Issue 19

M. Paggi, Frattura ed Integrità Strutturale, 19 (2012) 29-36; DOI: 10.3221/IGF-ESIS.19.03 32 least in 2D simulations. Interestingly, when the crack tip meets the bi-material interface, delamination of the rod cell takes place (path A-B). Since the interface fracture energy is higher than that of the PCD, the external applied load required for crack propagation has to be significantly increased with respect to the homogeneous case. Subsequently, crack deviates again into the rod (path B-C). A second peak is finally observed when the crack propagates through the binder between the cells (path C-D). Figure 4 : Dimensionless critical load for brittle crack propagation vs. dimensionless crack length. The response of a cellular microstructure is compared with that of a homogeneous layer. These results are important for two reasons. First, a crack would arrest its propagation at the first interface if the dimensionless applied load is lower than 2.0. This situation is substantially different from the case of a homogeneous layer, where the critical dimensionless load is a monotonic decreasing function of the crack length. Therefore, when the dimensionless applied load exceeds 1.5, then the crack cannot be arrested. Therefore, the use of a cellular microstructure acts as a crack-arrester , controlling the evolution of chipping failure modes. On the other hand, interfaces tougher than the rods is not always a desirable situation. In case of micro-chipping, weak interfaces may promote crack propagation along the rod boundaries. This would be suitable to activate a self-resharpening process of the tool tip, which progressively loses its cutting efficiency due to wear. Therefore, the optimal material microstructure would correspond to cellular rods embedded into a tougher matrix, with interface properties depending on the vertical coordinate on the cutting edge. The effect of a hierarchical assembly of interfaces It is also possible to quantify the effect of structural hierarchy by simulating the mechanical behaviour of a cellular microstructure using the finite element method and nonlinear fracture mechanics. To this purpose, let us consider the material microstructure depicted in Fig. 5. Figure 5 : Cross-section of cellular rods with bright cells and dark grey cell boundaries (adapted from [9]).