Sawan Kumar Guruva

Background: The mechanical performance assessment of orthopedic fixation systems is computationally and experimentally challenging due to the complex geometrical and mechanical features of bones. Non-contact experimental techniques, widely adopted in several engineering fields, is shown to overcome these issues. Objective: This work discusses a comparative experimental investigation into specimens mimicking healthy humerus bones and fractured bones subject to an innovative surgery procedure and to a classical technique referred to as the gold standard surgery. The new surgery consists in the installation of an external fixation mechanism that constrains, according to different spatial patterns, a certain number of titanium slender bars inserted and clamped into the fractured bones. Methods: The mechanical properties of artificial bones are characterized through compressive tests, while the morphology of the fracture surface is analyzed using a scanning electron microscope. A three-dimensional laser vibrometer is used to measure the resonance frequencies, mode shapes, damping ratios, and mechanical waves propagating from the actuators across the surface of the bones. Results: The results provide insights into which configuration of the fixator performs better for a fast recovery. Based on the observed dynamic behaviors, the optimal configuration of the fixator offers performance that is comparable to, or potentially better than, the gold standard surgical procedure. Conclusions: The novelty and feasibility of the adopted experimental approach paves the way towards the adoption of advanced non-contact techniques for the mechanical characterization of complex, non-homegenous and anisotropic materials and structures in biomedical applications enabling also data-driven models of the systems.

This work discusses a comparative experimental investigation into specimens mimicking healthy humerus bones and fractured bones subject to an innovative surgery procedure and to a classical technique defined as the gold standard surgery. The new surgery technique consists in the installation of an external fixation mechanism (fixator) that constrains in defined positions a certain number of titanium slender bars inserted and clamped to the fractured bones according to different spatial patterns. The mechanical properties of the material constituting the artificial bones are obtained by means of compressive tests while a SEM imaging is employed to characterize the morphology of the fracture surface. A 3D laser vibrometer is employed to acquire the resonance frequencies and the associated operational deflection shapes of the specimens subject to piezoelectric actuation. The same experimental configurations are employed to acquire the mechanical waves propagating from the actuators across the surface of the bones. The analysis of the results provides insights into which configuration of the fixator performs better for a fast recovery using as references the healthy bones and the gold standard surgery. The experimental campaign paves also the way towards the adoption of advanced techniques employed for the mechanical characterization of materials and structures in biomedical applications with the additional aim of extracting data-driven models of the systems.

The unusual features of the nonlinear dynamic response of nanocomposite beams made of polybutylene terephthalate (PBT) and branched carbon nanotubes (bCNTs) are documented experimentally. The frequency response curves for cantilever specimens with different bCNTs weight fractions (wt%) are obtained under harmonic base excitations by measuring the free tip displacement via 3D scanning laser vibrometry. The steady-state response acquired for different excitation levels exhibits a surprising nonlinear softening trend which is switched into hardening for higher bCNTs wt% and increasing oscillation amplitudes. The stick-slip hysteresis, caused by the interaction of the bCNTs with the thermoplastic hosting matrix, determines a softening nonlinearity of the material that counteracts the well-known geometric hardening associated with the nonlinear curvature of the first mode of the cantilever. However, for bCNTs weight fractions greater than 1%, the bridging of the branched CNTs gives rise to the formation of a strong network which contributes to the hardening response at higher oscillation amplitudes. This mechanical behavior is detected by the trend of the nonlinear harmonic spectra and by the estimation of the equivalent damping ratio using the half power bandwidth method. A nonlinear mathematical model of the nanocomposite cantilever samples derived from a 3D mesoscale hysteretic model of the PBT/bCNT material is shown to predict the observed unusual experimental behavior. The presence of bCNTs in a thermoplastic matrix seems to be the main driver of the highly tunable nonlinear stiffness and damping capacity.