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Boron-Doped Strontium-Stabilized Bismuth Cobalt Oxide Thermoelectric Nanocrystalline Ceramic Powders Synthesized via Electrospinning
(SPRINGER, 2014) Kocyigit, Serhat; Aytimur, Arda; Cinar, Emre; Uslu, Ibrahim; Akdemir, Ahmet
Boron-doped strontium-stabilized bismuth cobalt oxide thermoelectric nanocrystalline ceramic powders were produced by using a polymeric precursor technique. The powders were characterized by using x-ray diffraction (XRD), scanning electron microscopy (SEM), and physical properties measurement system (PPMS) techniques. The XRD results showed that these patterns have a two-phase mixture. The phases are face-centered cubic (fcc) and body-centered cubic (bcc). Values of the crystallite size, dislocation density, and microstrain were calculated by using the Scherrer equation. The lattice parameters were calculated for fcc and bcc phases. The SEM results showed that needle-like grains are formed in boron-undoped composite materials, but the needle-like grains changed to the plate-like grains with the addition of boron. The distribution of the nanofiber diameters was calculated and the average diameter of the boron-doped sample is smaller than the boron-undoped one. PPMS values showed that the electrical resistivity values decreased, but the thermal conductivity values, the Seebeck coefficients, and figure of merit (ZT) increased with increasing temperature for the two samples.
Bronz ve dökme demir talaşlarının geri dönüştürülmesiyle prizmatik şekilli makine elemanı üretimi ve mekanik karakterizasyonu
(2018) Aslan, Abdullah; Salur, Emin; Güneş, Aydın; Şahin, Ömer Sinan; Karadağ, Hakan Burak; Akdemir, Ahmet
Bu çalışmada küresel grafitli dökme demir (GGG-40) ve Bronz (CuSn10) talaşlarının izostatik sıcakpresleme yöntemiyle geri dönüştürülmesi ile üretilen metal matrisli kompozit malzemelerin (MMK) üretimive mekanik davranışları incelenmiştir. MMK malzemeler 4 farklı karışım oranında 3 farklı sıcaklıkta ve 3farklı basınçta üretilerek üretim parametrelerinin gözeneklilik, sertlik ve eğilme mukavemeti üzerine etkisiincelenmiştir. Prizmatik geometrili olarak üretilen numunelerin öncelikle gözeneklilik ve yoğunluk değerleritespit edilmiştir. Daha sonra Brinell ve Mikro Vickers ve 3 noktadan eğilme deneyleri ile MMKmalzemelerin mukavemeti belirlenmiştir. Optik mikroskopi tekniği ile mikro yapı görüntüleri incelenerektalaşların birleşme mekanizması incelenmiştir. Ayrıca XRD analizi aracılığıyla metalik talaşlar arasında arafaz oluşumu olup olmadığı sorgulanmıştır. Gözeneklilik deneyleri neticesinde %20 mertebelerindegözeneklilik elde edilmiştir. Aşırı plastik deformasyon gösteren CuSn10 sebebiyle mukavemeti artanCuSn10 talaşları sertlik değerini Bulk CuSn10 ile kıyaslanabilir seviyelere getirmiştir. Eğilme deneyineticesinde MMK malzemelerin eğilme mukavemetleri bulk CuSn10’dan düşük çıkmıştır. Sonuç olarakmetal talaşların başarılı bir biçimde metal matris kompozit haline getirilebileceği gösterilmiştir.
Characterization of styrene butadiene rubber and microwave devulcanized ground tire rubber composites
(WALTER DE GRUYTER GMBH, 2014) Karabork, Fazliye; Pehlivan, Erol; Akdemir, Ahmet
Ground tire rubber (GTR) was devulcanized by microwaves at the same heating rate (constant power) and different times of exposure. The devulcanized rubber (DVR) and untreated GTR were characterized physically and thermally. Composite materials were prepared from different proportions of the GTR, which was used as a filler, and the DV-R, which was used as part of the styrene butadiene rubber (SBR) matrix, and by varying the exposure time of the microwave power. These composites were compared with a control sample that was prepared from virgin SBR. The sol content (soluble part) and Fourier transform infrared spectroscopy (FTIR) analyses of the devulcanized samples were examined to define the efficiency of devulcanization. The cure characteristics and tensile properties of the SBR composites were researched. In this study, it was found that using DV-R as part of the rubber matrix produced much better properties than using GTR as a filler, thereby showing the significant benefits of microwave devulcanization. At the DV-R content of 50 phr, the elongation at break of the DV-R 5 min/SBR composites increased to 445.06% from 217.25% for the GTR/SBR composites, i.e., the elongation at break was enhanced by 105% by the devulcanization of GTR. Scanning electron microscopy (SEM) photographs displayed a better interface coherence between the DV-R 5 min and SBR matrix than the GTR/SBR composites.
