Multiscale Strain Field Characterization in Flexible Planar Auxetic Metamaterials with Rotating Squares

Abstract

Auxetic materials exhibit a negative Poisson's ratio when subjected to mechanical load. [1][2][3][4][5] For example, auxetic structures expand longitudinally and laterally when subjected to uniaxial tension instead of experiencing lateral contraction commonly seen in their conventional counterparts. Researchers have broadly considered the use of such materials and structures in many applications, including but not limited to biomechanics, flexible electronics, and soft robotics. [6][7][8][9] The growing interest in using auxetic structures in these applications is due to their improved mechanical properties, such as higher indentation resistance, improved vibration damping, resistance to shear, and enhanced fracture toughness, compared with their nonauxetic counterparts. [10][11][12][13] Auxetic structures are generally classified as a subset of "mechanical metamaterials" as their novel properties are not commonly found in nature or other conventional engineering materials. [3,14,15] The so-called mechanical metamaterials are highly influenced by the augmentation of their local morphology. Therefore, their physical properties can be controlled by certain microstructural and geometric features, thus allowing broader control of the behavior by design rather than the chemical composition of their constituent materials. [16,17] Property-adjustable auxetic materials with variable responses in changing environments have also been developed for various applications. [18][19][20][21][22][23] For instance, embedding magnetic insertions in auxetic metamaterials is shown to change the cellular configuration upon the application of a magnetic field, switching nonauxetic to an auxetic response. [24] Similar behaviors have been observed when a quadrilateral cellular structure consisting of bimaterial strips converts into a convex or concave shape with thermal triggers. [25] Researchers have explored various cell topologies that lead to auxetic behavior. [3,5,[26][27][28][29] The pioneering experimental studies in this field are credited to Lakes, who investigated auxetic foams. [30] In recent years and in line with the advancements in additive manufacturing, practical and cost-effective pathways to the realization of these architected structures have become more easily achievable. [31][32][33][34][35][36][37] Specifically, the fabrication of planar mechanical metamaterials with tunable properties has attracted great attention. For example, the cut and slit design strategy, wherein perforations are intentionally introduced into a sheet to promote auxeticity, has been explored in several studies. [35,38,39] These perforation patterns used to achieve auxetic behaviors include kagome lattices, [39] diamonds, [40] circles, [41] triangles, [42] ellipses, [43] stars, [31] self-similar squares, [44] and rectangles. [17,35,36] The auxetic response has also been achieved through randomly

Modular Soft Robotic Actuators from Flexible Perforated Sheets

“…The mode transition discussed above is governed by the interplay of local strain fields and their effect on cell rotation. [ 46 ] Controlling the rate of local cell rotations and strains upon pressurization and loading is at the core of the methodology discussed in this work. We directly utilize the unit cells investigated in Figure 1 in the development of actuators of various forms and functions in the following discussions.…”

Fabrication and Characterization of Electrically Conductive 3D Printable TPU/MWCNT Filaments for Strain Sensing in Large Deformation Conditions

“…Using planar auxetic metamaterials to develop flexible structures offers significant advantages, particularly in their ability to exhibit negative Poisson's ratios, leading to superior mechanical properties. [62][63][64] Such geometric configurations have potential in strain-sensing applications due to their unique deformation responses. [65][66][67] In this study, a unit cell design characterized by rectangular, center-symmetric cells was employed, also previously investigated by the authors.…”

ABC‐Auxetics: An Implicit Design Approach for Negative Poisson's Ratio Materials

“…An advantage of explicit approaches is that it is easy to tune and control the effective Poisson's ratio of a structure through simple parametric variations in the unit cell geometry, thereby allowing for controlled design exploration. [ 29–31 ] Several other works achieve similar results using a level‐set method to create a structure. [ 32,33 ] However, such explicit approaches lead to a design space that is fundamentally constrained by the specific geometric parameters of the cell that rely on the heuristic knowledge and creativity of the designer.…”