We propose an extension of the uniform approximation approach towards the simultaneous truncation of the associated dual energy. In the talk, we derive a hierarchy of approximating theories and corresponding a-priori error estimates for their solutions, for the special case of an isotropic, linear elastic, one-dimensional structural member with rectangular cross-section.
The uniform approximation approachClassical theories for thin structures like the Euler-Bernoulli beam and the Kirchhoff plate theory were developed by the use of disputable a-priori assumptions already in the 18 th and 19 th century. Rigorous mathematical justifications for these theories are available which proof them to appear as a limit of the three-dimensional theory of elasticity, if the thickness of the structure tends to zero. Recent results mostly use the method of Γ-convergence, which does not deliver quantitative error estimates directly. The development of refined theories, i.e., theories for moderately thick structures, or higher precision, started already in the middle of the 20 th century. Nowadays refined theories are still an open topic which is under intensive development. Mathematical justifications for refined theories are missing -even for the most established theories like the Reissner-Mindlin plate theory.The uniform approximation approach for the a-priori assumption-free derivation of theories for thin structures from the three-dimensional theory of linear elasticity goes back to pioneer treaties of Naghdi [1] and Koiter [2]. Kienzler [3] interprets the approach as a truncation of the elastic energy after a maximum power of a characteristic parameter that describes the relative thickness of the structure.We extend his ideas by truncating the associated dual energy as well, in order to provide a mathematical justification for refined theories by the proof of quantitative error estimates. By extending a standard result from duality theory (based on Korn's inequality) towards general anisotropy, the square error of an arbitrary displacement field v from the (weak) solution of three-dimensional elasticity can be estimated via the difference E pot (v) − E dual (µ) of the elastic potential and the dual energy of an admissible stress field µ. By using a dimensionless formulation and (abstract) Fourier-series expansions in thickness direction for the displacement field, the potential and dual energy appear as infinite power series in parameters describing the relative thickness of the structure. When truncating both series after a certain power, a finite set of field equations and stressboundary conditions can be derived from the Euler-Lagrange equations of the potential energy, whereas the dual energy's Euler-Lagrange equations deliver matching displacement-boundary conditions. This generates a hierarchy of theories with regard to the truncation power and by virtue of the mentioned result from duality theory the corresponding quantitative apriori error estimates.If smooth solutions are available, one is enabled to reduce the n...