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Multi criteria operating system selection using fuzzy replacement analysis and AHP
Full text linkBu çalışma Karar Alıcılar (KA) için İşlem Sistemi (İS) seçim sistemi kurmayı hedeflemektedir. Karar Alıcılar teknoloji seçiminde hem ekonomik hem de ekonomik olmayan unsurları göz önüne almak zorunda olduklarından, geliştirilen sistemde her iki unsura da yer verilmiştir. Karar alma sürecinin ekonomik yanı, Bulanık Yenileme Analizi kullanılarak geliştirilmiştir. Ekonomik olamayan unsurlar ve finansal veriler ise Bulanık Analitik Hiyerarşi Süreci (AHS) yaklaşımı kullanılarak biraraya getirilmiştir. Çalışma içerisinde aynı zamanda sayısal bir örneğe de yer verilmiştir. AHS yaklaşımının finansal yönü geliştirilen Bulanık Yenileme Analizi altyapısı tarafından desteklenmiştir. Bulanık AHS yönteminin Mühendislik Ekonomisi’nin ana konularından olan Yenileme Analizleri’nde kullanılması araştırmacılara yatırım alternatiflerinin değerlendirilmesinde etkin yollar sağlamaktadır.Anahtar Kelimeler: Bulanık kümeler, yenileme analizleri, analitik hiyerarşi süreci, teknoloji seçimi.This study aims at creating an Operating System (OS) selection framework for decision makers (DMs). Since DMs have to consider both economic and non-economic aspect of technology selection, both factors have been considered in the developed framework. The economic part of the decision process has been developed by Fuzzy Replacement Analysis. Non-economic factors ve financial figures have been combined using Fuzzy Analytic Hierarchy Process (Fuzzy AHP) approach. A real numerical application has also been demonstrated. This study developed a fuzzy AHP framework to select best OS alternative. While fuzzy AHP requires cumbersome computations, it is a more systematic method than the others are, and it is more capable of capturing a human?s appraisal of ambiguity when complex multi-attribute decision-making problems are considered. This is true because pairwise comparisons provide a flexible and realistic way to accommodate real-life data. The financial side of the framework is based on fuzzy replacement analysis. The results of fuzzy replacement analysis are included into fuzzy AHP analysis. Using Fuzzy AHP concept in Replacement Analysis investment decisions in fuzzy environment results a very effective way to evaluate alternatives. Using the very same developed framework, a subjective comparison, such as the comparison of diverse operating systems, has been conducted and demonstrated to readers.Keywords: Fuzzy sets, replacement analysis, AHP, technology selection
Açık işletmelerde optimum ekipman seçimi
Full text linkIn this study, an expert system for equipment selection in surface mining is developed. Expert system selects the optimum hydraulic excavator and off-highway truck combination, which is widely used in surface mining. A shell, developed by Intellicorp Company, KappaPC is selected for developing Equipment Selection Expert Systems. For KappaPC, rules and databases are prepared. Rules are determined according to equipment selection criteria, which are excavability, operating factors and material properties. Excavability is one of the most important parameter for equipment selection and a new excavability system is developed under the lights of the previously developed systems. Excavability and other parameters such as bearing capacity, bucket fill factor, bucket cycle time, etc. determines the best excavator. Truck selection is based on the remaining empty capacity after the excavator loads the truck with optimum number of bucket numbers (4-6). In this way, the expert system developed, selects the optimum hydraulic excavator and trucks. In order to determine optimum excavator and truck numbers the minimum capital cost of each combination is calculated. A combination, which has the minimum cost, is the optimum. In this fashion, an expert system, which optimally selects the hydraulic excavator and truck combination, employed in surface mining is