Abstract
The Poynting effect generically manifests itself as the extension of the material in the direction perpendicular to an applied shear deformation (torsion) and is a material parameter hard to design. Unlike isotropic solids, in designed structures, peculiar couplings between shear and normal deformations can be achieved and exploited for practical applications. Here, a metamaterial is engineered that can be programmed to contract or extend under torsion and undergo nonlinear twist under compression. First, it is shown that the system exhibits a novel type of inverted Poynting effect, where axial compression induces a nonlinear torsion. Then the Poynting modulus of the structure is programmed from initial negative values to zero and positive values via a pre‐compression applied prior to torsion. The work opens avenues for programming nonlinear elastic moduli of materials and tuning the couplings between shear and normal responses by rational design. Obtaining inverted and programmable Poynting effects in metamaterials inspires diverse applications from designing machine materials, soft robots, and actuators to engineering biological tissues, implants, and prosthetic devices functioning under compression and torsion.
Highlights
Introduction tensionThe response of metamaterials to direct shear[15] and inThe Poynting effect is a surprising non-linear elastic effect that makes, in the original experiment of Poynting,[1] a hanging piano wire under tension become longer when it is twisted (Figure 1a, left)
In a limited number of recent studies, induced linear torsion by compression have been uncovered in designed chiral structures[18,19,20,21,22]
We design a hollow cylindrical shell composed of an array of unit-cells, which provides a network of nonuniform beams (Figure 2a) capable of side-buckling and self-contacting under compression.[27]
Summary
We design a hollow cylindrical shell composed of an array of unit-cells, which provides a network of nonuniform beams (Figure 2a) capable of side-buckling and self-contacting under compression.[27]. We first conduct two series of compression experiments with different boundary conditions: in the first series the bottom side of the shell is clamped and the top side is free to rotate (“torsionfree”), while in the second series rotation is not allowed at both sides (“clamped”). We use the clamped boundary condition and perform two series of torsion experiments under fixed loads and fixed gaps. Compression stress is defined by σ = F/As, where As is the cross-section area. Φ, develops by applying shear force, Fs, and induces the axial deformation of δn under a fixed load or the normal force of Fn under a fixed gap. Normal and shear force responses of a cylindrical shell under torsion are given by Fn = GnJ(φ/h), and Fs = τ/R = GsJφ/(Rh), respectively, where τ is the torque around the axis of the.
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