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    Biogenic Silver Nanoparticles for Trace Colorimetric Sensing of Enzyme Disrupter Fungicide Vinclozolin
    (Mdpi, 2019) Hussain, Masood; Nafady, Ayman; Sirajuddin; Avci, Ahmet; Pehlivan, Erol; Nisar, Jan; Sherazi, Syed Tufail Hussain
    We report a novel, simple, efficient, and green protocol for biogenic synthesis of silver nanoparticles (AgNPs) in aqueous solution using clove (Syzygium aromaticum) extract as a reducing and protecting agent. Ultraviolet-visible (UV-Vis) spectroscopy was employed to monitor the localized surface plasmon resonance (LSPR) band of clove extract-derived AgNPs prepared under various conditions. Fourier-transform infrared (FTIR) spectroscopy analysis provided information about the surface interaction of the clove extract with the AgNPs. Ultrahigh-resolution transmission electron microscopy (UHRTEM) results confirmed the formation of spherical, uniformly distributed clove extract-capped AgNPs with sizes in the range of 2-20 nm (average size: 14.4 +/- 2 nm). Powder X-ray diffractometry analysis (PXRD) illustrated the formation of pure crystalline AgNPs. These AgNPs were tested as a colorimetric sensor to detect trace amounts of vinclozolin (VIN) by UV-Vis spectroscopy for the first time. The AgNP-based sensor demonstrated very sensitive and selective colorimetric detection of VIN, in the range of 2-16 mu M (R-2 = 0.997). The developed sensor was green, simple, sensitive, selective, economical, and novel, and could detect trace amounts of VIN with limit of detection (LOD) = 21 nM. Importantly, the sensor was successfully employed for the determination of VIN in real water samples collected from various areas in Turkey.
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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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    Effect of composite to aluminum single lap joints reinforced with graphene doped nylon 6.6 nanofibers
    (Sage Publications Ltd, 2021) Senyurt, Muhammet Ali; Ekrem, Mursel; Duzcukoglu, Hayrettin; Avci, Ahmet
    In this study was investigated the effect of nylon 6.6 (N6.6) nanofiber and nylon 6.6 by weight of 1%, 3% and 5% Graphene Nanoplates (GNPs) doped N6.6 nanofiber reinforced epoxy adhesives on single lap joint of AISI AA2024-T3 Aluminum Alloy (AA 2024)- Carbon Fiber Reinforced Composite (CFRC) plates. First of all, N6.6 nanofibers with/without GNPs in different proportions were produced by the electrospinning method with the help of a rotating and moving drum. Epoxy resin as the adhesive, nanofibers as the second interface to increase the strength of the epoxy and AA 2024 - CFRC were used as the bonded material and the plates cut in 101.6 x 25.4 mm according to ASTM D 1002-10 and ASTM D 5868-01, respectively. In this research was focused on the application of single lap joints by reinforcing with N6.6 nanofibres to decrease the brittleness and increase the toughness of the epoxy resin. When compared with epoxy resin, the shear strength of epoxy adhesive reinforced with N6.6 nanofibers with 3% GNPs by weight increased by 31.20%. Thermal analysis of epoxy resin and hybrid adhesives was examined by Differential Scanning Calorimeter (DSC). In addition, the physical properties of N6.6 nanofibers doped/undoped GNPs in different proportions by weight were observed with scanning electron microscopy (SEM) by damage analysis of fracture surfaces.
