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 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 Investigation of the Static and Dynamic Crushing Behavior of an Energy Absorbing Biomimetic Armor(Izmir Institute of Technology, 2017) Akbulut, Emine Fulya; Taşdemirci, Alper; Güden, MustafaIn this study, an innovative thin-walled energy absorbing structure was manufactured following by biomimicry rules and produced from AISI 304L stainless steel sheet material by deep drawing method. Manufacturing process was modelled in two stages to produce the numerical specimen containing residual stress/strain and thickness distribution using commercial software LS-DYNA. The balanus being a sea creature, consisting of an inner core structure and an outer shell structure, is the inspiration of this study. The balanus was compared to the other conventional geometries in terms of the energy absorption capacity and determined as highly advantageous configuration. Quasi-static crushing and drop weight experiments were conducted and modelled numerically. The observations indicated that the carried load by the balanus is greater than the arithmetic total of the carried load by the inner core and the outer shell separately due to the interaction effect. Besides, energy absorbing performance of the balanus improved under dynamic loading since the outer shell confines the inner core during the deformation and developed the energy absorption performance of it while the energy absorbing capacity of the other two decreased. After the end of the experimental studies, the energy absorption partitions between the components of the balanus were studied numerically and it was observed that the energy absorbing capacity of the balanus increases with increasing deformation velocity due to the strain rate sensitivity effect of the material and the differences of energy partition ratio between the two components decreases.