The loss factor cannot express frequency-dependent, nonperiodic time-dependent or nonlinear responses. We present a phenomenological model that describes the time- and frequency-dependent behavior of electromechanically active materials. Expanding rheology, we introduce an electrorheological model incorporating time constants corresponding not only to the viscoelastic response, i.e. strain vs. stress, but also: (1) polarization vs. field, (2) strain vs. polarization, (3) ferro-/antiferroelectric and ferroelastic domain switching under applied field and/or stress. A single set of constitutive equations is obtained that can adequately describe experimentally observed pure and mixed cases of ferroic, antiferroic, and nonferroic response.
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