Yazar "Eskizeybek, Volkan" seçeneğine göre listele

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    A facile and effective method for size sorting of large flake graphene oxide
    (Avestia Publishing, 2016) O¨zçakir, Ece; Eskizeybek, Volkan
    The size of the building blocks fundamentally governs physical performances of the macro-scale graphene based structures since larger building blocks usually yields better mechanical and electrical properties. Density gradient centrifugation method has emerged as a versatile and scalable method for sorting colloidal 2D nanomaterials. This paper provides a facile and effective size sorting approach to graphene oxide (GO) flakes with large sizes up to 40 ?m. The GO flakes were dispersed within distilled water. The centrifugation process parameters were calculated with respect to specific size ranges of GO flakes. Scanning electron microscopy utilized to prove the effectiveness of the separation process. Image processing analysis showed GO flakes with specific size ranges can be separate from aqueous suspension by controlling rotational speed and centrifugation time. The process was performed by using common benchtop centrifuges with low intensity centrifugal fields which requires low investment for a scalable process. © Avestia Publishing, 2016.
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    A novel polyaniline/NiO nanocomposite as a UV and visible-light photocatalyst for complete degradation of the model dyes and the real textile wastewater
    (Springer, 2021) Haspulat Taymaz, Bircan; Eskizeybek, Volkan; Kamış, Handan
    The textile processing industry utilizes enormous amounts of water. After the dying process, the wastewater discharged to the environment contains carcinogens, non-biodegradable, toxic, and colored organic materials. This study aimed to develop a nanocomposite material with improved photocatalytic activity to degrade textile dyes and without a need for a post-separation process after the use. For this, nickel oxide nanoparticles (NiO NPs) were synthesized by a simple method in aqueous media. Then, NiO-doped polyaniline (PANI/NiO) with efficient absorption in the visible region (optical band gap of 2.08 eV) synthesized on a stainless steel substrate with electropolymerization of aniline in the aqueous media. The photocatalytic activity of PANI/NiO film was also investigated by the degradation of model dyes. Under UV and visible light irradiation, the PANI/NiO film degraded methylene blue and rhodamine B dyes entirely in 30 min. Moreover, the PANI/NiO film was also utilized to degrade real textile wastewater (RTW) without applying any pre-process; it was entirely decomposed by the nanocomposite film in only 45 min under UV light irradiation. The photocatalytic reaction rate of the pure PANI film is increased as 2.5 and 1.5 times with the addition of NiO NPs under UV and visible light irradiations for degradation RTW, respectively. The photocatalytic efficiency was attributed to reduced electron-hole pair recombination on the photocatalyst surface. Furthermore, the photocatalytic stability is discussed based on re-use experiments. The photocatalytic performance remains nearly unchanged, and the degradation of dyes is kept 94% after five cycles.
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    A rational study on the hydrothermal aging of AFP manufactured CF/polyetherketoneketone composites with in situ consolidation supported by acoustic emission inspection
    (Wiley, 2022) Sukur, Emine Feyza; Elmas, Sinem; Seyyednourani, Mahsa; Eskizeybek, Volkan; Yildiz, Mehmet; Sas, Hatice S.
    In this study, carbon fiber (CF)/polyetherketoneketone (PEKK) composites with 5% void content, manufactured via an in situ consolidated automated fiber placement (AFP) lay-up process, are aged in hot water at 70 degrees C for 30 days. Firstly, a deep understanding of the deterioration in the mechanical performance is developed with a comprehensive and complementary set of material characterization strategies, including (i) microstructural characterization with Fourier-transform infrared spectroscopy (FTIR), (ii) thermal characterization with differential scanning calorimetry (DSC), and (iii) dynamic mechanical analysis (DMA). The material characterization concurrently highlights the plasticization and post-crystallization phenomena after aging with changes in the peak densities with FTIR, formation of second glass transition temperature (T-g) in DSC and DMA, and drop in storage modulus, loss modulus, and tan delta (delta) amplitudes. Then, acoustic emission (AE) is utilized as an inspection tool to identify the damage mechanisms regarding the 6.5%, 5.2%, and 4% decrease in tensile strength, strain at failure and modulus, respectively, in a comparative manner. The AE findings, remarking the weakening of the fiber-matrix interface after aging, are validated with scanning electron microscopy analysis. This study introduces an aging process-induced damage mechanism triggered with inhomogeneous water absorption for AFP manufactured CF/PEKK composites with in situ consolidation.
