# FBE- Uçak ve Uzay Mühendisliği Lisansüstü Programı - Doktora

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Deformation behavior of thin walled structures filled with auxetic and non-auxetic core materials
(Lisansüstü Bilimleri Enstitüsü, 2021) Usta, Fatih ; Türkmen, Halit Süleyman ; Scarpa, Fabrizio ; 670691 ; Uçak ve Uzay Mühendisliği
• Öge
Digital-twin flight modelling through machine learning for trajectory error estimation and recovery
(Lisansüstü Eğitim Enstitüsü, 2021) Uzun, Mevlüt ; İnalhan, Gökhan ; Demirezen, Mustafa Umut ; 670702 ; Uçak ve Uzay Mühendisliği Ana Bilim Dalı
In aerospace, automotive, marine, and military applications, low-density lightweight structures such as sandwich structures and filling materials have an important role in crashworthy applications due to their crush resistance during impact and blast situations. Large deformation can occur during the impact, blast and crush events; therefore sandwich structures and sacrificial crash tubes can bottom out and very high peak load can rise. For preventing this phenomenon, auxetic materials have drawn attention as a core and filling material due to their negative Poisson's ratio (NPR) specification, which provides lateral expansion under tensile loads and shrinkage under compressive loads. The purpose of this study is to examine the energy absorption characteristics of additively manufactured polymer and metal 2D auxetic lattice cells, in the edgewise direction where auxeticity can be experienced, subjected to axial quasi-static loads. Total mass and volume of lattice cell structures are kept almost equal for better comparison. % In the work presented in this dissertation, additive manufacturing (AM) is used to produce auxetic lattice structures. AM is an improved method for quick and complex productions, and layered manufacturing is the most common method for AM; aiming the design verification, visualization, and kinematic functionality testing. AM is a computer-controlled manufacturing method of needed parts which are directly transferred as a solid model from the computer. It is a very sufficient and proved way to reduce the time for product development. There are several methods for AM, which are selective laser sintering (SLS), electron beam melting (EBM), fused deposition modelling (FDM) and stereolithography (SLA). FDM is used in this study because of its device and consumption material are cheaper compared to other mentioned methods. On the contrary of advantages of FDM method, there are some uncontrollable production problems to be solved such as incomplete bottom layers, hanging strands, missing walls, pillowing, shifted layers, unfinished parts, delamination of layers, warping syndrome, burn marks and irregular walls. In this study, FDM production problems are listed and investigated. Furthermore, solution approaches are presented to prevent those production flaws. Zortrax M200 3D printing device and Acrylonitrile butadiene styrene (ABS) are used in this study. NX 12: Siemens PLM Software is used for designing the representation of a solid model then it is converted to a stereolithography (STL) file. The converted file is then imported to the machine software of Zortrax which is called Z-Suite. For the FDM productions, production variables are defined as high quality, maximum infill, normal seam, and no support usage. Different layer thicknesses of the productions are assigned as 0.09, 0.14, 0.19, 0.29 and 0.39 mm. Several specimens in each group are manufactured to determine the effect of the layer thickness on the tensile properties. As a result, stress-strain graphs are obtained, and the effect of printing options to mechanical properties are investigated to define the feasible layer thickness for producing structures. Consequently, it is understood that ABS is not enough ductile to maintain the auxeticity of the structures for a certain period. Thus, electron beam melting (EBM) is used in this study. It is one of the promising and sophisticated AM techniques that uses an electron beam to melt metallic powders. Titanium alloy (Ti6Al4V) is used by the reason of its outstanding mechanical properties of high specific strength, high corrosion resistance, excellent biocompatibility; hence titanium alloys are prominent material for aerospace and bioengineering fields. ARCAM EBM A2 3D printing machine and ARCAM Ti6Al4V ELI, which is a gas atomized prealloyed powder in a size range of 45-100 $\mu$m, are used in this study. To understand the mechanical behaviour and characterize EBM printed parts tensile tests are conducted. Each test specimens are produced in three different directions, 0, 45 and 90 due to characteristic anisotropic behaviour of EBM printed parts. Moreover, a study on the effect of inner defects, which are detected as the lack of fusion, in the EBM printed parts is conducted to define the mechanical performance of EBM printed components. The area of defect regions are observed using a scanning electron microscope and measured to investigate the defect area effect to mechanical performance. As a result, EBM printed Ti6Al4V parts experiences isotropic elasticity and yield stress, however, strain at break specifications show substantial differences according to build orientation which is dominated by the lack of fusion (LOF) problems. As an output, it is understood that the increase in the LOF region shows an almost linear decreases pattern with the strain at break value. Furthermore, firstly a comparative study with re-entrant and anti-tetrachiral auxetic is conducted to define the better auxeticity