Phd Degree / Doktora
Permanent URI for this collectionhttps://hdl.handle.net/11147/2869
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Doctoral Thesis Advanced Material Characterization and Modeling the Foreign Body Impact Damage Initiation and Progression of a Laminated Carbon Composite(01. Izmir Institute of Technology, 2023) Bayhan, Mesut; Taşdemirci, Alper; Güden, MustafaThe coupon level composite sample tests and the accompanying numerical models were carried out to predict the response of woven carbon fiber composite structures against impact. The numerical models of the coupon-level tests were implemented in LSDYNA software using the MAT_162 and MAT_58 composite material models. The results obtained by both quasi-static and dynamic tests were used to determine their constants. In addition to the tests that were used for the determination and calibration of the material model parameters, separate tests and their models were performed for the validation, including punch shear tests and low-velocity impact tests. It could be said that the material models examined were considered comprehensive and precise as the experimental results were well predicted by the numerical models. Also, the rate sensitivity of the woven carbon composite in the in-plane and thickness directions was investigated experimentally and numerically. In the tests, the DIC method was employed in the determination of the displacement and strain of the specimen. Based on the results obtained, it was concluded that the in-plane tensile properties are rate insensitive. Besides, the simulations of the component level tests, such as bird strike and drone impact, were established to investigate the damage initiation and propagation within the composite. It was found that the drone impact results in more severe damage compared to the bird impact. It is worth noting that the development of such precise composite material models to simulate dynamic loadings will definitely shorten the time between the beginning of designing and the component testing.Doctoral Thesis The Development of Constitutive Equations of Polycarbonate and Modeling the Impact Behavior(01. Izmir Institute of Technology, 2023) Sarıkaya, Mustafa Kemal; Güden, Mustafa; Taşdemirci, AlperThe Johnson and Cook (JC) flow stress and damage parameters of a polycarbonate were determined by the mechanical tests and numerical simulations. The experimental tests included quasi-static and high strain rate tension and compression, quasi-static notched-specimen tension, quasi-static indentation (QSI), low velocity impact (LVI) and projectile impact (PI). The flow stress equation determined from the experimental average true stress-true strain curve well agreed with the effective stress-strain obtained from the quasi-static numerical tension test. The numerical QSI force-displacement curve based on the experimental average true stress-true strain equation was further shown to be very similar to that of the experiment. The LVI and PI test simulations were then continued with the experimental average true stress-true strain equation using five different flow stress-strain rate relations: JC, Huh and Kang, Allen-Rule and Jones, Cowper-Symonds and the nonlinear rate approach. No strain rate sensitivity in the LVI tests was ascribed to low strain rate dependency of the flow stress at intermediate strain rates and large strains. On the other side, all the stress-strain rate relations investigated nearly predicted the experimental damage types in the PI tests, except the Cowper-Symonds relation which predicted the fracture of the polycarbonate plate at 140 m s-1. The absorbed energy at 160 m s-1 test was determined 1.6 times that of the QSI test, proving an increased energy absorption of the tested polycarbonate at the investigated impact velocities. The verified parameters were finally used to model the damages formed on a canopy against bird strike.Doctoral Thesis Biological Nano Silica Reinforced Polymeric Composites(01. Izmir Institute of Technology, 2022) Ülker, Sevkan; Güden, MustafaThe present thesis study focused on processing nano-silica powders of varying sizes and crystallinities through heat treatment (900–1200 °C), hydrofluoric acid leaching (1–7 N), and ball milling (1 h, 500 rpm) of natural diatom frustules. As-received frustules was composed of amorphous silica (88%) and quartz. The partially ordered crystalline low-quartz and/or precursor to low-cristobalite started to form at ~900 °C. As the heat treatment temperature increased, the crystallinity of the frustules increased from 9.3% at 25 °C to 46% at 1200 °C. Applying a ball milling reduced the mean particle sizes of the as-received and heat-treated frustules from 15.6–13.7 μm to 7.2-6.7 μm, respectively. Acid leaching of the as-received and heat-treated frustules resulted in a further increase in the crystallinity. Furthermore, ball milling applied after an acid leaching was very effective in reducing the particle size of the as-received and heat-treated frustules. The mean particle size of the acid-leached frustules decreased to 774-547 nm with a crystallinity varying between 12 and 48% after ball milling. A partially dissolved amorphous phase was observed in between crystalline silica grains after acid leaching, which