Doktora Tezleri

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Aerodynamic optimization of a transonic aero-engine fan module
(Izmir Institute of Technology, 2016-07) Kor, Orçun; Özkol, Ünver
Aerodynamic design of an aero-engine fan blade is a multi-step process with multi-variables. The general purpose in aerodynamic design is to obtain proper blade angles and flowpath geometry providing the necessary pressure ratio with maximum efficiency, while respecting the structural and aerodynamic constraints. The throughflow design in aerodynamic design procedure is a key step where one can obtain a basic aero-design which generally fixes 80% to 90% of the final fan geometry, by adjusting parameters like blade exit angle distribution, solidity, hub and shroud contour, meridional chord length, etc. Throughout this procedure, the aim of the designer is to obtain an optimum (i.e. light, reliable and robust) system with highest efficiency. Among optimization methods, zero order methods are reported to fit best for turbomachinery problems, due to their good performance in discrete and non-differentiable problems and their ability to find the global optimum. Genetic algorithm is the most widely used optimization method in turbomachinery optimization. Methods inspired by swarm intelligence are reported as promising global optimizers, whereas, to the author’s knowledge, there are no reported studies that employs such algorithms in turbomachinery throughflow optimization. These methods can find the neighborhood that provides the globally optimum design, rather than exactly finding the global design. This drawback is overcome by hybridizing genetic/swarm inspired algorithms by first order (gradient based) methods. Within this aspect, the present study focuses on developing genetic and swarm inspired algorithms hybridized with gradient based algorithms to find the optimum throughflow design of a transonic aero-engine fan module.
Analysis and synthesis of parallel manipulators
(Izmir Institute of Technology, 2008) Can, Fatih Cemal; Alizade, Rasim
In this study, novel parallel manipulators are introduced for industrial and medical applications. New methods are developed for the structural synthesis of Euclidean platform robot-manipulators with variable general constraints (EPRM). New mechanical structures such as serial, parallel and serial-parallel EPRM are designed along with proposed method.A new dimensional synthesis method of two DoF planar and spherical seven link mechanisms is presented. Interpolation and least square approximations are used to design the mechanism. In the solution of dimensional synthesis problems, nonlinear equations are converted to system of linear equations. The motion generation problem of a 3 DoF platform robot manipulator is solved for three, four and five precision poses. It is shown that the synthesis problem can be solved analytically for three prescribed poses. However, the solution is achieved by using a numerical method for four and five poses. The result, which is obtained from three prescribed poses, is used as an initial guess for four and five poses. Kinematic analysis of the manipulators is investigated. After the derivation of vector-loop equations, inverse and direct position analyses of the manipulators are presented. Constant orientation workspace of a three DoF spatial parallel manipulator is presented. The mechanical elements which are necessary for the construction of manipulators are introduced. The information about the motors which is needed for actuation of manipulators is given. Three DoF parallel manipulator is constructed for a industrial packaging system. Assembly of manufactured parts and mechanical elements are shown.
Biokinematic analysis of human body
(Izmir Institute of Technology, 2011) Gezgin, Erkin; Alizade, Rasim
This thesis concentrates on the development of rigid body geometries by using method of intersections, where simple geometric shapes representing revolute (R) and prismatic (P) joint motions are intersected by means of desired space or subspace requirements to create specific rigid body geometries in predefined octahedral fixed frame. Using the methodical approach, space and subspace motions are clearly visualized by the help of resulting geometrical entities that have physical constraints with respect to the fixed working volume. Also, this work focuses on one of the main areas of the fundamental mechanism and machine science, which is the structural synthesis of robot manipulators by inserting recurrent screws into the theory. After the transformation unit screw equations are presented, physical representations and kinematic representations of kinematic pairs with recurrent screws are given and the new universal mobility formulations for mechanisms and manipulators are introduced. Moreover the study deals with the synthesis of mechanisms by using quaternion and dual quaternion algebra to derive the objective function. Three different methods as interpolation approximation, least squares approximation and Chebyshev approximation is introduced in the function generation synthesis procedures of spherical four bar mechanism in six precision points. Separate examples are given for each section and the results are tabulated. Comparisons between the methods are also given. As an application part of the thesis, the most important elements of the human body and skeletal system is investigated by means of their kinematic structures and degrees of freedom. At the end of each section, an example is given as a mechanism or manipulator that can represent the behavior of the related element in the human body.
