There is an increasing interest towards the use of non-conventional material such as Functionally Graded Materials (FGM) for aerospace and automotive mechanical applications. Classical material models, e.g. Kelvin or Zener, can show some limitations in describing the viscoelastic behavior of these materials. A numerical and experimental approach to identify the optimal model order and the parameters of the constitutive material relationship in the frequency domain is proposed. The constitutive equation is modeled by means of a generalized Kelvin model and expressed in the form of a rational function. To describe the complex material behavior, high order polynomials are needed for the rational function and the problem of finding the function coefficients can be ill-conditioned. Different approaches for the rational function parameters identification are compared. A least square error identification technique adopting Forsythe orthogonal polynomials is proposed. The selected procedure is first applied on numerically estimated measurements with noise, and then on real measurement data obtained by forced vibration testing of Polytetrafluoroethylene specimens.

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