Plane-strain tensile loading numerical simulations of dynamic crack propagation in silicon nitride microstructures are conducted. The strength and toughness are evaluated as a function of strain rate and microstructural parameters, including grain size and density of needle-shaped grains. The silicon nitride microstructures are built using Voronoi tessellation for constructing regular grains and a merging procedure to generate elongated grains. Dynamic insertion of cohesive elements representing transgranular and intergranular cracking is a key feature of the modeling. The results show that inertia and elongated grains both contribute to the rate hardening of the specimen. The simulations reveal the existence of a threshold opening rate for intergranular cracks to transform into transgranular ones. Moreover, a higher percentage of transgranular fracture causes higher toughness. (c) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.