Master Degree / Yüksek Lisans Tezleri

Permanent URI for this collectionhttps://hdl.handle.net/11147/3008

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The Investigation of the Dynamic Compression Characteristics of a Layered Glass System
(01. Izmir Institute of Technology, 2023) Ağırdıcı, Burak; Taşdemirci, Alper
Layered glass structures are one of the most common material types used in air, land, and sea vehicles. Since these structures are exposed to external impact loads, it is important to determine their dynamic mechanical behavior. In this study, dynamic compression characteristics of the layered glass system were investigated numerically using the LS-DYNA finite element program. The Johnson Holmquist Ceramics material model was used for the glass layer, the Ogden Rubber material model, which is used in material models with high elastic structural behavior was used for the polyvinyl butyral (PVB) interlayer, and the SAMP-1 material model was used for the polycarbonate interlayer. Numerical studies were carried out to investigate the stress wave propagation, the amount of energy released, and the deceleration rate of the penetration velocity. Split Hopkinson Pressure Bar setup was used to numerically load the layered glass systems at high strain rates for a reliably easy controlled wave generation. The layered glass structure consisting of two interlayer types with different thicknesses was loaded in the SHPB system, and the effect of the interlayer material type and thickness on the stress wave propagation was investigated. Then, the projectile impact test was modeled at different impact velocities for a square plate of PVB-layered glass structure. The thickness of the PVB interlayer was kept constant, while the thickness and location of the glass layer varied. From the results, the slowing rate of the projectile, the amount of erosion energy, and the energy balance were determined.
The Development of Forming Simulation Methodology of a Plate Type Heat Exchanger
(01. Izmir Institute of Technology, 2023) Şimşek, İbrahim; Taşdemirci, Alper
In this study, the production process of plate type heat exchangers was developed as a simulation methodology. Within the scope of the study, first, the parameters in the production process were determined. Then, mechanical characterization studies were planned with the AISI 316L stainless steel material used during production and the alternative AISI 304 stainless steel material, and the tests were completed with the support of the relevant stakeholders. The tests were determined according to the requirements of the simulation methodology. In this context, uniaxial tensile test, biaxial hydraulic bulge test and Split Hopkinson tensile tests were performed to obtain the necessary inputs for the mechanical characterization of the material and creating the material model. The material models established with the information obtained from the tests were validated with the modeling of the test setups in the numerical environment. The simulation methodology was developed in the LS-DYNA environment in the light of the process parameters obtained from the production and the data obtained from the mechanical characterization tests. The simulation model created with the developed methodology was verified because of comparison with the sample produced from AISI 316L stainless steel material taken from production. After the verified model was obtained, a simulation model was created with AISI 304 stainless steel. In addition, for the model formed with AISI 316L stainless steel, process parameters optimization study was carried out, and preliminary work activities related to reducing production times were carried out in numerical environment. After these modeling activities, the knowledge of the license plate was increased. In addition, effective plastic stress during the process, springback effect, residual stress values after springback, effective plastic strain, thickness distribution and thickness reduction values were obtained for the plate. By using the forming limit diagram of AISI 316L stainless steel, information about the final formability behavior was obtained.
The Investigation of Energy Absorption Characteristics of Tpu Tpms Structures Subjected To Impact Loading
(01. Izmir Institute of Technology, 2023) Bakıcı, Çetin; Taşdemirci, Alper
In this thesis, the energy absorption capability of a schwarz based TPMS structure both experimentally and numerically was invetigated. In the product, TPU material and FDM printer was used. Instead of the regular schwarz primitive cell structure, which has been frequently examined in the literature, the sandwich structure design was prepared with the geometry selected from the region between two cells was used and its advantages were compared. In the selection of the TPMS structure, both its high energy absorption capability per unit weight and its geometry suitable for mass production in the future was important. A hyperelastic material TPU and a printer suitable for its production were selected to show deformation behaviour of the structure against multiple loading. After material characterization with TPU specimens, the determined printer parameters were kept constant, and single and multiple cell structures were produced. Static and dynamic tests were performed, and single and multiple-cell structures were modeled and validated in the LS-DYNA finite element package program. It was observed that as the strain rate increases, the structures densification point also decreased and the first peak force and the energy absorption per unit weight (SAE) increase. In addition, it was observed that the deformation behaviour of single and multiple-cell structures were rate dependent. It has been observed that the structure with 9 cells absorbs 20% more energy than the structure with unit cell, which is 9 times higher than the unit cell structure due to the interaction of cells. The developed structure was numerically exposed to blast loads following Nato Stanag 4569 standart. In this standart, from the defined of the injury criteria,on the lower and upper tibia joint should experienced force values lower than 2.6 kN and 5.4 kN respectively. From the numerial simulations, it was found that the structure was able to mitigate the blast load transmitted to the during the accaptable limits.
