Experimental Full-field Analysis of Size Effects in Miniaturized Cellular Elastomeric Metamaterials

Cellular elastomeric metamaterials are interesting for various applications, e.g. soft robotics, as they may exhibit multiple microstructural pattern transformations, each with its characteristic mechanical behavior. Numerical literature studies revealed that pattern formation is restricted in (thick) boundary layers causing significant mechanical size effects. This paper aims to experimentally validate these findings on miniaturized specimens, relevant for real applications, and to investigate the effect of increased geometrical and material imperfections resulting from specimen miniaturization. To this end, miniaturized cellular metamaterial specimens are manufactured with different scale ratios, subjected to in-situ micro-compression tests combined with digital image correlation yielding full-field kinematics, and compared to complementary numerical simulations. The specimens' global behavior agrees well with the numerical predictions, in terms of pre-buckling stiffness, buckling strain and post-buckling stress. Their local behavior, i.e. pattern transformation and boundary layer formation, is also consistent between experiments and simulations. Comparison of these results with idealized numerical studies from literature reveals the influence of the boundary conditions in real cellular metamaterial applications, e.g. lateral confinement, on the mechanical response in terms of size effects and boundary layer formation.Comment: 20 pages, 6 figures, Materials & Design, 11 May 202

Two-dimensional graded metamaterials with auxetic rectangular perforations

This work describes the in-plane uniaxial tensile mechanical properties of two-dimensional graded rectangular perforations metamaterials using numerical homogenization finite element approaches benchmarked by experimental results. The metamaterial configuration is based on graded patterns of centre-symmetric perforated cells that can exhibit an auxetic (negative Poisson's ratio) behavior. Global and local equivalent mechanical properties of the metamaterial are measured using digital image correlation techniques mapped over Finite Element models to identify strain patterns and related stress distributions at different scales. The samples and their numerical counterpart are parametrized against the spacing and aspect ratios of the cells. The overall stiffness behavior of the graded perforated metamaterial plates features a higher degree of compliance that depends both on the geometries of the cells of the graded areas, but also on the graded pattern used. Local Poisson's ratio effects show a general constraint of the auxetic behavior compared to the case of uniform plates, but also interesting and controllable shape changes due to the uniaxial tensile loading applied

Flexible planar metamaterials with tunable Poisson\u27s ratios

This research reports on the design, fabrication, and multiscale mechanical characterization of flexible, planar mechanical metamaterials with tailorable mechanical properties. The tunable mechanical behavior of the structures is realized through the introduction of orthogonal perforations with different geometric features. Various configurations of the perforations lead to a wide range of Poisson\u27s ratios (from −0.8 to 0.4), load-bearing properties, and energy absorption capacities. The correlations between the configuration of the perforations and the auxetic response of the structures are highlighted through computational and experimental characterizations performed at multiple length scales. It is demonstrated that the local in-plane rotation of the solid ligaments in a uniaxially loaded structure is the primary factor that contributes to its strain-dependent auxetic behavior at macroscopic scales. Confinement of these local rotations is then used as a practical strategy to activate a self-strengthening mechanism in the auxetic structures. It is further shown that the fabrication of planar flexible structures with controllable Poisson\u27s ratios is feasible through spatial adjustment of perforations in the structure. Finally, discussions are provided regarding the practical applications of these structures for a new generation of highly energy-absorbing protective equipment

Flexible planar metamaterials with tunable Poisson’s ratios

This research reports on the design, fabrication, and multiscale mechanical characterization of flexible, planar mechanical metamaterials with tailorable mechanical properties. The tunable mechanical behavior of the structures is realized through the introduction of orthogonal perforations with different geometric features. Various configurations of the perforations lead to a wide range of Poisson’s ratios (from −0.8 to 0.4), load-bearing properties, and energy absorption capacities. The correlations between the configuration of the perforations and the auxetic response of the structures are highlighted through computational and experimental characterizations performed at multiple length scales. It is demonstrated that the local in-plane rotation of the solid ligaments in a uniaxially loaded structure is the primary factor that contributes to its strain-dependent auxetic behavior at macroscopic scales. Confinement of these local rotations is then used as a practical strategy to activate a self-strengthening mechanism in the auxetic structures. It is further shown that the fabrication of planar flexible structures with controllable Poisson’s ratios is feasible through spatial adjustment of perforations in the structure. Finally, discussions are provided regarding the practical applications of these structures for a new generation of highly energy-absorbing protective equipment

