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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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    Öğe
    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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    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.