Role of slow dynamics in fast dynamics ultrasonic measurements

Nonlinear mesoscopic materials exhibit anomalous elastic nonlinearity in which fast and slow dynamics effects are mixed up. The former is an instantaneous nonlinear phenomenon due to the explicit strain dependence of velocity and damping. Slow dynamics is a non-equilibrium effect, governed by the dependence of the linear modulus and Q-factor on the dynamic strain level: when excited at constant strain, the sample properties vary in time (over minutes or more) until they reach a new equilibrium state. When excitation is removed, again slowly in time, the system recovers its original viscoelastic properties. The goal of this contribution is to show how slow dynamics might affect fast dynamics measurements and thus point out that care should be given to all time scales of the experiment, e.g. lag time between amplitude change and acquisition, duration of the acquisition and relaxation time between successive measurements. © 2020 Elsevier B.V.