ChipsLow-velocity impact behavior of porous metal matrix composites produced by recycling of bronze and iron chips
(SPRINGER, 2019) Şahin, Ömer Sinan; Güneş, Aydın; Aslan, Abdullah; Salur, Emin; Karadağ, Hakan Burak; Akdemir, Ahmet
In this study, the low-velocity impact behavior of porous metal matrix composites (MMCs) has been investigated. The MMCs consisting of spheroidal cast iron chips (GGG-40) and bronze chips (CuSn10) were produced by hot isostatic pressing. The MMCs were produced with different CuSn10 contents as 90-80-70-60%. The hot isostatic pressing was performed under three different pressures and temperatures. The produced MMCs were exposed to low-velocity impact loading under 2m/s by using a drop weight test stand. The test results were compared with the bulk CuSn10 and bulk GGG-40, separately. The test results revealed that hot-isostatic-pressed MMCs can successfully represent bulk material properties when subjected to low-velocity impact loading even these MMCs have porosity 2-8%.
Correlation Between Machinability and Chip Morphology of Austempered Ductile Iron
(AMER SOC TESTING MATERIALS, 2018) Yazman, Şakir; Gemi, Lokman; Uludağ, Muhammet; Akdemir, Ahmet; Uyaner, Mesut; Dışpınar, Derya
In this work, macrostructure, microstructure, and work hardening of as-cast and austempered ductile iron chips produced by different cutting speeds were investigated. Cutting forces, surface roughness, and tool wear were also studied. After austenitizing at 900 degrees C for 90 min, samples were austempered in a salt bath at 360 degrees C for 90 min. Machinability tests were conducted with uncoated cemented carbide tools on a CNC lathe. The tests were carried out without using any lubricant. In these tests, the cutting speed was varied in the range 50-200 m/min, while the feed rate (0.12 mm/rev) and depth of cut (2 mm) were kept constant. The tangential, feed, and radial cutting force components acting on the tool holder were measured with a three-component dynamometer. The chips obtained during machining were polished by embedding into epoxy resin, examined with optical microscope and micrographs were taken. The microhardness values were measured on each chip sample. As the cutting speed was increased, deformation strengthening of the microstructure was increased, which decreased the tool life. It was also detected that during the machining, with the effect of cutting forces and stress, spheroidal graphites were broken off in the chip and lost their sphericity, so the chip became fragile and unstable and the graphite displayed a lubricant feature. At low cutting speed, a built-up edge was observed. However, as the cutting speed was increased because of heating, built-up edges were unstable.
DIRECT RECYCLING OF BRONZE/STEEL CHIPS AND INVESTIGATION ITS MECHANICAL PROPERTIES
(INT SCIENTIFIC CONFERENCE SGEM, 2009) Karadag, Hakan Burak; Akdemir, Ahmet
Recycling of metals has been becoming an attractive subject, lately due to the fact that the depleted resources, high energy consumption and harmful gases emission. Metal machining chips are recycled conventionaly which consist of melting casting. In the paper the method of direct conversion of bronze and steel chips, into finished product without melting, as an alternative way to conventional method is described. Metal chips were comminuted by mechanical milling and for the next processing the granulated chips larger than 2 mm in length were eliminated. Bronz and steel chips were blended at previously designated percentage ratio according to their weight. In recent process, metal chips that pressed at room temperature were sintered. Finally the mechanical properties of the produced composite material were determined and compared with metallurgically produced bronze.
