Theses supervised by Prof. Dr. Fahrettin Öztürk
11 theses · Ankara Yıldırım Beyazıt University
Recycling of carbon fiber reinforced epoxy composites
Composite materials are extensively used across various industries due to their high strength-to-weight ratio, corrosion resistance, and durability. However, the increasing use of thermoset-based composites has raised critical concerns regarding end of life waste management, as they are difficult to recycle through conventional methods. Among emerging technologies, microwave-assisted acid digestion has gained attention as a promising, efficient, and environmentally friendly recycling approach. This study investigates the closed-loop recycling of carbon fiber reinforced polymer (CFRP) waste using a rapid microwave-assisted nitric acid digestion method. A parametric study was conducted by varying the process temperature (120 °C–150 °C) and microwave irradiation hold time (5–10 minutes) while keeping other parameters constant. In this working range, the most suitable condition was determined to be 10 minutes at 150 °C, and 99% resin removal efficiency was achieved. Recycled carbon fibers were characterized and compared to pristine fibers using Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), and Raman spectroscopy. The recycled fibers retained their structural integrity, showing no visible surface etching, no change in fiber diameter, and minimal resin residue. A comparison of the ID/IG ratios across different recycling processes indicates that there is no structural change occurred during recycling process. These findings highlight the significance of developing fast, clean, and scalable recycling technologies for thermoset composites, especially in high-performance applications like aerospace. The successful recovery of carbon fibers not only proves the feasibility of this method but also supports the advancement of closed-loop recycling systems. The recycling cycle defined by varying time and temperature parameters can serve as a foundation for future studies. As the recycled fibers preserved their structural integrity, they appear suitable for reuse, though further research is needed to evaluate their mechanical performance compared to virgin fibers for potential manufacturing applications.
Manufacturing and analysis of nano-fullerene reinforced peek matrix composite materials
Polymer-based materials are widely used across various industries, including automotive, medical, aerospace, electronics, and construction. While these materials are attractive due to their favorable performance-to-cost ratio, they also exhibit certain limitations, such as relatively low melting temperatures and variability in mechanical properties. These limitations restrict the use of polymers in certain applications; however, ongoing research focuses on developing polymer composite materials by incorporating various types of additives into the polymer matrix. Numerous studies in the literature have demonstrated that polymer matrix composites enhanced with nano- or micro-sized particles or fibers can significantly improve mechanical, thermal, optical, and electrical conductivity properties. In this study, the high-performance thermoplastic polymer PEEK, commonly used in the aerospace industry, was compounded with nano-sized fullerene at various ratios using the twin-screw extrusion method. The resulting composites were investigated in terms of their tensile strength, morphological structure, thermomechanical properties, and thermal stability behavior. The results revealed that the highest tensile strength and Young's modulus were achieved at a 0.5 wt% fullerene content, showing a 12.8% improvement compared to pure PEEK. At low filler concentrations, such as 0.2 wt%, a homogeneous and nanoscale dispersion was observed, contributing positively to the enhancement of material properties. However, with increasing filler content, microstructural heterogeneities in the form of particle agglomeration became evident. Dynamic Mechanical Analysis (DMA) indicated that the addition of fullerene enhances the thermomechanical properties of PEEK, with 0.5 wt% offering the most balanced performance. Thermogravimetric Analysis (TGA) results demonstrated that even small amounts of fullerene in the PEEK matrix can improve thermal stability by controlling the material's thermal degradation behavior.
Analysis of temperature effect on anisotropy of AZ31 sheet
AZ31 is a challenging alloy for aerospace, automotive, and biomedical industries. This study delves deep into the intricate relationship between build orientations, temperature, and the resulting mechanical behavior of AZ31 alloy. Utilizing tensile tests on specimens with distinct build orientations, the research evaluates the alloy's response to varying temperatures of 100, 200, and 300°C. The primary objective is to elucidate the nuanced effects of anisotropy and temperature gradients on the alloy's tensile strength, elongation, and overall mechanical performance. Furthermore, disc compression tests, a novel approach in this research, involve stacking multiple discs and subjecting them to compressive forces. This method, inspired by the findings of, aims to provide a comprehensive understanding of anisotropy, extract the Lankford values, and determine precise yield strengths. The study also ventures into the realm of computational simulations using the renowned MATLAB software. Several criteria, including the Hill48, the Barlat89, the Yld 2000, the Yld2004-2p, and the SHYqp2023 yield criteria are employed to simulate and predict the material's behavior under diverse conditions, a methodology that aligns with recent computational studies on similar materials. This research not only contributes to the existing body of knowledge but also paves the way for the development of advanced magnesium-based materials with tailored properties for specific applications. Results indicate that the mapped out biaxial anisotropy and the associated Lankford values and variations regarding temperatures and orientations. Older models, such as the Hill-48 and the Barlat-89 does not accurately predict anisotropic nature of AZ31. The Yld-2004 and the Bezier-SHYqp2023 models seem promising in predicting anisotropic features of AZ31 material, and materials of having the HCP crystal structures in general.
