Viscoelastic materials are commonly used to dissipate kinetic energy in case of impact and vibrations. Unfortunately, dissipating large amounts of energy in a monolithic material requires high combinations of two intrinsic properties – Young’s modulus and loss factor, which are generally in conflict. This limitation can be overcome by designing cellular materials incorporating negative stiffness elements within the unit cell.
Over the past few years, significant progress has been made on the analysis, fabrication and characterization of cellular metamaterials with non-convex strain energy landscape at the unit cell – or sub-unit cell level. However, nearly all the negative stiffness elements consist of arches, pre-buckled beams or rigid links connected by flexible hinges. As the performance of all these elements is highly sensitive to boundary conditions, all these designs present the inconvenience that a stiff frame is needed to constrain undesired deformations. Furthermore, in most cases a minimum number of unit cells in series is required to obtain energy dissipation.
In this study, we explore the use of magnetic forces to design negative stiffness elements and create bi-stable architected materials. As magnets exhibit a very steep force-separation relation, their incorporation in cellular architectures enables the realization of stretching dominated bi-stable lattices, which exhibit both high stiffness and energy dissipation even within a single unit cell. We design and fabricate elastomeric planar lattice designs with embedded permanent magnets, and characterize their mechanical properties – including energy dissipation and energy landscape – under quasi-static uniaxial loading conditions.
Interestingly, the morphologies of the two stable phases can be chosen in such a way that only one configuration possesses a band gap under elastic wave propagation, opening the possibility of using these bi-stable metamaterials as switchable mechanical filters.