Düşük karbonlu çelik tel takviyeli kır dökme demir kompozitlerin sertlik özelliklerinin incelenmesi
(Selçuk Üniversitesi Teknik Bilimler Meslek Yüksekokulu, 2007) Akdemir, Ahmet; Kuş, Recai
Bu çalışmada, takviye elemanı olarak düşük karbonlu çelik tel, matris malzemesi olarak ise kır dökmedemir kullanılarak metal matris kompozit üretilmiştir.Üretilen kompozit numunelerin bir kısmına gerilim giderme tavlaması diğerlerine de 800 0C, 850 0C ve 900 0C de normalizasyon ısıl işlemi uygulanmıştır. Numuneler Vickers ve Rockwell sertlik ölçme metodu ile sertlikleri ölçülmüştür. Kompozit numunenin dökmedemir bölgesinin sertliğinin ve dökmedemir numunenin de sertlik değerleri birbirine yakındır ve normalizasyon ısıl işlem sıcaklığından etkilenmediği görülmüştür. Kompozit numunede çelik bölgesinin sertliği dökmedemir bölgesi sertliğine ve dökmedemir numunenin sertliğine nazaran büyük çıkmıştır. Kompozit numunenin geçiş bölgesi sertliğinin diğer bölgelere göre diğer bölgelere göre en yüksek sertlik değerine sahip olduğu tesbit edilmiştir.
THE EFFECT OF BUILD UP EDGE FORMATION ON THE MACHINING CHARACTERISTICS IN AUSTEMPERED FERRITIC DUCTILE IRON
(AMER SOC MECHANICAL ENGINEERS, 2013) Yazman, Sakir; Akdemir, Ahmet; Uyaner, Mesut; Bakircioglu, Bari
In this study, chip formation mechanism during the machining of austempered ferritic DI and the effect of the emerging chip morphology on such machining properties as surface roughness and cutting forces has been scrutinized. After austenitizing at 900 degrees C for 90 min, DI specimens were austempered in a salt bath at 380 degrees C for 90 min. Chip roots were produced by using a quick stop device during the machining of austempered specimens in different cutting speeds. The metallographies of these specimens were performed and chip morphologies were examined. The fact that the cutting speed increased led to a decrease in built-up edge formation. Depending on this fact, it was detected that the change in built-up edge thickness substantially affected the surface roughness and cutting forces. It was also detected that during the machining, with the effect of cutting forces and stress, spheroidal graphites were broken off in the chip and lost their sphericity and so that the chip became fragile and unstable and grafites here displayed a lubricant feature.
Effect of non-penetrating impact damages of pre-stressed GRP tubes at low velocities on the burst strength
(ELSEVIER SCI LTD, 2014) Kara, Memduh; Uyaner, Mesut; Avci, Ahmet; Akdemir, Ahmet
In this study, failure behavior of pre-stressed Glass Fiber Reinforced Plastic (GRP) tubes subjected to low velocity impact was investigated. Filament wound E-glass/epoxy composite tubes with a winding angle of +/- 55 degrees were used in the experiments. The tubes pre-stressed to 32 bars internal pressure, one of the specified operating pressures according to ANSI/AWWA C950 standards. Both prestressed and non-prestressed GRP tubes were subjected to low velocity impact tests at 5, 10 and 15 J low energy levels. Plots of contact force and internal pressure with respect to time and those of force-displacement were obtained and damages occurred on the specimens were examined by means of reflected and transmitted light photography and light microscope. No marked fiber breakages are observed; however, debonding and some sort of delamination on the layers were observed. The GRP tubes with impact damages were subjected to monotonic burst pressures up to failure based on the ASTM D 1599-99 standards. Burst damages on the tubes were studied and variations of their corresponding burst strengths were spotted. Besides that, diameter changes with internal pressures were determined during the tests. Diameter changes with internal pressures for the impacted and the non-impacted tubes were found to be the same. (C) 2014 Elsevier Ltd. All rights reserved.
