Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis The Investigation of Static and Dynamic Compressive Deformation Behavior of a Paper Based Sandwich Material(01. Izmir Institute of Technology, 2022) İmrağ, Berkay Türkcan; Taşdemirci, Alper; Taşdemirci, AlperIn this study, dynamic and quasi-static compression behavior of paper-based honeycomb sandwich structures were investigated. It is known that the mechanical properties of paper-based honeycomb structures change with changing strain rate values. For this reason, dynamic and quasi-static loading conditions should be considered separately when investigating the compressive behavior of the structure. In the material characterization studies, a series of tests were conducted to examine mechanical properties of the paper layer material and sandwich structure. Using data from mechanical tests, numerical models were established in the finite element tool LS-DYNA. Outputs of numerical models were validated with mechanical test outputs. After the validation study, the effects that influence the dynamic compressive behavior of the paper-based honeycomb sandwich structure and their contribution percentages were investigated using the opportunities provided by the FE tool. The results showed a 150.48 % difference between the dynamic and quasi-static compressive behavior of the structure. The numerical results obtained from explicit and implicit solvers also showed good correlation with the experimental results. In addition, the micro-mechanical modeling approach in numerical models made it possible to investigate the effects such as strain rate sensitivity of the paper layer material, entrapped air inside the core cells, and micro-inertia individually. The contribution percentages of the effects were calculated by comparing the numerical and experimental results.Master Thesis The Effect of Deformation Rate on the Damage Tolerances of Nomex Honeycomb Cored Composite Sandwiches(01. Izmir Institute of Technology, 2021) Çelik, Muhammet; Güden, MustafaThe impact response and damage tolerance of E-glass/epoxy faces and Nomex honeycomb core sandwich were determined experimentally at different velocities (0-40 ms-1). Concentrated quasi-static indentation force (CQIF), low-velocity impact (LVI) and high-velocity impact (HVI) tests were performed sequentially using a universal test machine, a drop weight tester and a modified Split Hopkinson Pressure Bar system using a hemispherical indenter with a diameter of 16 mm. Velocity was increased by reducing the mass of the indenter in HVI. HVI was performed at the same impact energies (3-33 J) as LVI. Although CQIF and LVI showed similar damage modes, front face damage initiation and perforation occurred at higher energies in LVI, which was ascribed to the rate sensitivity of the face material. When the front face was penetrated at 10 J, residual strength was found to reduce 60%. The flexural waves and core shear were observed to become dominant above 40 J. Barely visible damage was identified below 10 J with a dent depth less than 1 mm, the damage area less than 50 mm2 and an NRS of ~0.8. Visible damage occurred between 50-400 mm2 damage areas when the front face was perforated (10-39 J). Discrete source damage was detected between 400-800 mm2 where full-penetration and core shear occurred (>40 J). Although damage areas in HVI were smaller than those of LVI at the same energies, compression after impact tests showed almost no effect of velocity on NRS, except HVI tested coupon showed a slightly higher mean NRS at 5.5 J.Master Thesis The Effect of Material Strain Rate Sensitivity on the Shock Deformation of an Aluminum Corrugated Core(Izmir Institute of Technology, 2018) Canbaz, İlker; Güden, Mustafa; Taşdemirci, AlperThe effect of the material model on the crushing behavior of a layered 1050 H14 aluminum corrugated sandwich structure was investigated numerically as function of velocity (0.0048, 20, 60, 150 and 250 m s-1) using three different material models; elastic-perfectly plastic (model I), elastic-strain hardening (model II) and elastic-strain and strain rate hardening (model III). Three-dimensional finite element models were developed in the explicit finite element code of LS-DYNA. Between 0.0048 m s-1 and 20 m s-1, the numerically calculated stresses at the impact and distal end were almost the same and in equilibrium, showing a “quasi-static homogenous mode”. The deformation mode at 60 m s-1 was a “transition mode” and between 150 and 250 m s-1 a shock mode in which the layers were crushed sequentially. The numerical study showed that the strain and strain rate hardening models tended to induce non-sequential layer crushing. The collective layer crushing was also more pronounced in the material model II and III than the material model I. For low strain hardening aluminum alloys and similar materials, the effect of strain hardening in increasing plateau stress was more significant than the strain rate hardening at the quasi-static velocity, while both strain hardening and strain rate hardening effect increased with increasing velocity. The stress reduction by the inclusion of imperfections however declined with the velocity since the samples started to deform near the impact end as the velocity increased.Master Thesis Dynamic Crushing Behavior of Sandwich Panels With Bio-Inspired Cores(Izmir Institute of Technology, 2017) Güzel, Erkan; Taşdemirci, Alper; Güden, MustafaIn the current study, a new approach was shown