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    A Systematic Characterization Approach for Vacuum Bag Only Prepregs towards an Accurate Process Design
    (Mdpi, 2022) Arikan, Muhammed H.; Eroglu, Fatih; Eskizeybek, Volkan; Sukur, Emine Feyza; Yildiz, Mehmet; Sas, Hatice S.
    Aerospace-grade composite parts can be manufactured using Vacuum Bag Only prepregs through an accurate process design. Quality in the desired part can be realized by following process modeling, process optimization, and validation, which strongly depend on a primary and systematic material characterization methodology of the prepreg system and material constitutive behavior. The present study introduces a systematic characterization approach of a Vacuum Bag Only prepreg by covering the relevant material properties in an integrated manner with the process mechanisms of fluid flow, consolidation, and heat transfer. The characterization recipe is practiced under the categories of (i) resin system, (ii) fiber architecture, and (iii) thermal behavior. First, empirical models are successively developed for the cure-kinetics, glass transition temperature, and viscosity for the resin system. Then, the fiber architecture of the uncured prepreg system is identified with X-ray tomography to obtain the air permeability. Finally, the thermal characteristics of the prepreg and its constituents are experimentally characterized by adopting a novel specimen preparation technique for the specific heat capacity and thermal conductivity. Thus, this systematic approach is designed to provide the material data to process modeling with the motivation of a robust and integrated Vacuum Bag Only process design.
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    Adsorption-Assisted Photocatalytic Degradation of Anionic Direct Yellow-50 and Cationic Methylene Blue Dyes by Chemically Synthesized Poly(1,5-diaminoanthraquinone)
    (Springer, 2025) Akilli, Aleyna; Ozler, Aysenur; Taymaz, Bircan Haspulat; Hanci, Ahmet; Eskizeybek, Volkan; Kamis, Handan
    Conducting polymers renowned for their exceptional photocatalytic activity, conductivity, and visible-light absorption capabilities present a compelling alternative for advanced photocatalytic applications. In this regard, the creation of conductive polymers of the next generation has enormous promise for improving energy efficiency as well as solving environmental issues. In this study, the conductive polymer poly(1,5-diaminoanthraquinone) (PDAAQ) with a band gap of 1.28 eV and an electrical conductivity of 1.23 S/cm was successfully synthesized via chemical oxidative polymerization using ammonium peroxydisulfate as an oxidant and perchloric acid as an initiator in an acetonitrile polymerization medium. The adsorption-assisted photocatalytic performance of PDAAQ has been investigated in cationic methylene blue (MB) and an anionic direct yellow (DY) dye under visible irradiation. The effect of polymerization medium, oxidant type, polymerization time, and monomer oxidant ratio on adsorption-assisted photocatalytic degradation of MB was investigated. The synthesized PDAAQ polymer demonstrates exceptional photocatalytic performance, completely degrading MB and DYE dyes under visible light illumination in 6 and 8 min through an adsorption-assisted photocatalysis mechanism. Besides, the photocatalytic dye degradation performance of PDAAQ was investigated for the degradation of synthetic wastewater (SWW) under visible light. The PDAAQ polymer proves to be an effective photocatalyst for photocatalytic applications, showcasing exceptional potential in degrading model dyes and treating synthetic wastewater.
