Prediction of crack initiation at blunt notches and cavities – size effects

Crack initiation at corners, V-notches and other situations such as interfaces breaking a free surface (delamination initiation) cannot be correctly predicted by the usual brittle fracture criteria (either Griffith or maximum stress). They give contradictory results and neither one nor the other agrees with the experiments. An additional characteristic length is required to define a satisfying criterion giving rise to the so-called “Finite fracture mechanics”. The crack is supposed to jump this length which depends both on the material properties and the local geometry of the structure; it is not a material parameter. In most cases this crack increment is small. The size effect arises with the interaction between the crack increment and another length characterising a microstructure such as a pore diameter, a notch root radius or an interface layer thickness. The remote load at failure depends on the actual value of this microstructure parameter whereas it was not expected in all cases. Assuming that the two interacting lengths remain small compared to the size of the global structure, an asymptotic procedure allows bringing into evidence the change in the apparent resistance of the structure due to this phenomenon. Results are compared with experiments in various domains: polymers, ceramics and rocks.