Investigation of the mechanical properties of auxetic tubular structures

This thesis investigates various applications and mechanical properties of auxetic structures in three key areas. The first section addresses the limitations of high-porosity peanut-shaped auxetic structures in engineering applications. Two approaches, introducing stiffeners or rotating the unit cell, are proposed to enhance the stiffness of tubulars. Finite element simulations using ANSYS/Static Structural Module are used to evaluate Poisson's ratio and stiffness of auxetic tubulars. Stiffened models show higher Poisson's ratios and stiffness. Rotating the unit cell amplifies stiffness and decreases Poisson's ratio in peanut-shaped auxetics. The second section discusses the energy absorption capabilities of auxetic tubulars made from 304L stainless steel with rotating deformation mechanisms. It focuses on perforation shapes, including elliptic, peanut, square, and stiffeners. The role of perforation shape and stiffener in crash parameters is evaluated with quasi-static testing and non-linear numerical modeling using ANSYS/Ls-Dyna. Elliptic perforations demonstrate superiority in energy absorption, and stiffened auxetic models perform well in crash applications. The third section examines the intersection of auxetic materials with civil engineering applications, specifically concrete-filled steel columns. For this purpose, the mechanical properties of composite columns formed with peanut-shaped auxetic tubulars with different Poisson ratios and porosity are examined under compression by using ANSYS/Static Structural module, and the results are compared with the behavior of conventional column. Analysis reveals that reducing tubular structure porosity and enhancing auxetic behavior increases the column's load-carrying capacity.