Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
Browse
7 results
Search Results
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 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.