A projection-based error analysis of HDG methods for Timoshenko beams

In this paper, we give the first a priori error analysis of the hybridizable discontinuous Galerkin (HDG) methods for Timoshenko beams. The analysis is based on the use of a projection especially designed to fit the structure of the numerical traces of the HDG method. This property allows to prove in a very concise manner that the projection of the errors is bounded in terms of the distance between the exact solution and its projection. The study of the influence of the stabilization function on the approximation is then reduced to the study of how they affect the approximation properties of the projection in a single element. Surprisingly, and unlike any other discontinuous Galerkin method, this can be done without assuming any positivity property of the stabilization function of the HDG method. We apply this approach to HDG methods using polynomials of degree k ≥ 0 in all the unknowns, and show that the projection of the error in each of them superconverges with order k + 2 when k ≥ 1 and converges with order 1 for k = 0. As a result, we show that the HDG methods converge with optimal order k + 1 for all the unknowns, and that they are free from shear locking. Finally, we show that all the numerical traces converge with order 2k + 1. Numerical experiments validating these results are shown.