Effect of porosity on the elastic properties of porcelainized stoneware tiles by a multi-layered model

Porcelainized stoneware represents a leading product in the world market of ceramic tiles, thanks to its relevant bending strength (with respectto other classes of tiles) and extremely low water absorption: these properties derive from its really low content of residual porosity. Nevertheless,an accurate investigation of the cross section of a porcelainized stoneware tile reveals a non-uniform distribution of the residual pores through thethickness, which results in a spatial gradient of properties. Porcelainized stoneware, therefore, may be looked at as a functionally graded material.In the present research, commercial porcelainized stonewares were analysed in order to define the effect of the residual porosity and its spatialdistribution on the mechanical properties of tiles. Polished cross sections of porcelainized stoneware tiles were investigated by optical and scanningelectron microscopy in order to define the content and distribution of residual pores as a function of distance from the working surface. For eachporcelainized stoneware, the local elastic properties of the ceramic matrix were measured by a depth-sensing Vickers micro-indentation technique,then the so-obtained microstructural images and elastic properties were used to model the stoneware tile mechanical properties. In particular, thecross section of each tile was described as a multi-layered system, each layer of which was considered as a composite material formed by a ceramicmatrix and residual pores. The elastic properties of each layer were predicted by applying analytical equations derived from the theory of compositematerials and, as a new approach, by performing microstructure-based finite element simulations. In order to validate the proposed multi-layeredmodel and identify the most reliable predictive technique, the numerical results were compared with experimental data obtained by a resonancebasedmethod.