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    The effect of halloysite nanotube modification on wear behavior of carbon-aramid fiber reinforced hybrid nanocomposites
    (Wiley, 2022) Cetin, Mehmet Emin; Bastosun, Yusuf; Tatar, Ahmet Caner; Cetin, M. Huseyin; Demir, Okan; Onal, Gurol; Avci, Ahmet
    In this study, the carbon-aramid fiber reinforced hybrid composites are fabricated using a vacuum-assisted hand lay-up method using halloysite nanotubes (HNTs) modified epoxy matrix. Ball-on-disk wear tests are performed to analyze the tribological effect of neat and HNTs-added specimens at 10, 15, and 20 N loads and 1 m/s sliding speed. Additionally, the wear rate and friction coefficient results are obtained to investigate the effect of the HNTs on the tribological behavior of hybrid composites. The wear mechanism of neat and nanocomposite specimens is specified by scanning electron microscopy (SEM) images, and the elemental analysis of worn surfaces is performed using EDX. Finally, the surface morphology is evaluated with 3D topography images. Additionally, thermal camera images are used to identify the thermal conductivity effect of HNTs on wear. The wear test results show that HNTs-addition to composite decreased the friction coefficient by 9%, 10%, and 11% for 10 N, 15 N, and 20 N loadings, respectively. The wear rate is also decreased average by 75% for wear loadings. Surface form images acquired from 3D topography support the enhancement in the friction coefficient and wear rate values. Furthermore, thermal camera images show that thermal conductivity improvement on the contact region is attributed to well thermal properties of HNTs. Furthermore, the solid-lubricant characteristic of HNTs as forming tribofilm is determined as the main reason for the enhanced tribological performance of nanocomposites. Finally, a detailed wear mechanism is proposed to explain the wear behavior of HNTs-added carbon-aramid hybrid composites based on SEM images.
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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 cryogenic and warm temperatures on quasi-static penetration resistance of carbon-aramid hybrid nanocomposites reinforced using halloysite nanotubes
    (Elsevier, 2021) Cetin, Mehmet Emin; Tatar, Ahmet Caner; Demir, Okan; Onal, Gurol; Avci, Ahmet
    The quasi-static penetration test of a composite structure aims to measure impact behavior under the out-of plane loadings without dynamic and rate effects. In this study, carbon-aramid reinforced nanocomposites were manufactured by vacuum-assisted resin transfer molding and hand lay-up techniques and characterized by quasi-static penetration tests. Halloysite nanotubes (HNTs) were added to epoxy resin as nanofillers. Quasi-static punch shear test (QS-PST) was performed using a 12.7-mm cylindrical punch with a 50.8-mm support span. QSPSTs tests were carried out at-50 degrees C,-25 degrees C, 0 degrees C, 25 degrees C, and 50 degrees C to determine the effect of temperature on quasi-static penetration behavior of neat and HNT-reinforced carbon-aramid nanocomposites. Absorbed energy values and penetration force-displacement curves were acquired from QS-PSTs for each sample. Photographs of the front and rear sides of the samples were taken and analyzed. Moreover, the samples were cut from the middle, and the damage throughout the thickness of the composite samples were examined. It was observed that for all test samples, damages increased when the temperatures decreased. For the same temperature, nano particle addition to the samples resulted in higher penetration force and less damage. We confirmed the possibility of increasing penetration resistance and energy absorption capacity by adding HNT to carbon-aramid fibers at cryogenic and warm temperatures.
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    Effects of extreme low temperatures on the impact behavior of boron nitride nanofillers added carbon fiber/epoxy composite tubes
    (Sage Publications Ltd, 2022) Kara, Memduh; Tatar, Ahmet Caner; Kirici, Muhammed; Kepir, Yusuf; Gunoz, Alper; Avci, Ahmet
    This study aims to investigate the low-velocity impact response of boron nitride nanoparticles (BNNPs) added carbon fiber reinforced pipes (CFRPs) at various temperature values. Carbon fiber/epoxy composite specimens with (+/- 55 degrees)(3) winding angles were manufactured with the filament winding method. Low-velocity impact tests were performed with a 15 J energy level at five different temperatures between -196 degrees C and 23 degrees C ranges to understand the cryogenic environment effect on BNNP added carbon fiber reinforced tubes and neat carbon fiber reinforced tubes. The force-displacement and contact force-time graphs were attained in consequence of low-velocity impact tests. The damage areas, their size and characteristics were scrutinized both visually and with the aid of a Microscope. The test results showed that when ambiance temperatures decreased, the damage areas considerably increased both filled with BNNP and without BNNP filament wound carbon fiber reinforced tubes.
