Mixed mode cohesive crack propagation

ABSTRACT A cohesive crack model is proposed to describe strain localization for the materials where strain-hardening is not prevailing over strain-softening (geomaterials, concrete-like materials, ceramics, etc.). Such a model is able to predict the size effects of fracture mechanics, i.e., the transition from ductile to brittle structure behaviour by increasing the size scale and keeping the geometrical shape unchanged. Whereas for Mode I, only untieing of the finite element nodes is applied to simulate crack growth, for Mixed Mode interelement crack propagation a 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. Experimentally, the fracture toughness parameter of concrete appears to be unique and represented by the Mode I fracture energy GF or the stress-intensity factor KIC, even for Mixed Mode problems.