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    An experimental implication of long-term hot-wet-aged carbon fiber/polyether ketone ketone composites: The impact of automated fiber placement process parameters and process-induced defects
    (John Wiley and Sons Inc, 2023) Şükür, Emine Feyza; Elmas, Sinem; Eskizeybek, Volkan; Sas, Hatice S.; Yıldız, Mehmet
    During the service life of aerospace-grade composites, process parameters and process-induced defects may become crucial. Most studies in this field have mainly focused on the relationship between process-induced defects and mechanical performance. However, the potential impact of process parameters and process-induced defects on the service life of composites serving under severe service conditions has received little attention. In this work, the effects of hydrothermal conditioning on the mechanical performance of carbon fiber/polyether ketone ketone (CF/PEKK) composites are examined, along with the correlation between automated fiber placement (AFP) process parameters and process-induced defects. For this, gap and overlap defects integrated CF/PEKK laminates were exposed to a long-term (90 days) hot-wet aging environment to simulate the actual service conditions. Defect-induced composite samples reached saturation point at the end of 30 days with a mass gain of 0.2 wt%. The aging process resulted in an increase in the degree of crystallization by almost 14% without a change in the chemical structure, indicating the postcrystallization of the PEKK matrix. Even though the thermo-mechanical performance diminished (~25%) with the aging process, storage modulus was slightly affected by process parameters and process-induced defects. Considering the flexural and shear test results after the aging process, the impact of gap and overlap defects on the service life of AFP composites can be minimized with higher compaction forces (600 N) and lower lay-up speeds (0.1 m/s).
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    An experimental study on low velocity impact performance of bolted composite joints part 1: Influence of halloysite nanotubes on dynamic loading response
    (Elsevier Ltd, 2021) Kaybal, Halil Burak; Ulus, Hasan; Eskizeybek, Volkan; Avcı, Ahmet
    Mechanical joints are a widely utilized to assembly fiber reinforced polymer composites in marine applications. Impact is one of the most encountered unpredictable loading types which significantly diminishes the mechanical properties of structures. The goal of this study is to investigate the dynamic loading response of bolted basalt-epoxy composite laminates under different impact energies. Unlike the existing low velocity impact tests of bolted composite joints, to reveal the effect of localized impact damage, the low-velocity impact tests were conducted on two different regions as the top of bolt (ToB) and the side of washer (SoW). In addition, the effects of HNTs reinforcement on the impact response and the damage propagation were also evaluated. It was obtained that ToB damage was comparatively severe for the composite joints due to the propagation of the damage through the hole center. Moreover, HNTs improved the impact resistance about %15, especially at lower impact energies. However, the nanoreinforcement efficiency diminished with increasing impact energy levels. The obtained results were further supported with macro-size images and scanning electron microscopy (SEM). Together with Part II, this study reports an extensive work of impact tests of bolted composite joints utilized in the marine industry.
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    An experimental study on low velocity impact performance of bolted composite joints-part 2: Influence of long-term seawater aging
    (Elsevier Ltd, 2021) Kaybal, Halil Burak; Ulus, Hasan; Eskizeybek, Volkan; Avcı, Ahmet
    In the first part of this two-part paper (Part 1), the low-velocity impact (LVI) response of bolted fiber-reinforced polymer joints was investigated considering with two scenarios based on the localized impact damage as the impactor hit on the top of the bolt (ToB) and the side of the washer (SoW). Moreover, the influence of halloysite nanotubes (HNTs) reinforcement of the epoxy matrix on the impact performance was also evaluated. As the second part of the research, this paper represents the effects of seawater aging on the LVI response of FRPs. For this, the composite joints were submerged in an artificial seawater environment for six months to accelerate aging. Afterward, as following the systematic experimental path exhibited in Part 1, LVI tests were conducted by dropping the impactor on ToB and SoW regions. The test results showed that the seawater aging impaired almost 30% of the composite joints' impact resistance, where HNTs reinforced multi-scale composite joints exhibited a 13% higher impact loading performance. The ToB impact scenario was considered as visually and quantitatively more detrimental than the SoW tests. The detrimental impact of seawater aging was validated by tracking the elemental evolution in the seawater environment. Based on the mechanical, morphological, and structural analyses, a novel damage mechanism was introduced to address seawater aging's progress, including the role of nanoreinforcements.
