Phd Degree / Doktora
Permanent URI for this collectionhttps://hdl.handle.net/11147/2869
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Doctoral Thesis Experimental and Numerical Investigation of the Impact Resistance and Impact Damage Tolerance of a Carbon Fiber Reinforced Thermoplastic Polyphenylene Sulfide (pps) Matrix Composite(01. Izmir Institute of Technology, 2024) Seven, Semih Berk; Güden, Mustafa; Taşdemirci, AlperThe impact resistance and impact damage tolerance of an aerospace grade high performance 5 Harness Satin woven fabric carbon fiber reinforced/polyphenylene sulfide matrix (CF/PPS) thermoplastic composite were investigated experimentally and numerically. The numerical modeling was performed using the experimentally determined parameters of material model MAT-58 and Hashin failure criteria in LS-DYNA using the single shell and stacked shell models. The numerical models of the low velocity impact (LVI) tests showed good correlations with the experimental tests while the stacked shell model showed nearer results with the experimental tests. The stacked shell model also estimated the LVI test delamination areas, which were comparable with the experimental damage areas. The LVI tested coupons were further subjected to the compression after impact (CAI) tests in order to determine the damage tolerance of CF/PPS composite. The CAI tests were modeled using the single shell model. The numerical models of the CAI tests showed very similar trends with the experimental CAI tests. The trends were shown to be more converging in the specimens tested at 3 m/s and above in the LVI tests. Lastly, three high velocity impact (HVI) tests were performed at around 100 m/s. The failure mode of the HVI tests was shown to be very different from that of the LVI tests. The long longitudinal and transverse cracks were formed in the HVI tests. The delamination damage in the HVI tests determined using the stacked shell model was found to be more comparable with the experimental delamination damage determined by the C-Scan.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.