Issue 19

M. Paggi, Frattura ed Integrità Strutturale, 19 (2012) 29-36; DOI: 10.3221/IGF-ESIS.19.03 31 In case of a standard PCD layer, where the size of the polycrystals is much smaller than the size of the layer, the material can be considered as homogeneous from the modelling point of view. As a consequence, crack propagation takes place under prevailing Mode I conditions and the crack path is curvilinear, as shown in Fig. 2. Figure 2 : Sketch of a PCD bit used in cutting tools [10]. The critical impact load is denoted by Pc and different possible failure modes ranging from micro- to macro-chipping are sketched. If heterogeneous cellular materials are used instead of a homogeneous layer, then a different crack path can be obtained. More specifically, considering the mechanical stress field due to a horizontal force acting at the tool tip, a magnification of the crack path for macro-chipping is shown in Fig. 3. Figure 3 : Fracture of a cutter with a cellular microstructure: scheme of the compact bit (left) and magnification of the crack path in the region inside the rectangular dashed box (right). This result is obtained by performing a finite element analysis of the bimaterial structure using linear elastic fracture mechanics as in [10]. The simulations are carried out exploiting the interface fracture mechanics features of FRANC2D [13]. More specifically, the fracture parameters of the bimaterial interfaces are assumed equal to the average value of those of the neighboring materials. The rod diameter is equal to 200  m and the thickness of the binder between the cells is 50  m (see [9] for more details about the geometry of the material microstructure). The critical load for crack propagation, P c , which corresponds to the condition of I IC K K  at the crack tip, is shown in Fig. 4 vs. the crack length a . The load P * represents the average load typically experienced during experimental tests and a max is the final crack length, when the crack meets the hardmetal substrate. At the beginning of the simulation, the crack propagates into a PCD road, and therefore there is no difference with respect to a standard homogeneous material, at