Compliant control of robotic co-workers in surgical applications
(01. Izmir Institute of Technology, 2023-01) Ayit, Orhan; Dede, Mehmet İsmet Can
In recent years, robots have taken place in surgical operations due to their advantages over humans, such as power, endurance, dexterity, and accuracy. Because of the lack of abilities, such as decision-making, adaptability, and creativity, human surgeons supervise the robots. The robots share the operation places with humans, called co-worker robots. Robots have the power to harm their environment; therefore, robots can generate dangerous situations for surgeons and patients. To deal with the issues, this dissertation aims to design active compliant control algorithms such as impedance control, admittance control, and hybrid position/force control to achieve safe interaction forces in surgical operations by considering the performance. The surgical co-worker robot’s type, actuation system, robot dynamics, and environment dynamics are important factors for designing the active compliant controller. Besides these, stability and robustness for safety, and agility and human effort for performance are considered for designing the controller. This dissertation takes into account three interaction scenarios encountered in surgical operations. In these scenarios, it is expected from the co-worker robot that it adapts to the sudden change in its environment dynamics. For instance, safe interaction is desired when the robot interacts with the stiff and soft tissues. To handle the issue, a switching control methodology is presented where the predefined control parameters are switched according to their environments. The methodology is implemented in a novel co-worker robot named NeuRoboScope, designed to assist the endoscopic pituitary gland surgery with the support of The Scientific and Technological Research Council of Turkey (TUBITAK). Moreover, active compliant control algorithms require a motion control algorithm as a low-level controller. In this dissertation, the computed torque method and independent joint controllers with gravity compensation are used as motion control algorithms. The computed torque method requires the dynamic model of the robot. Due to that, the dissertation proposes a simplified dynamic model with a correction coefficient for computational efficiency. ARM Cortex M4 processor runs the computed torque method with the proposed dynamic modeling method at 500 Hz. Also, this dissertation presents an independent joint controller which uses the simplified gravity matrix as a feedforward term for compensating the NeuRoboScope’s gravitational effect. The experimental results of both controllers are discussed in this dissertation.
Control of dynamics system behaviour by magnetorheological and varible orifice dampers
(Izmir Institute of Technology, 2013-07) Kınay, Gökçe; Özdemir, Serhan
Passive and semi-active control devices are widely utilized for response reduction in civil engineering structures subjected to strong earthquakes. These devices absorb energy from the system. They do not add energy into the system being controlled. Therefore, the system stays stable in the sense of bounded-input-bounded-output stability. In the current study, semi-actively controlled devices were investigated: magnetorheological dampers (MRDs) and variable orifice dampers (VODs). Various control schemes were applied to control the seismic response of a three-storey model structure. Some of these control systems were composed of MRDs applied to the bare model structure. Some of them consisted of hybrid application of MRD or VOD to the seismic isolated model structure. The hybrid control, which consisted of passive and semi-active controllers, was studied in order to benefit from advantages of both strategies and to compensate for their weak properties. In the simulations, different controllers were designed depending on the linear quadratic regulator (LQR), sliding mode control, H2/LQG, fuzzy logic, and linear quadratic Gaussian (LQG). The effectiveness of the control algorithms and the usefulness of semi-active dampers for response reduction were demonstrated through various numerical examples. Kalman-Bucy filter was designed due to the necessity of an observer in real-world applications with state feedback control. Additional damping at the base level reduced the base velocity directly and decreased the base displacement indirectly at the expense of larger drifts and floor accelerations of the superstructure. The study has shown that the hybrid control system can prevent or significantly reduce structural damage during a seismic event even in case of a frequency overlap of excitation and system. Additionally, vibration response of a truck seat was controlled by three different passive dampers and the MRD. The passive dampers could effectively reduce the oscillations of the truck seat. On the other hand, the capacity of the RD-1005-3 MRD was excessive for the suspension system of the current truck seat.