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    Boron nitride-MWCNT/epoxy hybrid nanocomposites: Preparation and mechanical properties
    (Elsevier, 2014) Ulus, Hasan; Ustun, Tugay; Eskizeybek, Volkan; Sahin, Omer Sinan; Avci, Ahmet; Ekrem, Mursel
    In this study, production and mechanical properties of hybrid nanocomposites have been investigated. Hybrid nanocomposites are consisting of boron nitride nanoplatelets (BN) and multiwall carbon nanotubes (MWCNT) embedded in epoxy resin. The BN and MWCNT were mixed to epoxy resin in different weight fractions and mixtures were utilized for tensile test specimen production. The synthesized BN and produced hybrid nanocomposites were characterized by SEM, TEM, XRD, FT-IR and TGA analyses. The elasticity modulus and tensile strength values were obtained via tensile tests. The fracture morphologies were investigated after tensile test by means of scanning electron microscopy. (C) 2013 Elsevier B.V. All rights reserved.
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    CdSO4-CdS Composite nanowires synthesized by arc discharge method
    (2022) Üstün, Tugay; Eskizeybek, Volkan; Toumiat, Amor; Avcı, Ahmet
    Herein, we successfully carried out the of CdS nanowires doped with CdSO4 nanoparticles by arc discharge method. For this, two cadmium rods with high purity as anode and cathode electrodes were submerged in a liquid nitrogen medium. The cathode electrode was filled with micro-sized sulfur powder to synthesize CdS nanowires. CdS nanowires were obtained by oxidation using low temperature with CdSO4 nanoparticles. After the arc discharge process, the resultant product was characterized to reveal structural and morphological properties. XRD and electron EDX analyses exposed the coexistence of the CdS and CdSO4 phases. Scanning electron microscopy (SEM) investigations revealed that the synthesized nanostructures formed in a 1D wire nanostructure morphology with a diameter of less than 10 nm and several micrometers in length. Besides, the transmission electron microscopy (TEM) analysis was utilized to monitor the CdSO4 nanoparticles located on CdS nanowires surface. It has been found that the absorption band blue-shifted with 0.53 eV. This blue shift originated from the quantum confinement in the nanoparticles.
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    CETP kompozitlerin ağaç matkabıyla delinme performanslarının deneysel incelenmesi
    (2021) Koyunbakan, Murat; Ünüvar, Ali; Eskizeybek, Volkan; Avcı, Ahmet
    Cam elyaf takviyeli polimer (CETP) kompozit malzemeler sahip oldukları üstün özelliklerinden dolayı yeni ve önemli mühendislik malzemeleridir. Mekanik özellikleri, rijitlik/ağırlık oranlarından dolayı özellikle hafiflik istenilen mühendislik yapılarında vazgeçilmez yapısal malzemelerdir. Bu sayede CETP kompozit malzemeler endüstride sıklıkla kullanılmaktadır. Delme işlemi endüstride CETP kompozitlerin montajı için gerekli deliklerin elde edilmesinde en önemli prosestir. Delik kalitesine kesme parametrelerinin yanı sıra matkap geometrisinin etkisi de ele alınmalıdır. Bu çalışmada, farklı kesme parametreleri (kesme hızı, ilerleme) ve matkap geometrileri (farklı çaplar) ile delme işlemi gerçekleştirilmiştir. İtme kuvveti ile giriş ve çıkış yüzeyleri için deformasyon faktörü sonuçları elde edilmiştir. Ayrıca kesme parametrelerinin sonuçlara olan etkilerinin belirlenmesi için Taguchi metodu ve varyans analizi kullanılarak analiz yapılmıştır. Taguchi analizi için L9(313) ortogonal deney tasarımı seçilmiş ve cevap tabloları kullanılmıştır. İtme kuvveti ve çıkış deformasyon faktörü için ilerlemenin, giriş deformasyon faktörü içinse matkap çapının daha etkin parametre oldukları tespit edilmiştir. Sonuçlar, CETP kompozit malzemelerin delinmelerinde bu yaklaşımın etkin bir şekilde uygulanabileceğini göstermiştir.
