Phd Degree / Doktora
Permanent URI for this collectionhttps://hdl.handle.net/11147/2869
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Doctoral Thesis Effects of Fiber Discontinuity in Fiber Reinforced Polymer Matrix Composites(01. Izmir Institute of Technology, 2024) Kılıçoğlu, Ahmet Süha; Tanoğlu, MetinBu tez, cam elyaf takviyeli kompozit plakaların mekanik davranışları üzerindeki yapısal süreksizliklerin etkisini araştırmaktadır. Bu süreksizlikler, kompozit mikro yapısındaki geometrik kısıtlamalar nedeniyle katmanlar arası (inter-ply) ve katman içi (intra-ply) olarak sınıflandırılmıştır. Süreksizlikler kürleme öncesi ilave edilmiştir. İlk olarak, malzeme özellikleri kupon seviyesinde testlerle belirlenmiştir. Daha sonra, bilgisayarlı sayısal kontrol kesimi ve manuel uygulama ile katman içi süreksizlikler oluşturularak iki tam ölçekli kompozit numune üretilmiştir. Bu numunelerin mekanik özellikleri, servo-hidrolik aktüatörler kullanılarak üç nokta eğme testi ile değerlendirilmiştir. Deneysel test sonuçları, kesitsel fiber hacim oranı değerlendirilerek CAE analiz tahminleriyle karşılaştırılmış ve yerel süreksizliklerin mikroskopik analizi ile desteklenmiştir. Çalışma, yer değiştirme bölgelerinin reçine açısından zengin alanlara yol açtığını ve ekzotermik kürleme sürecinin reçinenin rengini şeffaftan sarıya dönüştürdüğünü, bunun da mekanik dayanıklılığı azalttığını ortaya koymuştur. Ayrıca, fiber süreksizlikleri ve reçine boşlukları, cam elyaf takviyeli polimer (GFRP) kompozit yaprak yayların yapısal bütünlüğünü olumsuz etkilemektedir. Üretim sürecindeki düzensizlikler, malzeme dayanıklılığını ve boşluk doldurma kapasitesini etkilemektedir. Isı transferi ile ilgili sorunların ele alınması, reçine boşluklarını ve ısı kaynaklı çatlakları azaltmak için önemlidir. Bulgular, iç yapısal kusurlar ve reçine boşlukları arasındaki ilişkiyi anlamanın, kiriş tasarımı ve üretim süreçlerini önemli ölçüde iyileştirebileceğini önermektedir. Bu çalışma, mühendislik uygulamalarında kompozit malzemelerin yapısal performansını ve güvenilirliğini optimize etmek için kritik bilgiler sunmaktadır.Doctoral Thesis Improving the Joining Performance of Carbon Fiber Peek Based Thermoplastic Composites With Laser Surface Treatment(01. Izmir Institute of Technology, 2024) Türkdoğan Damar, Ceren; Tanoğlu, MetinBu doktora tezinin amacı, nanosaniye atımlı IR-Yb (kızılötesi-iterbiyum) fiber lazer yüzey ön işlemi kullanarak karbon fiber/polieter eter keton (KF/PEEK) kompozitlerinin birleşme bölgesi performansını arttırmaktır. Bu amaçla, sıcak presleme metodu ile üretilen, havacılık sektöründe oldukça yaygın kullanılan KF/PEEK termoplastik kompozitlerin yüzeyleri, farklı lazer parametreleri ile yapıştırıcı film kullanılarak birleştirme öncesi işlenmiştir. Ayrıca, ortalama lazer gücü, tarama hızı, frekans gibi değişen lazer parametrelerinin, yüzey yapısı, mikroyapı ve mekanik özellikler üzerindeki etkileri ortaya konmuştur. Lazerin yüzey üzerinde oluşturduğu etki, değişen parametrelere bağlı olarak, açığa çıkan enerjinin değeriyle kontrol edilebilmektedir. Bu çalışmada, bu üç lazer parametresinin değiştirilmesinin KF/PEEK kompozit yüzeyine etkisi SEM görüntüleri ile elde edilmiş ve PEEK matrisinin karbon fibere zarar vermeden yüzeyden seçici olarak uzaklaştırılması için gereken optimum enerji değeri için lazer parametrelerinin uygun çalışma aralıkları belirlenmiştir. Çalışmada istenilen yüzey kalitesini elde etmek için lazer parametrelerinin optimum çalışma aralıkları belirlendikten sonra, hazırlanan numuneler üzerinde tek turlu kesme (SLS), Charpy darbe ve çift konsol kiriş (DCB) testleri hem lazer hem de yüzeyleri lazer ile işlenmemiş referans numuneleri için gerçekleştirilerek, lazer yüzey ön işleminin mekanik özellikler üzerindeki etkileri net bir şekilde ortaya konmuştur. Birleştirme yüzeyleri uygun lazer parametreleriyle işlenen numunelerin mekanik dayanımlarında artış sağlanmıştır. Bu artış yüzey pürüzlülük parametreleri ve temas açısı ölçüm değerleri ile ilişkilendirilerek açıklanmıştır.Doctoral Thesis Designing Composite-Based Cylindrical Structures and Manufacturing Composite Prototypes by Filament Winding