Design of a robot assisted minimally invasive surgical system for pituitary tumor surgery based on safety features
(Izmir Institute of Technology, 2020-07) Maaroof, Omar Waleed Najm; Dede, Mehmet İsmet Can
The study is on the designing a robot assisted endonasal endoscopic surgical system; NeuRoboScope, the pituitary tumor resection surgery system. This system comprises a passive and an active arm. The passive arm positions the active arm in the surgery zone while the active arm assists the surgeon by positioning the endoscope during the surgery. The focus of this thesis is the mechanical and control safety features that can be implemented in the system. The safety enhancement methods of robot assisted minimally invasive surgery systems are investigated. Among the seventeen robot assisted endoscope holders, sixteen of them have been implemented in pituitary tumor and sinus surgeries. Safety is the main criterion that advances the progress of these systems and places them in operation rooms. Accordingly, two optimization procedures have been applied during the design of the NeuRoboScope system that have a direct effect on the suggested safety features. A novel optimization technique is proposed by employing a redundancy resolution method. The most suitable fixing point of the passive arm and its first link length is optimized to achieve the maximum manipulability with restrictions imposed by a modified condition number index and impedance of the passive arm. The active arm's partial gravity compensation is studied. Three spiral springs are used as counter-springs as the most compact and lightweight partial gravity compensation method. Particle swarm optimization method is employed for the optimization of the design parameters: spiral spring stiffnesses and preload angles. Consequently, at least 66% of actuator loads are compensated.
Design of dimensionally-stable laminated somposites subjected to hygro-thermo-mechanical loading by stochastic optimization methods
(Izmir Institute of Technology, 2011) Aydın, Levent; Artem, Hatice Seçil
The materials used in aerospace structures such as antenna, satellites and missiles should have such features as low density, high stiffness, low coefficients of thermal and moisture expansions simultaneously. Fiber reinforced polymer composite materials can satisfy these requirements with an appropriate stacking sequence using optimization methods and hence dimensionally stable composites are obtained. In this thesis, two different materials carbon/epoxy and E-glass/epoxy composites are considered. Both materials have been used for optimization, stress and failure analysis. However, only for E-glass/epoxy, experimental studies have been performed including determination of stiffness, strength characteristics, Poisson's ratio, fiber volume fraction, glass transition temperature (Tg) and coefficient of thermal expansion (CTE). The objective of optimization part is to design the stacking sequence of the carbon/epoxy and E-glass/epoxy laminated composites having low CTE and high elastic moduli. In design process, multi-objective genetic algorithm optimization of the carbon/epoxy composite plates are verified by single-objective optimization approach by using the Genetic Algorithm (GA), Generalized Pattern Search (GPS) and Simulated Annealing (SA) algorithms. MATLAB Optimization Toolbox is used to obtain Pareto-optimal designs and global optimum points for different model problems. Stress and strain distributions are presented through the thickness of the laminates subjected to mechanical, thermal, and hygral loadings. Stress analysis results showed that effect of mechanical loads dominate to hygral and thermal loads. All the stochastic search methods carried out in the present thesis have produced almost the same results with different stacking sequences.
Desing and production of light-weight pressure resistant composite tank materials and systems for hydrogen storage
(Izmir Institute of Technology, 2020-07) Kartav, Osman; Tanoğlu, Metin; Tanoğlu, Metin
This thesis focuses on the development of high-pressure resistant composite tanks for hydrogen storage. For this aim, composite tanks with aluminum liners were designed and manufactured by filament winding technique with various lay-up configurations and tested. The main objective of this study was to develop composite tanks with 700 bar working pressure and 1400 bar burst pressure. Furthermore, composite doily layers were incorporated into the filament winding technique and inserted at the front and end dome sections of the composite tanks to improve the burst pressure performance of the composite tanks and to develop the manufacturing process. Before the manufacturing process, the winding simulations were completed using CADWINDTM CAM software. The manufactured composite tanks were hydrostatically loaded with increasing internal pressure up to the burst pressure. During loading, the deformations over the composite tanks and liners were measured locally using strain gauges. Besides, composite plates were manufactured by filament winding technique to determine the mechanical and the thermo-mechanical properties, and the fiber mass fractions of composite sections were determined. Additionally, a preliminary study was carried out to investigate the effect of hybrid fiber usage on the burst pressure performance of steel liner based composite tanks. The effect of filament winding parameters on the burst pressure performance of composite tanks was investigated experimentally. The aimed burst pressure value of more than 1400 bar was obtained in this study for aluminum liner-based carbon fiber reinforced composite tanks. Also, a desired safe burst mode that is expected to occur in the mid-region of the composite tanks was successfully obtained. This study may be useful for the development of composite tanks for high-pressure hydrogen storage especially for the automotive industry and can be helpful to decrease the usage of fossil fuels.
