For materials with a crystalline structure, these shearing displacements are usually associated with defects within the crystal lattice.Such defects are called dislocations, and they give a crystalline structure the ability to sustain plastic deformations without fracturing.These internal shearing forces produce the characteristic behaviour of liquids such as treacle, heavy oils, or molten plastics.This characteristic motion, known as shearing flow, is an energy-dissipative process.Rigid materials such as metals, concrete, or rocks sustain large forces while undergoing little deformation, but if sufficiently large forces are applied, the materials can no longer sustain them by elastic deformation alone.
Rather, the solid gradually deforms and attains its steady-state deformation only after a significant period of time. Conversely, the sudden application of a fixed deformation to such a material produces initial stresses that can be very large; these stresses then slowly relax to a steady-state value as the material accommodates itself to the applied deformation.
This type of deformation is an energy-storing process, as exemplified by the compression of a spring.
(ductility (in which certain internal mechanisms permit shearing displacements to occur within the atomic structure).
This ability of ductile materials to flow plastically under load is fundamental to their usefulness in engineering.
As a consequence, an understanding of plastic flow is vital in technology, particularly in the production of large numbers of objects in a variety of complex shapes.