Abstract: In this paper, we numerically simulated the freeexpansion and fragmentation processes of an OFHC ring under an initialvelocity. The conventional Johnson-Cook thermo-viscoplastic constitutivemodel was used to describe the dynamic plastic behavior of the material. TheJohnson-Cook failure model incorporating a cohesive fracture criterion wasused to model the separation progress of the material. ABAQUS/Explicit codewith element erosion was used for the numerical simulations. Multiplesimulations were conducted with a same initial velocity on these meshes,creating a group of fragment samples. The average fragment sizes of eachsample group were obtained. It was found that the Grady-Kipp model based onthe momentum diffusion mechanism gives reasonably close predictions of thefragment sizes; The FEM simulation results show apparent unloading waves(the Mott wave) propagations, confirming that the momentum diffusion didcontrol the 1D ductile fragmentation process.