Method(01. Izmir Institute of Technology, 2024) Martin, Seçkin; Tanoğlu, MetinThis study reports the design, finite element modeling, optimization, fabrication and testing of relatively thick (radius/thickness ~ 7) and long carbon fiber reinforced polymers produced by filament winding against buckling damage under axial loading. The optimum winding angle and stacking sequence against Linear (Eigenvalue) buckling were determined in accordance with the predetermined design requirements utilizing genetic algorithm (GA) optimization via MATLAB. During the optimization process, the critical buckling load factor (λcr) was assigned as objective function, design constraints were natural frequency (fn) and angle of twist (Φ), and ply angles were considered to be variable and restricted with 20 to 87-degree continuous fiber angles in the laminate sequences. As a consequence of the test results, λcr of the proposed optimum model was found to be 3.2 times better than the reference model and both the analytical and finite element model satisfactorily predicted the critical buckling load for all CFRP rods consistent with the test results. The critical buckling loads calculated by applying a KDF of 0.95 for the finite element model and a KDF of 0.9 for the analytical solution were found to be reasonably appropriate for use in the preliminary design input. Additionally, results showed that a higher axial to the circumferential ratio of axial and bending stiffness (A11/A22, D11/D22) promises better buckling performance than other possible candidates. Finally, the microstructures of the produced rods were examined and the fiber volume ratios were calculated by means of chemical characterization.Doctoral Thesis Examination of Fatigue Behaviour of Carbon Fiber Reinforced Polymer Composites(Izmir Institute of Technology, 2021) Güneş, Mehmet Deniz; Tanoğlu, Metin; Tanoğlu, MetinThis PhD thesis aims to examine the fatigue behavior of sandwich panels fabricated from adhesively bonded aluminum honeycomb core and carbon fiber reinforced polymer composite face sheets. Initially, sandwich panels were manufactured with three different amounts of adhesive in their interface. Static flexural behavior was characterized with three-point bending tests. Load-displacement curves and static flexural failure modes were obtained and utilized to compare the static flexural behavior of fabricated sandwich. Fatigue behavior of sandwich panels were characterized with the three-point bending fatigue tests. Stiffness degradation curves were used to identify the failure cycles of sandwich panels. Fatigue failure modes and S-N curves were obtained to find out the effect of amount of adhesive on fatigue behavior of sandwich panels. The other study within this thesis was made to investigate the effect of core thickness on the fatigue behavior of the sandwich panels based on aluminum honeycomb core and carbon fiber reinforced polymer composite face sheets. Sandwich panels were fabricated by using three different aluminum honeycomb core thickness. Static flexural tests were carried out to determine the static flexural behavior of developed sandwich panels. Load-displacement curves and failure modes were obtained from flexural tests. In addition to this, core shear tests were performed to investigate the core shear strength of the honeycomb cores with different core thickness. Effect of core thickness on fatigue behavior of sandwich panels were characterized with fatigue failure modes and S-N curves. Stiffness degradation method was used to determine the fatigue failure cycles of the sandwich panels.Doctoral Thesis Development and Characterization of Innovative Fiber Reinforced Prepregs and Their Composites Containing Functional Fillers(Izmir Institute of Technology, 2021) Uz, Yusuf Can; Tanoğlu, MetinThis Ph.D. thesis aims to prepare laboratory-scale carbon fiber reinforced prepregs and improve the performance of their composites by incorporating functionalized single-wall carbon nanotubes (SWCNTs). The effect of nano-scale functional fillers on the characterization of prepregs and their composites was investigated to develop innovative