Fundamental Issues of Smeared Crack Models

For numerical simulation of fracture in concrete and rock the “smeared crack approach” is receiving increasing attention. On one hand renewed attempts in terms of the fixed and rotating crack models resort to fracture mechanics in order to refine the traditional orthotropic crack formulation. Along this approach the original concept involving Mode I type cracking is being broadened to include mixed mode fracture interpretation of the shear retention factor, if the crack memory is fully retained. On the other hand, fracture energy-based plasticity models are advocated by the authors, as well as other investigators, which describe the degradation of strength due to tensile cracking and decohesion in shear in terms of isotropic and anisotropic strain-softening concepts.The paper addresses the settled differences between elasticity and plasticity-based concepts which are currently used to describe localized and distributed failure due to tensile cracking and decohesion. In particular, the issues concerning the shear retention factor and rotation of principal directions will be scrutinized in view of recent discussions related to fixed versus rotating crack models. In the context of fracture energy-based plasticity the prevalent issues are also related to the problems of isotropic versus anisotropic strain-softening and the proper choice of appropriate directional fracture process variables.The treatise concludes with the analysis of two elementary model problems involving fixed and rotating principal coordinates in order to illustrate the subtle constitutive arguments pertaining to progressive fracture under Mode I and mixed mode fracture.