Experimental Testing of Bend-Twist Coupled Composite Shafts

Abstract

Bend-twist coupling is a property of certain specially designed composite beams that when subjected to loading conditions that would normally result in pure bending, undergo both bending and twisting. This phenomenon is called bend-twist coupling. To fully characterize the degree of bend-twist coupling in the beam, shear center per unit length (e y /L) must be determined. This paper proposes an experimental method for quantifying e y /L. The specific method for manufacturing these composite shafts is detailed. A special apparatus was constructed to load the tip of a cantilever sample with an adjustable torque. Digital image correlation (DIC) was used to measure tip rotation and deflection. From these measurements shear center values were determined. Repeating this process for different shaft lengths and orientations allows for a final e y /L measurement. Techniques are provided to reduce the various uncertainties in these measurements. The experiment utilizes three dimensional DIC, a rigid boundary condition such as a vice to simulate a cantilever condition, and an adjustable moment arm on which to load the specimen. The special case of a cylindrical carbon fiber beam is presented here, but the experimental methods can be generalized to tapered beams, non-circular cross-sections, and other orthotropic materials. The results are compared against finite element and analytical predictions. The final average experimental e y /L for all four shafts is 8 % higher than what the analytical method and FEA predict.