Numerical recreation of gap test

A gap test is a new experimental and numerical test proposed by Bažant et al. Its main goal is to show that the effective mode I (opening mode) fracture energy depends on the crack- parallel normal stress. Moreover, the authors of the test believe that the FE crack band model coupled with microplane model M7 and the lattice discrete particle model allow to fit results of the laboratory gap test satisfactorily. A specimen is in form of a concrete beam with a notch. A static scheme of the specimen changes when a gap between specimen and roller supports vanishes. The authors of this paper tried to recreate numerically the gap test using the concrete damaged plasticity (CDP) model. A finite element analysis was performed using Abaqus software. Main results compared with Bažant et al. were: a force-displacement relationship and a crack pattern. The numerical test will answer the question if the CDP model allows to recreate the gap test properly. To simplify computations the specimen was divided into elastic and plastic regions. The plastic region was fine meshed and a coarse mesh was assigned to the elastic regions. The gap modeled in the test was equal to 3 mm, so a pinned and a roller supports became active only when displacements of both ends of the specimen reached 3 mm. Displacement control was chosen in the FEM model in form of a displacement imposed on a top steel pad. Bottom pads were modeled as made of polypropylene. The force-displacement relationship was established using results obtained for the top steel pad and the crack pattern was presented with the equivalent plastic strain of concrete in tension.