developed.Keywords: Expert systems, equipment selection, surface miningBu çalışmada, açık işletmelerde son yıllarda kullanımı hızla artan bir üretim yöntemi olan hidrolik ekskavatör - kamyon yöntemi için en uygun ekipman seçimi yapan bir uzman sistem KappaPC kabuğu kullanılarak geliştirilmiştir. Kazıcı ekipman seçimindeki en önemli kriterlerden birisi olan kazılabilirliğin belirlenmesi için, daha önce yapılmış olan çalışmaların doğrultusunda uzman sistemde kullanılmak üzere bir kazılabilirlik sınıflaması oluşturulmuştur. Bununla birlikte, zeminin taşıma direnci, kepçe dolma faktörü, kepçe periyodu gibi diğer ekipman seçimi parametrelerinin de dikkate alınmasıyla hidrolik ekskavatör seçimi yapılmaktadır. Kamyon kapasitesinin belirlenmesi ise, ekskavatör tarafından doldurulan kamyonlarda ekskavatörün optimum kepçe sayısı (4-6) ile doldurduğu toplam hacim ile kamyonun kasa hacmi arasında oluşan farktan dolayı boş kalan kasa hacmine bağlı olarak gerçekleştirilmektedir. Ekskavatör ve kamyon satın alma maliyetleri kullanılarak, istenilen üretimi sağlayan her bir ekskavatör ve kamyon kombinasyonunun maliyetleri içinde en düşük maliyetin elde edildiği kombinasyon optimum ekskavatör ve kamyon sayısı olarak belirlenmiştir.Anahtar Kelimeler: Uzman sistemler, ekipman seçimi, açık işletmeler. 
HPFRCC for retrofitting of reinforced concrete members built with low-strength concrete
Full text linkBu çalışmada; düşük dayanımlı betonarme elemanların deprem yükleri altında davranışlarını iyileştirmek amacı ile kullanılabilecek bir malzeme geliştirilmesi hedeflenmiştir. Bu amaç için farklı özelliklerde Yüksek Performanslı Lif Donatılı Çimento Esaslı Kompozit (HPFRCC) numunelerin mekanik davranışı araştırılmıştır. Betonarme elemanların bu malzeme ile üretilecek ve kalınlıkları 20-50 mm arasında olması düşünülen ön üretimli HPFRCC ince plakların elamanların yüzeyine epoksi ve mekanik ankrajlar yardımıyla monte edilmesi ile güçlendirilmesi hedeflenmektedir. Bu plakaların mekanik özelliklerinin özellikle çekme dayanımlarının güçlendirilmesi düşünülen düşük dayanımlı panellere göre çok daha iyi olmasından dolayı güçlendirmede kullanılabileceği düşünülmüştür. Malzemenin mekanik özelliklerini geliştirmek amacı ile çeşitli parametrelerin etkilerinin incelendiği 27 adet standart silindir basınç (150×300 mm), 54 adet standart disk yarma-çekme (150×60 mm) ve 9 adet kiriş eğilme deneylerini (100×100×500 mm) kapsayan bir çalışma gerçekleştirilmiştir. Standart silindir ve standart disk yarma-çekme deneyleri sonuçları sıcak kürün çekme dayanımını arttırdığını göstermiştir. Liflerin az miktarda korozyona uğratılmasının mekanik özelikleri pratik olarak etkilemediği görülmüştür. Çelik lif oranının artışı çekme dayanımı, süneklik ve tokluk gibi mekanik özelikleri olumlu yönde etkilemesine karşın basınç dayanımı ve elastisite modülü gibi mekanik özelikleri pratik olarak etkilemediği görülmüştür. Eğilme deneyleri sonucunda çelik lif oranının % 4’e çıkması ile lifsiz numuneye göre tokluğunun çok büyük oranlarda ve net eğilme dayanımının yaklaşık altı kat arttığı görülmüştür. Anahtar Kelimeler: Betonarme, çelik lif, düşük dayanımlı beton, güçlendirme, HPFRCC.Improvements in concrete technology enabled engineers to produce special concretes with high strengths. However, as known high strength concrete exhibits a very brittle behavior. Many researchers have tried to change this brittle failure mode to a ductile type failure by adding various types of fibers in to the concrete. Consequently, different types of concrete with fibers were developed such as Fiber Reinforced Cementitious Composites (FRCC), Fiber Reinforced Concrete (FRC), Ductile Fiber Reinforced Cementitious Composites (DFRCC), High Performance Fiber Reinforced Cementitious Composites (HPFRCC) etc. Many researches on this topic have showed that fibers increased the ductility and energy dissipation capacity of the concrete members. In this research; the main goal is to develop a material that can be used to improve the behavior of reinforced