mechanism for 2D auxetic lattices. However, anti-tetrachiral auxetic structures show greater results than the re-entrant auxetic structures and, a modification for the re-entrant structure is needed to increase its energy absorption ability. Besides, another 2D auxeticity mechanism, which is called rotating rigid auxeticity, exists, however, this type of structure is not feasible for crushing applications due to bulky parts in the structure so it is not counted in this study. A comparative compression investigation of anti-tetrachiral and modified re-entrant lattices is conducted in-plane direction using experimental and numerical analyses. Lattice structures are manufactured using FDM 3D printing technology and crushed at the quasi-static condition. Non-linear finite element (FE) models of both structures are established, and the FE results are systematically compared with the experimental results. The onset of densification phases of both structures is determined numerically. Results indicate that deformation modes strongly affect the force-deflection response of both designs. In this manner, failure regions and buckling deformation in the tests are identified to find a relation with theory and to modify geometries. The anti-tetrachiral design exhibits higher specific energy absorption than modified re-entrant hexagonal lattices. Beyond the auxetic characteristics, deformation mechanism of the anti-tetrachiral lattices provides an opportunity to construct excellent crush absorption in-plane direction thanks to its high shear strength stem from its unique deformation mechanism. After having the validated constitutive equation for FDM printed ABS, a benchmark test is conducted using ABAQUS commercial finite element method (FEM) software to evaluate and define the energy absorption effective chiral mechanism among hexachiral, trichiral, anti-trichiral, tetrachiral, anti-tetrachiral and regular hexagonal topologies. As a result, hexachiral (chiral) auxetic lattice is selected, and a part of the study is devoted to the understanding of the energy absorption characteristics of filled chiral auxetic lattices cylindrical composite tubes subjected to a uniaxial and lateral quasi-static load. The lattice structures are manufactured using an FDM 3D printing technique and ABS material. Composite tubes without filling material are initially subjected to uniaxial and lateral quasi-static crushing loads at a rate of 10 mm/min. The same types of experiment are then performed on chiral lattices and chiral lattices filled composite tubes. For the different cases, the load-displacements curves are analyzed and the specific energy absorption (SEA) values are compared. The SEA capability for the axial quasi-static crushing of the chiral lattices filled composite tubes reach 43 J/kg with a 45\% decrease in comparison with the hollow composite design. On the contrary – and quite remarkably - the average SEA value in the case of lateral loading is 2.36 J/kg, with a 450\% increase in comparison with the hollow composite configuration.% Finally, chiral auxetic unit cell structures are produced from Titanium Alloy (Ti6Al4V) metallic powder using Electron Beam Melting (EBM) additive manufacturing technology. EBM printed chiral auxetic lattices are compressed with a three steps cyclic load profile in the edgewise direction numerically and experimentally. Also, a crush study is performed numerically and experimentally to evaluate the energy absorption ability of EBM printed metallic chiral lattice cells. For material characterization and understanding the material behaviour of EBM printed parts tensile and three-point flexural tests are conducted. Tensile and bending specimens are produced in two different thicknesses and orientations. Moreover, a surface roughness study is conducted due to high surface roughness of EBM printed parts, and an equation is offered to define load-carrying effective area for preventing cross-section measurement mistakes. In compliance with the equation and tensile test products, a constitutive equation is formed and used in the numerical analyses after selection and calibration process. The constitutive equation is verified by the comparison of experimental and numerical three-point bending test results. Furthermore, a compressive load profile is applicated to the EBM printed chiral lattice unit cells with two different thicknesses to track their Poisson's ratio variation, and displacement limit under large displacements without the formation of degradation, permanent deformations and failures. The utilization of a finite element code to verify a numerical model for optimum topology design and mechanical performance forecast, the same scenarios investigated in the compressive load profile experiments are then evaluated using non-linear computational models. In the computational models, a Fortran subroutine is used to define the load and displacement controls. In addition, in the numerical crush analysis, auxetic lattice cells are crushed between two rigid plates using damage and failure criteria for simulating failures. As a result, it is revealed that the thickness of the chiral lattice cells is the dominating parameter about its stiffness and auxeticity. Increase in the chiral node radius to thickness rate of 0.15 to 0.30 causes loss of auxetic behaviour. Lastly, their compression behaviours are compared, and the one showing auxeticity presents better energy absorption ability in terms of crush force efficiency, however, considering specific energy absorption value the results show the opposite.