materials for primary structures. To affect dispersion characteristics, SWCNTs were functionalized by oxidizing their surface with the carboxyl (-COOH) group using acid treatment. The modified resin system containing 0.05, 0.1, and 0.2 wt. % F-SWCNTs were developed with novel multi-step dispersion techniques. FTIR spectroscopy was performed to identify new bonding groups formed after the covalent functionalization. Unidirectional carbon fiber reinforced prepregs with/without F-SWCNTs were prepared using a drum-type winding technique by utilizing the solvent-dip (solution impregnation) process. The effect of F‐SWCNTs on the curing process and kinetic parameters of the carbon fiber/epoxy-based prepregs were investigated using non‐isothermal DSC. The activation energy of the curing reaction was calculated by the isoconversional methods. Also, the new numerical approach called GMN was developed to determine the activation energy of the thermosetting materials. For the fabrication of prepreg-based composite laminates, the vacuum bag-only (VBO) method was performed. The fiber volume fractions of the CFRP samples changed between 55.3% and 50.16%. The mechanical and thermomechanical properties of prepreg-based CFRP composites with/without F-SWCNTs were investigated. The optimum mechanical properties of F-SWCNTs filled CFRP composite was achieved at 0.05 wt.% of F-SWCNTs. However, mechanical properties were decreased due to the addition of higher content of F-SWCNTs, in comparison with neat CFRP.Doctoral Thesis Performance Improvement of Composite Materials Used as Hydrogen Storage Tanks by Microstructural Modifications(Izmir Institute of Technology, 2020) Ay, Zeynep; Tanoğlu, MetinThe goal of this Ph.D. thesis is to improve the performance of the cylindrical composites manufactured by filament winding method by using the toughened matrix resin with nano-sized filler (noncovalently functionalized with ethoxylated alcohol chemical-vapor-deposition-grown SWCNTs). The effect of SWCNT concentration on the mechanical performance of these composites was investigated and discussed. One of the main focus of this thesis is to examine the effect of nano-sized filler type and filler concentration on the performance of the epoxy-based composites. For this purpose, epoxy-based nanocomposites with different nano-sized filler types (SWCNT, TEGO, and HNT) at varying concentrations were developed by a calendaring (3-roll-mill) method. A series of mechanical tests were performed for reference composite and developed nanocomposites. The scanning electron microscopy (SEM) was used to reveal the morphology and toughening mechanisms by examining the fractured surface of nanocomposites. The rheological behaviors and contact angle measurements with glass fiber of the selected filler (SWCNT) incorporated epoxy suspensions were investigated to determine the suitability of suspensions for the filament winding process. The reference and SWCNT modified glass fiber (GF)-based cylindrical fiber-reinforced polymeric composites (CFRPCs) with an inner diameter of 60 and 275 mm were manufactured by the filament winding method. The split-disk and three-point bending tests were performed for GF-based CFRPCs. The double cantilever beam (DCB) test was also carried out for the reference and SWCNT modified GF-based CFRPCs to investigate the effect of SWCNT existence on the interlaminar fracture toughness of CFRPCs. The fractured surfaces after the DCB test were analyzed under the SEM to comprehend the toughening mechanisms, and micro-and nano-sized filler morphologies. Consequently, it was revealed that blending and hence toughening the epoxy resin with SWCNT improves the interlaminar properties of the GF-based composites.Doctoral Thesis Desing and Production of Light-Weight Pressure Resistant Composite Tank Materials and Systems for Hydrogen Storage(Izmir Institute of Technology, 2020) Kartav, Osman; Tanoğlu, MetinThis thesis focuses on the development of high-pressure resistant composite tanks for hydrogen storage. For this aim, composite tanks with aluminum liners were designed and manufactured by filament winding technique with various