ABSTRACT
Abstract Recent experimental observations indicate that softening of concrete and other brittle disordered materials is caused by a progressive process from microcracking to crack face bridging. The crack face bridges are intact ligaments connecting the two faces of a macrocrack, and appear at crack openings from 25-50 μπι. Precursory to the crack face bridging is isolated microcracking, which however is much more difficult to visualize in experiments. Numerical micromechanics can be used to further elucidate the various phenomena. Because the cracks have a characteristic dimension equal to the size of the specimen in the softening regime, boundary conditions play an important role in this stage of cracking. Therefore, boundary rotation effects were studied in uniaxial tension tests. Experiments both between fixed and rotating loading platens were carried out. The experiments were simulated with the lattice model as well. Next to this the test-geometry and special boundary conditions adopted in the experiments by Carpinteri & Ferro were analysed. The results indicate that crack face bridging, especially the type of bridging where flexural ligaments develop because two interacting crack tips avoid each other, may be further enhanced by the boundary conditions in the experiment. This point should be carefully considered in the future when tensile tests are carried out for the sole purpose of measuring the fracture energy of brittle disordered materials. Especially when tests are carried out between parallel travelling end platens, the fracture energy will be overestimated. Keywords: Micromechanics, Lattice Model, Crack face bridging, Softening, Concrete, Disordered materials, Experiments, Uniaxial tension, Boundary rotations, Fracture energy.
