15. Experimental Investigation of Bend-Twist Coupled Cylindrical Shafts

A new and unique way of orienting carbon fiber lamina in cylindrical shafts results in bend-twist coupling. A beam is said to possess bend-twist coupling when a pure bending moment applied to the beam results in simultaneous bending and twisting. This effect is normally associated with asymmetric cross-sections (such as c-shaped sections) but can result from a symmetric cross-section if the beam is anisotropic. In this case we study the bend-twist coupling resulting from a cylindrical shaft fabricated using a unique, non-obvious anisotropic lay-up. This research covers the design and manufacturing of these carbon fiber shafts to maximize the bend-twist coupling while simultaneously achieving acceptable torsional and flexural rigidity. Once constructed, the composite shafts are tested using DIC to measure the degree of rotation and deflection given an applied bending and twisting moment. The results are used to determine the shear center, degree of bend-twist coupling, torsional rigidity, and flexural rigidity as well as to verify the Finite Element Models. These models are used to predict the success of potential designs and to provide greater understanding of the phenomenon. Closed-form solutions provide a third means of verification, analysis, and optimization.