Radial fiber gradient design offers significant potential for optimizing high-performance composite shafts under geometric constraints. This study develops a 3D analytical model based on Lekhnitskii’s anisotropic elasticity theory and the Mori-Tanaka homogenization method to investigate the mechanical response of gradient fiber-reinforced composite shafts (GFRCSs) under combined bending and torsional loading. The results indicate that the functionally graded V-type (FGV) pattern maximizes the stiffness of GFRCSs compared to the uniform distribution, sustaining a persistent 14% bending stiffness gain even in thin-walled configurations. The strength advantage of the FGV design relative to the uniform benchmark exhibits significant sensitivity to the bending-to-torsion load ratio. Although the FGV pattern enhances localized material strength, the induced stress attraction effect may cause the rate of stress accumulation to outpace strength increments. This mechanism triggers failure boundaries prematurely under combined loading and leads to an inversion of strength benefits when the bending-to-torsion load ratio exceeds the critical threshold. The established critical load ratios and failure moment maps delineate the strength-advantaged regimes and load-bearing capacity of the gradient design, providing theoretical guidance and quantitative evidence for the preliminary design of high-performance gradient shafts.
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