Fluidity

the property whereby stressed bodies undergo plastic deformation or viscous flow. Fluidity is characterized by a value that is the reciprocal of viscosity. In viscous bodies (gases, liquids) fluidity is manifested at all stress levels. In plastic solid bodies, however, it occurs only at high stress levels that exceed the yield point.

Different bodies exhibit different fluidity mechanisms, which determine the resistance of the bodies to plastic or viscous flow. In gases, the mechanism governing fluidity is associated with the transfer of momentum from those layers where the gas molecules move predominantly in the direction of flow to layers where this motion is weaker. In liquids, the fluidity mechanism manifests itself in a diffusion that is predominantly in the direction of the stress effects. On a molecular level, the diffusion is explained by the spasmodic displacement of molecules, pairs of molecules, or segments of macromolecular chains (in substances of high molecular weight) accompanied by transition across an energy barrier. In crystalline solids, fluidity is associated with the motion of various kinds of crystal defects, including point defects (vacancies), line defects (dislocations), and volume defects (crowding). Flow can also arise from stress-induced twinning. The flow proceeding slowly over a long period of time in metals at high temperatures and in polymers and other materials is known as creep.

Fluidity phenomena occur both on earth and in outer space. On earth, fluidity manifests itself in the movement of continents, the motion in the atmosphere and hydrosphere, and the tectonic movements of mountain masses. In engineering, fluidity phenomena are encountered in the flow of gases and liquids through pipes and in various types of production equipment. Plastic flow and creep must be allowed for in construction, especially when structural members are subjected to great loads.