resulted in a rapid fracture/separation of the frustules in ball milling. The prepared nano-silica powders were further used as a filler in an epoxy matrix. The tensile strength, fracture strain, and modulus of epoxy increased with increasing the volume percent of nano-silica up to 2%. The increase in the yield strength and elastic modulus was about 50% and 30% with the addition of 2 vol% frustules, respectively. The rule of mixtures showed a very good agreement with the experimental elastic modulus values and a numerical model of the tensile test in LS-DYNA agreed well with the experimental tensile stress-strain behavior. The microscopic observations showed the presence of nano-silica powder, proving an efficient load transfer from matrix to powders on the fracture surfaces, confirming a strong interface between silica powders and matrix.Doctoral Thesis Processing Foam-Like Porous Glass Structure Using a Combined Process of Glass Powder Expansion in Aqueous Environment and Sintering Process(Izmir Institute of Technology, 2019) Zeren, Doğuş; Güden, Mustafa; Akdoğan, YaşarSoda-lime glass foams were formed by the controlled pore structure of inorganic particle-liquid suspensions at room temperature and then sintered at elevated temperatures between 650oC-800 oC. The slurries were prepared using the glass particles below 38 µm (fine), between 38 and 45 µm (medium) and between 45 and 56 µm (coarse) and with 50, 55, and 60 wt% solid content and 2, 3, and 4 wt% carboxymethyl cellulose (CMC) binder addition. The slurries were foamed using an Al-based foaming agent and a calcium hydroxide alkali activator with an amount of 1 wt%. An increase in CMC content and a decrease in particle size shifted the slurries from a Newtonian to a non-Newtonian behavior and slurry stabilization with the CMC addition. The extensively increased initial bubble pressure in high viscosity slurries resulted in higher linear expansion rate initially followed by a bursting of gas bubbles. The maximum foam linear expansion of the slurries increased with CMC addition until about ~5 Pa s and the expansions stayed almost constant over 400% expansion, while the slurries with the viscosity above 50 Pa s could not be foamed. The most effective factor on the maximum expansion was found the solid content followed by CMC content and the least effective factor was determined the particle size. Partial bonding of glass particles and excessive shrinkage of glass particles due to the melting of foam green bodies were seen at 650 and 800 oC sintering temperatures. Prepared foam glasses showed lower compressive strengths and thermal conductivities than the glass foams reported in the literature. Finally, foaming at room temperature with this technique was found to be more advantageous than conventional glass foam production techniques due to ease of pore formation controlling at room temperature.Doctoral Thesis The Penetration Behavior of Repeated Hemisphere Core Sandwich Structures: an Experimental and Numerical Study(Izmir Institute of Technology, 2018) Turan, Ali Kıvanç; Taşdemirci, Alper; Güden, MustafaIn this study, penetration behavior of novel core structure consisting hemispherical and cylindrical parts was investigated. Core units were manufactured with deep drawing method in two thicknesses to have monolithic form without any sort of assembly method or element. Produced specimens were then subjected to penetration tests at low and intermediate velocities against blunt, conical and hemispherical tipped indenters using special fixtures and apparatuses on conventional testing equipment. Effect of heat treatment on penetration behavior was investigated to observe whether residual stresses arise from manufacturing process changes the penetration behavior. Confinement effects were studied experimentally with a special fixture, allowing tested specimen to be radially confined with other core units as in an armor structure. Finally, experimental work was finished by conducting a case study where core units were subjected to spherical projectile impact up to impact velocities of 180 m.s-1 in a composite sandwich structure. Results show that each indenter geometry showed unique deformation characteristics in testing of both core units and both of the core geometries were able to hold a steel spherical projectile with mass of 110 g without full perforation at impact velocity of 180 m.s-1. Details of experimental results were presented in each chapter. Study also included modeling parts where core units were numerically produced with residual stresses and strains and good correlation was noted where thickness was compared with actual measurements on core units. Test conducted on single core structure in as-received and heat-treated condition were also repeated in numerical environment, where numerical study exhibited good correlation on both forcedisplacement curves and deformation of core units with tests. Correlation achieved with experimental study has led into further investigations of strain rate and micro-inertia where behavior of core units was studied at numerical impact velocities of 300 m.s-1. Results show that both strain rate and micro-inertia increase the local maximums and average of force levels. Effect of strain rate and micro-inertia is