The word “viscoelastic” is used to describe the mechanical response of materials exhibiting both the springiness associated with elastic solids and viscous flow characteristics associated with fluids. A familiar example of a material called viscoelastic is Silly PuttyTM. If a blob of Silly Putty is rolled into a ball and then dropped onto a hard surface, it will bounce like an elastic ball. If the ball is placed on a hard surface, its own weight will cause it to flow into a puddle. This behavior indicates that time is an intrinsic parameter in discussing viscoelastic response of materials. The elastic response is associated with a contact force of very short duration. The flow into a puddle occurs when forces act for a long period of time.

Viscoelastic response occurs in materials such as soils, concrete, cartilage, biological tissue, and polymers. Soils and cartilage can be thought of as porous solids filled with fluid. Viscous response is due to the flow of the fluid in the pores; elastic response is due to the distortion of the porous solid.