Lightweight mechanical metamaterials designed using hierarchical truss elements

Rotating unit systems constitute one of the main classes of auxetic metamaterials. In this work, a new design procedure for lightweight auxetic systems based on this deformation mechanism is proposed through the implementation of a hierarchical triangular truss network in place of a full block of material for the rotating component of the system. Using numerical simulations in conjunction with experimental tests on 3D printed prototypes, the mechanical properties of six types of auxetic structures, which include a range of rotating polygons and chiral honeycombs, were analysed under the application of small tensile loads. The results obtained show that there is almost no difference in the Poisson's ratios obtained from the regular, full structures and the ones made from triangular truss systems despite the latter, in some cases, being 80% lighter than the former. This indicates that these systems could be ideal candidates for implementation in applications requiring lightweight auxetic metamaterial systems such as in the aerospace industry

Experimental and numerical investigation on in-plane impact behaviour of chiral auxetic structure

This paper presents an experimental study on impact behaviour and plastic evolution of chiral structure subjected to in-plane impact loading using a Split Hopkinson pressure bar (SHPB). The finite element (FE) model developed in ABAQUS/Explicit was validated and utilized for parametric study, and further developed as an extension of experimental work. The impact scenarios from both structure itself and external input are considered, including relative density, topology parameter r/R and initial impact energy. Results indicate that chiral structure exhibits three critical failure modes corresponding to various impact velocities ranging from 5 m/s to 50 m/s. Interestingly, chiral structure occurs with two densification stages induced by ligaments-dominated and nodes-dominated crushing deformation, respectively, proving the capability of independent energy management mechanism. Increasing the value of relative density from 0.19 to 0.39 contributes to a maximum of 250% increase in the specific energy absorption (SEA). Although increasing the value of r/R from 0.04 to 0.2 can dramatically decrease Poisson’s ratio (PR) from 0.07 to -0.63 (significant negative PR), high strain-rate dependence of PR is also observed. In addition, the impact displacement is mostly influenced by initial impact energy but not by impact velocity and mass. The obtained results of this study provide a new insight into the impact performance of chiral structure, which contributes to the optimal design of auxetic crashworthiness system

Investigating the Effects of Topology on the Fracture and Failure Mechanisms of Low Density Metamaterials

Advances in additive manufacturing have enabled the creation of low density metamaterials with fine features and complex topographies. These new metamaterial topologies and size scales not previously possible broaden the spectrum of lightweight materials with unique properties that are advantageous in a variety of applications. There however is a lack of understanding of metamaterial failure and fracture behaviors. Studies tend to report only a few material properties rather than a comprehensive description of behavior. Due to this, there is a hesitancy to incorporate metamaterials into engineering designs despite proven remarkable properties. This work seeks to investigate in three parts the fracture and failure mechanisms controlling the deformation behavior of three different types of low-density metamaterials. The first part of the study explored increasing the fracture toughness of sheet-based metamaterials using designed porosity to redirect crack growth away from its original crack path to a less damaging direction. The crack was diverted into features in the metamaterial base topology, which served to toughen the material. It was identified that base material plays a role in the crack arrest mechanism activated. The added porosity was able to increase the fracture toughness of the metamaterial by a factor of three. The second part of the study calculated yield surfaces for common cellular material topologies that incorporates the anisotropy of tension, compression, and shear of cellular materials between different loading orientations. The shear component was the weakest of the topologies, atypical of monolithic material behavior. The third part of this study is currently on-going work to analyze the deformation of lattice metamaterials in compressive creep and compare the creep exponent and activation energy of the lattice to the base material as well as identify the mechanisms controlling the deformation of the lattice unit cell

Applicability of correlated digital image correlation and infrared thermography for measuring mesomechanical deformation in foams and auxetics

Cellular materials such as metal foams or auxetic metamaterials are interesting microheterogeneous materials used for lightweight construction and as energy absorbers. Their macroscopic behavior is related to their specific mesoscopic deformation by a strong structure-property-relationship. Digital image correlation and infrared thermography are two methods to visualize and study the local deformation behavior in materials. The present study deals with the full-field thermomechanical analysis of the mesomechanical deformation in Ni/PU hybrid foams and Ni/polymer hybrid auxetic structures performing a correlative digital image correlation and infrared thermography. Instead of comparing and correlating only the primary output variables of both methods, strain and temperature, also strain rates and temperature rates occurring during deformation were compared. These allow for a better correlation and more conclusive results than obtained using only the primary output variables