Experimental analysis of transient US waves inside plates from 3D vibration measurements along the surface: comparison between bone plate and aluminium plate

The diagnoses of the experts in forensic anthropology of the French Gendarmerie Nationale to estimate post-mortem interval (PMI) are usually carried out by physico-chemical methods that are sensitive to bone composition. Non-destructive methods are needed for expanding the PMI estimation and improve its accuracy. The present work proposes an experimental investigation of ultrasonic wave propagation on ex vivo bones and their potential use for the evaluation of the bone mechanical and geometrical properties as function of bone degradation. In this work, mechanical properties of bone are studied by ultrasonic guided propagation. The bone samples are extracted from bovine femur as plate samples of cortical bone (5x40x80mm). The preliminary ultrasonic experiments are performed on theses samples using a 1D laser vibrometry to monitor the wave propagation along the main axis of the bone. In order to determine the nature (i.e. wave type and polarization) of the observed normal vibrations and to separate the aspects that could be linked to the bone anisotropy from pure geometrical aspects, the same propagation aspects are studied on an isotropic aluminum plate. Using a 3D laser vibrometer (Polytec PSV 3D 500), both the aluminum plate and the bone plate are scanned, providing the measurement of the vibrations along the 3 directions and for each point of the scanned surface. The signal differences between the two plates are analyzed pointing out a possible gradient of material properties along its thickness. A geometrical model is proposed to link the space-time signals to their Fourier domain wavenumber-frequency and Lamb modes dispersion curves. A future ellipsometry study will be done on human bones for several PMI to follow the evolution of mechanical properties of the bone according to the time after the death.