J. Eberhardsteiner:
"Synthese aus konstitutivem Modellieren von Beton mittels dreiaxialer, nichtlinear-elastischer Werkstoffgesetze und Finite-Elemente-Analysen dickwandiger Stahlbetonkonstruktionen";
Supervisor, Reviewer: H.A. Mang, H. Reiffenstuhl; Institute for Strength of Materials, TU Wien, 1989.
This thesis contains a comparison and a critical review of nonlinear-elastic material models for the pre-peak behavoir of concrete under triaxial compressive loading, disregarding thermal and time effects. On the basis of results obtained from tests with nonmonotonic and nonproportional loading, four constitutive models are investigated, partially modified and evaluated. The selected nonlinear-elastic models are: the originally total material model by Kotsovos and Newman, the incremental orthotropic model by Elwi and Murray, the incremental constitutive law by Stankowski and Gerstle and the hypoelastic concrete model by Shafer and Ottosen. In order to achieve a realistic description of the deformational behavior of concrete under nonmomotonic loading conditions, an unloading criterion, suggested by Stankowski and Gerstle, was added to the two first-mentioned constitutive models. With exception of problems concerning the unloading behavior for the case of using the material model by Shaper and Ottosen, the agreement between experimental and analytical results is satisfactory.For the mathematical description of the failure envelope in the stress space, the five-parameter model by Willam and Warnke is used. If the failure is reached, concrete is treated as a softening material, irrespective of the mode of failure (tensile or compressive failure). A survey of experimental results and mechanical models concerning the post-failure behavior of concrete gives an overview of the state-of-the-art. A criterion for distinguishing between tensile and compressive failure is suggested.
In the first part of this thesis the free-of-restraint region of a concrete specimen, characterized by uniform stress distributions in all loading directions, is considered. Program modules concerning the investigated nonlinear-elastic material models are implemented in the FE program system MARC for the purpose of employing these models for ultimate load analyses of thick-walled structural members made of reinforced-concrete. Two structural members are investigated. Firstly, a reinforced concrete corbel, characterized by local tensile failre is studied. Secondly, a radially loaded reinforced concrete ring, primarily subjected to compression, is considered. A comparison of analytical and experimental results indicates that different constitutive models for the pre-peak behavior of concrete do not cause significantly different analytical results. However, consideration of the softening behavior of concrete is very important. The improvement of the mechanical models for softening of concrete, which have been used in this work, should be a topic for further research activities.