to develop an innovative loadcarrying and energy absorbing structure which can fulfill the requirements in the fields of automotive, defense and aerospace. Two different topics which have been in great demand in the recent times were combined: sandwich structures and bio-inspiration. Balanus which is a barnacle living along the seashores and on the ships’ surfaces was taken under examination to design a novel sandwich structure core geometry. The designed geometry was manufactured with deep drawing process. The sandwich structures were produced with different face sheets using a pattern to ensure the repeatability of the crushing tests. Firstly, the advantage of the bio-inspired core over the conventional core geometries was shown with a numerical study. Then, the crushing tests were conducted at both quasi-static and dynamic loading rates. Further, the effects of foam filling, confinement, inertia and strain rate sensitivity on the crashworthiness performance of the proposed structure were investigated. In addition to the experimental studies, numerical analyses were also performed using LS-DYNA 971. In the numerical studies, manufacturing process of the core geometry was also modeled to count in the residual stress/strain so that a good proximity was obtained between the experimental and numerical results. Moreover, the penetration and perforation behaviors were inspected. Utility of the proposed geometry where a high resistance is needed against dynamic crushing was demonstrated. Finally, several suggestions were proposed for the future works to elaborate the present study.Master Thesis The Investigation of Blast Response of Sandwich Panels With Bio-Inspired Cores(Izmir Institute of Technology, 2017) Tüzgel, Fırat; Taşdemirci, Alper; Güden, MustafaIn this thesis, blast response of sandwich structures with bio-inspired cores having applicable potential for protection against blast loading, balanus, were investigated in detail. The proposed geometry consists of outer shell and inner core which separately manufactured using deep-drawing method. Commonly used blast simulation methods which are pure Lagrangian, Arbitrary Lagrangian Eulerian (ALE), and hybrid (coupled other two approaches) approaches were comparatively investigated as finding their main outstanding features and drawbacks after investigation of blast phenomenon. Calibration study with facesheet of sandwich structure was conducted to demonstrate practically performance of blast simulation methods and tune essential parameters. Well proximity between results was obtained in calibration study. Converge analysis which is especially mandatory in ALE approach was also implemented employing Grid Converge Index (GCI) for selection of mesh density of air and plate in calibration study. Pure Lagrangian approach is conservative approach among the studied blast simulation methods was shown. Direct Pressure Pulse (DPP) experiments were separately conducted for facesheets of sandwich and complete sandwich structures to reveal dynamic performance of them. Equivalent blast loading conditions corresponding to each DPP experiment were found as considering deformation levels of the structures. Therefore, DPP experiment as lab-scale experiment effectively mimicked blast-type loading was revealed. Effect of heat treatment and placement of proposed geometries subjected to blast loading were also examined creating large scaled sandwich structures. Finally, it was demonstrated sandwich structure with balanus cores revealed good blast mitigation performance even at low-scaled distance and would be able to satisfied requirement of defence industry.Master Thesis Development of Aluminum Honeycomb Cored Carbon Fiber Reinforced Polymer Composite Based Sandwich Structures(Izmir Institute of Technology, 2016) Okur, Mehmet Ziya; Tanoğlu, MetinLightweight composite sandwich structures are composed of composite structures that are laminated between thin stiff facesheets bonded to a thicker lightweight core. These structures have high potatial to be used in civil engineering applications, marine, aerospace industry etc. applications due to their high strength to weight ratios and energy absorption capacity. In these structures, the bending loads are generally carried by the force couple formed by the face sheets while the shear loads are carried by the lightweight core materials. Main purpose of the core material is to provide a high moment of inertia. Therefore, under flexural loading, sandwich panels have higher specific mechanical properties relative to the monocoque structures. Also, the core resists transverse forces and stabilizes the laminates against global buckling and local buckling. The resulting structure provides increased buckling resistance and its rigidity. In this study, sandwich composite structures were developed with carbon fiber reinforced polymer composite facesheets and the cores made by Aluminum (Al) based honeycomb with various thicknesses. Carbon fiber/epoxy composite facesheets were fabricated with non-woven unidirectional (UD) fabrics (with 0o/90o orientation) and epoxy resin by vacuum infusion technique. Al honeycomb layers were sandwiched together with carbon/epoxy facesheets using a thermosetting adhesive. Mechanical tests were carried out to determine the mechanical behavior of face sheets, aluminum cores and the composite sandwich structures. Effect of core thickness on the mechanical properties of the sandwich structures was investigated.