation is recoverable as it disappears after the removal of applied forces.
- Temporary deformation is also called elastic deformation, while permanent deformation is called plastic deformation.
Time-dependent recoverable deformation under load is known as elastic deformation, while the characteristic recovery of temporary deformation after the removal of the load as a function of time is called elastic after-effect.
Progressive, permanent deformation under constant load is called creep. For visco-elastic materials, both recoverable and permanent deformations occur together which are dependent on time.
When a material is subjected to applied forces, the material experiences elastic deformation followed by plastic deformation. The transition from elastic state to plastic state is characterized by the yield strength of the material. Microscopically, plastic deformation involves breaking of original atomic bonds, movement of atoms and the restoration of bonds. In other words, plastic deformation is based on irreversible displacements of atoms through substantial distances from their equilibrium positions.
The plastic deformation mechanism is different for crystalline and amorphous materials. For crystalline materials, deformation is accomplished through a process called slip that involves the motion of dislocations. In amorphous materials, plastic deformation takes place by a viscous flow mechanism in which atoms or ions slide past one another under applied stress without any directionality.
Describing the behavior of metals under practical conditions is vital. Therefore, the theory of plasticity neglects the following aspects:
- Anelastic Strain: Anelastic strain is a time-dependent recoverable strain.
- Hysteresis Behavior: Hysteresis behavior results from the loading and unloading of material.
- Bauschinger Effect: Bauschinger effect shows the dependence of yield stress on loading path and direction.
Plastic deformation is indeed uniform but only up to some extent of strain, whereafter plastic deformation is concentrated to the phenomenon called necking. The changeover from uniform plastic deformation to non-uniform plastic deformation is characterized by ultimate tensile strength (σu).