Development and characterization of innovative fiber reinforced prepregs and their composites containing functional fillers
(Izmir Institute of Technology, 2021-07) Uz, Yusuf Can; Tanoğlu, Metin; Tanoğlu, Metin; Izmir Institute of Technology
This Ph.D. thesis aims to prepare laboratory-scale carbon fiber reinforced prepregs and improve the performance of their composites by incorporating functionalized single-wall carbon nanotubes (SWCNTs). The effect of nano-scale functional fillers on the characterization of prepregs and their composites was investigated to develop innovative materials for primary structures. To affect dispersion characteristics, SWCNTs were functionalized by oxidizing their surface with the carboxyl (-COOH) group using acid treatment. The modified resin system containing 0.05, 0.1, and 0.2 wt. % F-SWCNTs were developed with novel multi-step dispersion techniques. FTIR spectroscopy was performed to identify new bonding groups formed after the covalent functionalization. Unidirectional carbon fiber reinforced prepregs with/without F-SWCNTs were prepared using a drum-type winding technique by utilizing the solvent-dip (solution impregnation) process. The effect of F‐SWCNTs on the curing process and kinetic parameters of the carbon fiber/epoxy-based prepregs were investigated using non‐isothermal DSC. The activation energy of the curing reaction was calculated by the isoconversional methods. Also, the new numerical approach called GMN was developed to determine the activation energy of the thermosetting materials. For the fabrication of prepreg-based composite laminates, the vacuum bag-only (VBO) method was performed. The fiber volume fractions of the CFRP samples changed between 55.3% and 50.16%. The mechanical and thermomechanical properties of prepreg-based CFRP composites with/without F-SWCNTs were investigated. The optimum mechanical properties of F-SWCNTs filled CFRP composite was achieved at 0.05 wt.% of F-SWCNTs. However, mechanical properties were decreased due to the addition of higher content of F-SWCNTs, in comparison with neat CFRP.
Development and mechanical characterization of anti-blast sandwich composites for explosive effect
(Izmir Institute of Technology, 2011) Baştürk, Suat Bahar; Tanoğlu, Metin
Composite sandwich structures have high potential to be used in anti-blast armour systems due to their lightweight and resistance to explosive effects. This study focuses on the production and mechanical characterization of sandwich structures with aluminium (Al) foams of various thicknesses in conjunction with skins composed of Al/GFPP fibre/metal laminates. The bonding between the components of the sandwich was achieved by various surface modification techniques such as silane surface treatment, polypropylene (PP) adhesive film addition and their combination. The Al sheet/Al foam sandwiches were also prepared to investigate the effect of GFPP addition on the performance of sandwich structures. The energy absorption capacities together with compressive and flexural behaviour of both Al foams and FML/Al foam sandwiches were evaluated by flatwise compression and three point bending tests. The samples with higher elastic modulus usually exhibited higher collapse strength for each thickness set of foam and foam based sandwiches. Also, the core thickness increase led to the increase of overall flexural collapse load and GFPP presence promoted the strength of the sandwiches and dissipated energy values. In order to investigate the blast response of the sandwich panels, the quasi-static sandwich panel analysis was related to dynamic blast loadings. For this purpose, the sandwich composites were subjected to compression loading with a specially designed loading fixture and the corresponding test method is called as “simulated blast test”. The sandwiches were assumed as single degree of freedom mass-spring systems to include the dynamic effect. The peak deflections and survivability of the panels under blast loading were predicted based on the formulations reported in the literature. To evaluate the blast response of the monolithic materials, composites and sandwich panels, blast testing was performed using specially designed blast test frame system and 0.5 to 6 kg TNT explosives. Test results revealed that composites such as GFPP exhibited successful results against blast explosions.
