Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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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.Master Thesis Experimental and Numerical Analysis of the Strain Rate Dependent Compressive Strength of a Cellular Concrete(Izmir Institute of Technology, 2019) Akyol, Burak; Güden, Mustafa; Taşdemirci, AlperExperimental and numerical quasi-static and high strain rate tests, including compression, indentation and direct impact, were performed on a cellular concrete in order to investigate the effect of strain rate on the compressive strength. The results of compression tests indicated three distinct regions of the compressive strength dependence on strain rate. A relatively lower strain rate dependent compressive stress was found in the quasi-static strain rate-regime, 2x10-3-2x10-1 s-1, a relatively high strain rate dependent compressive stress in the dynamic strain rate-regime, 180-103 s-1 and a cut-off strength above 103 s-1. The dynamic increase factor (DIF=dynamic/static fracture strength) varied between 1 and 2.5 from quasi-static to dynamic strain rate-regime with a sharp increase after about 100 s-1. The indentation tests using 25 and 30 mm-diameter indenters in the quasi-static strain rate-regime (uniaxial state of strain) and resulted in moderate DIF values (1-1.13), very similar with those of the quasi-static compression tests (1-1.15). In the indentation tests, the DIF values significantly and also confirmed the numerically determined DIF values of concrete at 1000 s-1 (~1.30) without radial and axial inertia. The compression and direct impact tests in the Split Hopkinson Bar (SHPB) set-up were implemented numerically in LS-DYNA using an anisotropic strain rate insensitive material model, MAT_096 (MAT BRITTLE DAMAGE). The stress readings were performed at the specimen different locations of the SHPB and indicated that radial and axial inertia were dominant between 1 and 30 m s-1 (30-1000 s-1).Master Thesis Residual Strength Analysis of an E Glass/Polyester Composite Subjected To Impact(Izmir Institute of Technology, 2018) Bayhan, Mesut; Taşdemirci, Alper; Güden, MustafaIn this thesis, residual strength analysis of an E-Glass/polyester laminate was carried out for multiple impact loading. MAT_162 material model in LS-DYNA finite element code was used to model the constitutive behavior of the composite and material model parameters were determined using the results of the mechanical characterization. Experimental and numerical multiple loading were performed for two cases, namely Split Hopkinson Pressure Bar (SHPB) and projectile impact testing device. Numerical models were simulated DYNAIN file method strategy in which a composite laminate was impacted multiple times, which was very close to the actual case. A numerical ballistic test model using conventional strategy (in the same simulation all the loads applied sequentially) was employed to check the accuracy of that using the proposed new methodology. After the first hit the SHPB results revealed that delamination occurred at the interface between the first and second layers. Following the second hit, delamination propagated along the inside layers of the composite and occurred at the interface between the eighth and ninth layer. As far as the bar responses concerned, the reflected pulse increased from zero to a maximum value then gradually decreased at the first impact. However, a sharp rise was seen in the reflected pulse of the second impact due to failure corresponding to catastrophic failure. In projectile impact tests, delamination was also found to be increased with the increasing number of hits at both front and back surfaces. Similar results were obtained for both DYNAIN and conventional strategies. It was concluded that simulations showed well agreement with experimental results.Master Thesis The Effect of Strain Rate on the Dynamic Mechanical Behaviour of Concrete(Izmir Institute of Technology, 2018) Uysal, Çetin Erkam; Taşdemirci, Alper; Güden, MustafaThe fast-growing population of mankind has brought out household needs and working structures that might be subjected to static and dynamic loads. Impact loads and repetitive dynamic loads can produce an overload on the structures in a very short period that causes relentless casualties and unfortunate property losses. The response of the concrete material on strain rate increase is critical. The dynamic characterization of concrete, lack of adequate and consistent study causes disagreement about strain rate sensitivity of concrete, so a consensus has not been reached. In this study, quasi-static (3.55x10-5, 3.23x10-4, and 2.97x10-3 s-1) and high strain rate (140-250 s-1) tests were conducted and the effect of strain rate on the mechanical behavior of concrete was investigated both experimental and numerical. A modified Split Hopkinson Pressure Bar test setup was used, by using an EPDM (Ethylene Propylene Diene Monomers) rubber pulse shaper, non-oscillatory results and nearly constant strain rate were reached, and premature failure was prevented. Modeling the test setup was conducted in Ls-Dyna and the Holmquist-Johnson Cook material model parameters were found. A good agreement between experimental and numerical results was reached. The strength enhancements of concrete material, while increasing strain rate was noticed. Using both experimental and numerical studies, the total strength increase is due to inertia effect and strain rate sensitivity effects were observed.