Enhanced assumed strain elements and locking in membrane problems

The assumptions which guarantee convergence of EAS elements first introduce d by Simo and Rifai, can be reduced to two postulates. Specifically, the st ability condition refers to a relaxed stability of an associated Hu-Washizu principle. We investigate the problem of volume locking of membrane proble ms. Estimates of a certain type which for instance imply Volume locking of the Q1-element, are fortunately not present with EAS elements. Finally we v erify an equivalence of the EAS concept with a Hellinger-Reissner method wh ich was observed in numerical computations by Andelfinger and Ramm. (C) 199 8 Elsevier Science S.A. All rights reserved.