Identification and Interpretation of Microplane Material Laws

The present paper addresses the so-called microplane formulation which became recently more and more popular for the description of quasi-brittle materials. The essential feature of this material formulation is a split of the local microplane strains and stresses allowing one to resort to simplified or in certain cases even unidirectional constitutive laws. The main attraction of the microplane concept is that an initial or evolving anisotropic material behavior can be described in a natural and simple way. Motivated from a macroscopic viewpoint, it is advocated to restrict the microplane concept to the pure volumetric-deviatoric split, as a constraint subset of the most often applied volumetric-deviatoric-tangential split. This variant has the particular advantage that typical macroscopic responses are directly reflected on the mesoscale. It will be shown that in certain cases the present version of a microplane formulation is closely related to well-known macroscopic models although being much more general than those macroscopic formulations. This close relation is exploited to derive physically sound microplane constitutive laws. Therefore the characteristic damage mechanisms of materials at two levels of observation, (1) at the macroscale in the sense of classical continuum damage mechanics, and (2) at the mesoscale utilizing the so-called microplane concept, are examined. The comparison of the microplane formulation to a well-known macroscopic one-parameter damage model enables the identification and interpretation of the microplane constitutive laws. The constitutive formulations are embedded in a thermodynamically consistent framework. Finally, the performance of the attained microplane formulation is analyzed in a mixed-mode fracture simulation of concrete.