clearly distinguished for a threshold displacement level where micro-inertia is further dominant on behavior.Doctoral Thesis Experimental and Numerical Evaluation of the Blast-Like Loading of Fiber Reinforced Polymer Composites and Aluminum Corrugated Core Composite Sandwiches Through Projectile Impact Testing Using Aluminum Corrugated Projectiles(Izmir Institute of Technology, 2015) Odacı, İsmet Kutlay; Güden, Mustafa; Taşdemirci, AlperThis thesis develops and validates a laboratory scale blast-like testing method that can simulate explosive blast tests in air and under water without using explosives. The study has mainly focused on the shock loading potential of 1050 H14 trapezoidal corrugated core aluminium sandwich structures on E-glass/polyester composite plates and corrugated core composite sandwich structures experimentally, numerically and analytically. The composite plates were modelled using MAT_162 material model in LS-DYNA finite element code. Quasi-static and high strain rate tests were performed to determine the material model parameters of composite and corrugated structure. The resultant parameters were calibrated and validated by comparing the numerical results with the experimental results. The planar shock wave formation and propagation in corrugated core sandwich structures were shown experimentally using a direct impact Split Hopkinson Pressure Bar test set-up. Rigid-perfectly-plastic-locking material model and Hugoniot jump relations revealed the shock loading potential of the tested corrugated core sandwich structures. The shock loading response of composite plates and sandwich structures were investigated by firing the corrugated sandwich projectiles on the targets. These impact tests were also simulated numerically and an analytic model was used to predict the plate deflections. The experimentally, numerically and analytically determined back face deflections were compared with the deflections of the Conwep blast simulations in LS-DYNA. The results have shown that the corrugated core sandwich structures can generate shock loading as in the explosive blast tests and can be used to produce shock loads in laboratory scale experiments.Doctoral Thesis Experimental and Numerical Approaches To Evaluate the Crushing Behavior of Combined Geometry Core Sandwich Structures Against Blast(Izmir Institute of Technology, 2015) Kara, Ali; Taşdemirci, Alper; Güden, MustafaIn this study, novel sandwich structures containing combined geometry structures as core materials were designed and developed for blast protection applications. The proposed combined geometries consist of a hemispherical geometry attached seamlessly to a cylindrical segment. Deep drawing method was used to obtain four different types of combined geometries having two different radii from blanks with two different initial thicknesses. The mechanical properties of the blank material were obtained by conducting tensile experiments at quasi-static and high strain rate regimes. Thereafter, crushing and energy absorption behavior of core units were determined by tests at quasi-static and low velocity regimes, experimentally. Before crushing simulations, manufacturing method was simulated to have realistic residual stress/strain and thickness variations of numerical specimens. Having accurate deformation history, crushing experiments were simulated and a good agreement was reached proving the realistic modeling of the manufacturing effects. The effect of heat treatment on the crushing behavior of combined geometry shells was also investigated both experimentally and numerically and there was a good agreement noted. After, cross-shaped sandwich structures of one type of combined geometry were prepared. Static, low velocity and high velocity crushing behavior of sandwiches were investigated. Study on sandwich structures also included confined experiments in order to account for the interaction between the core units and between the core units and surrounding environment; such a case might be a bigger sandwich in which adjacent cores could exert forces to each other. Numerical study was validated by comparing experimental and numerical results of three different loading regimes for sandwiches. Having well-verified numerical models, numerical study was extended to investigate strain rate and inertial effects on sandwich structures by simulations at high crushing velocities. With complete knowledge on crushing and energy absorption of single geometries and sandwiches, behavior of sandwiches under blast was investigated by using ConWep function. Various types were proposed for arrangements of sandwiches to have higher energy absorption and lower transmitted forces to the protected structures.Doctoral Thesis Experimental and Numerical Investigation of the Quasi-Static and High Strain Rate Crushing Behavior of Single and Multi-Layer Zig-Zag 1050 H14 Al Trapezoidal Corrugated Core Sandwich Structures(Izmir Institute of Technology, 2014) Kılıçaslan, Cenk; Güden, Mustafa; Taşdemirci, Alper; Güden, Mustafa; Taşdemirci, AlperThe quasi-static and dynamic crushing behavior of single, double and multi-layer zig-zag 1050 H14 Al trapezoidal corrugated core sandwich structures in 0°/0° and 0°/90° core orientations and with and without interlayer sheets