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    Effects of nanomaterials on the mechanical properties of epoxy hybrid composites
    (Springer Int Publ Ag, 2020) Erkendirci, Omer Faruk; Avci, Ahmet
    In this study, the effects of nanoparticles added to a newly developed hybrid composite material are examined. We investigated enhancing mechanical properties of plain weave (PW) carbon and glass fiber fabric/epoxy resin hybrid composite material we had developed by adding nanomaterials into the composition. For this purpose, we selected silica (SiO2) nanoparticles and carbon nanotubes (CNT). The two different nanomaterials are added into epoxy resin (L160) in four different combinations, which are SiO2, CNT + SiO2 and CNT together and none, and in five different thickness, which are laminations of three, five, seven, eleven and fifteen layers. We then tested these four mixtures in five thickness making up twenty combinations in the area of mechanical properties using tension test methods. Results of the tests in stress-strain graphs are shared in detail by displaying the mechanical properties of epoxy hybrid composite materials in tables and load-extension figures. The results and their causes are discussed giving a general conclusion of the nanomaterial enhancement properties of the new hybrid composite.
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    Effects of polyvinyl alcohol nanofiber mats on the adhesion strength and fracture toughness of epoxy adhesive joints
    (Elsevier Sci Ltd, 2018) Ekrem, Mursel; Avci, Ahmet
    The brittle nature of the polymer based adhesive joints is the major drawback limiting the service life. In this study, electrospun polyvinyl alcohol (PVA) nanofiber mat were introduced within the epoxy adhesive joint region to improve mechanical performance of the joints. The epoxy resin wetted electrospun PVA nanofiber mat were placed in between aluminum adherends and cured uncer vacuum conditions to remove air bubbles and volatiles. The mechanical performance of the reinforced aluminum joints was investigated by utilizing single lap joint (SIJ) and double cantilever beam (DCB). To reveal nano- and micro-scale toughening mechanics of the nanofiber reinforcements, the fracture surfaces were analysed using scanning electronmicroscope (SEM). Mod I fracture toughness and lap shear strength of the adhesively bonded joints were found to increase with addition of PVA nanofiber mats into epoxy adhesive.
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    Effects of seawater corrosion environment on the impact behavior of multi-walled carbon nanotube and SiO2 reinforced basalt/epoxy hybrid nanocomposites
    (Sage Publications Ltd, 2022) Demirci, Ibrahim; Avci, Ahmet
    In this study, the impact behaviors of nano silica (SiO2 nanoparticles), MWCNTs (multi-walled carbon nanotube), and SiO2+MWCNTs (nano silica vs multi-walled carbon nanotube hybrid) nanoparticle additives in basalt fiber reinforced composites exposed to seawater corrosion were investigated. The Mediterranean was chosen as the corrosive sea water due to its high salinity. Basalt fiber reinforced composites immersed in seawater have quick mass absorption during the first 30 days. However, it can also vary between the first 28 and 40 days depending on the structure of the composite and environmental conditions. Immersion times were determined as 0, 10, 20, and 40 days. Tensile and low velocity impact tests were performed to evaluate the mechanical performance after seawater corrosion. Low velocity impact tests were carried out at 10 and 20 J energy levels. The aim of this study was to examine the effects of corrosion caused by quick mass absorption on the tensile and impact behaviors of basalt fiber reinforced composites. And also to investigate the contribution of SiO2 and MWCNT nanoparticle additives to the mass absorption mechanism and mechanical performance of basalt fiber reinforced composites. SiO2 and MWCNT nanoparticle additives increased the tensile and impact strength of basalt fiber reinforced composites. However, the tensile and impact behaviors of nanoparticle filled and nanoparticles unfilled basalt fiber reinforced composites were adversely affected by the seawater corrosion environment. Due to the geometric structure of SiO2 nanoparticles, the best mechanical performance was observed in SiO2 filled basalt fiber reinforced composites.
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    Electrospinning of polymeric nanofiber (nylon 6,6/graphene oxide) for removal of Cr (VI): synthesis and adsorption studies
    (Springer International Publishing Ag, 2019) Parlayici, Serife; Avci, Ahmet; Pehlivan, Erol
    Polymer nanofibers are valuable materials that are of great importance because they are inexpensive and they display high strength. In this study, nylon-6,6 (N6,6)/graphene oxide (GO) nanofibers were developed and characterized. GO was synthesized from graphene using the known Hummer's method. GO is obtained by reaction of the graphene with strong oxidants and was characterized using Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The characterizations showed that the graphene was suitably oxidized. GO was added to N6,6 to produce N6,6/GO nanofibers at loadings of 0.5, 1.0, 1.5, and 2.0 wt% through the melt mixing method. The batch adsorption model for Cr (VI) adsorption was applied as a function of time, initial Cr (VI) concentration, adsorbent dosage, and pH to examine nanofiber activity. N6,6/GO a exhibited high adsorption capacity for Cr (VI) at pH 2.0. The maximum adsorption capacity for Cr (VI) ion was found to be 47.17 mg/g of N6.6/GO using the related isotherm curves and equations. When the equilibrium data were examined, it was concluded that Langmuir adsorption isotherm was more suitable.
