Visokohitrostna karakterizacija mehanskih lastnosti celičnih metamaterialov : doktorska disertacija

slovenian

doktorska dela

2022

cellular metamaterials   computer simulation   direct impact test   high strain rate   spatially graded density

Univerza v Mariboru, Fakulteta za strojništvo

In the doctoral dissertation, unique cellular metamaterials with an axisymmetric chiral auxetic structure were fabricated and characterized. Structures geometry was based on a 3D tetra chiral square unit cell with sinusoidal struts and was spatially graded in the radial direction. Samples of three axisymmetric geometrical structures with different relative densities were manufactured by advanced 3D printing technology. Due to the strain rate sensitivity, an extensive characterization of axisymmetric metamaterials in all loading regimes (quasi-static, transitional and shock regime) was required. Thus, advanced experimental and computer simulation methods were used for extensive characterization of novel metamaterial. Comprehensive mechanical characterization is achieved when the cellular metamaterial passes all the deformation phases, which are: an initial stage, the stress plateau stage and densification stage. In a preliminary study, chiral metamaterials were experimentally tested at classical SHPB test apparatus. Experimental results reviled inadequacy of SHPB apparatus, which only enables partial characterization of cellular metamaterials. By determination of the SHPB apparatus limits, classical SHPB apparatus was modified to enable direct impact test on improved DIHB apparatus. The performance of DIHB apparatus was validated by mechanical characterization of the well-known open cell metal foam m-pore®. In addition, a DIC optical method was implemented to determine the displacement fields, strain fields, and Poisson’s ratios at higher accuracy on meso-scale. DIHB test apparatus enabled extensive mechanical characterization of novel axisymmetric metamaterials with an auxetic structure. A comprehensive characterization of all tested axisymmetric structures was achieved in all loading regimes. It has been proven that with a graded internal chiral structure of axisymmetric cellular metamaterials, a constant plateau stress without oscillations and a gradually increasing Poisson ratio can be achieved. In addition, the geometric structure with the highest auxetic response, the highest stiffness with respect to the relative density of the structure, and the lowest stress oscillations in the plateau phase was determined.

08.08.2023