Effect of Microstructure on the Fracture Toughness of Polycrystalline Diamond and Polycrystalline Cubic Boron Nitride Declan Carolan a , 1 , Declan McNamara 1, b , Patricia Alveen 1, c , Neal Murphy 1, d and Alojz Ivankovic 1, e 1 School of Mechanical and Materials Engineering, University College Dublin, Ireland a declan.carolan@ucd.ie, b declan.mc-namara@ucdconnect.ie, c patricia.alveen@ucdconnect.ie, d neal.murphy@ucd.ie, e alojz.ivankovic@ucd.ie Keywords: Brittle fracture, scale bridging, finite volume method, cohesive zone model Abstract. Voronoi tessellation is employed to generate two-dimensional microstructures of single phase and dual interpenetrating phase materials. A novel finite volume based arbitrary crack propagation solver implemented in OpenFOAM is described. This solver allows for specification of different cohesive zone models for each phase within the microstructure as well as unique cohesive zone formulations at the interfaces of any material pair. Initial results suggest that the developed model is capable, at least qualitatively, of capturing the features of both inter-granular and trans- granular fracture. Introduction Over the past quarter century, both Polycrystalline Diamond (PCD) and Polycrystalline Cubic Boron Nitride (PCBN) have become the tool material of choice for both the oil and gas industry and high speed machining of aerospace alloys. PCD is used primarily in the oil and gas industry whereas PCBN is used for high speed machining of ferrous alloys. An ideal cutting tool material should be both hard and tough. The mechanical performance of PCD and PCBN tools is significantly affected by their microstructure [1, 2]. The microstructure is often complex, consisting of grains of different sizes and morphologies as well as significant quantities of other phases. For this reason it is important to be able to accurately model a material microstructure. Early work by Ghosh and Yunshan [3] and Ghosh et al [4] developed a Voronoi cell finite element model (VCFEM). One should also refer to significant work by Espinosa and Zavattieri [5, 6] and Zhou and Zhai [7] who analysed dynamic fragmentation of ceramic composites by inserting cohesive elements along the grain boundaries. More recently, work by Zhou et al. [8] and Zhang et al. [9] have used Voronoi tessellation and embedded grain boundary cohesive elements to study inter-granular crack growth on a micro- structural level. Of particular note is work by Nittur et al. [10] who incorporated a procedure to handle the complex contact of fragmenting grains and fracture surfaces. Microstructure Generation Consider a pair of seed points, a and b, in ℝ 2 as shown in Figure 1. The perpendicular bisector of ab divides the plane into two halves. All points in the plane on one side of the perpendicular bisector are closer to a than to b, while all points on the opposite side of the perpendicular bisector are closer to b than to a. Next consider a third seed point, c. The perpendicular bisectors of ac and bc can be drawn as before. There are now three regions surrounding each seed point. Each region coincides at