19. Viscoelasticity

Viscoelasticity is a type of material behavior with memory, that is to say the strain history affects the current stresses. For this type of behavior creep and relaxation experiments are used. In creep experiments a stress is applied at time zero and the strains are recorded as a function of time, the creep function. In relaxation experiments a strain is applied at time zero and the stresses are recorded as a function of time, the relaxation function.

Based on the principle of superposition, the creep or the relaxation function can be used respectively to calculate the strain as function of the stress history or the stress as function of the strain history. This principle reduces the applicability of the formulation to linear viscoelasticity.

A vital part of the finite element implementation of the viscoelastic models is to find an algorithm in which it is not necessary to `remember' the complete strain or stress history, because this would require too much computer memory for real structures.