Possibility of Phase Transition in Mechanical Metamaterials

Based on the available knowledge from numerous simulation studies regarding the occurrence of metamaterial phase transition, structures were produced that exhibit a real auxetic-non-auxetic tran­sition. The occurrence of this transition was possible thanks to the modified structure of rotating rectangles, showing significant dimensional changes. Through analysis and experimental evidence, it has been shown that there are limiting values of parameters, above which the Poisson’s ratio be­comes positive, and the structure loses its auxetic properties. It has been demonstrated that through the stretching of such structures, with a gradually increasing linear elongation horizontally, there is a curvilinear increase in elongation vertically. For rectangular unit cells with a parameter a/b>1.5 and x=0.1, the relationship between elongation in the horizontal and vertical directions (ΔX2/X2=f(ΔX1/ X1)) has a maximum and a zero point (ΔX2/X2=0), which corresponds to the occurrence of a phase transition. The Poisson’s ratio in this area ranges from -∞ to +∞. Such a huge auxetic effect can be achieved through the anisotropy of the structure. In the phase diagram, the Poisson’s ratio – degree of deformation shows a specific area corresponding to a sudden transition in auxetic properties, where it concerns a phase transition without changes in symmetry. Morphologically, instead of two phases, there is only one with the same spatial arrangement of unit cells, differing only in linear dimensions.