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    CNT-PAN hybrid nanofibrous mat interleaved carbon/epoxy laminates with improved Mode I interlaminar fracture toughness
    (Elsevier Sci Ltd, 2018) Eskizeybek, Volkan; Yar, Adem; Avci, Ahmet
    Interleaving laminated composites with electrospun nanofibrous mats comes out as a promising micro scale strategy to strengthen interlaminar regions of laminated composites. The aim of this study is to evaluate the synergetic contribution of nano- and micro-scale mechanisms on interlaminar delamination. For this, carbon nanotubes (CNTs) reinforced polyacrylonitrile (PAN) electrospun hybrid mats were successfully fabricated and utilized as interleaves within the interlaminar region of carbon/epoxy laminated composites. The Mode I interlaminar fracture toughness values were enhanced up to 77% by introducing CNTPAN nanofibrous interleaves. Specifically, the nano-scale toughening mechanisms such as CNTs bridging, CNTs pull-out, and sword-sheath increased the Mode I fracture toughness by 45% with respect to neat PAN nanofibrous interleaves. The related micro- and nano-scale toughening mechanisms were evaluated based on the fracture surface analysis. Atomic force microscopy was also utilized to quantify the magnitude of surface roughness changes on the interlaminar region with respect to multi scale interleaving reinforcement and correlate surface roughness changes due to crack deflection to increased fracture toughness. (C) 2018 Elsevier Ltd. All rights reserved.
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    Coating graphene nanoplatelets onto carbon fabric with controlled thickness for improved mechanical performance and EMI shielding effectiveness of carbon/epoxy composites
    (Elsevier Ltd, 2023) Mutlu, Gökhan; Yıldırım, Ferhat; Ulus, Hasan; Eskizeybek, Volkan
    Coating nanostructures on fiber reinforcement is a facile and scalable technique to manufacture next-generation fiber-reinforced polymer composites with tailored physical properties. Optimizing the nanomaterial coating thickness on fibers is vital in tailoring the multifunctionality of fiber-reinforced composites without sacrificing the mechanical performance since it relies on the fiber–matrix interface, where interlaminar and other physical properties are governed. This paper investigates the impact of graphene nanoparticle (GNP) coating thickness on the mechanical properties, fracture behavior, thermo-mechanical, and electromagnetic interference (EMI) shielding effectiveness (SE) of composite structures. We grafted GNPs on carbon fabrics using a solution coating method with various thicknesses (10, 20, and 30 µm), and GNPs grafted fabrics were impregnated with an epoxy resin. The 20 µm GNPs coating thickness exhibited the highest mechanical performance, increasing the tensile and interlaminar shear strength by 32% and 26%, respectively, compared to pristine samples. Storage modulus and transition (Tg) temperature values increased by 18.6% and 13.6% for 20 µm coating thickness, respectively. Besides, the unstable crack growth at the fiber–matrix interface was stabilized when the GNPs coating thickness reached 20 µm according to delamination toughness tests. While mode-I fracture toughness increased up to 22%, an improvement of 13.5% was obtained in mode-II fracture toughness. The underlying toughening mechanisms at the interfacial region were identified using scanning electron microscopy. The EMI-SE was slightly increased by the GNPs grafting, whereas thinner GNPs coatings exhibited higher shielding efficiency.