lay-up configurations and tested. The main objective of this study was to develop composite tanks with 700 bar working pressure and 1400 bar burst pressure. Furthermore, composite doily layers were incorporated into the filament winding technique and inserted at the front and end dome sections of the composite tanks to improve the burst pressure performance of the composite tanks and to develop the manufacturing process. Before the manufacturing process, the winding simulations were completed using CADWINDTM CAM software. The manufactured composite tanks were hydrostatically loaded with increasing internal pressure up to the burst pressure. During loading, the deformations over the composite tanks and liners were measured locally using strain gauges. Besides, composite plates were manufactured by filament winding technique to determine the mechanical and the thermo-mechanical properties, and the fiber mass fractions of composite sections were determined. Additionally, a preliminary study was carried out to investigate the effect of hybrid fiber usage on the burst pressure performance of steel liner based composite tanks. The effect of filament winding parameters on the burst pressure performance of composite tanks was investigated experimentally. The aimed burst pressure value of more than 1400 bar was obtained in this study for aluminum liner-based carbon fiber reinforced composite tanks. Also, a desired safe burst mode that is expected to occur in the mid-region of the composite tanks was successfully obtained. This study may be useful for the development of composite tanks for high-pressure hydrogen storage especially for the automotive industry and can be helpful to decrease the usage of fossil fuels.Doctoral Thesis Modeling, Simulation and Analysis of Type-Iii Composite Overwrapped Pressure Vessels for High-Pressure Gas Storage(Izmir Institute of Technology, 2019) Kangal, Serkan; Tanoğlu, MetinIn this thesis, multi-layered composite overwrapped pressure vessels (COPVs) for high-pressure gaseous storage were modeled by finite element (FE) method and manufactured by filament winding technique. Two liners with distinct geometries were utilized for containing gas and forming a basis for composite filament winding. 34CrMo4 steel as a load-sharing metallic liner was selected for investigation of hybridization effects. Glass and carbon filaments were overwrapped to the liner with a winding angle of [±11°/90°2]3 to obtain a fully overwrapped composite reinforced vessel with non-identical front and back dome endings. The other type of liner was made of Al 6061-T6 and chosen for containing high-pressure gas such as hydrogen and its better strength-to-weight ratio suitable for onboard applications. Doily layers were implemented to the structure for inducing safe burst modes and increasing the burst pressure of the aluminum-based COPVs. All vessels were hydrostatically loaded with increasing internal pressure up to the burst pressure. The mechanical performances of pressure vessels were investigated by both experimental and numerical approaches. In numerical approaches, FE analysis was performed featuring a simple progressive damage model available in ANSYS for composite section. The metal liners were modeled as elastic-plastic material with two different hardening approaches; bilinear and multilinear hardening. The results from steel based COPV indicate that the FE model provided a good correlation between experimental and numerical strain results for the vessels with indications that the composite interlayer hybridization has positive effects on radial deformation of the COPVs. The constructed model for aluminum-based COPVs was also able to predict experimental burst pressures within a range of 8%.Doctoral Thesis Toughening of Carbon Fiber Based Composites With Electrospun Fabric Layers(Izmir Institute of Technology, 2017) Beylergil, Bertan; Tanoğlu, Metin; Aktaş, EnginThe objective of this PhD thesis is to investigate interlaminar Mode-I fracture toughness of carbon fiber/epoxy composites interleaved by micro and/or nano scaled PA66 nonwoven veils. Also, the effects of electrospun PVA nanofibers