Alexios Papacharalampopoulos

A numerical study on the propagation of Rayleigh and guided waves in cortical bone according to Mindlin’s Form II gradient elastic theory

Cortical bone is a multiscale heterogeneous natural material characterized by microstructural effects. Thus guided waves propagating in cortical bone undergo dispersion due to both material microstructure and bone geometry. However, above 0.8 MHz, ultrasound propagates rather as a dispersive surface Rayleigh wave than a dispersive guided wave because at those frequencies, the corresponding wavelengths are smaller than the thickness of cortical bone. Classical elasticity, although it has been largely used for wave propagation modeling in bones, is not able to support dispersion in bulk and Rayleigh waves. This is possible with the use of Mindlins Form-II gradient elastic theory, which introduces in its equation of motion intrinsic parameters that correlate microstructure with the macrostructure. In this work, the boundary element method in conjunction with the reassigned smoothed pseudo Wigner-Ville transform are employed for the numerical determination of time-frequency diagrams corresponding to the dispersion curves of Rayleigh and guided waves propagating in a cortical bone. A composite material model for the determination of the internal length scale parameters imposed by Mindlins elastic theory is exploited. The obtained results demonstrate the dispersive nature of Rayleigh wave propagating along the complex structure of bone as well as how microstructure affects guided waves.

Dematerialisation of products and manufacturing-generated knowledge content: relationship through paradigms

Dematerialisation refers to the decline, over time, of material usage in industrial products. The subject of this paper is to identify the relationship between the material and knowledge content of several products. The issue is whether the knowledge content, through time, will be affecting the products, in terms of dematerialisation. Specifically, a study has been conducted on widely used products in order to determine the way they have changed through their life-timeline. Two different indicators have been introduced for the quantification of the trend, in terms of material and knowledge content.

Wave propagation of Rayleigh waves in bones: A gradient viscoelastic approach

Bone is a inhomogeneous hierarchical material with different microstructure at different length scales. On the other hand, the constituent materials (mineral, proteins, water and polysaccharides) and the fluid in bone pores give rise of viscoelastic behavior. In the present work, the effect of bones mi-crostructure and its viscoelastic properties on the propagation of Rayleigh waves is investigated. Microstructural effects are taken into account through Mindlins general gradient elastic theory. A composite material model proposed by Ben-Amoz is employed for the determination of the intrinsic parameters imposed by Mindlins theory. Viscoelasticity is introduced via convolution integrals and proper relaxation functions valid for bones. All the gradient viscoelastic problems are solved via the Boundary Element Method and exploiting the correspondence principle. The results reveal that both microstructure and viscoelastic properties affect significantly the propagation and dispersion of Rayleigh waves in long bones.