Viscoelasticity is the discipline that seeks to explain both the elastic and viscous behavior of materials. Elastic deformation can be seen in the way a rubber band behaves, though metals and other materials behave in a fundamentally similar way. Viscosity refers to how strongly fluids resist rapid changes in shape; honey, for example, is very viscous because it tends to deform slowly. All materials show some elastic and viscous effects when changing shape.
Viscoelasticity is fundamentally a rearrangement of molecules in a substance. For small deformations of metals, viscoelastic effects can often be ignored. High temperature metals, woods, plastics, and biological tissue generally show significant viscoelastic properties that cannot be overlooked.
In addition to rubber bands, elastic behavior is also demonstrated in springs. The further a spring is compressed, the more force is required to hold it there. Springs are called linear if doubling the amount of compression will require double the force. While it may not be noticeable to the eye, metals get linearly compressed or stretched out when forces are applied to them. Elastic materials quickly return to their original sizes when all forces are removed.
Purely viscous behavior can be understood in the way honey responds to stresses from gravity. Honey can be poured from a jar, but it moves very slowly. This results because internal stresses between molecules increase with relative velocity between molecules. Faster movement of molecules is met with greater resistance to that movement. Viscous materials exhibit a time-dependent response to deformation.
James Clerk Maxwell, a Scottish physicist and mathematician, came up with a model to describe the phenomenon of viscoelasticity. It uses a spring for elastic effects and a dashpot, or device that resists motion based on its velocity, for viscous effects. A car’s suspension system uses this same process using shocks as the dashpots. Large deformations in the system are resisted by the springs, while rapid changes in deformation are resisted by the dashpots. Viscoelasticity is also commonly modeled using electric circuits.
Other parts of viscoelasticity are the effects of creep and stress relaxation. Creep is when a material tends to slowly sag, or deform, when it is subjected to a force for a long time. Engineers must take creep into account when designing buildings, because creep can cause materials to weaken to the point of breaking. A related effect, the phenomenon of stress relaxation, refers to a reduction in internal stress for a material held in a particular shape.