Development of a streamline curvature throughflow design method for fan module of turbofan engines
(Izmir Institute of Technology, 2015-07) Acarer, Sercan; Özkol, Ünver
Through-flow modeling of turbomachinery flows is the principle tool for inverse design, off-design analysis and post-processing of test data, due to its capability to simulate the principal aspects of turbomachinery flows, swirling flow with rotors and stators, in the axisymmetric meridian plane with minimum two orders of magnitude smaller computational time compared to three-dimensional analysis methods. Turbomachine energy equation and empirical models for incidence, deviation, pressure loss and blockage are used to define source terms for an axisymmetric compressible flow solution. Even though the subject has been studied in numerous aspects for compressors and turbines, open literature on fully coupled fan and splitter design of turbofan engines is still limited. The present study addresses this void by developing a new split-flow method for inverse streamline curvature flow solution methodology in the course of this thesis. Hybridized empirical models that are compiled from the literature are implemented as a baseline to be calibrated. The method is validated both experimentally and numerically on a total of six different test cases within a three-step validation strategy. Firstly, split-flow solutions of the developed method for three representative duct geometries, but without a turbomachinery, are validated. Secondly, two different single-stream transonic fans, NASA 2-stage fan and a custom-designed fan stage are used to experimentally and numerically validate the empirical models, respectively. Thirdly, experimental data of GE-NASA by-pass fan is used to validate the complete models. It is shown that the accuracy of solutions in the tested cases are within less than 1.6% in pressure ratio, 2.3% in efficiency, 8% in velocity and 1.8 degree in flow angle. With this accuracy level, the proposed method is shown to be valid and can be implemented into existing compressor streamline curvature methodologies with minimal numerical effort.
Development of energy-efficient personalized thermal comfort driven control in HVAC systems
(Izmir Institute of Technology, 2018-12) Turhan, Cihan; Gökçen Akkurt, Gülden; Simani, Silvio
Increasing thermal comfort and reducing energy consumption are two main objectives of advanced HVAC control systems. Studies conducted in the last decade show that intelligent HVAC systems can geatly affect thermal comfort, health, satisfaction, and productivity of building occupants while decreasing the energey consumption. Also, personelized thermal comfort driven control of the HVAC systems is the most effective way of saving energy and maintaining thermal comfort. In this thesis, an energy-efficient personalized thermal comfort control algorithm is developed to improve HVAC control systems. The thesis presents a complete system to control algorithm which includes the deployment of wireless sensor network. First a novel control algorithm is developed to perceived comfort conditions of occupants and to save energy. Then, a prototype of the personalized thermal comfort driven controller (PTC-DC) is manufactured an tested in a case building at İzmir Institute of Technology Campus, İzmir/Turkey. The proposed control strategy is tested betwen July 3rd, 2017 and November 1st, 2018, and compared with conventional controller in terms of energey saving and boath energetic and exergetic approaches of thermal comfort. The results showed that PTC-DC satisfies neutral thermal comfort for 92% of total measurements days while AM=0 for only 6% of total measurement days for conventional controller. From energy consumption point of wiev, PTC-DC decreased energy consumption by 13.2% compared to conventional controller.
Development of multi and double walled carbon nanotubes (CNTs) / vinylester nanocomposites
(Izmir Institute of Technology, 2008) Seyhan, Abdullah Tuğrul; Tanoğlu, Metin; Tanoğlu, Metin
This study focuses on development and characterization of thermosetting resin based nanocomposites containing multi and double walled carbon nanotubes with and without amine functional groups (MWCNT, DWCNTs, MWCNT-NH2 and DWCNTNH2).A novel 3-roll milling technique was conducted to prepare the resin suspensions with carbon nanotubes (CNTs). Rheological measurements performed on the resin suspensions showed that addition of very low contents (0.05, 0.1 and 0.3 wt. %) of MWCNTs and MWCNT-NH2 affected the flow characteristic of the resin, significantly.Further, the curing behavior of a vinylester-polyester hybrid resin suspensions containing 0.3 wt % of MWCNTs and MWCNT-NH2 was intensively studied. It was found that regardless of amine groups, presence of CNTs affected the polymerization of the hybrid matrix resin. Final individual fractional conversion rates of styrene and vinylester monomers were found to be vastly dependent on the type of CNTs. Glass transition temperature (Tg) values of the nanocomposites with MWCNTs and MWCNTNH2 were found to increase with filler content. Moreover, nanocomposites containing MWCNTs and MWCNT-NH2 were found to possess higher tensile strength, elastic modulus as well as fracture toughness and fracture energy as compared to the neat hybrid resin. On the other hand, electrical properties of the nanocomposites were also investigated and it was found that nanocomposites with MWCNTs exhibited the lowest percolation threshold value. In addition, nanocomposites with amino functionalized CNTs were found to exhibit lower electrical conductivity as compared to those with untreated CNTs. Nanocomposites with AC electric field induced aligned CNTs were also prepared. Finally, based on the findings obtained for CNT/ resin suspensions, as a case study, electrically conductive glass fiber reinforced composite laminates were successfully produced, using the CNT modified resin suspensions as matrix material, via Vacuum Assisted Resin Transfer Molding (VARTM) and Resin Transfer Molding (RTM) methods.
