Which of the following is not classified as a viscoelastic material?

Viscoelastic materials exhibit both viscous and elastic characteristics when deformed. They exhibit time-dependent strain, meaning their response to stress can change over time. Typically, polymers such as plastics and certain rubbers are classified as viscoelastic due to their ability to undergo both elastic deformation (reversible) and viscous flow (time-dependent).

Metals, on the other hand, are primarily characterized by their elastic behavior. When a metal is subjected to stress, it will deform elastically up to its yield point, after which it will undergo plastic (permanent) deformation. This behavior does not include the time-dependent strain characteristics that define viscoelastic materials. For this reason, metal is not classified as a viscoelastic material, making it the correct answer to the question.