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    Electrospinning of polymeric nanofiber (nylon 6,6/graphene oxide) for removal of Cr (VI): synthesis and adsorption studies
    (Springer International Publishing Ag, 2019) Parlayici, Serife; Avci, Ahmet; Pehlivan, Erol
    Polymer nanofibers are valuable materials that are of great importance because they are inexpensive and they display high strength. In this study, nylon-6,6 (N6,6)/graphene oxide (GO) nanofibers were developed and characterized. GO was synthesized from graphene using the known Hummer's method. GO is obtained by reaction of the graphene with strong oxidants and was characterized using Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The characterizations showed that the graphene was suitably oxidized. GO was added to N6,6 to produce N6,6/GO nanofibers at loadings of 0.5, 1.0, 1.5, and 2.0 wt% through the melt mixing method. The batch adsorption model for Cr (VI) adsorption was applied as a function of time, initial Cr (VI) concentration, adsorbent dosage, and pH to examine nanofiber activity. N6,6/GO a exhibited high adsorption capacity for Cr (VI) at pH 2.0. The maximum adsorption capacity for Cr (VI) ion was found to be 47.17 mg/g of N6.6/GO using the related isotherm curves and equations. When the equilibrium data were examined, it was concluded that Langmuir adsorption isotherm was more suitable.
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    Enhanced Salty Water Durability of Halloysite Nanotube Reinforced Epoxy/Basalt Fiber Hybrid Composites
    (Korean Fiber Soc, 2019) Ulus, Hasan; Kaybal, Halil Burak; Eskizeybek, Volkan; Avci, Ahmet
    In this study, we report the effect of halloysite nanotube (HNT) modification on salty water aging durability of epoxy (Ep)/basalt fiber (BF) hybrid composites. For this, various amounts of HNTs were introduced into the Ep matrix, and the HNTs/Ep mixture was used to impregnate basalt fabrics to fabricate hybrid laminated composites. The hybrid composites were exposed substantial increases in the tensile strength and the fracture toughness. Besides, after salty water aging for 6 months, the hybrid composites exhibited remarkably improved aging performance with almost 10 % less reduction in both tensile and flexural strengths and fracture toughness compared to the neat basalt-epoxy composites. SEM analysis showed relatively less number of cracks, micro-voids and better interfacial bonding for the 2 wt% HNTs reinforced hybrid composite specimens in comparison to the neat counterpart, similarly conditioned in all cases. The consequences of salty water aging on micro-scale morphology were discussed based on the fracture morphologies to reveal degradation mechanisms in the existence of HNTs reinforcement.
