Fatigue fracture in concrete structure

The safety of cracked concrete dams is fundamentally affected by the mechanical behaviour of the material under seismic excitation. In this case the crack can grow at a lower load level compared to the monotonic case. This phenomenon is called subcritical crack propagation and depends on the mechanical behaviour of the fracture process zone (FPZ), where the material, albeit damaged, is still able to transfer stresses. In this zone, which appears at the tip of the macroscopic crack, many non- linear phenomena occur: microcrack interaction with aggregate, porosity and other microcracks. These phenomena are analysed with the Huang-Li micro-mechanical model (see Huang & Li (1989)). In this paper, the FPZ is modelled as a fictitious extension of the real crack (stress free crack), where tensile stresses are a decreasing function (softening) of the displacement discontinuity (see Hillerborg et al. (1976)). Through this approach, by assuming the fracture energy as a material property, it is possible to predict many size effects, which become evident in laboratory tests. The local behaviour under unloading and reloading conditions is modelled through Hordijk's continous function model (CFM, see Hordijk (1991)). At any point of the FPZ, the stress path vs. crack opening displacement (COD) is not fixed a-priori, but depends on the interaction between hysteretic cycle and crack growth (Barpi & Valente (2004)).The loading process is based on a first monotonic step stopped before reaching the peak load. Afterwards a series of cyclic loading phases is applied, at increasing load levels, until collapse occurs. In numerical simulations the collapse condition is achieved when the effective tangent stiffness matrix loses the positiveness condition. The results of numerical analyses appear in good agreement with the experimental data obtained by Slowik et al. (1996) in the case of wedge splitting test