Models for coated elastic bodies

Several technologies involve the coating of a bulk material with a thin layer made up of another material, so as to achieve enhanced mechanical properties for the composite system. The use of coated solids embraces a broad field of applications, so that a strong research effort has been devoted to these systems. From a mechanical point of view, a coating layer diffuses the load on an attached solid in a non-local way, thus introducing a characteristic length, and profoundly affects the mechanical response and failure mechanisms of the coated object. Therefore, the development of mechanical models to describe the behaviour of coated materials plays an important role in engineering design. In the framework of linear elasticity, the case of an elastic thin layer, perfectly bonded to an elastic disk, is analyzed in the present thesis by providing a mathematical tool with which to determine the mechanical response of the coating/bulk complex, which may find application in micro and nano technologies, for instance in the characterization of nanowires via nanoindentation. The coating is modelled by means of an Euler-Bernoulli curved rod, assumed to be perfectly bonded on the boundary of a circular elastic disk. The elastic rod acts as a coating for the disk and its axial inextensibility imposes an isoperimetric constraint on the internal disk, which is constrained to maintain its perimeter constant during the deformation process. The mechanical model for the coating/disk system is formulated for general loading, using the complex potential formalism. The elastic rod becomes equivalent to a Benveniste-Miloh interface characterized by the bending stiffness of the rod; in this way the problem can be solved entirely on the disk through the complex potential formalism and Kolosov- Muskhelishvili potentials. The kinematics and statics of the rod, together with its axial inextensibility, lead to the formulation of a 5th-order differential equation governing the mechanical state at every point on the boundary of the disk. The solution of this equation is obtained by means of a complex Fourier series expansion for the unknown fields on the boundary of the disk, when a particular distribution of the external load is prescribed. The complex variables method shows that the unknown complex coefficients involved in the series expansion depend only on the external load. Hence, all the elastic fields become known on the coating and on the boundary and within the disk. The analytical results are complemented with experiments related to a load distribution which models two equal and opposite concentrated forces. In this regard, two coated disks were designed and then manufactured (with a CNC engraving machine) from a single block of polymethyl methacrylate so that the bonding between the coating and disk was perfect and residual stresses were absent. The samples were tested in a circular polariscope and the results strongly supported the coated disk model, so the photoelastic fringes were very well captured by the elastic solution. Different situations were investigated in order to study the non-local stress diffusion of the coating. The limit case of an isoperimetric disk was also investigated by imposing a vanishing bending stiffness for the coating. This limit situation corresponded to a disk equipped with a device able to preserve the perimeter of the disk during the deformation. Exploiting the framework developed, the bifurcation problem of the coated disk was analyzed, assuming that the coating was subject to a radial pressure of three different types. A closed-form analytical solution was obtained for the bifurcation pressure and modes, showing that the presence of the disk profoundly changed the bifurcation landscape of the coating, forming a circular elastic rod. In fact, the circular rod admits only oval modes, while the coating/disk system displays high-frequency circumferential undulations. The experimental, analytical, and numerical results presented open new possibilities for the design of coated solids of cylindrical geometry, which may find applications in micro and nano technologies, for instance in the characterization of nanowires via nanoindentation.