concrete members built with low-strength concrete (f´c 10 MPa, in which f´c is the compressive strength of the concrete) under seismic loads. HPFRCC may be used for this purpose. The main idea is to produce precast thin panels made of HPFRCC and retrofitting reinforced concrete members by attaching these panels to the surface of the reinforced concrete members. It is clear that as the mechanical properties of the HPFRCC improve, the effectiveness of HPFRCC panels in retrofitting will also increase. In order to investigate the mechanical characteristics of the HPFRCC, two series of experimental studies were carried out. The first series of tests included compression tests of 27 standard cylinder specimens and tension tests of 54 standard splitting disc specimens. For compression tests nine sets and for each set three identical cylinder specimens were produced. For tension tests six standard splitting disc specimens were produced for each of nine sets. The diameter of cylinder is 150 mm and height is 300 mm. The diameter and height of the standard splitting discs are 150 and 60 mm, respectively. In the tests of the first series, the testing parameters included hot water curing, decomposition of steel fibers before adding them into mixture, corrosion and the amount of steel fibers. Second series of tests included flexural tests of beams with dimensions of 100×100×500 mm. The only parameter in beam tests is the amount of steel fibers. Three different mixtures and for each mixture there identical beams were produced. Compression tests of standard cylinder specimens were carried out either under static monotonic or cyclic axial compression loads by using Amsler machine with 5000 kN loading capacity. Two linear displacement transducers were used to measure axial deformations. To determine the elastic modulus, the specimens were subjected to one loading and unloading cycle in the linear range of the stress-strain curve before the full test and then elastic modulus was obtained from the slope of the linear best-fit line. During loading and unloading cycle axial deformations were measured both in the full length (300 mm) and in 150 mm at midheight of the specimens. For the tests of standard disc specimens a 1000 kN capacity Amsler machine was used. For the beam tests 100 kN capacity Instron 5500R machine was used. Fracture energy, bending strength and toughness of the specimens were obtained from load-displacement curves. Three point loading tests were carried out for beams and loading rates were 0.05 mm/minute and 0.1 mm/minute for the ascending and descending branch of load-displacement curves, respectively. The displacements were measured at the middle point of the beams. In the tests of specimens without steel fibers the loading rate was 0.0175 mm/minute during the entire test. The maximum target displacement was 10 mm, which corresponded to 2.5% drift ratio for specimens with steel fibers. For the tests of specimens without steel fibers, there was no maximum target displacement since the behavior was very brittle. The brittle failures occurred around 0.03% drift ratio. As results of the compression and splitting tension tests, the hot water curing was found to increase the tensile strength of concrete significantly. It was also found that small amount of the corrosion of steel fibers have no effect on the mechanical characteristics. It was clearly seen that as the amount of steel fibers increased, the mechanical properties such as tensile strength, toughness and ductility also increased while compressive strength and elastic modulus did not change. Results of beam tests showed that the bending strength and toughness increased significantly with increasing amount of steel fibers. Keywords: HPFRCC, low strength concrete, reinforced concrete, retrofitting, steel fibers
Effect of mixing proportions on the compressive strength and permeability of concretes containing mineral admixtures
