Multi-layered materials are everywhere, from fiber-reinforced polymer composites (FRPCs) to plywood sheets to layered rocks. When in service, these materials are often exposed to long-term environmental factors, like moisture, temperature, salinity, etc. Moisture, in particular, is known to cause significant degradation of materials like polymers, often resulting in loss of material durability. Hence, it is critical to determine the total diffusion coefficient of multi-layered materials given the coefficients of individual layers. However, the relationship between a multi-layered material's total diffusion coefficient and the individual layers' diffusion coefficients is not well established. Existing parallel and series models to determine the total diffusion coefficient do not account for the order of layer stacking. In this paper, we introduce three parameters influencing the diffusion behavior of multi-layered materials: the ratio of diffusion coefficients of individual layers, the volume fraction of individual layers, and the stacking order of individual layers. Computational models are developed within a finite element method framework to conduct parametric analysis considering the proposed parameters. We propose a new model to calculate the total diffusion coefficient of multi-layered materials more accurately than current models. We verify this parametric study by performing moisture immersion experiments on multi-layered materials. Finally, we propose a methodology for designing and optimizing the cross-section of multi-layered materials considering long-term moisture resistance. This study gives new insights into the diffusion behavior of multi-layered materials, focusing on polymer composites.
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