Variational Formulation of Curved Beams in Global Coordinates
In this paper we derive a variational formulation for a linear curved beam which is natively expressed in global Cartesian coordinates. During derivation the beam midline is assumed to be implicitly described by a vector distance function which eliminates the need for local coordinates. The only geometrical information appearing in the final expressions for the governing equations is the tangential direction, and thus there is no need to introduce normal directions along the curve. As a consequence zero or discontinuous curvature, for example at inflection points, pose no difficulty in this formulation. Kinematic assumptions encompassing both Timoshenko and Euler--Bernoulli beam theories are considered. With the exception of truly three dimensional formulations, models for curved beams found in literature are typically derived in the Frenet frame defined by the geometry of the beam midline. While it is intuitive to formulate curved beam models in these local coordinates, the Frenet frame suffers from ambiguity and sudden changes of orientation in straight sections of the beam. Based on the variational formulation we implement finite element models using global Cartesian degrees of freedom and discuss curvature coupling effects and locking. Numerical comparisons with classical solutions for both straight and curved cantilever beams under a tip load are given, as well as numerical examples illustrating curvature coupling effects.