Resonant ultrasound spectroscopy for viscoelastic characterization of anisotropic attenuative solid materials.

Resonant ultrasound spectroscopy (RUS) is an accurate measurement method in which the full stiffness tensor of a material is assessed from the free resonant frequencies of a small sample, and the viscoelastic damping is measured from the resonant peaks width. High viscoelastic damping causes the resonant peaks to overlap and therefore complicate the measurement of the resonant frequencies and the inverse identification of material properties. For that reason, RUS has been known to be fully applicable only to low damping materials. The purpose of this work is to adapt RUS for the characterization of highly attenuating viscoelastic materials. Spectrum measurement using shear transducers combined with dedicated signal processing is employed to retrieve the resonant frequencies despite overlapping. A probabilistic (Bayesian) formulation of the inverse problem, tackling the problem of correctly pairing the measured and predicted frequencies, is proposed. Applications to polymethylmethacrylate (isotropic) and glass/epoxy transversely isotropic samples are presented. The full set of viscoelastic properties is obtained with good repeatability. Particularly, elastic moduli of the isotropic samples are obtained within 1%.