In the last decades the research community has shown an increasing interest in the engineering applications of fractional calculus, which allows to accurately characterize the static and dynamic behaviour of many complex mechanical systems, e.g. the non-local or non-viscous constitutive law. In particular, fractional calculus has gained considerable importance in the random vibration analysis of engineering structures provided with viscoelastic damping. In this case, the evaluation of the dynamic response in the frequency domain presents significant advantages, once a probabilistic characterization of the input is provided. On the other hand, closed-form expressions for the response statistics of dynamical fractional systems are not available even for the simplest cases. Taking advantage of the Residue Theorem, in this paper the exact expressions of the spectral moments of integer and complex orders (i.e. fractional spectral moments) of linear fractional oscillators driven by acceleration time histories obtained as samples of stationary Gaussian white noise processes are determined.
IntroductionThere is an increasing amount of research on the use of fractional operators to describe viscoelastic properties of materials in structural dynamics [1]. In fact, it has been shown that fractional integrals and derivatives are suited to mathematically model constitutive equations of viscoelastic materials, returning creep and relaxation functions whose general shapes well fit experimental data [2][3][4][5][6][7][8][9]. In civil and mechanical engineering, when fractional operators are used to model dissipative forces in dynamic systems, the latter are indicated with the term fractional oscillators. Several numerical methods to integrate the equations of motion of fractional systems have been proposed. A comprehensive review of papers dealing with the dynamic behaviour of fractional linear and nonlinear systems, single and multi-degrees-of-freedom, including vibration of rods, beams, plates and shells, among others, can be found in [1] and [10]. Among the various studies on fractional oscillators in literature, Rüdinger [11] proposes their use as viscoelastic tuned mass dampers to reduce vibrations of systems excited by an external white noise. The fractional oscillator is constituted by a mass linked to the main structure through a linear spring placed in parallel to a viscoelastic damping element exerting a force