Two new locking-free nonconforming finite elements for the pure displacement planar elasticity problem are presented. Convergence rates of the elements are uniformly optimal with respect to A. The energy norm and L2 norm errors are proved to be O（h2） and O（h3）, respectively. Numerical tests confirm the theoretical analysis.
We consider a singular perturbation problem which describes 2D Darcy-Stokes flow. An H（div）- conforming rectangular element, DS-R14, is proposed and analyzed first. This element has 14 degrees of freedom for velocity and is proved to be uniformly convergent with respect to perturbation constant. We then simplify this element to get another H（div）-conforming rectangular element, DS-R12, which has 12 degrees of freedom for velocity. The uniform convergence is also obtained for this element. Finally, we construct a de Rham complex corresponding to DS-R12 element.
In this paper,we construct a tetrahedral element named DST20 for the three dimensional Darcy-Stokes problem,which reduces the degrees of velocity in [30].The finite element space Vh for velocity is H(div)-conforming,i.e.,the normal component of a function in Vh is continuous across the element boundaries,meanwhile the tangential component of a function in Vh is average continuous across the element boundaries,hence Vh is H^1- average conforming.We prove that this element is uniformly convergent with respect to the perturbation constant s for the Darcy-Stokes problem.At the same time,we give a discrete de Rham complex corresponding to DST20 element.
In the use of finite element methods to the planar elasticity problems,one diffculty is to overcome locking when elasticity constant λ→∞.In the case of traction boundary condition,another diffculty is to make the discrete Korn＇s second inequality valid.In this paper,a triangular element is presented.We prove that this element is locking-free,the discrete Korn＇s second inequality holds and the convergence order is two.
