A viscoelastic material is one in which:

• hysteresis is seen in the stress-strain curve.
• stress relaxation occurs: step constant strain causes decreasing stress
• creep occurs: step constant stress causes increasing strain
• small dependence on strain-rate

Viscoelastic material models are frequently used to describe the behaviour of (soft) human tissue. Commonly used viscoelastic models are the Kelvin Model (standard linear model ), the Voigt Model and Maxwell model . Each model can be represented by springs and dashpots set in series of parallel.

Linear viscoelasticity is when the function is separable in both creep response and load.