  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 vital to understand
moisture diffusion mechanics 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 measuring 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 micromechanical 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
moisture environments.