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    Çok duvarlı karbon nanotüp katkılı poliamit 6 polimerinin mekanik özelliklerinin incelenmesi
    (2020) Kuş, Gözde; Koyunbakan, Murat; Yetgin, Salih Hakan; Yıldırım, Ferhat; Eskizeybek, Volkan; Genç, Abdurrahman
    Bu çalışmada, poliamit 6 (PA6) polimerine çok duvarlı karbon nanotüp (ÇDKNT) takviye edilerekPA6/ÇDKNT nanokompozitleri üretilmiştir. Elde edilen nanokompozitlerde değişen ÇDKNT miktarınınçekme, eğilme ve darbe dayanımlarına etkisi incelenmiştir. ÇDKNT nanotakviyelerinin PA6 matriksiçerisindeki homojen dağılımlarının sağlanması için PA6/ÇDKNT karışımı çift vidalı ekstruderdengeçirilmiştir. Elde edilen karışım enjeksiyonla kalıplama tekniği kullanılarak, çekme, darbe ve eğilmenumunelerinin üretiminde kullanılmıştır. Çalışma sonucunda, ÇDKNT miktarının PA6 matriksinin mekaniközellikleri üzerinde önemli etkisi olduğu belirlenmiştir. Buna göre, artan ÇDKNT miktarına bağlı olarakPA6/ÇDKNT nanokompozitlerin çekme ve eğilme dayanımları ile elastisite modülleri sırasıyla %5.79, %16ve %7.8 oranlarında artarken darbe dayanımı ve şekil değiştirme değerleri sırasıyla %11.2 ve %988oranlarında azalmıştır.
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    Çok Duvarlı Karbon Nanotüp Katkılı Poliamit 6 Polimerinin Termal, Termo-Mekanik Ve Tribolojik Özelliklerinin İncelenmesi
    (2020) Kuş, Gözde; Yetgin, Salih Hakan; Koyunbakan, Murat; Yıldırım, Ferhat; Eskizeybek, Volkan; Genç, Abdurrahman
    Bu çalışmada, çok duvarlı karbon nanotüp (ÇDKNT) katkısının Poliamit 6 (PA6) nanokompozitlerin termal, termo-mekanik ve tribolojik özelliklere etkisi incelenmiştir. ÇDKNT katkılı PA6 nanokompozit granüller çift vidalı ekstruder ile üretilmiştir. Elde edilen granüllerden enjeksiyonla kalıplama tekniği kullanılarak standartlara uygun numuneler üretilmiştir. PA6 nanokompozitlerin ergime sıcaklığı ve kristallenme oranı diferansiyel taramalı kalorimetre (DSC) analizi ile depolama modülü ve kayıp modülü ise dinamik mekanik analiz (DMA) ile belirlenmiştir. Aşınma testleri kuru ortam şartları altında çelik diske karşı pim-disk sistemi ile gerçekleştirilmiştir. Çalışma sonucunda, ÇDKNT katkılı PA6 polimerinin ergime sıcaklığı sabit kalırken kristallenme oranı artmıştır. Artan ÇDKNT miktarına bağlı olarak PA6 kompozitlerin depolama modülü ve kayıp modülü artmıştır. Uygulanan yükün 10 N’dan 40 N’a artması ile katkısız PA6 polimerinin sürtünme katsayısı %62,5 oranında artarken, PA6 nanokompozitlerin sürtünme katsayıları sırasıyla %41,3; %34,6; %45,4 oranında artmıştır. Genel olarak, PA6 ve PA6 nanokompozitleri için aşınma oranı 10-13 -10-14 m2 /N elde edilmiştir. En yüksek aşınma oranı 2,5×10-13 m2 /N değeri ile katkısız PA6 polimerinde elde edilirken en düşük aşınma oranı 2,0×10-14 m2 /N değeri ile %0,3 ÇDKNT katkılı PA6 nanokompozitinde elde edilmiştir.