were investigated both experimentally and numerically at varying impact velocities. The numerical simulations were conducted using the finite element code of LS-DYNA. The effect of fin wall imperfection was assessed through the fin wall bending and bulging. The numerical homogenization of the single layer corrugated structure was performed using MAT26 honeycomb material model. The buckling stress of single- and double-layer corrugated sandwich structures increased when the strain rate increased. The increased buckling stresses were ascribed to the micro inertial effects. The initial buckling stress at quasi-static and high strain rate was numerically shown to be imperfection sensitive. Increasing the number of core layers decreased the buckling stress and increased the densification strain. The panels tested with spherical and flat striker tips were not penetrated and experienced slightly higher deformation forces and energy absorptions in 0°/90° corrugated layer orientation than in 0°/0° orientation. However, the panels tested using a conical striker tip were penetrated/perforated and showed comparably smaller deformation forces and energy absorptions, especially in 0°/90° layer orientation. The homogenized models predicted the low velocity compression /indentation and projectile impact tests of the multi-layer corrugated sandwich with an acceptable accuracy with reduced computational time.Doctoral Thesis The Effects of Diatom Frustule Filling on the Quasi-Static and High Strain Rate Mechanical Behavior of Polymer Matrices(Izmir Institute of Technology, 2010) Gültürk, Elif; Güden, MustafaIn this study quasi-static tension and quasi-static (1x10-3 and 1x10-1 s-1) and high strain rate (300-600 s-1) compression and quasi-static tensile behavior of diatom frustules-filled, Diatomaceous earth (CD) and Kieslguhr (ND), epoxy matrices were investigated experimentally and microscopically. For comparison, the compression and tensile behavior of the neat epoxy was also determined. Compression results showed that diatom frustules filling increased both modulus and yield strength of the epoxy matrix at quasi-static and high strain rates. ND frustules filled epoxy samples showed a higher strain rate sensitivity compare with CD filled samples. Tensile test results showed that the modulus of filled epoxy increased with increasing frustule content. The frustule filling, however, decreases the tensile failure strains of the epoxy and increased the tensile strength slightly. Microscopic observations on the fracture surfaces and the mounted cross-sections of deformed samples showed that the failure mechanisms were debonding of the frustules-epoxy interface and the fracture of the frustules at quasistatic strain rates while the failure of the filled composite at high strain rate was dominated by the fracture of the matrices. These results confirmed that significant benefits might anticipated from the use of diatom frustules as reinforcements and fillers in polymeric materials. Various methods; acid leaching, thermal shock and ball milling were further applied to process nano size silica powder from frustules. Projectile impact tests indicated that frustule addition increased the ballistic resistance of epoxy matrices. Finally, the strength and modulus of the filled epoxy matrices were predicted using analytical models developed for short fiber composites.Doctoral Thesis In Vitro Bioactivity of the Surface-Treated Ti6a14v Open Cell Foams(Izmir Institute of Technology, 2010) Türkan, Uğur; Güden, MustafaThe effects of commonly used chemical surface treatment methods including alkali and nitric acid treatment and acid etching on biomimetic CaP deposition and bacterial adhesion (S. epidermidis) of an open cell Ti6Al4V foam using two different powders, powder 1 (P1) and powder 2 (P2) were investigated in a simulated body fluid (SBF) solution up to 14-day. The optimum conditions of nitric acid and alkali surface treatment for the biomimetic CaP coating of the studied foams were also determined using the surface response methodology. Alkali treatment induced a relatively thin layer of porous Na2Ti5O11 on the flat surface and inside the pores, while nitric acid treatment did not affect the surface roughness; it increased surface area difference significantly by introducing nano scale undulations on the surface. A uniform CaP layer formation was found on the flat surface and interior of the pores of untreated foam samples after 14-day of SBF immersion. However, alkali treatment and nitric acid treatment reduced the immersion time of CaP layer formation for P2 foam samples. The GIXRD, SEM and FTIR analysis showed that the CaP layer was in the form of carbonated hydroxyapatite (CHA). The porous Na2Ti5O11 surface layer formed in alkali treated foam specimens promoted the bacterial retention on the foam particles. A correlation between the nano metric scale surface roughness and the associated bacterial colonization was further shown. Based on response surface methodology, the following experimental design conditions were found to induce a uniform coating in alkali treated P1 and P2 foam specimens: 1M NaOH at 20 C for 12.5h. For nitric acid treated P1 and P2 foams, the optimum condition was found as 20% nitric acid solution at 40 C for 1h.