Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis Development of material flow stress and damage models for 304 stainless steel(01. Izmir Institute of Technology, 2024) Akdoğan, İbrahim Berk; Güden, MustafaPaslanmaz çelik 304 alaşımı üzerine yapılan önceki deneysel ve sayısal çalışmalar çoğunlukla martensitin hacim kesrine bağlı akış gerilimi davranışının belirlenmesine odaklanmıştır. Alaşımın hasar davranışı darbe ile ilgili uygulamalarda eşit derecede önemlidir. SS 304 alaşımının dinamik yükleme davranışını simüle etmek amacıyla hasar modellerinin belirlenmesi konusunda henüz sistematik bir çalışma yapılmamıştır. Bu tezde, Johnson ve Cook (JC) akış gerilimi ve JC hasar denklemlerinin parametreleri bir paslanmaz çelik 304 alaşımı için deneysel olarak belirlenmiştir. Belirlenen parametreler daha sonra bu parametreleri çıkarmak için kullanılan deneysel testlerin modellenmesiyle doğrulanmış ve kalibre edilmiştir. Sayısal modeller LS-Dyna'da uygulanmıştır. B4C kaplı ve kaplanmamış SS 304 plakalar üzerinde 800 m s-1'de gerçekleştirilen deneysel balistik testler, belirlenen model parametreleri kullanılarak Ls-Dyna'da simüle edilmiştir. Son olarak, mikroskobik çalışmalar test edilen alaşımdaki martensitik dönüşümün ve yüksek gerinim oranlarındaki adiabatik ısının martensit oluşumunun kapsamını azalttığını açıkça göstermiştir. Son olarak, martensit fraksiyonu literatürdeki denklemler kullanılarak farklı gerinim oranlarında analitik olarak tahmin edilmiştir.Master Thesis Determination of the Equivalent Stress-Strain Curves of Ductile Metals Through Image Analysis(01. Izmir Institute of Technology, 2024) Çakmak, Mehmet; Güden, Mustafa; Taşdemirci, AlperThis thesis presents a methodology for determining the equivalent stress-strain and the failure strain-stress triaxiality curves of ductile metallic materials using the advanced computing and image analysis methods. The determined curves were then used to calculate the parameters of the Johnson and Cook (JC) flow stress and damage models. A code was developed in Python to perform the numerical calculations and image analysis using the Python's libraries and image analysis tools. The main entries to the code were the experimental force-displacement curves at different strain rates, the experimental failure strain-stress triaxiality curve at a reference quasi-static strain rate, the experimental failure strain-strain rate curve at a constant stress triaxiality and the video images of the deforming test specimens. The correctness and reliability of the developed code in predicting the equivalent stress-strain curves and the parameters of the JC flow stress and damage models were clearly demonstrated for the selected 316L and AISI 4340 alloys. The code could also be easily adopted to other well-now constitutive equations commonly used in the finite element software. Finally, the results of present study contribute to the field of mechanical engineering by providing a robust tool for the materials characterization essential for designing and optimizing engineering components subjected to complex states of stresses.Master Thesis The Effect of Strain Rate on the Deformation Behavior of Additively Manufactured Short Carbon Fiber Reinforced Polyamide Composites(Izmir Institute of Technology, 2022) Zeybek, Mehmet Kaan; Güden, MustafaThe compression behavior of Polyamide 6 (PA6- nylon 6) and short carbon fiber reinforced polyamide 6 (Onyx) produced by the Fused Deposition Modelling (FDM) method was investigated at both quasi-static and dynamic strain rates. The effects of layer height, specimen shape and dimensions, build direction and the orientation of the layers on the compression behavior were also determined. The results showed that the addition of short carbon fibers to the PA6 matrix increased the compression strength by 3-4 times and the determined flow stress has a strong correlation with the porosity amount in the specimens. The compression test results showed that cylindrical specimens had slightly higher flow stress than the cubic specimens. The compression tests on the specimens produced with different lengths showed almost no difference regarding stress-strain behavior. Furthermore, the specimens produced with 90 degrees showed the highest elastic modulus and yield strength and the specimens produced with 30 and 60 degrees the lowest modulus and yield strength. The Concentric infill specimens exhibited higher elastic modulus values and flow stresses than Cross raster infill specimens at all strain rates. In the layers of concentric rings, the outer rings prevented the lateral expansion of inner rings, leading to higher flow stresses than the cross raster [0/90] lay-up. The flow stress of both PA6 and Onyx specimens increased with increasing strain rate. The rate sensitivities of PA6 and Onyx specimens were shown to be similar to each other.Master Thesis The Deformation Rate Sensitivities of Additively and Conventionally Fabricated 316l Alloys(01. Izmir Institute of Technology, 2021) Enser, Samed; Güden, MustafaThe compression stress-strain behavior of a Scanning Laser Melt 316L (SLM-316L) and an annealed and extruded commercial 316L (C-316L) were determined between 1x10-3 s-1 and 2500-3150 s-1. SLM-316L deformed by twinning and slip, while C-316L by martensitic transformation and slip with no fracture