Size and boundary effects during failure in quasi-brittle materials: experimental and numerical investigations

摘要

The degradation of quasi-brittle materials encompasses micro-crack propagation, interaction and coalescence in order to form a macro-crack. These phenomena are located progressively within the so-called Fracture Process Zone (FPZ). The shape and growth of the FPZ, and its interaction with boundaries lead to typical phenomena such as size effects, boundary effects and shielding effects. Classical failure constitutive models involve strain softening due to progressive cracking and a regularization technique for avoiding spurious strain and damage localization. Different approaches have been promoted in the literature such as integral-type non-local models, gradient damage formulations, cohesive cracks models or strong discontinuity approaches. Such macroscale failure models have been applied on a wide range of problems, including the description of damage and failure in strain softening quasi-brittle materials, softening plasticity, creep or composite degradation. An important element of validation of failure models is that they should be able to capture size and boundary effects for various geometries. However, numerical predictions of size effect on different geometries or the description of boundary effects are quite rare in the literature because experimental data on different specimen geometries and on the same material are not available for comparison. If experiments involving size effect are numerous in the literature, they are restricted to a specific geometry and barely consider structures made of the same material, with different geometries. Most of the time, the notch-to-depth ratio tends to zero without reaching zero and unnotched specimens are studied separately, with different materials compared with size effect tests on notched specimens. This paper aims at presenting new experimental and numerical investigations of failure for geometrically similar notched and unnotched concrete specimens made of the same mix. Different geometries (four depth and three notch sizes) have been considered to obtain results involving size and boundary effects at the same time. A mesomodel is used to study the FPZ evolution upon damage depending on the geometry and boundary conditions. A very good agreement with the experimental results is obtained. An analysis of the correlations involved during the fracture process at the mesoscale is performed and a good agreement with acoustic emissions data is revealed.