Admissible matrix formulation is a patch test approach for efficient construction of multi-field finite element models. In hybrid stress and strain elements, the formulation employs the patch test to identify the constraints on, respectively, the flexibility and stiffness matrices which are most detrimental to the element efficiency. Admissible changes are introduced to the matrices so as to reduce the computational cost while the element accuracy remains virtually intact. In this paper, a comprehensive review of admissible matrix formulation is presented. Finite element techniques seminal to the formulation are also discussed.
Sze, K. Y. (1996). Admissible matrix formulation - From orthogonal approach to explicit hybrid stabilization. Finite Elements in Analysis and Design, 24(1), 1–30. https://doi.org/10.1016/0168-874X(95)00026-P