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    Combined effect of fiber hybridization and matrix modification on mechanical properties of polymer composites
    (SAGE Publications Ltd, 2023) Demir, Okan; Yar, Adem; Eskizeybek, Volkan; Avcı, Ahmet
    Glass/carbon fiber reinforced hybrid composites are great candidates for wind turbine blade manufacturers to make larger blades. Variation of stacking sequences ensures design freedom to the composite engineers to optimize the composite structure's mechanical performance. On the other hand, matrix modification of polymer composites with nanoparticles is also of interest to introduce multifunctional properties. This research aims to scrutinize the influence of simultaneous fiber hybridization and matrix modification on polymer composites’ tensile, flexural, and low-velocity impact properties. Hybrid glass/carbon epoxy composites and hybrid glass/carbon/multi-walled carbon nanotube (MWCNT) multiscale polymer composites of stacking sequences [GCGCGC]S, [CGCGCG]S, and [G6C6] were manufactured. Fiber hybridization dramatically improved tensile strength between 51% and 76% compared to glass fiber composite. Depending on the stacking sequence, the flexural strength of the hybrid composites was improved between 10% and 16% concerning carbon fiber composite. With the introduction of MWCNTs, a slight increase in the tensile strength for unsymmetrical hybrid composites by around 5% and decreases by 7% for symmetrical ones were observed. Similar behavior was seen for bending characteristics. Additionally, low-velocity impact tests showed that it is achievable to bring greater impact peak forces up to 70% for hybrid composites than carbon fiber epoxy composites. MWCNTs modification of the matrix restrained the impact damage propagation, as proved by C-scan analysis.
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    Compression after Impact and Charpy Impact Characterizations of Glass Fiber/Epoxy/MWCNT Composites
    (Korean Fiber Soc, 2020) Demircan, Ozgur; Kadioglu, Kemal; colak, Pinar; Gunaydin, Erdinc; Dogu, Mustafa; Topalomer, Neslihan; Eskizeybek, Volkan
    In this study, glass fiber/epoxy resin/multi-walled carbon nanotubes (MWCNTs) were used to fabricate hybrid composites with biaxial warp-knitted fabrics. The biaxial warp-knitted fabrics were grafted with various amounts of MWCNTs and the hybrid composites were fabricated using the resin transfer molding (RTM) method, subsequently. The fabricated samples were subjected to compression after impact and Charpy impact tests. The hybrid composites exhibited higher compression after impact modulus and strength with 26 % and 17 % compared to the samples without nanotubes, respectively. Moreover, the MWCNTs integrated specimens showed 17 % improvement of Charpy impact strength against specimens without carbon nanotubes in 0 degrees degree direction. Fracture surface analysis revealed lower number of cracks and shorter crack propagation lengths in the MWCNTs reinforced specimens. The improvement in mechanical properties of the hybrid composites can most likely be attributed to an increase in interfacial adhesion due to the presence of the carbon nanotubes.