Mechanical and physicochemical performance for recycled CF/PEKKthermoplastic composite materials
The use of the composite materials has recently been increased due to their lightness and high strength. Due to the increasing interest in composite materials, the amount of composite waste accumulated creates a problem for the environment. Therefore, the areas where the collection and incineration processes required for disposal are carried out are not sufficient. For a more sustainable future, the European Union has taken decisions about waste storage and recycling. In addition, various studies have been initiated to increase the use of thermoplastic resin composites, which are recyclable materials. If the necessary studies and infrastructure are not created, there will be a rapid resource consumption and the raw material reserves will gradually decrease. The increase in consumption and the decrease in reserves cause to the danger about the continuity of the sector. In this study, recycled and unprocessed composite materials were compared to reduce the environmental impact of materials during the production phase, reduce economic costs, and ensure product continuity. Carbon/PEKK unidirectional types were consolidated with autoclave curing oven. The samples taken from the final product are mechanical such as bending and interlaminar shear sample (ILSS); It has been tested with physicochemical tests such as differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and fiber void ratio (FVC)-porosity test. The sample was observed under scanning electron microscopy (SEM) to interpret the changes in the structure before and after recycling. All the results obtained were compared with each other and evaluated with the results obtained from the existing studies in the literature. Results indicate that the virgin thermoplastic showed 68.05% higher flexural strength and 7.85% flexural modulus values compared to the recycled thermoplastic composites. the average interlaminar shear strengths were measured as 81.8 MPa and 64.7 MPa for the virgin and the recycled thermoplastic composites, respectively.
Sürekli elyaf takviyeli termoplastik kompozitlerin sonlu elemanlar analizi
In recent years, polymer matrix composites have been preferred much more than traditional materials, such as metal, especially in the aerospace industry. The interest in thermoplastic composites is increasing day-by-day due to certain advantages, such as a very short process time when compared to thermoset composites, and the production of more durable integral part due to its weldability and recyclability. One of the most important thermoplastic composite part production methods is thermoforming. Although it has several advantages, such as a low cycle time and the easier production of complex parts, it is not yet widely used in the aerospace industry, where part accuracy is at the highest level. The aim of the study was to develop a method for manufacturing defect-free thermoplastic composite parts by means of the thermoforming. First, optimum process parameters were determined by finite element analysis performed via Ls-Dyna. Some of the mechanical properties of the fabric material were obtained through characterization tests in order to obtain more realistic results. At the same time, the first trial composite part with thermocouples was formed with the intent of standardizing the processing times for the experimental studies. Despite the determination that there were no shape distortions, it was observed that the surface quality was quite bad. In order to eliminate these defects, different spring configurations were developed and blank composite plate structures were designed. Thermoform analysis was then performed using the optimum parameters for all of the different configurations. The part with the U-beam structure was thermoformed in accordance with the analysis. It was seen that the experiments made for the validation were in very good agreement with the analysis. Comparisons were made of the fiber orientations and deformations. The effect of the homogeneous tool temperature, blank plate geometry, and gripper configurations on the manufacturing defects was evaluated. Experimental results showed that the homogeneous tool temperature is of great importance in thermoforming. Finally, it was concluded that none of the material models were sufficient to take into account all of the deformation mechanisms and deformations that occurred during and after the process.
