Experimental research on mechanical behaviors of additively manufactured cellular structures from titanium and polymer based materials

In the industry, there has been an increasing trend towards designs with featured behavior, light materials, and new manufacturing methods. With topological design, the additive manufacturability of cellular structures with lighter, acceptable strength, specific energy absorption, flexibility, and geometric complexity has been the focus of the researchers. There are many scientific studies in the literature on the behavior of materials with a positive Poisson's ratio, but there is limited research on the mechanical behavior of new cellular structures with a negative Poisson's ratio. In this PhD thesis, cellular structures with different topologies such as honeycomb, re-entrant, chiral and hybrid were designed and manufactured with optimum parameters from PLA and Ti6Al4V materials to investigate the negative Poisson's ratio and its effect on mechanical behavior. Tensile and compression test specimens in accordance with ASTM D638-14 and E8 / E8M-16a standards were manufactured from PLA and Ti6Al4V material by fused deposition modelling (FDM) and direct metal laser sintering (DMLS) respectively. The effect of geometric variables such as the positioning direction of the unit cell of the cellular structures and the thickness, length, height, and angle of the rib in the unit cell on the mechanical performance were determined. As a result, the dimensional difference in the unit cell of the cellular structures made of Ti6Al4V by DMLS and PLA material by FDM between design and manufacturing was measured as 1,6 % and 22,5 % respectively. The surface roughness of PLA samples was measured in the range of 10-20 µm, and 3-4 µm for Ti6Al4V. With the increase in rib thickness, elastic modulus, tensile and compression strengths increased. It was determined that honeycomb structures absorb more energy under tension and re-entrant structures in compression. Especially, honeycomb structures have a positive Poisson value, while re-entrant, chiral and hybrid structures exhibited negative Poisson behavior up to a certain amount of deformation. It is concluded that this behavior contributes to the energy absorption of the re-entrant structures.