In this paper, impact testing of auxetic structures filled with strain rate sensitive material is presented. Two dimensional missing rib, 2D re-entrant honeycomb, and 3D re-entrant honeycomb lattices are investigated. Structures are divided into three groups according to type of filling: no filling, low expansion polyurethane foam, and ordnance gelatine. Samples from each group are tested under quasistatic loading and dynamic compression using Split Hopkinson Pressure Bar. Digital image correlation is used for assessment of in-plane displacement and strain fields. Ratios between quasistatic and dynamic results for plateau stresses and specific energy absorption in the plateau are calculated. It is found out that not only the manufactured structures, but also the wrought material exhibit strain rate dependent properties. Evaluation of influence of filling on mechanical properties shows that polyurethane increases specific absorbed energy by a factor of 1.05-1.4, whereas the effect of gelatine leads to increase of only 5-10%. Analysis of the Poisson's function reveals influence of filling on achievable (negative) values of Poisson's ratio, when compared to unfilled specimens. The results for the Poisson's function yielded apparently different values as the assessed minima of quasi-static Poisson's ratio in small deformations are constrained by a factor of 15.

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In the presented paper, three types of auxetic structures were produced with direct 3D printing and their compressive mechanical properties were tested. Samples were prepared from acrylic material suitable for high-resolution direct printing. Three different structures exhibiting in-plane and volumetric negative strain-dependent Poisson's ratio were selected for the analysis: two-dimensional missing-rib cut, two-dimensional inverted (re-entrant) honeycomb and three-dimensional inverted (re-entrant) honeycomb. The samples were subjected to quasi-static compression, from which stress-strain relationships were established. For a proper strain evaluation, digital-image correlation was applied to measure full-field displacements on the sample surfaces. From the displacement fields, true strain/true stress curves were derived for each sample. Furthermore, for each structure a three-dimensional FE model was developed using beam elements and subjected to identical loading conditions. Then, experimentally obtained stress-strain relationships were compared with numerically obtained results. For all the tested auxetic structures, the compressive behaviour was predicted well by the FE models. This demonstrates that parametric FE models can be used to tune the design parameters of the structures with a negative Poisson's ratio to optimize their overall properties. Keywords: auxetics, cellular materials, quasi-static testing, finite-element method V prispevku so predstavljene tri vrste struktur materialov z negativnim Poissonovim razmerjem, ki so proizvedene z neposrednim tridimenzionelnim tiskanjem. Preizku{ene so bile njihove mehanske lastnosti pri stiskanju. Vzorci so bili pripravljeni iz akrilnih materialov, ki so primerni za visoko resolucijsko neposredno tiskanje. Za analizo so bile izbrane tri razli~ne strukture, ki prikazujejo negativno odvisno Poissonovo razmerje v ravnini in v prostoru: dvodimenzionalni prerez z manjkajo~im rebrom, dvodimenzionalni obrnjeni (navznoter usmerjeni) vzorec satovja in tridimenzionalni obrnjeni (navznoter usmerjeni) vzorec satovja. Vzorci so bili izpostavljeni kvazi-stati~nem stiskanju pri katerem smo ugotavljali razmerja sileraztezek. Za primerno oceno sile obremenitve je bila uporabljena metoda korelacije digitalne slike in s tem izmerjeni odmiki na povr{ini vzorcev. Glede na te odmike so bile za vsak vzorec izpeljane dejanske obremenitvene krivulje. Nadalje je bil za vsako strukturo izdelan tridimenzionalni FE model, z uporabo matemati~nega modela podpornih struktur in izpostavljen identi~nim pogojem obremenitve. Nato smo primerjali eksperimentalno pridobljena razmerja med silo in obremenitvijo, z ra~unsko pridobljenimi rezultati. S pomo~jo primerjalnih diagramov sile in raztezka lahko ugotovimo, da FE modeli dobro napovedujejo obna{anje pri stiskanju vseh preizku{enih struktur z negativnim Poissonovim razmerjem. To prikazuje mo`nost uporabe parametri~nih FE modelov za prilagoditev zasnovnih parametrov struktur z negativnim Poissonovim razmerjem za optimiziranje njihovih splo{nih lastn...

Time-lapse X-ray computed microtomography was employed to quantify the deformation behaviour of closed-cell aluminium foam. The specimen was incrementally loaded and tomographically scanned using a custom X-ray tomographic device to capture the deforming microstructure. Because of the very small thickness of the cell walls and the high ratio between pore size and cell wall thickness cone-beam reconstruction procedure was applied. A finite element (FE) model was developed based on the reconstructed three-dimensional data. The FE model was used for two purposes: i) the nodal points were used for tracking the displacements of the deforming structure, ii) verification of the material model for description of the foam's deformational behaviour. Digital volumetric correlation (DVC) algorithm was used on data obtained from the time-lapse tomography to provide a detailed description of the evolution of deformation in the complex structure of aluminium foam. The results from DVC demonstrate the possibility to use the complex microstructure of the aluminium foam as a random pattern for the correlation algorithm. The underlying FE model enables easy comparison between experimental results and results obtained from numerical simulations used for evaluation of proposed constitutive models. KEYWORDS: Computerized Tomography (CT) and Computed Radiography (CR); Pixelated detectors and associated VLSI electronics; X-ray radiography and digital radiography (DR)

The mechanical behaviour of three different auxetic cellular structures, hexa-chiral 2D, tetra-chiral 2D and tetra-chiral 3D, was experimentally investigated in this study. The structures were produced with the powder bed fusion method (PBF) from an austenitic stainless steel alloy. The fundamental material mechanical properties of the sample structures were determined with classic quasi-static compressive tests, where the deformation process was captured by a high-resolution digital camera. The Split Hopkinson Pressure Bar (SHPB) apparatus was used for dynamic impact testing at two impact velocities to study the strain-rate dependency of the structures. Two synchronised high-speed cameras were used to observe the impact tests. The captured images from both quasi-static and dynamic experiments were processed using a custom digital image correlation (DIC) algorithm to evaluate the displacement/strain fields and the Poisson’s ratio. Predominant auxetic behaviour was observed in all three structures throughout most of the deformation process both under quasi-static and impact loading regimes. The tetra-chiral 2D structure showed the most significant auxetic behaviour. Significant stress enhancement in all tested structures was observed in dynamic testing. The Poisson’s ratio strain-rate dependency was confirmed for all three auxetic structures.