Design and analysis of a new variable incidence wing and ruddervator mechanisms for UAVs
In scope of this study, a new special design of wing and ruddervator mechanisms are developed in order to take off and land UAVs from navy carriers with short runway. The angle of attack of wing of the UAV is gradually increased during take-off and landing in order to enhance the generated lift by wings. The design of these mechanisms are performed within "fail-safe design mentality" as these mechanism are Category-A class structures of UAVs. In order not to change the static stability of UAV while angle of attack of wing changes, it is mandatory to keep neutral point position unchanged, to ensure this, ruddervator angle of attack is also changed. It is aimed to shorten the take-off distance of aircraft by changing flight mechanics characteristics of UAV by dint of special mechanism design of wing and ruddervator.
Evaluation of hole filling repair of carbon fiber reinforced polymer composite laminates
Carbon-Fiber Reinforced Polymer (CFRP) composites have been recently used in many industries specifically aerospace due to their advantages such as rigidity, high strength, low density, and corrosion resistance. The use of composite materials is increasing, in parallel with the damage to which these materials are exposed is also increasing in number. However, despite all the benefits they provide, these materials are not easy to repair. Consequently, various damage repair methods have been developed for composite materials and this issue gaining importance. In this thesis, minor damage to aircraft structures was repaired by the plug repair technology. Firstly, prepreg carbon fiber composite HEXPLY M91/IM7/34RC/UD/194/12K plies were used to produce composite laminate plates. These plies were laid in the atmosphere-controlled environment. Throughout the lay-up process, peel plies were used as backing materials after compaction and vacuum operations were implemented. Then, holes were drilled on these composite laminate plates for plug repairing. The holes were divided into three groups of six specimens and were filled with different materials containing Loctite 9396 adhesive and various amounts of chopped glass fibers. After repairs, Manual Ultrasonic Pulse Echo (MUPE), a non-destructive test method, was used for quality control of the repaired specimens. MUPE results indicated that the repaired specimens were high quality and suitable for testing. Subsequently, tensile tests were carried out for 24 specimens by using an Instron 5985 machine with 250 kN capacity. The results of this study demonstrated that an improvement by up to 5% was achieved with the repair using 50% chopped glass fiber and Loctite 9396 adhesive. It was observed that the mechanical properties of the composite structure were enhanced with the plug repair technology.
Performance analysis of carbon fiber reinforced polymer against bird strike
Since the invention of powered flight, aviation safety has been affected by bird strikes, a significant risk worldwide. The aviation industry is turning to composite materials that provide high specific strength, superior corrosion resistance, and enhanced fatigue performance, with the objective of extending the service life of aircraft components while also achieving cost and weight advantages. The primary objective of this study is to investigate the structural effects of high velocity bird impacts on the leading edge of a composite wing. The bird and composite material models are validated with experimental data from the literature on titanium and carbon fiber-reinforced composite plates, respectively. The numerical analyses accurately predicted strain results. A wing leading edge structure with M40J composite skin was designed and subjected to bird strike simulations. Bird strike simulations were then conducted on the structure using different bird masses. The results showed that as the bird's size and mass increased, the severity of structural damage to the composite skin also increased. Furthermore, the effects of impact angle were analyzed, and it was observed that a decrease in the yaw impact angle reduced structural damage, whereas angled impacts along the pitch axis caused more severe damage. Furthermore, the influence of laminate stacking sequences on structural performance was investigated. In this context, M40J composite skins with cross-ply, angle-ply, anti-symmetric, and symmetric stacking sequences were analyzed and compared. The results showed that the cross-ply laminate exhibited the lowest performance, whereas angle-ply laminates with intermediate fiber angles and symmetric configurations provided superior impact resistance. These findings emphasize the critical role of stacking sequence in composite structural design and highlight the importance of optimizing fiber orientation to achieve lightweight yet resilient solutions.
