Material dynamic behaviour can be explained from the mechanical model represented in the figure shown above. It has a spring with stiffness E governing the material behaviour in the linear elastic range. Connected in series with the spring, a second component describes material behaviour in plastic deformation range and rate dependency. This second element is a parallel configuration with a friction plane which defines yield stress and a non-linear system integrated by a spring (strain hardening) and a damper (strain rate effect).
The friction plane does not get deformed by stresses applied below yield point. So, in this case, the spring governs the system behaviour (linear elastic material).
If yield stress is reached, the friction plane gets deformed, and also the non-linear element starts to work. In the case of static load, the damper does not influence the global response of the system (elastoplastic material behaviour with strain hardening).
In the case of the dynamic load, the damper generates a resistant force proportional to velocity, so the material behaviour becomes stiffer (in dynamic range a higher load level is necessary than in static range in order to obtain the same strain level).
The limit case would be when the load is applied with infinite speed. In this situation the damper generates an infinite resistant force, and the non-linear element that governs the plastic range becomes rigid. As a consequence, the resultant material behaviour in this limit case is equivalent to the linear elastic one.