Orthotropic fluid diffusion in composite marine structures. Experimental procedure, analytical and numerical modelling of plates, rods and pipes

Finite elements simulations of fluid diffusion for composite structures are becoming more common in marine and offshore applications. Input parameters for such simulations are the orthotropic diffusion constants for the materials. Diffusivity in fiber direction is faster than diffusivity in transverse direction; it is therefore important to determine all these constants to predict long-term fluid diffusion in composite structures. Current standards for the determination of the diffusion constants prescribe a sample geometry that enforces 1-D diffusion, but thin samples are not easy to fabricate, especially from thick laminates. Edges with metallic coatings are also prescribed, which can influence the calculation of the moisture saturation content and may detach during the experiments. Furthermore, axial symmetrical structures are often used offshore, requiring 3-D diffusion analysis in cylindrical coordinates. The aim of this paper is to provide a 3-D theory and methods for obtaining diffusion constants for plates, rods and pipes.