Visco-elastic Solid o Kelvin Material :
Stress Relaxation Behaviour : Kelvin model does not produce stress relaxation throughout the time when it is applied to a strain step (constant strain ε0 ). The viscous element generates an infinite stress in t=0 which tends quickly to zero, whereas the spring maintains a constant stress. If H(t) and &delta(t) are the functions of Heaveside and Dirac respectively, the equation of the model is :



Behaviour with oscillating loads: If the applied load is F=F0·sen(ωt), stress and a strain are:

Substituting in the Kelvin model:


The dissipated energy by unit of volume in a cycle is ηωε02