This paper presents a novel micromechanical unit cell model for continuous fiber reinforced composites, which features a fiber with an octagonal cross-section embedded in surrounding matrix, and was named as octagonal fiber model. The cross-section of octagonal fiber model was subdivided into five by five sub-regions, and the conditions of equilibrium and deformation compatibility were applied to derive expression of effective ply properties, and stress amplification factors, which correlate microstresses in sub-regions with ply stresses. For E-glass/epoxy and carbon/epoxy material systems with different fiber volume fractions, effective ply properties and stress amplification factors in sub-regions were evaluated using derived formulae. Results from octagonal fiber model were then compared with those from multiple analytical methods and finite element unit cell model. It was shown that effective ply properties predicted by octagonal fiber model were generally in good agreement with those from finite element model, and octagonal fiber model outperformed other analytical counterparts in estimating stress amplification factors, demonstrating the potential of octagonal fiber model.
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