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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
    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).
  • [ X ]
    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.