Diagnosis and recovery of hardware faults encountered during operation of mobile robots
(Izmir Institute of Technology, 2020-12) Şahin, Osman Nuri; Dede, Mehmet İsmet Can; Özdemir, Sehan; Izmir Institute of Technology
Mobile robots are used in many critical tasks. In such tasks, it is of great importance to tolerate the faults that the robot may encounter during the operation in order to complete the task successfully. This dissertation focuses on tolerating the faults that occur in the hardware of the mobile robots. To tolerate these faults, it is necessary to be prepared for the faults that the robot may encounter during the operation and to determine an appropriate fault toleration strategy. The mobile robot considered in this dissertation has holonomic motion ability in the plane thanks to its omnidirectional wheels. The types of faults focused on are the slippage of one of the wheels of this mobile robot and the performance degradation in the motor that actuates one of the wheels. To tolerate these two faults, an active fault tolerant control method is developed. A model-based fault diagnosis algorithm is developed for fault diagnosis algorithm, which is one of the two main parts of active fault tolerant control. To obtain the dynamic model of the mobile robot that is used in this algorithm, firstly, the friction between the wheel and the ground used is modeled. The parameters of the friction model are identified via the developed test setup. As a result of the tests performed for fault diagnosis, it is seen that these two types of faults occurring in the holonomic mobile robot can be diagnosed with developed fault diagnosis algorithm. In order to tolerate these faults, two different fault recovery algorithms which make use of kinematic redundancy of the mobile robot are developed, and the developed algorithms are tested. As a result of the fault recovery tests performed for the motor performance degradation, it is observed that the motion performance of the mobile robot improved despite the presence of the fault. Thanks to the developed recovery algorithm in the recovery tests for wheel slippage, it is observed that there is a significant decrease in the amount of slippage occurring on the faulty wheel and accordingly the mobile robot performs the desired motion more accurately.
The effects of diatom frustule filling on the quasi-static and high strain rate mechanical behavior of polymer matrices
(Izmir Institute of Technology, 2010) Gültürk, Elif; Güden, Mustafa
In this study quasi-static tension and quasi-static (1x10-3 and 1x10-1 s-1) and high strain rate (300-600 s-1) compression and quasi-static tensile behavior of diatom frustules-filled, Diatomaceous earth (CD) and Kieslguhr (ND), epoxy matrices were investigated experimentally and microscopically. For comparison, the compression and tensile behavior of the neat epoxy was also determined. Compression results showed that diatom frustules filling increased both modulus and yield strength of the epoxy matrix at quasi-static and high strain rates. ND frustules filled epoxy samples showed a higher strain rate sensitivity compare with CD filled samples. Tensile test results showed that the modulus of filled epoxy increased with increasing frustule content. The frustule filling, however, decreases the tensile failure strains of the epoxy and increased the tensile strength slightly. Microscopic observations on the fracture surfaces and the mounted cross-sections of deformed samples showed that the failure mechanisms were debonding of the frustules-epoxy interface and the fracture of the frustules at quasistatic strain rates while the failure of the filled composite at high strain rate was dominated by the fracture of the matrices. These results confirmed that significant benefits might anticipated from the use of diatom frustules as reinforcements and fillers in polymeric materials. Various methods; acid leaching, thermal shock and ball milling were further applied to process nano size silica powder from frustules. Projectile impact tests indicated that frustule addition increased the ballistic resistance of epoxy matrices. Finally, the strength and modulus of the filled epoxy matrices were predicted using analytical models developed for short fiber composites.
