Yazar "Sas, Hatice S." seçeneğine göre listele

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    Öğe
    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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    Öğe
    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.
  • Küçük Resim
    Öğe
    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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    Öğe
    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.
  • Küçük Resim
    Öğe
    Feasibility and performance evaluation of randomly oriented strand recycled composite skins in sandwich structures: A green cost-effective solution for aerospace secondary load-bearing applications
    (Wiley, 2025) Özbek, Yağız; Al-Nadhari, Abdulrahman; Elmas, Sinem; Eskizeybek, Volkan; Yıldız, Mehmet; Sas, Hatice S.
    Despite the advantages of recycled randomly oriented strand (ROS) composites over recycled grinded ones, the warpage issue hinders their adaptation in the industry due to tolerance requirements. To address this challenge, ROS composites are incorporated into secondary bonded sandwich structures such that the core material ensures the straightness of the ROS composite skins. Additionally, atmospheric plasma activation (APA) is utilized to enhance the skin/core bonding to prevent skin separation under loading. The ROS composite skins are manufactured via vacuum-assisted hot press to achieve a cost-effective aerospace-grade quality. The structural integrity of the sandwich structure is assessed through flatwise tensile and edgewise compression tests, while the mechanical and thermomechanical performance is evaluated using flexural, impact, and dynamic mechanical analysis (DMA) tests. The flatwise tensile and edgewise compression tests confirm that APA effectively prevents core detachment, as evidenced by an average tensile strength of 2.28 MPa and an average compressive strength of 171.7 MPa. Moreover, the flexural and impact tests show that no premature skin failure occurs, supported by an average facing strength of 59.23 MPa in flexural testing and an average impact energy of 49.96 kJ/m(2). The DMA test indicates that most of the stiffness loss is due to the core material. This comprehensive analysis highlights recycled ROS composites as a sustainable and cost-effective alternative for quasi-isotropic skins in aerospace secondary load-bearing sandwich structures such as floors, doors, engine cowls, and spoilers.
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    Öğe
    Multi-walled carbon nanotube grafted 3D spacer multi-scale composites for electromagnetic interference shielding
    (Wiley, 2022) Yildirim, Ferhat; Kabakci, Elif; Sas, Hatice S.; Eskizeybek, Volkan
    The development of structural fiber reinforced polymer composites with various additional functionalities is becoming a hot research area to achieve the application of multi-functional composites in the aerospace and automotive industries. An innovative material solution is 3D spacer composites with distinctive anisotropic structural characteristics. Herein, we report the manufacturing of multi-walled carbon nanotubes (MWCNTs) grafted of 3D spacer glass/epoxy multi-scale composites and their electromagnetic interference shielding efficiencies (EMSE). To manufacture multi-scale composites, we utilized dip coating, vacuum filtering, and vacuum infusion methods to introduce MWCNTs of the woven fabric, while we also modified the epoxy resin with MWCNTs to increase electrical conductivity of intrinsic insulator epoxy resin. Owing to the rectangular-shaped channel structure, which is beneficial for multiple reflection and scattering between top and bottom face sheets, the resultant 3D spacer multi-scale composite represented a good EMSE performance of -18.3 dB in the frequency range of 8.2-12.4 GHz with an increase of 107% comparing the corresponding neat composite counterpart. Moreover, we measured the in-plane conductivity as 1.89E-2 S/m after MWCNTs grafting, while the out-of-plane conductivity remained three times lower than the in-plane conductivity. Dynamic mechanical analysis revealed that the storage modulus increased almost three times with the MWCNTs grafting, while glass transition temperature shifted to higher temperatures (from 77.5 to 89.7 degrees C). Therefore, we anticipate that our study will expand the use of 3D spacer composites in the aviation and automotive industries.