  Granular impact -- the dynamic intrusion of solid objects into granular media
-- is widespread across scientific and engineering applications including
geotechnics. Existing approaches for simulating granular impact dynamics have
relied on either a pure discrete method or a pure continuum method. Neither of
these methods, however, is deemed optimal from the computational perspective.
Here, we introduce a hybrid continuum-discrete approach, built on the coupled
material-point and discrete-element method (MP-DEM), for simulating granular
impact dynamics with unparalleled efficiency. To accommodate highly complex
solid-granular interactions, we enhance the existing MP-DEM formulation with
three new ingredients: (i) a robust contact algorithm that couples the
continuum and discrete parts without any interpenetration under extreme impact
loads, (ii) large deformation kinematics employing multiplicative
elastoplasticity, and (iii) a trans-phase constitutive relation capturing
gasification of granular media. For validation, we also generate experimental
data through laboratory measurement of the impact dynamics of solid spheres
dropped onto dry sand. Simulation of the experiments shows that the proposed
approach can well reproduce granular impact dynamics in terms of impact forces,
intrusion depths, and splash patterns. Further, through parameter studies on
material properties, model formulations, and numerical schemes, we identify key
factors for successful continuum-discrete simulation of granular impact
dynamics.
