Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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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.Master Thesis The Development of a New Testing Methodology in Dynamic Mechanical Chracterization of Concrete(Izmir Institute of Technology, 2018) Seven, Semih Berk; Güden, Mustafa; Taşdemirci, Alper; Taşdemirci, Alper; Güden, MustafaConcrete is one of the most used material types in the world. Due to its structural complexity and insufficient testing techniques, the dynamic mechanical behavior of concrete has not yet been revealed sufficiently. This thesis aims to develop reliable and accurate mechanical characterization methodology for concrete using the combination of experimental and numerical methods together. The dynamic mechanical characterization of concrete at quasi-static and high strain rates was performed implementing unique techniques for both experimental and numerical studies. In quasi-static testing, universal compression test machine was used with strain gage mounted specimen for better strain measurements. In high strain rate tests, two modifications were implemented on the conventional Split Hopkinson Pressure Bar (SHPB) test apparatus. The first modification is the usage of pulse shaper to obtain nearly constant strain rate and dynamic stress equilibrium in the specimen. Second, piezo-electric quartz crystal force transducers were implemented on the specimen-bar interfaces to increase accuracy and sensitivity of the force measurement on the front and back forces of the specimen. Experimental results were validated constituting numerical study using finite element tool LS-DYNA. Concrete was modeled using Holmquist-Johnson-Cook (MAT_111) material model. HJC material model parameters were determined using experimental results coupling with the numerical analysis and the mechanical behavior of concrete was constituted. It was concluded that using pulse shaper and quartz crystals pretty useful when testing concrete and other brittle materials at high strain rates. Modification of new specimen geometries on numerical analysis showed better understandings of the effect of geometry on the dynamic stress equilibrium.Master Thesis The Effect of Material Strain Rate Sensitivity on the Shock Deformation of an Aluminum Corrugated Core(Izmir Institute of Technology, 2018) Canbaz, İlker; Güden, Mustafa; Taşdemirci, AlperThe effect of the material model on the crushing behavior of a layered 1050 H14 aluminum corrugated sandwich structure was investigated numerically as function of velocity (0.0048, 20, 60, 150 and 250 m s-1) using three different material models; elastic-perfectly plastic (model I), elastic-strain hardening (model II) and elastic-strain and strain rate hardening (model III). Three-dimensional finite element models were developed in the explicit finite element code of LS-DYNA. Between 0.0048 m s-1 and 20 m s-1, the numerically calculated stresses at the impact and distal end were almost the same and in equilibrium, showing a “quasi-static homogenous mode”. The deformation mode at 60 m s-1 was a “transition mode” and between 150 and 250 m s-1 a shock mode in which the layers were crushed sequentially. The numerical study showed that the strain and strain rate hardening models tended to induce non-sequential layer crushing. The collective layer crushing was also more pronounced in the material model II and III than the material model I. For low strain hardening aluminum alloys and similar materials, the effect of strain hardening in increasing plateau stress was more significant than the strain rate hardening at the quasi-static velocity, while both strain hardening and strain rate hardening effect increased with increasing velocity. The stress reduction by the inclusion of imperfections however declined with the velocity since the samples started to deform near the impact end as the velocity increased.Master Thesis The Deformation Behavior of a Multi-Layered Aluminum Corrugated Structure at Increasing Impact Velocities(Izmir Institute of Technology, 2017) Sarıkaya, Mustafa Kemal; Güden, Mustafa; Taşdemirci, AlperThe compression impact deformation of a layered 1050 H14 aluminum corrugated sandwich structure was determined both experimentally and numerically under low, intermediate and high velocities to investigate the validity of the perfect and imperfect models. Three-dimensional finite element models of the tested specimens were developed using the LS-DYNA. At increasing velocities from quasi-static velocity to 200 m s-1, the tested corrugated structures showed three distinct deformation modes: between 0.0048 and 22 m s-1 the deformation was quasi-static homogenous mode; between 22 and 60 m s-1 a transition mode and above 90 m s-1 a shock mode. These observations were also confirmed by the camera records and model layer strain profiles. The imperfect models predicted the deformation behavior in homogeneous and transition modes, while the imperfect and perfect models both well predicted the shock mode. Layer strain profiles showed that as the velocity increased, the crushed layer densification strains increased. The numerical models and experiments of direct impact tests showed that distal end crushing stress increased with increasing velocity. The increase of the stress within the homogeneous and transient mode velocities was ascribed to the micro-inertia effect and the tested corrugated structure showed a Type II behavior. The rigid perfectly plastic locking (r-p-p-l) model prediction using quasi-static plateau stress and densification strain and quasi-static plateau stress and numerically determined densification strain at that specific velocity resulted higher velocities and full densification, while the r-p-p-l model based on varying plateau stress and densification strain well predicted in the shock mode.Master Thesis Modelling of Pore Formation in Porous Materials(Izmir Institute of Technology, 2017) Ülker, Sevkan; Güden, Mustafa; Akdoğan, YaşarThe purpose of this thesis is to model the expansion behavior of aqueous slurries. Foamed or cellular material made using such method is known, especially in the concrete industry. What appears to be lacking in the literature is the knowledge of pore formation and pore growth in inorganic particles based on aqueous slurry systems that result in the formation of cellular structures. The motivation of this study is to provide a scientific view in identifying and explaining the critical parameters that govern the pore growth and expansion of such slurry based systems. Bubble growth and pore formation are also studied experimentally. Experimental results are used to compare with the empirical study conducted by Kanehira at al. (Kanehira, et al., 2013), and mathematical modeling of pore formation plotted with Wolfram Mathematica software. Experimental procedure consists of three types of aluminum and calcium ratios which provide information about bubble growth and pore formation. These types are 50% aluminum – 50% calcium hydroxide (50/50), 70% aluminum – 30% calcium hydroxide (70/30), and 80% aluminum – 20% calcium hydroxide (80/20). According to the results of studies, mathematical modeling system consists of the pressure difference between the inside and outside of a spherical bubble as the driving force for defining growth. While aluminum ratio increases, bubble growth rate decreases due to release of hydrogen gases which affect bubble expansion phenomenon. In the experimental and mathematical modeling, 50/50 ratio has maximum bubble growth rate compared to 70/30 and 80/20 ratios. The results of experimental and mathematical modeling suggest that viscosity is a very significant parameter which controls the bubble growth rate.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.