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    Damage tolerance of basalt fiber reinforced multiscale composites: Effect of nanoparticle morphology and hygrothermal aging
    (Elsevier Sci Ltd, 2024) Sukur, Emine Feyza; Elimsa, Selen; Eskizeybek, Volkan; Avci, Ahmet
    Barely visible impact damages of fiber-reinforced polymers (FRPs) have been the subject of much systematic investigation, specifically with the combination of the service conditions. Introducing nanoparticles into the polymer matrix is an effective strategy to improve the impact resistance and aging performance of FRPs. However, the effect of nanoparticle morphology on the mechanical performance and damage tolerance of hygrothermally aged FRPs has yet to be extensively investigated. Here, we report the effect of silica (SiO2, 0D), halloysite (HNT, 1D), and montmorillonite clay (NC, 2D) nanoparticles on the damage tolerance of basalt fiberreinforced epoxy composites, considering their environmentally harsh service conditions. The ceramic nanoparticle-modified epoxy represented the highest mechanical performance in the case of 2 wt% nanoparticle addition for all nanoparticle types. The efficiency of ceramic nanoparticles altered with the loading type in the epoxy nanocomposites. SiO2 nanoparticle-modified epoxy demonstrated the highest tensile strength (44 % increase), while HNT nanoparticle-modified epoxy demonstrated the highest flexural strength (30 % increase). The hygrothermal aging resulted in a slight increase in the impact performance of multi-scale FRPs. In contrast, the HNT nanoparticle-modified multi-scale FRPs exhibited the highest impact resistance with an increase of 8 % in impact load. Dynamic mechanical analysis revealed the multi-scale composite's crosslinking density increased drastically (47 %) with hygrothermal aging, which increased the storage modulus (14 %) and glass transition temperature (15.7 %) due to physical aging effects as revealed by FTIR analysis. Compression after impact tests showed that the compression strength of HNT-modified multi-scale composites increased 17.8 % after the aging. This study provides valuable insights into developing and performing multiscale composites for demanding aviation and wind energy applications.
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    Effect of long-term stress aging on aluminum-BFRP hybrid adhesive joint's mechanical performance: Static and dynamic loading scenarios
    (Wiley, 2022) Ulus, Hasan; Kaybal, Halil Burak; Cacik, Fatih; Eskizeybek, Volkan; Avci, Ahmet
    Composite-aluminum hybrid adhesive joints represent an ideal solution for designing lightweight structures for the marine industry. However, seawater aging is a serious concern, limiting the safe service life of the joint. Notably, efforts to understand the impact of aging have largely focused on the short-term periods without considering actual operating conditions. Here, we report the mechanical performance of hybrid joints subjected to the long-term stress aging. Besides, we modified the epoxy adhesive with halloysite nanotubes (HNTs) to limit the aging driven adhesive degradation and improve the adhesive's rigidity. We evaluated mechanical performances of hybrid joints by performing tensile, flexural, and drop-weight impact tests. While we increased the load-carrying capacity by over 25% with the HNTs modification before the stress aging process, modified adhesive withstood almost 55% higher tensile load than the neat epoxy adhesive after six-month stress aging. The modified adhesive also absorbed 41% less impact energy, indicating the efficiency of HNTs on limiting the degradation due to the stress aging. Furthermore, the damage mode transformed from adhesion to cohesion, thanks to the improved adhesive-composite interface performance. We envisage that these exciting results will pave the way for designing robust hybrid joints for the marine industry.
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    Effects of meso- and micro-scale defects on hygrothermal aging behavior of glass fiber reinforced composites
    (Wiley, 2022) Sukur, Emine Feyza; Elmas, Sinem; Seyyednourani, Mahsa; Eskizeybek, Volkan; Yildiz, Mehmet; Sas, Hatice S.
    Design and process-induced defects in fiber-reinforced polymers (FRPs) lead to fracture nucleation due to the stress concentrations. In addition to the degradation in mechanical properties, defects can accelerate aging of FRPs and limit their service life. Efforts to understand the impact of defects have largely focused on the mechanical performance of FRPs. However, their impact on aging performance has not yet been extensively investigated. Here, we report the effect of the meso-scale (missing yarn) and micro-scale (micro-crack) defects on the hygrothermal aging behavior of FRPs. Missing yarn defects were generated by pulling-out yarns in warp and weft directions of glass fabric. Then, micro-cracks were induced in composite laminates by acoustic emission controlled tensile loading/unloading. After exposing samples to the hygrothermal aging, we found that meso-scale defects deteriorate mechanical/thermomechanical performance, reaching 30% decrease in the flexural strength. Notably, even though increasing micro-crack density reduces the moisture saturation time, the aging time is reported as a more predominant design parameter, deteriorating the mechanical performance for micro-crack-induced FRPs.