Variational study of a Maxwell–Rayleigh-type finite length model for the preliminary design of a tensegrity chain with a tunable band gap

Abstract

In this study, a Maxwell–Rayleigh-type model is investigated, describing a unidimensional lattice with a finite length, where the unit cell includes hosting and resonant masses mutually connected by elastic springs. The configuration selected is inspired by the specific engineering design to be discussed: however, the theoretical approach pursued is rather general and can be easily generalized to different scenarios. By the heuristic homogenization based on a Piola’s Ansatz, an equivalent continuum is specified: through a variational approach, resting on the minimization of the Hamilton’s action functional, a dispersion relation is deduced, revealing the existence of a band gap. Such a continuous interval of frequencies, inside which the propagation of waves is inhibited, can be tuned controlling the features of the original system. Thereafter, exploiting the same equations, the stationary elasto-dynamic response for a chain with a finite length is also deduced, corresponding to standing waves. On the basis of such results, the preliminary design of a linear chain with a finite number of cells is carried out. By the present strategy proper boundary conditions are deduced to be prescribed at the ends of the 1D lattice sample, and not over the unit cell (as it occurs in Bloch–Floquet approach with periodicity conditions). To realize the mutual elastic connections between unequal masses fitting the problem constraints, tensegrity prisms are selected, including compressed bars and tensioned cables, for which it is possible to govern the tangent axial stiffness through the cable pre-tensioning. We analyze the scenario in which the band gap coincides with the interval $1-10\mathrm{Hz}$, indicating an appropriate geometry and suitable engineering materials for the tensegrity elements.