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    Enhancement of PCL/PLA Electrospun Nanocomposite Fibers Comprising Silver Nanoparticles Encapsulated with Thymus Vulgaris L. Molecules for Antibacterial and Anticancer Activities
    (Amer Chemical Soc, 2022) Cimen, Cansu Gunes; Dundar, Mehmet Akif; Kars, Meltem Demirel; Avci, Ahmet
    Silver nanoparticles (AgNPs) have been recognized for their outstanding antibacterial activities, which are required for antibacterial coating materials in therapeutic applications. A bacterial resistant electrospun nanofibrous mat made of polycaprolactone (PCL) in combination with polylactide acid (PLA) containing silver nanoparticles encapsulated with Thymus vulgaris L. (thyme) extract (eAgNPs) was fabricated in order to assess the potential of applicability in biomedical applications such as cancer treatment, wound healing, or surgical sutures. In the current study, PCL and PLA used as the basis polymers were blended with biosynthesized eAgNPs, pure AgNPs, and thyme extract (TE) to observe the effects of additives in terms of antibacterial and anticancer activity and morphologic, thermal, mechanical, biocompatibility, and biodegradability properties. The biological characteristics of fabricated electrospun nanofibrous mats were evaluated in vitro. Physicochemical characteristics of the nanofibrous mats were examined by UV-vis spectrophotometry, scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), Fourier-transform infrared spectroscopy (FTIR), mechanical tensile testing, X-ray diffraction (XRD), thermogravimetric examination (TGA), and water contact angles (WCAs). The results showed that a biodegradable nanofiber scaffold with a mean fiber diameter of 280 nm is morphologically homogeneous and highly hydrophobic, has higher tensile strength than PCL/PLA nanocomposite fiber, and is resistant to Escherichia coli and Staphylococcus aureus. The cytotoxic and anticancer properties of nanomaterials were defined using L929 and SK-MEL-30 cells. The developed material inhibited cell proliferation and led to apoptosis of cell lines. It can be suggested that the use of Thymus vulgaris L. extract-encapsulated silver nanoparticle-doped PCL/PLA nanofibers produced by the electrospinning method has the potential for cancer therapy in skin tumor cell lines.
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    Enhancement of Wear and Friction Characteristics of Epoxy Resin by Multiwalled Carbon Nanotube and Boron Nitride Nanoparticles
    (Taylor & Francis Inc, 2015) Duzcukoglu, Hayrettin; Ekinci, Serafettin; Sahin, Omer Sinan; Avci, Ahmet; Ekrem, Mursel; Unaldi, Mahmut
    Epoxy resins are widely used in engineering applications. However, their low thermal stability limits their usage at high sliding velocities and loads. The mechanical properties and thermal stability of a machine element subjected to friction and wear are very important. In this study, friction and tribology behaviors of multiwalled carbon nanotubes (MWCNTs) and boron nitride (BN)-modified epoxy resin have been investigated. Epoxy resin modified by three different nanoparticle configurations, 0.3% MWCNT, 0.5% BN, and 0.5% MWCNT/0.3% BN, was investigated. The tribological characteristics of nanoparticle-modified epoxy resin were compared with properties of neat resin. The friction and tribological behavior of modified epoxy resin were tested using a ball-on-disc test stand at 1.2 and 1.5 ms(-1) sliding velocities under 10 N applied load. The tests were done under dry condition and 1,800 m distance. The friction coefficient, wear loss, and temperature increase during testing were recorded and compared with that obtained for neat epoxy. It is observed that nanoparticle modification resulted in enhanced wear resistance and a reduction in friction coefficient and working temperatures.
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    Evaluation of boron nitride nanoparticles on delamination in drilling carbon fiber epoxy nanocomposite materials
    (Sage Publications Ltd, 2020) Kaybal, Halil Burak; Unuvar, Ali; Kaynak, Yusuf; Avci, Ahmet
    The reinforcements of nanoparticles have an important role in improving the machinability of nanocomposite materials. Except for the known nanoparticles such as carbon nanotube, graphene, nanoclay, etc., the effect of boron nitride reinforcement on the machinability of composite materials are a recent research topic. In this study, boron nitride nanoparticle was introduced to the matrix resin that brings about additional strength and enhancement in thermal and mechanical properties of the composite. Though it was confirmed that this composition enhances the focused properties, it is necessary to investigate drilling performance of these composite and identify the effects of this boron nitride nanoparticle on machinability of carbon fiber epoxy nanocomposite considering thrust force, delamination factor, etc. Accordingly, while the thrust force is increased by reinforcement of the boron nitride nanoparticles, on the contrary of literature, delamination factor is tend to reduce as compared with reference composite. This experimental study shows the addition of boron nitride nanoparticles help to reduce delamination factor of carbon fiber epoxy nanocomposite. In addition, hole surfaces and drilling mechanism analyzed with optical and scanning electron microscope about damage estimation.
