Fiber reinforced polymer composite (FRPC) materials are extensively used in lightweight applications due to their high specific strength and other favorable properties including enhanced endurance and corrosion resistance. However, these materials are inevitably exposed to moisture, which is known to drastically reduce their mechanical properties caused by moisture absorption and often accompanied with plasticization, weight gain, hygrothermal swelling, and de-bonding between fiber and matrix. Hence, it is important to understand the mechanics of moisture diffusion into FRPCs. The presence of fibers, especially impermeable like Carbon fibers, introduce tortuous moisture diffusion pathways through polymer matrix. In this paper, we elucidate the impact of fiber packing and morphology on moisture diffusion in FRPC materials. Computational models are developed within a finite element framework to evaluate moisture kinetics in impermeable FRPCs. We introduce a tortuosity factor for calculating the extent of deviation in moisture diffusion pathways due to impermeable fiber reinforcements. Two-dimensional micromechanical models are analyzed with varying fiber volume fractions, spatial distributions and morphology to elucidate the influence of internal microscale fiber architectures on tortuous diffusion pathways and corresponding diffusivities. Finally, a relationship between tortuosity and diffusivity is established such that diffusivity can be calculated using tortuosity for a given micro-architecture. Tortuosity can be easily calculated for a given architecture by solving steady state diffusion governing equations, whereas time-dependent transient diffusion equations need to be solved for calculating moisture diffusivity. Hence, tortuosity, instead of diffusivity, can be used in future composites designs, multi-scale analyses, and optimization for enabling robust structures in extreme moisture environments.

纤维增强聚合物复合材料 (FRPC) 材料因其高比强度和其他有利特性（包括增强的耐久性和耐腐蚀性）而广泛用于轻量化应用。然而，这些材料不可避免地暴露在水分中，众所周知，水分会显着降低由吸湿引起的机械性能，并且通常伴随着塑化、重量增加、湿热膨胀以及纤维与基体之间的脱粘。因此，了解水分扩散到 FRPC 中的机制非常重要。纤维的存在，尤其是像碳纤维这样不可渗透的纤维，会通过聚合物基质引入曲折的水分扩散路径。在本文中，我们阐明了纤维堆积和形态对 FRPC 材料中水分扩散的影响。计算模型是在有限元框架内开发的，以评估不可渗透 FRPC 中的水分动力学。我们引入了一个曲折系数来计算由于不可渗透的纤维增强材料而导致的水分扩散路径的偏差程度。分析具有不同纤维体积分数、空间分布和形态的二维微机械模型，以阐明内部微尺度纤维结构对曲折扩散路径和相应扩散率的影响。最后，建立了曲折度和扩散率之间的关系，以便可以使用给定微体系结构的曲折度来计算扩散率。通过求解稳态扩散控制方程，可以轻松计算给定架构的曲折度，而需要求解瞬态扩散方程来计算水分扩散率。因此，在未来的复合材料设计、多尺度分析和优化中，可以使用曲折度而不是扩散度来实现极端潮湿环境中的坚固结构。