The Investigation of Static and Dynamic Compressive Deformation Behavior of a Paper Based Sandwich Material
(01. Izmir Institute of Technology, 2022) İmrağ, Berkay Türkcan; Taşdemirci, Alper; Taşdemirci, Alper
In this study, dynamic and quasi-static compression behavior of paper-based honeycomb sandwich structures were investigated. It is known that the mechanical properties of paper-based honeycomb structures change with changing strain rate values. For this reason, dynamic and quasi-static loading conditions should be considered separately when investigating the compressive behavior of the structure. In the material characterization studies, a series of tests were conducted to examine mechanical properties of the paper layer material and sandwich structure. Using data from mechanical tests, numerical models were established in the finite element tool LS-DYNA. Outputs of numerical models were validated with mechanical test outputs. After the validation study, the effects that influence the dynamic compressive behavior of the paper-based honeycomb sandwich structure and their contribution percentages were investigated using the opportunities provided by the FE tool. The results showed a 150.48 % difference between the dynamic and quasi-static compressive behavior of the structure. The numerical results obtained from explicit and implicit solvers also showed good correlation with the experimental results. In addition, the micro-mechanical modeling approach in numerical models made it possible to investigate the effects such as strain rate sensitivity of the paper layer material, entrapped air inside the core cells, and micro-inertia individually. The contribution percentages of the effects were calculated by comparing the numerical and experimental results.
On the Selection of Material Model for the 3d Printed Plastics
(01. Izmir Institute of Technology, 2021) Yorulmazlar, Berika; Taşdemirci, Alper
In this study, the behavior of suitable material models which fulfil the need of representation of static and dynamic constitutive behavior ABS plastic produced with Fused Deposition Modeling (FDM) method was investigated. The accuracy of material model strongly depends on the accurate determination of its constants. These constants were obtained by conducting quasi-static and high strain rate experimental studies. The high strain rate tests of FDM built ABS samples were performed using split-Hopkinson pressure bar (SHPB) and split-Hopkinson tension bar (SHTB) and gas gun set-ups. Numerical models were conducted by using the commercial explicit finite element code LS-DYNA 971. Raw data obtained from experiments at low and high strain rates, were reduced and defined in material models. ΜΑΤ_24, ΜΑΤ_81, ΜΑΤ—187 material models were considered in numerical models to investigate the constitutive behavior of the FDM b^ilt ABS material. Good correlation was observed between the numerical and experimental data with the use of selected material models. Then, Generalized Incremental Stress-State dependent damage Model (GISSMO) was selected to characterize the failure behavior of the FDM built ABS. Parameters and curves that defines the state necking and failure occurs at, were found by using optimisation tool, LS-OPT. After observing successful match between the numerical and experimental forcedisplacement curves, GISSMO parameters were defined in SHTB and gas gun numerical models. The results showed good correlation for also the gas gun and SHTB tests in terms of failure behavior, eventually. These imply that GISSMO has the potential to predict necking and localization of deformation of the 3D-printed ABS plastics for different load cases.
Dynamic Crushing Behaviour of Cactus Geometry Inspired Core Structure
(Izmir Institute of Technology, 2019) Balya, Ozan; Taşdemirci, Alper; Güden, Mustafa
Cactus 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.
The Dynamic Mechanical Characterization of a Bio-Inspired Sandwich Structure
(Izmir Institute of Technology, 2019) Ramyar, Ayda; Taşdemirci, Alper; Güden, Mustafa
In 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.
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, Alper
Experimental 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).
Residual Strength Analysis of an E Glass/Polyester Composite Subjected To Impact
(Izmir Institute of Technology, 2018) Bayhan, Mesut; Taşdemirci, Alper; Güden, Mustafa
In 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.
The Effect of Strain Rate on the Dynamic Mechanical Behaviour of Concrete
(Izmir Institute of Technology, 2018) Uysal, Çetin Erkam; Taşdemirci, Alper; Güden, Mustafa
The 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 Degree / Yüksek Lisans Tezleri

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