Properties of mechanical metamaterials with the focus on magnetic inclusions

Abstract

Mechanical metamaterials are man-made systems having anomalous macroscopic mechanical properties originating primarily from the geometry of the subunits rather than composition of the material at the molecular level. Even though, over the years, mechanical metamaterials have been thoroughly studied, it does not mean that there are not any new aspects related to their behaviour which remain to be discovered such as the capability of these systems to induce their own rotational motion as a result of internal deformation. In this thesis, this novel phenomenon was analysed, optimised and confirmed both by means of a theoretical model and experimental prototype for particular mechanical metamaterials deforming via the rotation of their subunits. It was also proposed that potential prototypes utilising this concept could prove to be useful in applications where control over the rotational motion of the system is of particular importance. The role of magnetic inclusions inserted into standard mechanical metamaterials was also thoroughly investigated. It is proposed that, as a result of the interaction between subunits constituting the system, such magnetic inclusions have a very important role in modifying the stiffness characteristics of the systems. More specifically, it was shown via a theoretical model as well as by means of experimental testing that the smart insertion of magnetic inclusions permits control of the mechanical behaviour of such metamaterials as these inclusions interact with each other as the system deforms. Results suggest that the considered system may not only exhibit negative Poisson’s ratio but also negative stiffness, which effect could not be possible without the use of magnetic inclusions. In order to further investigate different physical phenomena which can be exhibited by mechanical metamaterials with magnetic inclusions, through the use of the Ising model, it was shown that such systems make it possible to induce the magnetocaloric effect even in the absence of an external magnetic field. By means of computer simulations, it was also shown that the rate at which such systems are deformed has a large impact on the evolution of magnetic domains within the system. It is also proposed that magnetic inclusions could be useful in inducing the deformation process instead of the ‘standard’ external forces. In addition to all this, this thesis has also looked into two metamaterials constructs which are particularly amenable to negative properties. In particular, a mechanical system incorporating a hierarchical design which was investigated through a dynamics approach, where it was shown that such a system makes it possible to obtain a wide range of mechanical properties and deformation patterns solely as a result of the control over the resistance of structural units to the motion promoting the deformation process. Also studied was a novel structure composed of appropriately connected generic triangles which system was reported to exhibit both negative linear compressibility and negative thermal expansion.