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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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    Evaluating the effectiveness of nanofillers in filament wound carbon/epoxy multiscale composite pipes
    (Elsevier Sci Ltd, 2016) Ustun, Tugay; Ulus, Hasan; Karabulut, Salim Egemen; Eskizeybek, Volkan; Sahin, Omer Sinan; Avci, Ahmet; Demir, Okan
    The performance of filament wound (FW) composite pipes is considered to be fundamentally governed by fiber properties and winding angles; however, matrix dominated properties such as axial and hoop strengths are also responsible in design of FW composite pipes. This paper presents the experimental results of a project aiming to assess the benefits of addition of carbon nanotubes (CNTs) and/or boron nitride nanoplates (BNNPs) as nanofillers within epoxy matrix of FW carbon fiber composite pipes. The nanofillers enhance the burst and hoop strengths up to 17.0% and 31.7%, respectively, over the control samples. Failure analysis revealed that the morphologies of nanofillers play an important role on the matrix toughening and strengthening the fiber matrix interface. Highest mechanical performance of the multiscale composite pipes was obtained with the addition of CNTs and BNNPs within the epoxy matrix concurrently related with the synergetic effect of the two different nanofillers. (C) 2016 Elsevier Ltd. All rights reserved.
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    Selectively Reinforced Functionally Graded Composite-like Glass/Carbon Polymer Nanocomposites: Designed for Efficient Bending and Impact Performance
    (Korean Fiber Society, 2022) Demir, Okan; Tatar, Ahmet Caner; Eskizeybek, Volkan; Avcı, Ahmet
    Offshore wind turbine blades (OWTBs) are exposed to various types of loadings during their service life. Moreover, due to their tremendous size, huge investment costs are established, including advanced engineering materials and production process solutions. To decrease their investment cost without sacrificing their mechanical performances, advanced engineering solutions in the view of material selection and design should be implemented. With this motivation, we aimed to develop a novel laminated composite design considering reducing investment costs without compromising the bending and impact resistance of an OWTB. For this, an efficient and cost-effective design of a functionally graded composite (FGM)-like glass/carbon fibers reinforced hybrid polymer composite with a specific stacking sequence was presented. To evaluate mechanical performance of the composite structure, tensile, flexural, and to simulate environmental conditions, low-velocity impact tests were conducted. Furthermore, multi-walled carbon nanotubes (MWCNTs) were also introduced into the polymer matrix to evaluate their effectiveness in the hybridized composite. Drastic improvements in the bending strength (55.8 %) and strain (39.7 %) were obtained compared to the neat carbon fiber reinforced epoxy composites (CFs), especially with the aid of MWCNTs. According to impact tests, it was pointed out that it is possible to obtain higher impact peak forces (around 15 %) compared to neat CFs. However, MWCNTs contributed with slight increments in impact resistance but effectively restricted the impact damage propagation. This study reveals it is possible to tune the bending performance, the absorbed energy, and the damage extension by utilizing glass and carbon fiber laminates in an FGM-like structure.