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Öğe 3D printed PLA/copper bowtie antenna for biomedical imaging applications(Springer, 2020) Avsar Aydin, Emine; Torun, Ahmet RefahThis study aims to increase the performance of the microwave antenna by using 3D printed conductive substrates, which is mainly used in biomedical imaging applications. Conventional antennas such as Horn and Vivaldi have coarse dimensions to integrate into the microwave imaging systems. Therefore, 3D printed Bowtie antenna structures were developed, which yield low cost and smaller sizes. PLA, PLA/copper, and PLA/carbon substrates were produced with a 3D printer. These materials were tested in terms of their dielectric constants between 1 and 10 GHz. The conductive part of the antenna was copper, with a thickness of 0.8 mm, which was embedded in the substrate parts. The reflection coefficients of the antennas were tested within 0-3 GHz frequency range via miniVNA network analyzer. The results show that the 3D printed PLA/copper and PLA/carbon antenna are highly suitable for the usage in biomedical imaging systems.Öğe Dynamic response of filament winding angle on complex shaped mandrels(Inst Natl Cercetare-Dezvoltare Textile Pielarie-Bucuresti, 2020) Torun, Ahmet RefahAerodynamic parts such as rocket nose and heat shield related composites are mainly produced with filament winding machines. Winding angle of the reinforcement filament is the main parameter to determine the thermomechanical properties of the final composite part. The angle adjustments on the machine cause the temporary response of the filaments. This study derives an analytical method to determine the real angle of the filaments on the mandrel.Öğe Evaluation of recycled Al-LDPE-Al sandwich panels as ballistic protection material(Ice Publishing, 2020) Torun, Ahmet Refah; Kaya, Seyma Helin; Choupani, NaghdaliMetal-polymer-metal hybrid sandwich panels are gaining importance in civil, automotive and aerospace applications due to their light weight and damping properties. Compared with composite materials, hybrid materials consisting of separate metal and thermoplastic parts can be recycled much more easily. Besides their applications as covering material on buildings as well as general insulation material, recycled aluminum (Al)-low-density polyethylene (LDPE)-aluminum hybrid panels yield a potential usage of light ballistic protection. In this study, a standard hybrid panel of 3.2mm polyethylene filling and two 0.4mm aluminum metal sheets was experimentally tested under ballistic impact. A finite-element model was used with a commercial software program and validated against the experimental results. The finite-element results show that stacking of multiple layers of panels absorbs more energy than an equivalent single-layer panel. Six layers of stacked hybrid aluminum-LDPE-aluminum panels are capable of absorbing the impact energy of a 9mm pistol projectile, and they can be utilized as recyclable inexpensive ballistic protection materials.Öğe Fracture characteristics of mixed-mode toughness of dissimilar adherends (cohesive and interfacial fracture)(Taylor & Francis Ltd, 2020) Abadi, Ramak Hossein; Torun, Ahmet Refah; Fard, Arash Mohammadali Zadeh; Choupani, NaghdaliThis paper investigates the fracture behavior of aluminum/polymer dissimilar adhesively bonded joints at different mixed-mode ratios. For this purpose, a series of experiments were carried out using a modified Arcan fixture to obtain critical fracture load in mode I, mode II and mixed-mode, for two types of cohesive and interface starter cracks. Experimental data, along with the Virtual Crack Closure Technique (VCCT) were employed to extract the strain energy release rates. The average values of strain energy release rates were found and for cohesive cracks, and and for interface cracks. Considering the relations between the strain energy release rate and the stress intensity factor, and were also calculated for cohesive and interface starter cracks.Öğe Fracture characterization and modeling of Gyroid filled 3D printed PLA structures(Walter De Gruyter Gmbh, 2021) Torun, Ahmet Refah; Dike, Ali Sinan; Yildiz, Ege Can; Saglam, Ismail; Choupani, NaghdaliPolylactic acid (PLA) is a commonly used biodegradable material in medical and increasingly in industrial applications. These materials are often exposed to various flaws and faults due to working and production conditions, and increasing the demand for PLA for various applications requires a full understanding of its fracture behavior. In addition to ABS, PLA is a widely used polymeric material in 3D printing. The gyroid type of filling is advantageous for overcoming the relatively higher brittleness of PLA in comparison with conventional thermoplastic polymers. In this study, the effects of various filling ratios on the fracture toughness of 3D printed PLA samples with gyroid pattern were investigated numerically and experimentally for pure mode I, combined mode I/II, and pure mode II. Two-dimensional finite element modeling was created, and the two-dimensional functions of stress intensity coefficients were extracted in loading mode I, mode I/II, and mode II at varied filling ratios of the gyroid PLA samples. Mixed-mode fracture tests for 3D printed PLA samples with a gyroid pattern at various filling ratios were performed by using a specially developed fracture testing fixture. The results showed that the amount of fracture toughness of the samples under study in tensile mode was much higher than those values in shear mode. Also, as the percentages of the filling ratios in the samples increased, both tensile and shear fracture toughness improved.