on the mechanical performance of these composites were investigated. Additionally, this thesis also deals with the effects of aramid nonwoven veils on the mechanical properties of CF/EP composites. The produced nanofibers produced by electrospinning were directly deposited on carbon fiber fabrics. Then, reference and nano-modified laminates were manufactured by vacuum infusion method. A series of mechanical tests such as tensile, compression, three point bending, Charpy-impact, interlaminar shear strength and open hole tensile tests (OHT) were carried out on the prepared specimens. Double cantilever beam (DCB) tests were conducted on reference and interleaf-modified laminates. The effect of PA 66 nanofiber areal weight density was also evaluated with varying electrospinning time. Scanning electron microscopy (SEM) was used to investigate the fiber morphology and to understand the toughening mechanisms. Dynamic mechanic analysis (DMA) was used to investigate the thermo-mechanical behavior of reference and interleaf-modified composite specimens. Differential scanning calorimetry (DSC) was used to determine the thermal properties of micro and electrospun PA66 nonwoven veils. Comparing the mechanical test results, the most effective nonwoven interleaving system was determined in terms of higher delamination resistance and in-plane mechanical properties. Finite element method (FEM) was used to evaluate the effects of electrospun PA66 nonwoven veils on the CF/EP composites. Numerical simulations of Mode-I fracture toughness tests were carried out using ANSYS Workbench. The results showed that the most effective material was electrospun PA66 nonwovens considering the higher delamination resistance. Additionally, the electrospun PA 66 nonwovens also improved Charpy-impact and interlaminar shear strength of the reference CF/EP composites. Numerical results showed good agreement with the experimental ones.Doctoral Thesis Development of Multi and Double Walled Carbon Nanotubes (cnts) / Vinylester Nanocomposites(Izmir Institute of Technology, 2008) Seyhan, Abdullah Tuğrul; Tanoğlu, MetinThis study focuses on development and characterization of thermosetting resin based nanocomposites containing multi and double walled carbon nanotubes with and without amine functional groups (MWCNT, DWCNTs, MWCNT-NH2 and DWCNTNH2).A novel 3-roll milling technique was conducted to prepare the resin suspensions with carbon nanotubes (CNTs). Rheological measurements performed on the resin suspensions showed that addition of very low contents (0.05, 0.1 and 0.3 wt. %) of MWCNTs and MWCNT-NH2 affected the flow characteristic of the resin, significantly.Further, the curing behavior of a vinylester-polyester hybrid resin suspensions containing 0.3 wt % of MWCNTs and MWCNT-NH2 was intensively studied. It was found that regardless of amine groups, presence of CNTs affected the polymerization of the hybrid matrix resin. Final individual fractional conversion rates of styrene and vinylester monomers were found to be vastly dependent on the type of CNTs. Glass transition temperature (Tg) values of the nanocomposites with MWCNTs and MWCNTNH2 were found to increase with filler content. Moreover, nanocomposites containing MWCNTs and MWCNT-NH2 were found to possess higher tensile strength, elastic modulus as well as fracture toughness and fracture energy as compared to the neat hybrid resin. On the other hand, electrical properties of the nanocomposites were also investigated and it was found that nanocomposites with MWCNTs exhibited the lowest percolation threshold value. In addition, nanocomposites with amino functionalized CNTs were found to exhibit lower electrical conductivity as compared to those with untreated CNTs. Nanocomposites with AC electric field induced aligned CNTs were also prepared. Finally, based on the findings obtained for CNT/ resin suspensions, as a case study, electrically conductive glass fiber reinforced composite laminates were successfully produced, using the CNT modified resin suspensions as matrix material, via Vacuum Assisted Resin Transfer Molding (VARTM) and Resin Transfer Molding (RTM) methods.