The effects of SiC particle addition on the foaming and mechanical behavior of aluminum closed-cell foams produced by foamming of powder compacts
(Izmir Institute of Technology, 2010) Yüksel, Sinan; Güden, Mustafa
The maximum and linear expansions of a large number of SiC particle/Al powder compacts of varying average SiC particle size (0.03-67 .m), weight percentage (wt%) and size distribution and Al compacts without particle addition were experimentally determined. The powder compacts showed varying expansion values depending on the size, wt% and size distribution of the particles. The linear and maximum expansions for small size SiC particle additions were found to be relatively high at relatively low wt%'s (5 wt%) and decreased with increasing wt% of the particles from 5 wt% to 10 and 15 wt%. The compacts with small average particle size but wider particle size distribution showed higher expansions than the compacts with the similar average particle size but narrower particle size distribution, showing the importance of the particle size distribution on the expansions of Al compacts. The foam expansions were further shown to increase with SiC particle addition until about a critical cumulative particle surface area; however, the expansions decreased significantly at increasingly high cumulative particle surface areas due to the excessive increase in the compact viscosity. For the investigated powder compacts, the optimum wt% of SiC addition was determined, as a function particle size, based on the critical cumulative particle surface area. Compression tests showed that the density of the foam was the most effective parameter in increasing the plateau stresses. Microscopic analysis showed that the main deformation mechanism in Al and SiC/Al foams was the cell wall bending, i.e. cell edges buckled over cell walls. This resembled the deformation characteristics of the open cell foams. It was finally shown that SiC particle addition increased the foam plateau stresses over those of Al foam without particle addition, which was mainly attributed to the reduced fraction of the metal on the cell edges.
Examination of fatigue behaviour of carbon fiber reinforced polymer composites
(Izmir Institute of Technology, 2021-07) Güneş, Mehmet Deniz; Tanoğlu, Metin; Tanoğlu, Metin; Izmir Institute of Technology
This PhD thesis aims to examine the fatigue behavior of sandwich panels fabricated from adhesively bonded aluminum honeycomb core and carbon fiber reinforced polymer composite face sheets. Initially, sandwich panels were manufactured with three different amounts of adhesive in their interface. Static flexural behavior was characterized with three-point bending tests. Load-displacement curves and static flexural failure modes were obtained and utilized to compare the static flexural behavior of fabricated sandwich. Fatigue behavior of sandwich panels were characterized with the three-point bending fatigue tests. Stiffness degradation curves were used to identify the failure cycles of sandwich panels. Fatigue failure modes and S-N curves were obtained to find out the effect of amount of adhesive on fatigue behavior of sandwich panels. The other study within this thesis was made to investigate the effect of core thickness on the fatigue behavior of the sandwich panels based on aluminum honeycomb core and carbon fiber reinforced polymer composite face sheets. Sandwich panels were fabricated by using three different aluminum honeycomb core thickness. Static flexural tests were carried out to determine the static flexural behavior of developed sandwich panels. Load-displacement curves and failure modes were obtained from flexural tests. In addition to this, core shear tests were performed to investigate the core shear strength of the honeycomb cores with different core thickness. Effect of core thickness on fatigue behavior of sandwich panels were characterized with fatigue failure modes and S-N curves. Stiffness degradation method was used to determine the fatigue failure cycles of the sandwich panels.
