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    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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    Damage tolerance of basalt fiber reinforced multiscale composites: Effect of nanoparticle morphology and hygrothermal aging
    (Elsevier Sci Ltd, 2024) Şükür, Emine Feyza; Elimsa, Selen; Eskizeybek, Volkan; Avcı, 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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    Impact of basalt powder on the rheological, thermal, mechanical, and tribological properties of natural and polychloroprene rubber composites: A study on aging and filler interaction
    (Wiley, 2025) Yıldız, Enes; Değirmenci, Talha; Esen, Melih; Göksüzoğlu, Mert; Kuru, Gözde; Eskizeybek, Volkan; Şükür, Emine Feyza
    This study investigates the effects of basalt powder (BP) as a filler in natural rubber (NR) and synthetic polychloroprene rubber (CR) composites, focusing on their structural, mechanical, and thermal properties before and after aging. The NR/BP and CR/BP composites were prepared with varying basalt content levels of 0, 5, 10, 15, 50, and 100 parts per hundred rubber (phr) and characterized accordingly. Mechanical properties such as tensile strength, Young's modulus, and abrasion resistance were evaluated, alongside thermal properties, using thermogravimetric analysis. The results showed that increasing the basalt content reduced dispersion quality and weakened the filler and matrix interaction, decreasing tensile strength and Young's modulus. The reduction in tensile strength is substantial, with a decrease of 71.7% in NR/BP composites and 64.1% in CR/BP composites at maximum additive ratios. Aging significantly improved the tensile strength and modulus of the materials, which are attributed to an increased crosslink density and the transformation of polysulfide bonds into disulfide bonds. Shore A hardness increased with basalt content, reaching 60.3 for NR/BP and 74.1 for CR/BP at 100 phr, while abrasion resistance decreased, with abrasion loss rising by 130% and 88% for NR/BP and CR/BP composites, respectively. Scanning electron microscopy (SEM) analysis revealed increased surface roughness and filler aggregation at higher basalt contents, contributing to reduced mechanical performance. Fourier-transform infrared spectroscopy (FTIR) and energy-dispersive X-ray spectroscopy (EDX) analyses confirmed the presence and dispersion of basalt powder within the composites.