We derive upper and lower bounds for the torsional rigidity of host shafts containing a number of cylindrical fibers. The transverse cross-sections of the host shaft and the fibers are simply connected, but could be arbitrary in shape. Utilizing the fact that the torsion solution of a homogeneous host shaft with simply connected cross-section can be known, we propose a method to construct statically and kinematically admissible fields interior to the matrix and to the fibers. Previous developments on bounding the torsional rigidity of composite shaft so far are confined to circular fibers. Here we try to simulate fibers with non-circular cross-section and incorporate the interactions of the cross-sectional shapes of the host shaft and the fibers at the same time. Proceeding from extremal principles of elasticity, together with propositions of some domain integration procedures, we provide a universal expression for bounds on the torsional rigidity of the composite shaft. The exact expressions depend on the constituent information of the fibers and the host shaft, which could offer useful information to tailor the shape and the arrangement of the constituents to achieve an optimal value.
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