Effect of silica/graphene nanohybrid particles on the mechanical properties of epoxy coatings
(SPRINGER HEIDELBERG, 2019) Özcan, Ümit Esra; Karabörk, Fazliye; Yazman, Şakir; Akdemir, Ahmet
Epoxy resins are used as coating materials, but the practical use of epoxy coatings in industries is limited due to their weak mechanical properties. In the present paper, different amounts of silica nanoparticles (SiO2) and graphene nanoplatelets (GNPs) were introduced separately and together into an epoxy coating matrix as reinforcements. Graphene, a newly discovered carbon allotrope, has been found to improve the mechanical properties of the polymer composites in which it is dispersed. Silica particles are also known to improve the mechanical properties of composites. In this study, mechanical, physical and thermal properties of the epoxy coatings are considered as multidimensional by the macro- and microanalyses. The experimental results showed that after the addition of GNPs into the epoxy matrix, the flexibility and impact resistance of the coatings increased by 8.3 and 157.1%, respectively, in relation to neat epoxy. The microhardness increased by 53.8% and penetration depth, which is indicative of the scratch resistance, decreased by 29.7%, with the addition of SiO2-GNPs nanohybrid. A remarkable synergistic effect was observed between the GNPs and SiO2, which improved the hardness and the scratch resistance of the epoxy coatings.
Effect of the size on the friction characteristics of brake friction materials: a case study with Al2O3
(EMERALD GROUP PUBLISHING LTD, 2018) Sugoezue, Banu; Daghan, Behcet; Akdemir, Ahmet
Purpose The purpose of this paper is to study the potential of alumina (Al2O3) in nanometer size in automotive brake friction materials. Design/methodology/approach Four brake linings containing alumina differing in particle size (355 mu m and 80 nm) and various amount (5 and 10 Wt.%) were designed and produced. The hardness, density and porosity of the samples were measured. All samples were tested on a full-scale brake dynamometer with gray cast iron disc to determine the tribological properties. Detailed examinations on the worn surface were analyzed using a scanning electron microscopy. Findings It was concluded that all performance parameters were beneficially affected because of nano alumina. Originality/value This paper emphasizes the importance of nano-composites in the automotive industry and helps industrial firms and academicians working on wear of materials.
The Effects of Cutting Speed and Depth of Cut on Machinability Characteristics of Austempered Ductile Iron
(ASME-AMER SOC MECHANICAL ENG, 2012) Akdemir, Ahmet; Yazman, Sakir; Saglam, Haci; Uyaner, Mesut
Ductile iron can acquire enhanced thermal and mechanical properties from austempering heat treatment. The present study aims to identify the function of different cutting parameters affecting machinability and to quantify its effects. Turning was performed to test machinability according to the ISO3685-1993 (E) standard. After austenitizing at 900 degrees C for 90 min, austempered ductile iron (ADI) specimens were quenched in a salt bath at 380 degrees C for 90 min. The cutting force signals along three directions were measured in real time, whereas flank wear and surface roughness were measured offline. For the cutting parameters, the cutting speed and depth of cut were varied, but the feed rate was kept constant. In the flank wear tests, machining length was corresponded to tool life. In addition, in order to find out the effect of cutting parameters on surface roughness (R-a), tangential force (F-t), and flank wear (VB) during turning, response surface methodology (RSM) was utilized by using experimental data. The effect of the depth of cut on the surface roughness was negligible but considerable in the cutting forces. The increased cutting speed produced a positive effect on surface roughness. It is found that the cutting speed was the dominant factor on the surface roughness, tangential force, and flank wear. [DOI:10.1115/1.4005805]
Effects of production parameters and conditioning upon ballistic characteristics of para aramid light armors
(SAGE PUBLICATIONS LTD, 2008) Akdemir, Ahmet; Candan, Can; Sahin, Oemer Sinan
Composite materials are increasingly used in armors design due to their high strength to weight ratios and energy absorption capacities. It is very important to determine the best production parameters for armor production. On the other hand, the armors must show superior durability against harsh working conditions. So it is crucial to know the ballistic characteristics of such composite armors. In this study the effect of production parameters upon the terminal ballistic properties of para-aramid composite armor were examined under different conditionings. The composite armor plates were produced by hot pressing with four different pressing times. Before ballistic test, the armor plates conditioned at +21, +63, and -35 degrees C at 50% of relative humidity and +21 degrees C immersed into water temperature for 24 h, in which weapons, ammunition, and equipment designed for use in the battlefield, are expected to fully operate.
