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...

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In this study, it was aimed to increase the piezoelectric and pyroelectric properties of electrospun polyvinylidene fluoride (PVDF) nanofibers simultaneously by using specific nanofillers. Graphene oxide (GO), graphene, and halloysite nanotubes with different concentrations (0, 0.05, 0.4, and 1.6% wt/wt) were combined with PVDF solution and were fabricated in the form of nanofibers through electrospinning. Pyroelectric properties of samples were measured by submerging sealed samples in hot water (360°K) and ice (270°K). The piezoelectric properties of the samples were evaluated through bending tests. The microstructural, mechanical, and thermal properties of the electrospun PVDF nanocomposite were investigated using scanning electron microscope, Instron instrument, and thermogravimetric analysis, respectively. To further support the experimental observations for generating electric voltage in the bended nanogenerator, the PVDF nanogenerator (PNG) was also modeled by a finite element analysis based on the theory of linear piezoelectricity using COMSOL Multiphysics simulation software. Experimental results showed that adding nanofillers could improve the piezoelectric and pyroelectric properties of all samples, associated with the increment of β-phase in the nanofibers. It was concluded that adding nanofillers could increase pyroelectricity about 50% more than piezoelectricity in pristine PVDF nanofiber web. The PNG containing 1.6 wt% GO showed the highest efficiency in terms of piezoelectricity and pyroelectricity. In addition, the results showed that the ratio of piezoelectric to pyroelectric coefficients was constant (~1.5) and it was independent of the nanofiller type and content. The effect of external force and vibration frequency on the output voltage was also investigated. Increasing the compressive force and vibration frequency caused a greater output voltage. Finally, the fabricated nanogenerator was integrated on insole and elbow to investigate its energy harvesting capabilities from body movement.

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