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    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) Ozbek, Yagiz; Al-Nadhari, Abdulrahman; Elmas, Sinem; Eskizeybek, Volkan; Yildiz, 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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    Influence of strand size and morphology on the mechanical performance of recycled CF/PEKK composites: Harnessing waste for aerospace secondary load-bearing applications
    (Elsevier Sci Ltd, 2025) Ozbek, Yagiz; Al-Nadhari, Abdulrahman; Eskizeybek, Volkan; Yildiz, Mehmet; Sas, Hatice Sinem
    The flexibility and precision of automated fiber placement (AFP) have made it a standard methodology in the aviation industry. However, the use of continuous slit tapes along component lengths generates significant waste. This waste presents an opportunity for recycling into secondary load-bearing structures, particularly in applications where components are not subjected to extreme working conditions. In this study, carbon fiber-reinforced polyetherketoneketone (CF/PEKK) strands are recycled into randomly oriented strand (ROS) panels using a costeffective, vacuum-assisted hot press process while maintaining aerospace-quality standards. Both long and short strand lengths, as well as shredded strands mimicking real-life industrial waste, are analyzed for their mechanical performance and geometric stability. Mechanical properties of the recycled CF/PEKK composites are evaluated through tensile, shear, compression, Izod impact, and dynamic mechanical analysis (DMA), using digital image correlation (DIC) for precise measurements. Additionally, topological 3D scanning is used to assess the geometric stability of the panels. Results indicate that short strands offer superior mechanical properties, while shredded strands perform comparably. This study makes a unique contribution by demonstrating the effective recycling of slit tape waste into high-performance composite materials, advancing sustainable practices in aerospace applications.