Metamaterials with Poisson’s ratio sign toggling by means of microstructural duality

Neither positive nor negative Poisson’s ratio materials are fully advantageous across a wide range of load bearing applications. In some cases, it is advantageous for structures to possess duality in material properties so as to take advantage of auxetic behavior under one loading condition as well as conventional behavior when the loading condition changes. This paper proposes two metamaterials—the hybrid rhombic–re-entrant metamaterial and the hybrid kite-arrowhead metamaterial—in which the prescription of either compressive or tensile load always leads to lateral expansion. Specifically, the Poisson’s ratio of these metamaterials range from negative infinity to zero upon tensile load, and from zero to positive infinity upon compressive load. This duality of auxetic and conventional behavior for each of these metamaterial is achieved by employing a simultaneous lock and slide mechanism that is interchangeable upon load reversal. When loaded in one direction, the locked part takes effect while the sliding part becomes redundant. Upon load direction reversal, the previously locked and sliding parts convert to sliding and locked parts, respectively, thereby activating the redundant part while deactivating the functional part. The change in effective microstructural components, therefore, facilitates Poisson’s ratio sign toggling with load reversal. Due to the indiscriminate lateral expansion regardless of the axial loading direction, such behavior in a fiber is useful to resist both fiber pull-out as well as fiber push-out from the matrix material.