Abstract
Acerbi, Buttazzo and Percivale gave a variational definition of the nonlinear string energy under the constraint “det∇u>0” (see [E. Acerbi, G. Buttazzo, D. Percivale, A variational definition of the strain energy for an elastic string, J. Elasticity 25 (1991) 137–148]). In the same spirit, we obtain the nonlinear membrane energy under the simpler constraint “det∇u≠0”.