The　microstructure　of　materials　has　an　important　effect　on　the　chip　formation　and　fracture　process　during　metal　cutting.　Its　heterogeneity　makes　the　dynamic　mechanical　response　of　materials　complicated,　which　would　become　more　notable　with　the　increase　of　cutting　speed.　In　this　paper,　a　spring-network　discrete　element　model　was　established　to　characterize　the　heterogeneous　material　for　cutting　simulation.　The　dynamic　stress　propagation　was　investigated　during　the　cutting　tool　impacting　on　the　workpiece　at　high　speeds.　Voronoi　tessellation　method　was　utilized　to　create　polycrystalline　structures　in　the　model　and　the　stress　distribution　was　discussed.　Compared　with　the　results　by　the　homogeneous　model,　the　distribution　of　dynamic　stress　in　polycrystalline　model　is　obviously　discontinuous.　Furthermore,　the　normal　stress　and　shear　stress　propagates　in　the　workpiece　as　longitudinal　and　transverse　stress　waves,　which　are　reflected　and　transmitted　at　interfaces.　The　fluctuation　of　stress　triaxiality　is　more　obvious　and　changes　significantly　with　the　increase　of　cutting　speed,　which　provides　an　idea　for　explaining　the　fracture　mode　transition　of　material　in　high　speed　cutting　process.　©　2018　The　Authors.　Published　by　Elsevier　Ltd.