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    An experimental evaluation on the dynamic response of water aged composite/aluminium adhesive joints: Influence of electrospun nanofibers interleaving
    (Elsevier Sci Ltd, 2022) Ulus, Hasan; Kaybal, Halil Burak; Berber, Nihat Erdem; Tatar, Ahmet Caner; Ekrem, Mursel; Ataberk, Necati; Avci, Ahmet
    The impact response of adhesives is a critical design parameter considering their lifetime. Additionally, environmental effects such as water or moisture may cause to degrade of the polymer-based adhesive and shorten its service life. This study aimed to investigate the impact response of water-aged aluminium-composite adhesively bonded single lap joints (SLJs). Nylon 6.6 nanofibers modified with graphene nanoplatelets (GNPs) were introduced in the adhesion areas to increase adhesive performance. The water aging resulted in decreased impact resistance in all cases. However, nanofiber-modified SLJs exhibited comparatively higher impact performance under both non-aged and water-aged conditions. Further, the GNP reinforced nylon 6.6 nanofibers increased the maximum impact load by 15 and 19% compared to neat nanofibers before and after aging, respectively. The fracture surfaces were examined via scanning electron microscopy (SEM) to understand damage and toughness mechanisms. A schematic model has been developed to explain the mechanisms leading to improved bonding performance by applying N6.6 nanofiber reinforcement to the adhesion zone.
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    Experimental Investigation of Tensile and Impact Response of Nano-Alumina-Filled Epoxy Hybrid Composites Reinforced with Carbon-Kevlar and Carbon-Glass Fabrics
    (Springer Heidelberg, 2022) Erkendirci, Omer Faruk; Avci, Ahmet; Dahil, Lutfiye; Kaya, Kenan; Kilictek, Serkan; Sezgin, Abdullah
    The present study investigates tensile strength and impact resistance of alumina (Al2O3) filled carbon-S2-glass (CG) and carbon-Kevlar-49 (CK) reinforced epoxy hybrid composites, for three, five and seven layers. Nanoparticles of 40-nm size are dispersed in the epoxy matrix at a concentration of 2 wt.% of the matrix. Composites are manufactured employing the vacuum-assisted resin transfer molding method. Mechanical properties are determined by tensile and Charpy-V-notch impact tests. It is shown that number of layers affects tensile and impact characteristics. Ultimate tensile stress of CK laminates increases by approximately 35%; however, it decreases for CG laminates by 20%, per addition of a single layer of a particular fiber, whereas a similar trend in elongation at break and tensile modulus is observed for both types of composites. Stress-strain curves follow a particular pattern, characterized by unstable delamination and gradual fragmentation of a single laminae. With the addition of nanoparticles, both modulus of elasticity and ultimate tensile stress are greatly decreased by approximately one and two orders of magnitude. Values of ultimate tensile stress and strain increase with number of layers for CK laminates, while it is the other way around for CG laminates. Charpy impact tests reveal that impact energy and impact strength of the CG laminates are larger than those of the CK laminates; and impact strength increases with number of layers. Micro-scale damage analysis is conducted via scanning electron microscope (SEM) technique. Accordingly, fiber breakage, fiber pull-out and matrix cracks occur in the composites.
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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 Sci Ltd, 2021) Kaybal, Halil Burak; Ulus, Hasan; Eskizeybek, Volkan; Avci, 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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    Fabrication of novel chitosan-humic acid-graphene oxide composite to improve adsorption properties for Cr(VI)
    (Springer Heidelberg, 2019) Parlayici, Serife; Avci, Ahmet; Pehlivan, Erol
    A novel chitosan-humic acid-graphene oxide (Cts-HA-GO) composite was produced and tested to remove hexavalent chromium (Cr(VI)) during the adsorption process. The percentage adsorption was 85.28% by using 200 ppm Cr(VI) and 2 g/L composite. Four hours of contact time was enough to remove Cr(VI) from the aquatic medium. The extent of adsorption of Cr(VI) on the surface of Cts-HA-GO composite was examined on the basis of increasing quantity of the composite. The functional group characterization and morphology of composite was characterized using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. After reaching the equilibrium, the residual concentration of the Cr(VI) solution was measured with an UV-visible spectrophotometer. The results showed that the adsorption behaviour was in accordance with the Langmuir isotherm model, and the maximum adsorption capacity of Cr(VI) determined by using this model was 83.64 mg/g at the pH of 2. A pseudo-second-order model equation has justified the equilibrium between Cr(VI) and the composite for the kinetic studies.