until about 0.51 strain. The higher yield strength of SLM-316L than C-316L was attributed to the higher dislocation density of SLM-316L. The higher work hardening rate of C-316L alloy was proved due to the higher resistance of martensite plate than twin boundary to the dislocation motion. As the strain rate increased, both alloys showed increased flow stresses. However, the rate sensitivities declined as the strain increased due to the adiabatic heating at high strain rates. The Johnson and Cook flow stress material models of both alloys were further determined for the adiabatic and isothermal conditions. The martensite formation in C-316L specimens and twinning formation in SLM-316L alloys decreased at high strain rates compared to quasi-static strain rates. The XRD spectra of C-316L also confirmed the reduced martensite formation at high strain rates. The reduced twin and martensite formation at high strain rates were attributed to the increased stacking fault energy due to the adiabatic heating of the test specimens. The increase of stacking fault energy at high strain rates promoted a higher fraction of the deformation by slip. Lastly, the reloading tests revealed a strain-rate history effect in SLM-316L and no strain-rate history effect in C-316L.Master Thesis The Constitutive and Damage Models of Additively Manufactured Ti6al4v Alloy(01. Izmir Institute of Technology, 2021) Hızlı, Burak; Güden, MustafaElectron Beam Melting (EBM) is one of the metal additive manufacturing methods that enable the fabrication of Ti6Al4V alloy parts with intended shapes in where this alloy is of significant interest such as aerospace and biomedical industries due to its outstanding properties. In this study, the microstructural and mechanical properties of EBM-produced Ti64 were comprehensively investigated. Microstructural analysis was conducted on as-built specimens. Microstructural analysis showed that EBM-produced Ti64 possesses α+β duplex phase with directional microstructural alterations and high porosity fraction in the part volume. Mechanical properties were investigated under tension loadings at quasi-static rates (0.001-0.1 1/s) and compression loading at quasi-static and high strain rates (0.001-2154 1/s). Thereafter, Johnson-Cook (JC) strength and damage models were individually calibrated from the experimental results of tension and compression behaviors and experimental fracture strains in order to numerically predict the material flow behavior of EBM-produced Ti64 considering the strain, strain rate, and temperature effects in the case of various loadings combined with temperature changes. EBM-produced Ti64 exhibited proximate mechanical properties in terms of tension and compression behaviors, however extremely low ductile behavior under tension loadings resulting premature failure without necking. Eventual fracture of this material occurred via tearing of the scanned layers for tension loadings and shear crack following the shear band formation propagation on 45° to loading axis for compression loadings. Calibrated JC strength and damage models for EBM-produced Ti64 were able to predict flow behavior and fracture strains within strain rate range between 0.001 and 1000 1/s. However, the JC strength model could not predict the flow behavior at excessively high strain rates (2154 1/s) due to complex deformation mechanisms including adiabatic heating.Master Thesis Modelling the Damage Formation of Bolted Carbon Fiber Reinforced Epoxy Composite Joints at Increasing Strain Rates(01. Izmir Institute of Technology, 2021) Albir, Çağatay; Güden, MustafaThe bearing strength of a carbon fiber reinforced/epoxy unidirectional composite joint incorporating a single hex bolt fastener was investigated under quasi-static and dynamic loads experimentally and numerically with two different bolt torques, 2.5 N m and 10 N m. The tests were conducted with neat fit clearance and without washer. The quasi-static tests were conducted at 3.33x10-5 and 1.66 x10-3 m s-1 according to the ASTM D5961 Procedure C. The dynamic tests were conducted in at Tension Split Hopkinson Pressure Bar (TSHPB) at 12.68 m s-1 using a specially designed specimen grip to ensure the same conditions as the quasi-static tests. Three dimensional explicit finite element models of bearing tests were developed in the LS-DYNA and the composite was modelled using the MAT_162 composite material model incorporating the strain rate effects. At the quasi-static velocities, a relatively low strain rate dependence of bearing peak force was found with almost no effect of applied bolt torque. In the TSHPB tests, the bearing force increased by 57% of those of quasi-static tests. The deformation mode also altered in dynamic tests and the increase of the bolt torque resulted with increasing the bearing peak force by 5%.