Öğe Fracture characterization of bonded composites: A comparative study(Growing Science, 2022) Choupani, Naghdali; Torun, Ahmet RefahBonded joints have important benefits over conventional joining techniques such as rivets, welding, bolts and nuts in structural applications, particularly for components prepared of composite or polymeric materials. Due to the involvement of many geometric, material and construction variables, and the complex fracture and mechanical modes offered in the bonded joints, a proper consideration of fracture behavior is required to fully achieve their benefits. The fractures in bonded joints are mainly of three types; interlaminar (delamination), adhesive (interfacial) and cohesive crack. For a particular defect, crack propagation may occur in the tensile (mode I), the shear (mode II), and the tear (mode III) and their combinations (mixed mode). This study deals with topics such as theories of bonded composite joints and repairs, finite element analysis and fracture-based analysis and tests of mixed-mode cohesive, interfacial and interlaminar fracture mechanics. By employing geometrical factors extracted from finite element analysis and experimental results obtained from a modified Arcan test fixture, the mixed-mode cohesive, interfacial, and interlaminar fracture toughness are determined and fracture surfaces obtained are discussed. © 2022 Growing Science Ltd. All rights reserved.Öğe Fracture Toughness Assessment of Longitudinally Seam-Welded Gas Pipelines at Low Temperatures(Asce-Amer Soc Civil Engineers, 2020) Yengejeh, Ehsan Alipour; Torun, Ahmet Refah; Khajedezfouli, Mohammadreza; Choupani, NaghdaliAmerican Petroleum Institute (API) 5L X65 and X70 steel pipelines are employed widely for natural gas transportation and typically experience internal high transmission pressures and low temperatures in cold regions. Investigations show that in spite of strict inspections and controls, welding joints are commonly subjected to the presence of defects, with cracking being the most important one. Due to the many geometrical, material, and manufacturing variables involved, the failure mechanisms and procedures presented in the welded gas pipelines are complex. In order to achieve the benefits of gas pipelines, a deep understanding of the failure behavior is needed, especially in mixed mode loading conditions and at low temperatures. In this work, fracture toughness evaluation of longitudinal seam-welded gas transportation pipes at low temperatures and under mixed mode loading was studied based on experimental and numerical analysis. The experimental procedure included preparing the API X65 steel pipes, making the butterfly shaped experimental specimens, and encapsulating and insulating specimens in order to control and stabilize the temperature of test. By utilizing the Arcan fixture, the experiments carried out under different loading conditions at low temperatures and fracture forces by using force-displacement diagrams were obtained. Stress intensity factors (SIFs) for tensile and shear loading were found by numerical analysis and geometry correction factors were determined. The results indicated that the fracture toughness of the seam-welded samples decreased at lower temperatures, and it is strong to the tensile mode loading but weak to the shearing mode loading. The tensile and shear values of fracture toughness of seam weld under consideration for the temperature of -25 degrees C were found 203.3 and 11.8 kJ/m(2), respectively. (c) 2020 American Society of Civil Engineers.Öğe Mixed-mode fracture behavior of 3D-printed PLA with zigzag filling(Ice Publishing, 2021) Torun, Ahmet Refah; Yildiz, Ege Can; Kaya, Seyma Helin; Choupani, NaghdaliPolylactic acid (PLA) is a widely used biomaterial in medical applications as a biodegradable and renewable aliphatic polyester type of material. This material is often subjected to different defects and damages from in-service and manufacturing conditions, and the increasing demand for PLA for different applications requires a thorough understanding of its fracture behavior. In this work, a numerical and experimental study of the mixed-mode fracture behaviors of three-dimensional (3D)-printed PLA samples with a zigzag pattern of different filling ratios was performed using a recently developed special loading fixture. The 3D-printed samples were produced with a 200 degrees C nozzle at 60 degrees C bed temperature and 50mm/s printing speed. Mixed-mode fracture tests from pure tensile to pure shear loading were performed by varying the loading angle, alpha, from 0 to 90 degrees. Finite-element analyses were conducted by using the Abaqus software program, and geometrical factors were obtained at different loading angles. As a result, the fracture toughness values of pure tensile loading, pure shear loading and mixed modes were determined.Öğe Multilayer Perceptron Approach to Different Parameter Bow-tie Antennas Produced with PLA / Carbon / Copper Mixtures(Institute of Electrical and Electronics Engineers Inc., 2020) Saribas, Gozde; Aydin, Emine Avsar; Torun, Ahmet RefahThe study aims to observe the operating efficiency of seven different Bowtie Antennas produced with PLA/Carbon mixture material under the desired conditions. The constructed antennas' desired feature is that the return loss (s11) in the low-frequency range is -10dB and below. This value is that the antenna's s11 value is of great importance in detecting the tumor in the breast tissue. Parameters of antennas are the antenna substrate's thickness, the antenna substrate area, and the antenna legs' length. With these parameter values, s11 data were obtained in the frequency range 0-3 GHz. Another purpose of the study is to observe the effect and accuracy of the parameters used based on the return loss estimation of the designed antennas. The data obtained were evaluated using the Multilayer Perceptron approach. © 2020 IEEE.