Fiber-reinforced polymers (FRPs) are sensitive to moisture diffusion. Deterioration caused by moisture can limit their service lives considerably. In this work, a three-dimensional finite element modeling and analysis framework is presented to investigate the moisture diffusion kinetics inside fiber-reinforced inside polymer matrix composites by considering different angle and cross-ply orientations. A small localized representative volume element considering a few fibers in the neighborhood of three-layer stacks has been analyzed. The emphasis is on the effect of different fiber orientations over moisture saturation time and diffusion-induced stresses. Stresses induced due to moisture diffusion in FRPs are evaluated on the free fiber ends. The numerical results from finite element approximations are compared with theories of composite micromechanics such as rule of mixtures, Halpin–Tsai model and concentric cylinder assemblage framework. It is observed that the orientation of fiber layers can greatly influence the moisture ingress inside the matrix and resulting stresses. At intermediate time durations of moisture progression, the cross-ply orientation had ~ 25% lower weight gain in comparison with the unidirectional ply orientations. The overall von Mises stresses at the fiber matrix interface were also lower in cross-ply orientations by ~ 40% in comparison with the other orientations with similar fiber volume fraction. The three-layered cross-ply, 90/90/90 orientation took almost 50% more time to fully saturate with moisture in comparison with the unidirectional, 0/0/0 orientation. The interpretations from the smaller local microstructural models presented in this work can be extended to study and design the structure scale composite layups for the improved moisture durability.

纤维增强聚合物（FRP）对水分扩散敏感。水分引起的劣化可能会大大限制其使用寿命。在这项工作中，提出了三维有限元建模和分析框架，以通过考虑不同的角度和交叉层方向研究纤维增强的内部聚合物基复合材料内部的水分扩散动力学。分析了考虑到三层堆叠附近的一些纤维的小局部代表体积元素。重点是不同的纤维取向对水分饱和时间和扩散引起的应力的影响。在自由纤维末端评估因FRP中水分扩散而引起的应力。将有限元近似的数值结果与复合微力学的理论进行了比较，例如混合规则，Halpin-Tsai模型和同心圆柱体组装框架。可以看出，纤维层的取向会极大地影响基质内部的水分进入并产生应力。在水分持续时间的中间时间段，与单向层定向相比，交叉层定向的增重低约25％。与具有相似纤维体积分数的其他取向相比，纤维基质界面处的整体von Mises应力在交叉层取向上也降低了约40％。与单向相比，三层交叉层（90/90/90取向）要花更多的时间才能完全浸透水分，0/0/0方向。这项工作中提出的较小局部微观结构模型的解释可以扩展为研究和设计结构尺度的复合铺层，以提高耐湿性。