Impacts of bi-layered elastic projectiles

In contact mechanics the study of the impact with a substrate remains a challenge for heterogeneous systems, from clusters of particles at the microscopic level [1] to the collisions of macroscopic vehicles [2]. We investigate experimentally the impact of a bi-layered elastic projectile with a substrate. Our projectiles are made of a soft part on top of a rigid one. Velocities before and after the impact are measured to characterize the coefficient of restitution (Cor).

The rigid part follows the Hertz law of contact with a Cor close to 1; the soft part is elastic. We show that the layering of the soft part to the rigid one will modify the restitution which exhibits a complex trend in function of the soft material proportion: a minimum of 0.2 is found for a peculiar proportion of 0.4 and several local extrema are observed as well. We also highlight the occurrence of multiple micro-bounces during the impact.

A theoretical model is proposed to account for the specific geometry of the problem and considers both the Hertz contact and the elastic waves propagation inside the soft layer. This study shows that the wave propagation is of paramount importance for the energy restitution.

[1] Basile & Dumont. Phys. Rev. E 61.2 (2000):2015.

[2] Vangi. Int J Impact Eng. 36.3 (2009): 512-521.