UC San Diego

This research is conducted to study the propagation of stress pulses with wave length comparable to the size of a unit cell in an one-dimensional chain composed of steel cylinders alternating with toroidal Nitrile rubber O-rings under different levels of precompression or without precompression force by using experiments, numerical simulations and theoretical analysis. Nitrile O-rings are making the system more tunable than the traditional metamaterials composed of steel spherical particles and this metamaterial has better dissipative properties especially for high energy repeatable impact loading. This strongly nonlinear discrete metamaterial can be potentially useful for acoustic and shock/impact mitigating applications.

Investigated metamaterial is strongly nonlinear system with unique properties, where the strong nonlinearity arises from the Hertzian contact interaction or from more general double power interaction law under large compression. If the initial precompression is zero, such a system cannot support a classical sound waves and for this reason it is called a “sonic vacuum”.

The static force-displacement relationship for toroidal O-ring is described by a combination of two power-law terms being in satisfactory agreement with experimental observations. This suggests unique tunability and it was demonstrated that the pulse speed indeed is strongly tunable with applied static stress. A strong attenuation of stress pulse was observed at a relatively short distances from the entrance. It is attributed to the viscous dissipation during dynamic deformation of strongly nonlinear viscoelastic O-rings, its dependence on the initial precompression was studied. Several models for this nonlinear metamaterial were numerically investigated to clarify the mechanism of the observed strongly nonlinear dissipation by representing O-rings in the frame of Kelvin-Voigt model: strongly nonlinear massless spring with parallel dashpot with nonlinear damping coefficient being function of initial precompression. Single bell shape waves of different durations and amplitudes were generated in the system without precompression by an impact of strikers with different masses. The viscous dissipation prevents the incoming pulse from splitting into trains of solitary waves typical for non-dissipative strongly nonlinear discrete systems. Very unusual behavior of strikers with relatively large masses in comparison with mass of steel cylinders in the metamaterial was observed in experiments – strikers did not recoil after impact with investigated metamaterial. Results of numerical modeling were in agreement with speed of stress pulses, their attenuation and strikers behavior observed in experiments.