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 Process Parameters and Mechanical Properties of Geopolymer Glass Foam Structures(01. Izmir Institute of Technology, 2020) Polat, Dilan; Güden, MustafaThe effects of waste-glass powder particle size (23 and 72 μm), solid/liquid ratio (S/L=1, 1.5 and 2) and aluminum foaming agent content (2-20 wt%) on the expansion behavior of geopolymer slurries were investigated experimentally. Geopolymer slurries were prepared using an activation solution of NaOH (8M) and sodium silicate (10% NaOH, 27% SiO2). The expansions and temperatures of the slurries were measured in-situ using a laser distance meter and a thermocouple, respectively. Few geopolymer foams were sintered at 600, 700, 725 and 750 °C. The compression strengths and thermal conductivities of foam samples were also determined. The expansion of slurries continued until the temperature increased to 85-90 °C. At this temperature, the slurry evaporation; hence, increased S/L ratio limited both the hydrogen release rate and geopolymerization reaction. As the content of Al increased, the final foam density decreased, while the coarse powder slurries resulted in lower densities (240-530 kg m-3) than the fine powder slurries (280-530 kg m-3). Three crystal phases, muscovite, sodium aluminum silicate hydrate and thermonitrite, were determined after the geopolymerization. The muscovite formation was noted to be favored at higher S/L ratios. The partial melting of glass particles started after ~700 °C, while sintering above this temperature decreased the final density. The reduced density above 700 °C was ascribed to the release of carbon dioxide by the decomposition of thermonitrite. Both the compressive strength and thermal conductivity of geopolymer and sintered foams increased at increasing densities and were shown to be comparable with those of previously investigated geopolymer and glass foams. The geopolymer foams sintered at 750 °C exhibited the lowest density and the highest compressive strength.Master Thesis Dynamic Crushing Behaviour of Cactus Geometry Inspired Core Structure(Izmir Institute of Technology, 2019) Balya, Ozan; Taşdemirci, Alper; Güden, MustafaCactus geometry inspired core structure was manufactured with the fused deposition modelling method by a 3D printer using Acrylonitrile Butadiene Styrene (ABS) material filament. The characterization of ABS was made by performing compression tests to take some parameters for numerical models. Numerical preliminary studies were carried out by using the areal density concept and direct-impact Hopkinson pressure bar test method to compare the cactus geometry with the conventional ones in point of the specific energy absorption capacity (SEA). It was understood that from the preliminary work, the cactus inspired structure is intriguing to investigate the dynamic crushing behaviour at least. Quasi-static, drop weight and direct-impact Hopkinson pressure bar tests were conducted to comprehend the energy absorption and crushing behavior in all cases, then to investigate the strain rate and inertia effects on the structure. Implicit and explicit numerical models were made by using LS-DYNA software to validate experiments and to set a precedent for future works. It was seen that the result of numerical models is in harmony with that of experiments excluding the non-fracture structure at the quasi-static implicit model. Moreover, although quasi-static compression gave the structure more stable deformation behavior compared to drop weight impact, higher energy absorption capability was observed on drop-weight tests. In addition, the strain rate effect is more forceful in point of loading carrying capacity compared to the inertia effect despite the fact that it provides the development of buckling and damage formation.Master Thesis The Dynamic Mechanical Characterization of a Bio-Inspired Sandwich Structure(Izmir Institute of Technology, 2019) Ramyar, Ayda; Taşdemirci, Alper; Güden, MustafaIn this study, the sandwich structure consisting of novel-3D-printed-polymeric base core was examined in terms of crashworthiness. The designed core structure for energy absorption purpose is inspired by the geometry of the human fingerprint. The fingerprint geometry is a spiral-shaped, asymmetrical and complex structure; therefore, the manufacturing of the geometry is difficult by conventional manufacturing methods. Fused Deposition Modeling (FDM) which is one of the additive manufacturing (AM) methods was used for fingerprint core preparation by layer by layer production technique with low-density material. After the material characterization of 3D printed thermoplastic specimens, optimum geometric parameters of fingerprint were determined via experimental and numerical studies by changing the height and thickness. The fingerprint performed better crushing performance compared to other conventional geometries. Quasi-static and dynamic crushing experiments were conducted, and the results were verified with models by non-linear finite element code LS-DYNA. The results showed that the energy absorption capacity and peak crushing force of fingerprint geometry increases with strain rate increment. However, the deformation behavior of the structure under dynamic loads changes and the material becomes more brittle. This is caused by the change in deformation mechanism due to AM and material itself. It was found that the 3-D printed core structure is suitable to be employed at low-to-medium strain rates due to its multi-stage deformation behavior. It was observed that the bio-inspired sandwich structure consisting of 4 fingerprint-core can absorb 10% more impact energy than fourfold individual 3-D printed core geometry, which indicates the promising potential of the novel sandwich structure for crashworthiness applications.