Less is better: Coated spherical vessels over-perform their entirely graded counterparts

The thermomechanical stress analysis of thick-walled spherical vessels is considered. The thickness of the vessel is assumed to be constituted of an internal functionally graded (FG) coating and an external homogeneous layer. In the graded internal layer the property variations along the thickness are linked to the constituents volume fractions by means of widely used rules of mixture. The stress behavior of these two-layered vessels is numerically assessed within the linear theory of elasticity by employing the finite difference method. A stress analysis is carried out varying the thickness of the coating and the heterogeneity index to optimize an indicator of the equivalent stress strictly linked to the load carrying capacity. In particular, it is surprisingly found that two-layered FG vessels structurally perform better than vessels that are FG in the whole thickness, leading to improved performance and nontrivial implications on the design of the coating.