Performance analysis of carbon fiber reinforced polymer against bird strike
Since the invention of powered flight, aviation safety has been affected by bird strikes, a significant risk worldwide. The aviation industry is turning to composite materials that provide high specific strength, superior corrosion resistance, and enhanced fatigue performance, with the objective of extending the service life of aircraft components while also achieving cost and weight advantages. The primary objective of this study is to investigate the structural effects of high velocity bird impacts on the leading edge of a composite wing. The bird and composite material models are validated with experimental data from the literature on titanium and carbon fiber-reinforced composite plates, respectively. The numerical analyses accurately predicted strain results. A wing leading edge structure with M40J composite skin was designed and subjected to bird strike simulations. Bird strike simulations were then conducted on the structure using different bird masses. The results showed that as the bird's size and mass increased, the severity of structural damage to the composite skin also increased. Furthermore, the effects of impact angle were analyzed, and it was observed that a decrease in the yaw impact angle reduced structural damage, whereas angled impacts along the pitch axis caused more severe damage. Furthermore, the influence of laminate stacking sequences on structural performance was investigated. In this context, M40J composite skins with cross-ply, angle-ply, anti-symmetric, and symmetric stacking sequences were analyzed and compared. The results showed that the cross-ply laminate exhibited the lowest performance, whereas angle-ply laminates with intermediate fiber angles and symmetric configurations provided superior impact resistance. These findings emphasize the critical role of stacking sequence in composite structural design and highlight the importance of optimizing fiber orientation to achieve lightweight yet resilient solutions.
Manufacturing and analysis of nano-fullerene reinforced peek matrix composite materials
Polymer-based materials are widely used across various industries, including automotive, medical, aerospace, electronics, and construction. While these materials are attractive due to their favorable performance-to-cost ratio, they also exhibit certain limitations, such as relatively low melting temperatures and variability in mechanical properties. These limitations restrict the use of polymers in certain applications; however, ongoing research focuses on developing polymer composite materials by incorporating various types of additives into the polymer matrix. Numerous studies in the literature have demonstrated that polymer matrix composites enhanced with nano- or micro-sized particles or fibers can significantly improve mechanical, thermal, optical, and electrical conductivity properties. In this study, the high-performance thermoplastic polymer PEEK, commonly used in the aerospace industry, was compounded with nano-sized fullerene at various ratios using the twin-screw extrusion method. The resulting composites were investigated in terms of their tensile strength, morphological structure, thermomechanical properties, and thermal stability behavior. The results revealed that the highest tensile strength and Young's modulus were achieved at a 0.5 wt% fullerene content, showing a 12.8% improvement compared to pure PEEK. At low filler concentrations, such as 0.2 wt%, a homogeneous and nanoscale dispersion was observed, contributing positively to the enhancement of material properties. However, with increasing filler content, microstructural heterogeneities in the form of particle agglomeration became evident. Dynamic Mechanical Analysis (DMA) indicated that the addition of fullerene enhances the thermomechanical properties of PEEK, with 0.5 wt% offering the most balanced performance. Thermogravimetric Analysis (TGA) results demonstrated that even small amounts of fullerene in the PEEK matrix can improve thermal stability by controlling the material's thermal degradation behavior.
Recycling of carbon fiber reinforced epoxy composites
Composite materials are extensively used across various industries due to their high strength-to-weight ratio, corrosion resistance, and durability. However, the increasing use of thermoset-based composites has raised critical concerns regarding end of life waste management, as they are difficult to recycle through conventional methods. Among emerging technologies, microwave-assisted acid digestion has gained attention as a promising, efficient, and environmentally friendly recycling approach. This study investigates the closed-loop recycling of carbon fiber reinforced polymer (CFRP) waste using a rapid microwave-assisted nitric acid digestion method. A parametric study was conducted by varying the process temperature (120 °C–150 °C) and microwave irradiation hold time (5–10 minutes) while keeping other parameters constant. In this working range, the most suitable condition was determined to be 10 minutes at 150 °C, and 99% resin removal efficiency was achieved. Recycled carbon fibers were characterized and compared to pristine fibers using Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), and Raman spectroscopy. The recycled fibers retained their structural integrity, showing no visible surface etching, no change in fiber diameter, and minimal resin residue. A comparison of the ID/IG ratios across different recycling processes indicates that there is no structural change occurred during recycling process. These findings highlight the significance of developing fast, clean, and scalable recycling technologies for thermoset composites, especially in high-performance applications like aerospace. The successful recovery of carbon fibers not only proves the feasibility of this method but also supports the advancement of closed-loop recycling systems. The recycling cycle defined by varying time and temperature parameters can serve as a foundation for future studies. As the recycled fibers preserved their structural integrity, they appear suitable for reuse, though further research is needed to evaluate their mechanical performance compared to virgin fibers for potential manufacturing applications.