Physically accurate deformable models based on the finite element method (FEM) are being used for a wide range of applications, from entertainment to medicine. This article describes how we applied this method in the CAD/CAM area that is concerned with reconstructing 3D models of teeth. We simulated the process of mastication by employing a deformable model that represented the substrate, and a rigid model that represented the teeth. We extended a recent approach for substrate deformation by also modelling the fracture of the substrate by the mastication process. Although including fracturing into the process allowed us to assess a mastication result, it posed new technical challenges such as defining the start of fracturing, propagating fracture through the substrate, detecting collisions between substrate pieces after fracturing, and resolving such collisions. We developed an approach that solved these challenges. The resulting simulation allowed us to compare the functionality of different occlusal designs in a mastication process. We are convinced that these simulations are an interesting tool that could be used to improve occlusal performance, especially in complete dentures, which are nowadays being more and more digitally designed. Keywords: occlusion, masticatory performance, CAD/CAM, complete dentures, chewing simulation, substrate