Classification of deformation

The response of a material to applied stress can occur in a variety of ways. Under some conditions, an equilibrium state can be achieved upon applying a stress, whereas time-dependent or steady-state deformation can also be achieved. To understand the microscopic basis for this difference, it is useful to consider the nature of atomic displacement caused by an applied stress. At static equilibrium, each atom occupies a position corresponding to the minimum potential energy (Fig. 3.1a). Upon applying a stress (a force per unit area), atoms move their positions from their stable positions. If the stress is small, or the temperature is low (or time is short), then only small, instantaneous displacement will occur. Consequently, when the stress is removed, atoms go back to their original positions. This type of instantaneous and recoverable deformation is called elastic deformation.