Auxetic mechanical metamaterials: from soft to stiff

Auxetic mechanical metamaterials are artificially architected materials that possess negative Poisson’s ratio,demonstrating transversal contracting deformation under external vertical compression loading.Their physical properties are mainly determined by spatial topological configurations.Traditionally,classical auxetic mechanical metamaterials exhibit relatively lower mechanical stiffness,compared to classic stretching dominated architectures.Nevertheless,in recent years,several novel auxetic mechanical metamaterials with high stiffness have been designed and proposed for energy absorption,load-bearing,and thermal-mechanical coupling applications.In this paper,mechanical design methods for designing auxetic structures with soft and stiff mechanical behavior are summarized and classified.For soft auxetic mechanical metamaterials,classic methods,such as using soft basic material,hierarchical design,tensile braided design,and curved ribs,are proposed.In comparison,for stiff auxetic mechanical metamaterials,design schemes,such as hard base material,hierarchical design,composite design,and adding additional load-bearing ribs,are proposed.Multi-functional applications of soft and stiff auxetic mechanical metamaterials are then reviewed.We hope this study could provide some guidelines for designing programmed auxetics with specified mechanical stiffness and deformation abilities according to demand.

Efficient model for the elastic load of film-substrate system involving imperfect interface effects

In this paper,an efficient calculation method based on discrete Fourier transformation is developed for evaluating elastic load induced elastic deformation fields of film-substrate system.Making use of 2 D discrete Fourier transformation,the elastic fields induced by Hertz load is harvested in frequency domain,and the displacement and stress fields across the interface are enforced to satisfy the elasticity conditions for each Fourier modes.Given arbitrary distributed stress field at free surface plane of the three types of film-substrate systems,unique resultant elastic field within the can be harvested.Hertz load of half space,elastic film on elastic substrate,elastic film on rigid substrate system and elastic film-substrate system with three types of imperfect interface models are investigated:(1)the spring-like imperfect interface model which can be described as:u^fk|zf=-h-u^sk|z^s=0=KTσKZ and u^fz|zf=-h-u^sz|z^s=0=KNσZZ;(2)the dislocation-like interface model,where interface displacement and stress components relation can be described as:u^fi|zf=0=k^uiju^si|z^s=0 andσ^fiz|z^f=0=σ^siz|zf=0=σ^siz|z^s=0;(3)the force-like interface model,where interface displacement and stress components relation can be described as:u^fi|z^f=0=u^si|z^s=0 andσ^fiz|z^f=0=k^tijσ^siz|z^s=0 respectively.Finally,several simulation examples are performed for verification of the reliability and efficiency of the proposed semi-analytical methods.

Damage evolution of PμLSE additive-manufactured micro-lattice metastructures: Synchrotron radiation 3D tomography image-based analysis

The manufacturing of additives with projection micro litho stereo exposure(PμLSE)has provided an opportunity for the fabrication of metastructures with complex microstructures at micro-nano resolutions.However,the performance evaluation of as-fabricated metastructures is challenging.The benefit of synchrotron radiation-based 3 D imaging techniques and advanced image processing methods makes it is feasible to study fabrication defects and damage processes of micro-nanoscale bodycentered cubic(BCC)lattices manufactured with PμLSE.First,synchrotron radiation technology is used to capture the structural features inside the micro-lattice samples.Subsequently,several types of statistical defects-based image finite element models are adopted to analyze the failure process of the structure under compression loading.Finally,comparisons between in situ experiments and numerical simulation results are performed for verification.The method of the combined non-destructive testing of synchrotron radiation and image finite element technology provides a robust technique for evaluating the performances of additive-manufactured micro-lattice with complex microstructures.

Mechanics and Wave Propagation Characterization of Chiral S-Shaped Auxetic Metastructure

Auxetic metastructures have attracted tremendous attention because of their robust multifunctional properties and promising potential industrial applications.This paper studies the in-plane mechanical behaviors of a chiral S-shaped metastructure subjected to tensile loading in both X-direction and Y-direction and wave propagation properties using the finite element(FE)method.The relationships between structural parameters and elastic behaviors are also discussed.The results indicate that the orientation of chiral S-shaped metastructure under tensile loading in the X-direction exhibits higher auxeticity and stiffness.Then,the band structures and the edge modes of each band gap of the chiral S-shaped metastructure are explored,and the relations between band gap properties and structural parameters are also systematically analyzed.Moreover,we explore the wave mitigation of the chiral S-shaped metastructures by regulating the structural parameters.Finally,the transmission properties of the finite chiral S-shaped periodic metastructures are studied to confirm the results of band gap simulation.This study promotes the engineering application of vibration isolation of chiral structures based on the band gap theory.