Global description of crack propagation in ceramics

The subject of crack propagation in brittle materials is sometimes confusing, since many terms or concepts are not always clearly defined, as toughness, sub-critical crack propagation, threshold, R  -curve, etc. However, crack propagation can be simply and rigorously analysed by considering the equilibrium point in terms of energy release rate, as defined by the Griffith criteria, based on fundamentals of physics. The departure from this equilibrium leads to crack velocities related to dissipative mechanisms. Reinforcement mechanisms of any type can shift the equilibrium towards higher driving force values, giving an increase of crack resistance. The degradation of this reinforcement, as the case of cyclic fatigue, simply reduces the resistance to propagation. The total description of the behaviour is therefore included in the crack velocity—energy release rate (v−Gv−G) graph. However, the major point is the equilibrium, because in addition to its theoretical importance, it is of practical interest for brittle materials, since it corresponds to an energy value below which no propagation occurs.