The material we saw so far responded the stretching imparted on the material. In other words stress is a function of strain; completely insensitive to the rates of loading. However, there is another class of materials which responds to not the level of stretching but the rate at which loading occurs. Material responding to rates of loading are called viscous materials. Viscous denotes the rate sensitivity. Most of the fluid respond this way. However, some metals under some conditions also respond similarly (e.g. superplastic materials). A combination of viscous and elastic behavior called viscoelastic behavior is exhibited by many polymers and polymeric composites.