Elasto-Plastic Stress Analysis of Thermoplastic Matrix Composite Laminated Plates Under in-Plane Loading
(Technomic Publ Co Inc, 1999) Yapıcı, Ahmet; Tarakçıoğlu, Necmettin; Akdemir, Ahmet; Avcı, Ahmet
Thermoplastic matrix reinforced with metal fiber, composite laminated plates were manufactured by using moulds. The symmetric and antisymmetric laminated plates were loaded by in-plane forces. An elastic-plastic numerical solution has been carried out by finite element technique (FEM) for some load steps. Residual stresses and expansion of plastic zone have been illustrated in tables and figures.
Elektro-eğirme yöntemiyle nanofiber tabakalı hava filtresi üretimi ve karakterizasyonu
(Selçuk Üniversitesi Mühendislik Fakültesi, 2016) Dinçer, Kevser; Önal, Gürol; Akdemir, Ahmet; Selbes, Mehmet
Bu çalışmada, makro boyuttaki gözeneklere sahip farklı malzemelerin, yüzeylerininfiltreleme performansını arttırmak amacıyla, elektro-eğirme yöntemiyle nanofiber tabaka ile kaplanması amaçlanmıştır. Çalışma sonunda sadece mikrofiberfiltrelerin yüzeyine kaplanan nanofiber tabakaların filtre olarak kullanılabileceği görülmüştür. Mikro partiküllerin filtrasyonunda kullanılan 25-85g/m2 gramaj arasındaki mikrofiber filtrelerin yüzeyi, polimer solüsyonu besleme debisi 21-30ml/h, uygulanan gerilim 28-32kV ve elektrotlar arası mesafe 19-23cm aralığında elektro-eğirme deney parametreleri ile kaplanmıştır. Nanofiber tabaka ağırlığı 2-4g/m2, nanofiber çapı 77-200nm arasında değişen beş farklı hava filtresi numuneleri elde edilmiştir. Elektro-eğirme deney parametrelerindeki değişim ile nanofiber tabaka ağırlığı, nanofiber çapı ve yüzey morfolojisindeki değişimler gözlenmiş ve üretilen numunelere etkisi incelenmiştir. Sonuçta, makro gözenekli malzemelerin nanofiber tabakalı filtre olarak kullanılamayacağı görülmüştür.
Experimental investigation of fatigue damage formation of hybrid pipes subjected to impact loading under internal pre-stress
(ELSEVIER SCI LTD, 2017) Gemi, Lokman; Sahin, Omer Sinan; Akdemir, Ahmet
In this study, the effect of impact loading upon fatigue behavior of hybrid composite pipes has been investigated. The composite pipes were subjected to tangential pre-stress as 3-48 MPa according to ANSI/AWWA C950 standard than, subjected to low velocity impact at 20 J. The impact pre-damaged pipes then subjected to cyclic loading at 50% sigma ult in accordance with ASTM D 2992-06. It is also observed that increasing the tangential pre-stress decreases impact damage area and increases fatigue life of the pipe. It is also showed that impact pre-damage are not effective on the fatigue life of the pipe at the pre-stress level of 24 MPa. (C) 2017 Elsevier Ltd. All rights reserved.
Fabrication and characterisation of boron doped barium stabilised bismuth cobalt oxide nanocrystalline ceramic composite
(TAYLOR & FRANCIS LTD, 2013) Uslu, İbrahim; Çınar, Emre; Koçyiğit, Serhat; Aytimur, Arda; Akdemir, Ahmet
In this paper, the fabrication and characterisation processes of both boron doped and undoped barium stabilised bismuth cobalt oxide nanocrystalline ceramic powders using polymeric precursor were reported. Obtained boron doped barium stabilised bismuth cobalt oxide nanocrystalline ceramic powders, which have been synthesised by polymeric precursor technique at temperatures below 900 degrees C and at atmospheric condition, were characterised by X-ray diffraction, Fourier transform infrared and scanning electron microscopy techniques. According to X-ray results, fcc and bcc phases coexist in the samples of the nanocrystalline ceramic powders. Crystallite sizes for body centred cubic structure were calculated using Scherrer equation for both boron doped and undoped samples. In addition, lattice parameters were calculated for all samples.
