Size effect in the mixed mode crack propagation: softening and snap-back analysis

Strain softening and localization in concrete are described by means of a cohesive crack model. Whereas for mode I, only untying of the finite element nodes is applied to simulate crack growth, for mixed mode interelement crack propagation topological variation is required at each step. In the case of four point shear testing, the load vs deflection diagrams reveal snap-back instability for large sizes. By increasing the specimen sizes, such instability tends to reproduce the classical LEFM instability, predicted by the Maximum Circumferential Stress Criterion. The fracture toughness parameter of concrete appears to be unique and represented by the mode I fracture energy or the stress-intensity factor , even for mixed mode problems.