Öğe Multilayer Perceptron Approach to PLA, PLA /Copper and PLA / Carbon Substrate Bow-tie Antenna Production(Institute of Electrical and Electronics Engineers Inc., 2020) Saribas, Gozde; Aydin, Emine Avsar; Torun, Ahmet RefahThe study aims to observe the operating efficiency of Bowtie Antennas produced with three different materials under the desired conditions and to compare between materials. The produced antennas' desired feature is that the return loss (s11) in the low-frequency range is -10dB and below. This value is that the s11 value of the antenna is of great importance in detecting the tumor in the breast tissue. Parameters of antennas produced with different materials; The dielectric coefficient of the material used is the antenna diameter, the antenna substrate's thickness, and the length of the antenna legs. With these parameter values, s11 data at different frequency values in the frequency range of 0-3 GHz were obtained. Another purpose of the study is to observe the effect and accuracy of the parameters used on the result of the return loss estimation of the designed antennas. The data obtained were evaluated using the Multilayer Perceptron approach. © 2020 IEEE.Öğe Oblique impact behavior of Al-LDPE-Al sandwich plates(Walter De Gruyter Gmbh, 2020) Kaya, Seyma Helin; Karaoglu, Furkan Nuri; Saglam, Ismail; Choupani, Naghdali; Torun, Ahmet RefahMetal-polymer-metal hybrid sandwich panels are gaining importance in various industrial applications due to their light weight and damping properties. When compared with composite materials, hybrid materials consisting of separate metal and thermoplastic parts can be recycled more easily. In addition to their applications in civil engineering, the aluminum-low density polyethylene-aluminum (Al-LDPE-Al) sandwich panels yield a potential use as light ballistic protection material. In this study, a standard hybrid panel of 3.2 mm polyethylene filling and 0.4 mm of two aluminum metal sheets was experimentally tested under ballistic impact. A finite element model was constructed via commercial software and validated through shooting experiments with a rifle under real conditions. The finite element model was used to simulate the oblique impact behavior of Al-LDPE-Al sandwich panels as a single layer, as 5 layers stacking and as a single layer equivalent of the stacked 5 layer. Results showed that the oblique impact does not have a significant effect on the single layer panel. Stacked layers, however, and the equivalent single layer of a stacked layer have the highest energy absorption under a 30 degrees hitting angle.Öğe The Effect of Weekly Distributed Mathematics Homework and Quizzes on the Learning Performance of Engineering Students(Tempus Publications, 2019) Torun, Ahmet RefahEducation should promote learning which goes beyond short-term memory and lasts for years. Many research results suggest that a repeated practice over time is necessary to achieve long-lasting and long-term learning. Engineering mathematics consists of the basic skills which every kind of engineer must master. Within the scope of this study, we developed a learning system for mathematics lectures of engineering students. Instead of evaluating the students with midterm and final examinations, we evaluted a group of 89 engineering students with weekly homework and quizzes and compared it with two control groups of 85 and 114 engineering students. Our hypothesis for this study is: Learning by doi ng with guidance and distributed practices leads to better learning results than just watching the lecturer explaining and solving exercises. The midterm and final examinations are analogous to massed practices which cannot penetrate into the field of long-term leaning. The comparison with two control groups showed that the grades and long-term learning effects are significantly higher for the group with weekly homework and quizzes.Öğe The effect of weekly distributed mathematics homework and quizzes on the learning performance of engineering students(Tempus Publications, 2019) Torun, Ahmet RefahEducation should promote learning which goes beyond short-term memory and lasts for years. Many research results suggest that a repeated practice over time is necessary to achieve long-lasting and long-term learning. Engineering mathematics consists of the basic skills which every kind of engineer must master. Within the scope of this study, we developed a learning system for mathematics lectures of engineering students. Instead of evaluating the students with midterm and final examinations, we evaluted a group of 89 engineering students with weekly homework and quizzes and compared it with two control groups of 85 and 114 engineering students. Our hypothesis for this study is: Learning by doing with guidance and distributed practices leads to better learning results than just watching the lecturer explaining and solving exercises. The midterm and final examinations are analogous to massed practices which cannot penetrate into the field of long-term leaning. The comparison with two control groups showed that the grades and long-term learning effects are significantly higher for the group with weekly homework and quizzes. © 2019 TEMPUS Publications.