Fabrication and characterization of bismuth oxide-holmia nanofibers and nanoceramics
(ELSEVIER SCIENCE BV, 2013) Aytimur, Arda; Kocyigit, Serhat; Uslu, Ibrahim; Durmusoglu, Senol; Akdemir, Ahmet
In this article, a novel and simple method to produce both boron doped and undoped holmia stabilized bismuth oxide nanoceramic materials has been put forward. Boron doped and undoped poly ( vinyl alcohol)/bismuth-holmia acetate nanofibers were produced using the electrospinning technique and were calcined at 850 degrees C afterward in order to obtain nanopowder. The characteristics of the nanofibers were investigated with FT-IR, XRD, and SEM. XRD analyses showed that boron undoped holmia stabilized bismuth oxide nanopowders have the face-centered cubic structure (delta-phase), and that the incorporation of boron atoms into the composite prevents the nucleus formation and turns the structure into a more amorphous glassy form. The SEM micrographs of the fibers showed that the addition of boron results in the formation of cross-linked bright-surfaced fibers. The average fiber diameters for electrospun boron doped and undoped PVA/Bi-Ho acetate nanofibers were calculated using the ImageJ software as 102 nm and 171 nm, respectively. (C) 2012 Elsevier B.V. All rights reserved.
FARKLI ORANLARDA DPDS KULLANILARAK DEVULKANİZE EDİLMİŞ ATIK TAŞIT LASTİKLERİ İLAVE EDİLEN SBR MALZEMENİN MEKANİK ÖZELLİKLERİNİN İNCELENMESİ
(2015) Karabörk, Fazliye; Akdemir, Ahmet
Oda sıcaklığında öğütülen atık taşıt lastiği tozu mikrodalga yöntemiyle devulkanize edilmiştir. İşlemde devulkanizasyon ajanı olarak, kükürt bağlarının (S-S) koparılmasında etkinliği çeşitli çalışmalarda ortaya konulmuş olan, DPDS (difenildisülfür) kullanılmıştır. Mikrodalga gücü sabit tutularak, zamanın ve DPDS miktarının prosese etkileri incelenmiştir. İşlem görmemiş atık lastik tozu fiziksel ve termal olarak karakterize edilmiştir. 20 phr oranında atık lastik tozu ve aynı oranda devulkanize kauçuk (DVR) stiren bütadien kauçuğa (SBR 1723 ve SBR 1502) katılarak kompozit malzemeler hazırlanmıştır. Bu kompozitler orijinal SBR'den hazırlanan kontrol numunesiyle karşılaştırılmıştır. Devulkanizasyonun etkinliğini ortaya koyabilmek için devulkanize numunelerin çözünme oranları (sol content) belirlenmiş ve Fourier Transform Infrared Spektrofotometre (FTIR) analizi yapılmıştır. Elde edilen kompozit malzemelerin mekanik özellikleri incelenmiştir. Çalışmada, DVR katılan malzemelerde atık lastik tozu katılan malzemelere göre daha iyi devulkanizasyonunun önemli avantajlar sağladığını göstermiştir. Kopma uzaması değeri, işlem görmemiş atık lastik tozu katılan kompozit malzemede %481 iken, 1 g DPDS katılarak 4 dk. süreyle devulkanize edilen kauçuğun katıldığı DVR/SBR kompozitte %578 olarak elde edilmiştir, böylece kopma uzaması değeri atık lastik tozunun devulkanizasyonuyla %20 oranında artmıştır.
Fatigue and fracture behavior of woven steel reinforced polyethylene composite
(SAGE PUBLICATIONS LTD, 2008) Erkendirci, O. Faruk; Avci, Ahmet; Akdemir, Ahmet
The objective of this study is to investigate the fatigue and fracture behavior of woven steel reinforced high density polyethylene (HDPE) thermoplastic composite having a middle tension (MT) specimen with through-thickness crack. The composite material was manufactured from bi-directional woven Cr-Ni steel fibers and HDPE with a fiber volume fraction (V-f) of 0.04. The crack initiation, damage behavior, fatigue crack-growth rate (da/dN), stress intensity factors (Delta K), strain energy release rates (Delta G), and fracture toughness (K-IC) were determined in the study. Moreover, Delta G values were obtained by using linear elastic fracture mechanics (LEFM) and compliance methods, and the results were compared with each other. Also, da/dN-Delta K and da/dN-Delta G were plotted for determining fracture behavior under fatigue loading of the composite material.

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