Aim of this doctoral dissertation is the study of the nonlinearities affecting the human body response under multi-axial whole-body vibrations (WBV). The research was entirely addressed to the characterization of the response for standing persons using a novel approach for the study of the nonlinearities of the body-transmitted vibrations.

The study of the nonlinearities was performed for both single-axis and multi-axial vibration. The reference parameter for the biodynamic response of the human body was the apparent mass, i.e. the frequency response function between the transmitted force and the applied acceleration.

In the first part of this work, nonlinearities were identified by conditioning the apparent mass deriving from the vertical WBV with a set of nonlinear functions of the acceleration. In the first part of the work both the acceleration and the force were measured only along the vertical direction.

Afterwards, the full (three-by-three) matrix was identified with a purposelydesigned excitation system composed by two electrodynamic shakers and a triaxial force plate. The excitation was initially mono-axial and the force was measured along the three coordinated axes. Both the symmetry of the apparent mass matrix and the effect of the vibration magnitude were assessed with paired t-student and Wilcoxon signed-rank tests. In the last part of the research, the response (forces along three mutually perpendicular directions) was measured with the uncorrelated excitation along two axes. The apparent mass derived in these conditions has been compared with the one obtained upon exciting a single axis.

The contributions of the nonlinear terms to the apparent mass were negligible and the nonlinearity was associated to the variation of the modal parameters in time (low frequency motion during the tests and involuntary muscular actions). The individual’s response (i.e. APMS matrix) was more dependent on the vibration magnitude. Magnitude dependent effects may be overlaid by the uncertainty introduced by a large scatter in the population’s biometric data. The conditioned APMS matrix (both population and individual) was comparable to that derived using linear estimators. The biodynamic response was influenced by the addition of a secondary transversal acceleration. In case of dual-axis excitations, the overall magnitude had a marginal contribution since dual-axis APMSs did not differ.