Experimental and numerical characterization of fracture behavior for quasi-brittle materials

A hybrid experimental-numerical analysis was extensively utilized in this study to characterize the static and dynamic mixed mode I and II fracture behavior of concrete. The development of a fracture process zone associated with stable/rapid crack growth in single edge-notched (SEN), three-point bend concrete specimens was monitored with static/dynamic moire interferometry. The orthogonal displacement fields were recorded simultaneously by a technique developed in this study. The crack extension history together with the loading history were then used to drive a static/dynamic finite element model of the fracture specimen in its generation/propagation mode and to determine the crack closure stress due to aggregate bridging, the energy release and dissipation rates and the resistance curves in terms of the stress intensity factors. The crack closing stress (CCS) versus crack opening displacement (COD) relation was obtained by matching the computed and measured crack opening and sliding displacements in addition to the far field parameters, such as load versus load-line displacement. Additional verifications were made through the match between the computed and measured strain gage data and the predicted and measured crack kinking angles at onset of fracture and during the crack extension.; This study showed that the crack closing stress (CCS) was inversely proportional to the crack opening displacement (COD) and that the crack shearing stress within the fracture process zone was negligible. A comparison with mode I static and dynamic fracture showed that the presence of mode II fracture increased the crack closing stress presumably through increased aggregate interlocking of the crack surfaces. The fracture energy dissipated at the FPZ increased with crack extension and reached a constant dissipation rate after the fracture process zone was fully developed. It also showed that the fracture energy dissipation inside the fracture process zone was the major energy sink of concrete fracture. Under mixed mode I and II loading, the crack extended in the dominant mode I and kinked according to the Ramulu-Kobayashi criterion. As expected, the presence of the fracture process zone increased loading carrying capacity of concrete.