Illuminating Origins of Impact Energy Dissipation in Mechanical Metamaterials

Abstract

Elastomeric mechanical metamaterials have revealed striking ability to attenuate shock loads at the macroscopic level. Reports suggest that this capability is associated with the reversible elastic buckling of internal beam constituents observed in quasistatic characterizations. Yet, the presence of buckling members induces non-affine response at the microscale, so that clear understanding of the exact energy dissipation mechanisms remains clouded. In this report, the authors examine a mechanical metamaterial that exhibits both micro-and macroscopic deformations under impact loads and devise an experimental method to visualize the resulting energy dissipation mechanisms. By illuminating the dynamic distribution of strain in the metamaterial, the authors uncover a rational way to program the macroscopic deformation and enhance impact mitigation properties. The results emphasize that mechanical metamaterials clearly integrate materials science and structural engineering, encouraging future interdisciplinary studies to capitalize on the opportunities.The blow of an impact can rapidly harm people and engineered systems. [1,2] This challenge has fueled development of resilient engineered material systems able to suppress the transmission of impact energy to the shielded body or structure. Recent attention has turned to buckling-based material system concepts, whereby elastic and/or plastic buckling structural members on a microscale level [3][4][5][6][7] are harnessed to absorb shock at the system level. Preferring the reusability of elastic buckling phenomena and the practicality of elastomers, classes of elastomeric mechanical metamaterials have been devised with internal cellular geometries that absorb elastic energy via reversible buckling events in periodic, microscale elastomeric beam constituents. [8][9][10][11][12] Under low frequency cycling of applied strain or stress, these new generations of cellular materials [13] are shown to exhibit exceptional elastic energy dissipation properties [14][15][16] due to a "reversible plasticity" effect [17] that has an analogy to energy management in cytoskeletal actin networks. [18][19][20][21] Yet, a common vision for elastomeric mechanical metamaterials is to mitigate impact, in which case high strain rates are induced. In this light, experimental evidence suggests these metamaterials are effective to dissipate sudden accelerations [9] and shocks, [11] although the exact energy mitigation mechanisms are unknown. While there are inherent ties between shock absorption mechanisms and the elastic buckling of the beam constituents in such elastomeric metamaterials, [22] when composed into material systems the presence of buckling members gives rise to non-affine response at the microscale that ultimately governs macroscopic energy dissipation properties of the system. [23,24] These internal instabilities break the direct proportionality between micro-and macroscale behaviors. [25] Although high-speed video has shed light on energy transfer and dissipation in similar problem...

Multifunctional Mechanical Metamaterials with Embedded Triboelectric Nanogenerators

“…As a recently emerged topic, mechanical metamaterials take advantage of deformation of localized geometric features rather than physical or chemical characteristics of the constituent material to achieve exotic and tunable mechanical or acoustic properties that do not occur naturally. [ 31–36 ] Typical designs include auxetic metamaterials that have negative Poisson's ratios, [ 37–43 ] metamaterials employing local multistabilities for negative stiffness or tailorable constitutive relations, [ 44–50 ] as well as acoustic metamaterials capable of manipulating directions and frequency components of mechanical wave propagations. [ 51–54 ] Two archetypal geometries, namely the beam‐array [ 49 ] and the bi‐circular‐hole [ 55 ] mechanical metamaterials, are selected here for illustration of TENG‐embedded metamaterials given the large deformations during their operations and the simplicity of their 2D shapes.…”

“…[ 31–36 ] Typical designs include auxetic metamaterials that have negative Poisson's ratios, [ 37–43 ] metamaterials employing local multistabilities for negative stiffness or tailorable constitutive relations, [ 44–50 ] as well as acoustic metamaterials capable of manipulating directions and frequency components of mechanical wave propagations. [ 51–54 ] Two archetypal geometries, namely the beam‐array [ 49 ] and the bi‐circular‐hole [ 55 ] mechanical metamaterials, are selected here for illustration of TENG‐embedded metamaterials given the large deformations during their operations and the simplicity of their 2D shapes. The beam‐array geometry is a straightforward representative of shock absorbing metamaterials that utilize nonaffine deformation [ 49,56 ] due to buckling of its local members to yield a near‐zero‐stiffness plateau on their global macroscopic stress–strain curves, while the bi‐circular‐hole topology is an example of programmable metamaterials that can not only meet specific structural criteria by variations of design parameters but also adapt to updated requirements by external confinements or local geometric changes that can be easily applied to the already fabricated product.…”

Foundations for Soft, Smart Matter by Active Mechanical Metamaterials

“…Adapted with permission. [ 21 ] Copyright 2018, John Wiley & Sons, Inc. c) Metamaterial geometry transformations to tune band gaps. Adapted with permission.…”

Harnessing Magnets to Design Tunable Architected Bistable Material

“…Elastic metamaterials are artificial composites and they have various classifications and unusual properties, like negative Poisson's ratio, negative and zero thermal expansion, negative effective modulus, negative effective mass density, shape memory effect, etc, which makes them quite attractive in advanced engineering applications, for example, energy absorption, vibration isolation, wave guiding, invisible cloaking, and subwavelength focusing . One notable characteristic is the capability to generate bistable or multistable properties .…”