Snapping Mechanical Metamaterials under Tension

Abstract

We present a monolithic mechanical metamaterial comprising a periodic arrangement of snapping units with tunable tensile behavior. Under tension, the metamaterial undergoes a large extension caused by sequential snap-through instabilities, and exhibits a pattern switch from an undeformed wavy-shape to a diamond configuration. By means of experiments performed on 3D printed prototypes, numerical simulations and theoretical modeling, we demonstrate how the snapping architecture can be tuned to generate a range of nonlinear mechanical responses including monotonic, S-shaped, plateau and non-monotonic snap-through behavior. This work contributes to the development of design strategies that allow programming nonlinear mechanical responses in solids.Mechanical metamaterials are man-made materials, usually fashioned from repeating unit cells which are engineered to achieve extreme mechanical properties, often beyond those found in most natural materials.[1] They gain their unusual, sometimes extraordinary, mechanicalproperties from their underlying architecture, rather than the composition of their constituents. Metamaterials exhibit interesting mechanical properties, such as negative Poisson's ratio, [2,3] negative incremental stiffness, [4] negative compressibility [5] and unusual dynamic behavior for wave propagation. [6] As Ron Resch (artist and applied geometrist) points out in his statement "the environment responds by collapsing quite often", [7] instabilities can be exploited to design advanced materials with innovative properties. [8] Recently, harnessing elastic instabilities played a central role in the rational design of novel 2D [9][10][11][12][13][14] and 3D [15][16][17] mechanical metamaterials with either significantly enhanced mechanical properties or equipped with new functionalities, e.g. programmable shape transformations. [14] In most of the examples mentioned above, elastic instabilities are exploited to trigger a pattern switch by a broken rotational symmetry, mostly governed by Euler buckling. In these works instabilities are induced by an applied compressive load. [16] This observation naturally leads to the question of whether one can either benefit from other mechanical instability mechanisms for metamaterial design or extend current concepts to other loading conditions. In this work, we exploit mechanical instabilities triggered by snap-through buckling to create a metamaterial which experiences a pseudo pattern switch in tension and exhibits a programmable mechanical response. Our design is inspired by a monolithic bistable mechanism, [18] i.e. two curved parallel beams that are centrally-clamped as schematized in Figure 1a. A normal force applied in the middle of the double-beam mechanism can prompt it to snap through to its second stable state ( Figure 1a, dashed lines). We release clamped conditions at both ends to create a repeatable unit cell (Figure 1b), composed of two centrally connected cosine-shaped slender segments, which can be tessellated in plane to form a periodic ar...

HCI meets Material Science

“…The beams can then return to their initial configuration when a sufficient reverse force is applied, enabling multiple changes in shape. In [189] the authors created a bistable metamaterial that moves through several metastable states via snap-through buckling when stretched, until it reaches full extension, achieving strains of up to 150%.…”

Exploiting Microstructural Instabilities in Solids and Structures: From Metamaterials to Structural Transitions

“…When a mechanical metamaterial is built by arranging such elastic blocks on a square lattice, an instability is triggered under equibiaxial tension resulting in a checkerboard pattern of pores with two different sizes [161,187,188]. Moreover, sequential snap-through instabilities triggered under tensile loading in mechanical metamaterials comprising 1D arrays of curved beams [189] (see Fig. 14(a)) and 2D arrays of rotating units [190] result in large extension and a range of nonlinear mechanical responses.…”

“…Adapted from Ref. [189]. (b) Response of an elastic sheet perforated with a square array of mutually orthogonal cuts under uniaxial tension.…”

Buckling Pneumatic Linear Actuators Inspired by Muscle

“…[12][13][14] An emerging opportunity in the engineering of soft machines is to harness these instabilities to achieve new functionalities. [15][16][17] For example, buckling has proven useful in the design of stretchable soft electronics, [18,19] tunable materials, [20][21][22][23] and fluidic actuators. [24] We have previously demonstrated that reversible buckling of elastomeric beams can be used to make a torsional soft actuator.…”