We present a numerical model for the formation of fractures in rocks at grain scale. The model is based on a discrete approach using a spring network in combination with the "Elle" platform for the simulation of microstructures. In the model grains are defined by clusters of particles that are themselves connected by linear elastic springs that can break. In order to test the model we performed simulations of extension and shortening of a single layer, pure shear deformation of an aggregate with a statistical distribution of grains and studied fracture networks around expanding grains. We investigated the dynamics of fracture propagation, the geometry of fracture patterns and stress distributions as well as the rheological behavior of different materials. The model produces patterns found in natural systems and shows dynamics and behavior of fractures that is in agreement with theoretical models. This type of numerical model offers therefore a useful tool to study specific problems concerning fracture development in polycrystalline rocks.