Material and particle size sensitivity analysis on coefficient of restitution in low-velocity normal impacts

In many granular processes, impacts play a crucial role. These impacts are often described by the coefficient of restitution (COR). This COR does not only depend on impact velocity but also on the material pairing, the shape of impacting bodies, number of impacts, etc. This paper analyzes and compares the sensitivity of the COR for often seen material pairings metal–metal and metal–polymer. For experimental investigations, a steel sphere impacts different planar material probes in a defined manner, e.g., a sphere–wall contact is reproduced. While the metal–metal impacts show a significant dependency on impact velocity, the metal–polymer impacts show only little influence of the impact velocity. Also, repeated impacts onto the same spot have a significant influence on metal–metal impacts, while metal–polymer impacts are not affected. To gain insights not only about the macroscopic behavior of impacts but also about the microscopic behavior, finite element simulations are performed using an efficient 2D axisymmetric model and viscoelastic and elastic–viscoplastic material models. A good agreement between experiments and FEM simulations are achieved for the utilized material pairings. Then, the influence of the sphere’s size is studied. Afterward, a deeper look into the energy dissipation process during contact is investigated. Finally, the contact duration and normal force in the contact zone are studied experimentally.