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Characterization of SLS printed chiral auxetic cellular structures subjected to high strain ratesThursday (08.10.2020) 14:20 - 14:40
Auxetic cellular structures are used to build modern engineering materials with advanced mechanical properties, such as negative Poisson's ratio and increased load capacity at low density , . The purpose of this study was to systematically evaluate the dynamic response of auxetic structures with different chiral cell arrangement. The study focused on high-strain rate characterization of structures at two different deformation rates, where the change of the Poisson's ratio and deformation fronts were observed. Three different chiral structures were evaluated: a) 2D hexa-chiral, b) 2D tetra-chiral and c) 3D tetra-chiral. The samples were produced with SLS printing technique from austenitic steel SS316L with outer dimensions of approx. 14 mm × 14 mm × 14 mm. Two additional plates were extruded on the contact surfaces of the samples to enable appropriate load transfer from the loading device to the sample structure. Quasi-static compression tests of each chiral structure were performed using a servo-hydraulic loading device to evaluate the basic material parameters, such as Young’s modulus, Poisson's ratio, plateau stress and densification strain. The dynamic experiments were performed using the Split Hopkinson Pressure Bar (SHPB) at the lower (21 m/s) and higher (45 m/s) velocity of the striker . The corresponding strain rates were 1000 s^(-1) and 2300 s^(-1), respectively. Two pairs of foil strain-gauges connected by a half Wheatstone bridge on each bar were used to obtain the experimental stress/strain variations on the SHPB. A pair of the Fastcam SA-Z (Photron, Japan) high-speed cameras was used for the observation of the deformation mechanism and the front propagation from two sides of the specimen. The first camera was operated at frame-rate of 252 kfps (image resolution 256 x 168 px) while the second camera was operated at frame-rate of 280 kfps (image resolution 256 x 144 px). The displacement field, strain field and Poisson's ratio were evaluated with the Digital Image Correlation (DIC) algorithm. The strain rate dependency of the structures was anticipated and confirmed by the observed stress enhancement under high strain rate impacts. A good correlation was found between the experimental and DIC determined results. Interestingly, a double stress plateau region was observed in 2D tetra-chiral and 3D tetra-chiral structures, which is a consequence of the deformation mechanism of these structures.