Experimental and numerical approaches to evaluate the crushing behavior of combined geometry core sandwich structures against blast
(Izmir Institute of Technology, 2015-07) Kara, Ali; Taşdemirci, Alper; Güden, Mustafa
In this study, novel sandwich structures containing combined geometry structures as core materials were designed and developed for blast protection applications. The proposed combined geometries consist of a hemispherical geometry attached seamlessly to a cylindrical segment. Deep drawing method was used to obtain four different types of combined geometries having two different radii from blanks with two different initial thicknesses. The mechanical properties of the blank material were obtained by conducting tensile experiments at quasi-static and high strain rate regimes. Thereafter, crushing and energy absorption behavior of core units were determined by tests at quasi-static and low velocity regimes, experimentally. Before crushing simulations, manufacturing method was simulated to have realistic residual stress/strain and thickness variations of numerical specimens. Having accurate deformation history, crushing experiments were simulated and a good agreement was reached proving the realistic modeling of the manufacturing effects. The effect of heat treatment on the crushing behavior of combined geometry shells was also investigated both experimentally and numerically and there was a good agreement noted. After, cross-shaped sandwich structures of one type of combined geometry were prepared. Static, low velocity and high velocity crushing behavior of sandwiches were investigated. Study on sandwich structures also included confined experiments in order to account for the interaction between the core units and between the core units and surrounding environment; such a case might be a bigger sandwich in which adjacent cores could exert forces to each other. Numerical study was validated by comparing experimental and numerical results of three different loading regimes for sandwiches. Having well-verified numerical models, numerical study was extended to investigate strain rate and inertial effects on sandwich structures by simulations at high crushing velocities. With complete knowledge on crushing and energy absorption of single geometries and sandwiches, behavior of sandwiches under blast was investigated by using ConWep function. Various types were proposed for arrangements of sandwiches to have higher energy absorption and lower transmitted forces to the protected structures.
Experimental and numerical evaluation of the blast-like loading of fiber reinforced polymer composites and aluminum corrugated core composite sandwiches through projectile impact testing using aluminum corrugated projectiles
(Izmir Institute of Technology, 2015-09) Odacı, İsmet Kutlay; Güden, Mustafa; Taşdemirci, Alper
This thesis develops and validates a laboratory scale blast-like testing method that can simulate explosive blast tests in air and under water without using explosives. The study has mainly focused on the shock loading potential of 1050 H14 trapezoidal corrugated core aluminium sandwich structures on E-glass/polyester composite plates and corrugated core composite sandwich structures experimentally, numerically and analytically. The composite plates were modelled using MAT_162 material model in LS-DYNA finite element code. Quasi-static and high strain rate tests were performed to determine the material model parameters of composite and corrugated structure. The resultant parameters were calibrated and validated by comparing the numerical results with the experimental results. The planar shock wave formation and propagation in corrugated core sandwich structures were shown experimentally using a direct impact Split Hopkinson Pressure Bar test set-up. Rigid-perfectly-plastic-locking material model and Hugoniot jump relations revealed the shock loading potential of the tested corrugated core sandwich structures. The shock loading response of composite plates and sandwich structures were investigated by firing the corrugated sandwich projectiles on the targets. These impact tests were also simulated numerically and an analytic model was used to predict the plate deflections. The experimentally, numerically and analytically determined back face deflections were compared with the deflections of the Conwep blast simulations in LS-DYNA. The results have shown that the corrugated core sandwich structures can generate shock loading as in the explosive blast tests and can be used to produce shock loads in laboratory scale experiments.
Experimental and numerical investigation of the quasi-static and high strain rate crushing behavior of single and multi-layer zig-zag 1050 H14 Al trapezoidal corrugated core sandwich structures
(Izmir Institute of Technology, 2014) Kılıçaslan, Cenk; Güden, Mustafa; Taşdemirci, Alper
The quasi-static and dynamic crushing behavior of single, double and multi-layer zig-zag 1050 H14 Al trapezoidal corrugated core sandwich structures in 0°/0° and 0°/90° core orientations and with and without interlayer sheets were investigated both experimentally and numerically at varying impact velocities. The numerical simulations were conducted using the finite element code of LS-DYNA. The effect of fin wall imperfection was assessed through the fin wall bending and bulging. The numerical homogenization of the single layer corrugated structure was performed using MAT26 honeycomb material model. The buckling stress of single- and double-layer corrugated sandwich structures increased when the strain rate increased. The increased buckling stresses were ascribed to the micro inertial effects. The initial buckling stress at quasi-static and high strain rate was numerically shown to be imperfection sensitive. Increasing the number of core layers decreased the buckling stress and increased the densification strain. The panels tested with spherical and flat striker tips were not penetrated and experienced slightly higher deformation forces and energy absorptions in 0°/90° corrugated layer orientation than in 0°/0° orientation. However, the panels tested using a conical striker tip were penetrated/perforated and showed comparably smaller deformation forces and energy absorptions, especially in 0°/90° layer orientation. The homogenized models predicted the low velocity compression /indentation and projectile impact tests of the multi-layer corrugated sandwich with an acceptable accuracy with reduced computational time.

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