  This paper deals with the estimation of the distance between the solution of
a static linear mechanic problem and its approximation by the finite element
method solved with a non-overlapping domain decomposition method (FETI or BDD).
We propose a new strict upper bound of the error which separates the
contribution of the iterative solver and the contribution of the
discretization. Numerical assessments show that the bound is sharp and enables
us to define an objective stopping criterion for the iterative solver