Full text linkBetonda dayanıklılığın (dürabilite) en az dayanım kadar önemli olduğu son yıllarda daha iyi anlaşılmaya başlamıştır. Dayanıklı beton üretmek için düşük su/çimento oranının yanında puzolan (mineral katkı) kullanımının da gerekli olduğu bilinmektedir. Bu deneysel çalışmada farklı oranlarda uçucu kül veya silis dumanı mineral katkıları içeren iki seri beton üretilmiştir. ‘Merkezi Kompozit Tasarım’ adındaki deneysel tasarım ve analiz yapan istatistiksel yöntem kullanılarak her seride karışım parametrelerinden toplam bağlayıcı miktarı, su/bağlayıcı oranı ve puzolan/bağlayıcı oranı bağımsız değişken olarak seçilmiştir. Değişken sınırları puzolan özelliklerine ve betonların belirli bir kıvamda bulunması koşuluna göre belirlenmiştir. Her seride, kullanılan istatistiksel programın belirlediği 20 adet karışım hazırlanmıştır. Betonların basınç dayanımı ve klorür geçirimliliğini ölçmek üzere alınan numunelerin yarısı laboratuvar ortamında havada diğer yarısı da kirece doygun su içerisinde 90 gün boyunca saklanmıştır. Basınç dayanımının yanında ASTM C 1202 standardına uygun olarak hızlı klorür geçirimliliği deneyleri gerçekleştirilmiştir. Tüm deney sonuçları varyans analizi ile değerlendirilmiş ve her iki kür koşulu için ölçülen beton özelliklerinin karışım parametreleri cinsinde ayrı ayrı modelleri belirlenmiştir. Betonun seçilen bileşim parametrelerinin (toplam bağlayıcı miktarı, su/bağlayıcı oranı ve puzolan/bağlayıcı oranı) basınç dayanımı ve klorür geçirimlilği üzerine etkisi elde edilen istatistiksel modellere dayanılarak değerlendirilmiştir. Hızlı klorür geçirimliliği deneyinin havada saklanan betonların karşılaştırılmasında kullanılabileceği ancak su kürü uygulamasında mineral katkılı betonlarla katkısız betonların karşılaştırılmasında hatalı sonuçlara neden olabileceği belirlenmiştir. Anahtar Kelimeler: Uçucu kül, silis dumanı, kür koşulları, istatistiksel analiz, merkezi kompozit tasarım.Concrete is a material which contains different amounts of pores in various sizes. On this account, penetration of liquid and/or gas into the concrete occurs by diffusion, absorption under pressure or capillary sorption through the pores. In no matter what way the transfer of liquid and gas occurs, the phenomenon is called as permeability. Permeability depends on amount, distribution, size and connectivity of pores within concrete for as much as rate of penetration of aggressive substances into concrete mainly related to the continuity of the pores and their shape and distance. It has been widely accepted for many years that mechanical strength of concrete is adequate enough for a structure and little maintenance would be sufficient to overcome any problem occurred during the service life. However, in recent years it is well known that in order to obtain a durable concrete, which shows the expected performance throughout its service life, selection of suitable materials in accordance with environmental conditions, the best proportion of concrete mixture and sufficient curing after casting are the most important steps to pay attention. Ingredients of concrete should be selected according to their participation in durability. Pozzolanic materials, most of which are by products of industries, are widely used in concrete production, especially to increase the durability properties. Pozzolanic materials react with Ca(OH)2, generated during the hydration of C3S and C2S components of cement, to form calcium silicate hydrate (C-S-H), hence reduce a weak part of concrete against durability. Furthermore, pozzolans improve the impermeability of concrete which is the main parameteraffecting the durability of concrete. In this experimental research, two series of concrete incorporated with fly ash and silica fume were produced. The statistical program of 'Central Composite Design' was used to design experiments and to analyze the testing results. In each series, total binder content, water/binder ratio and pozzolan/binder ratio were chosen as independent variables and the ranges of the variables were adjusted according to the properties of pozzolanic material and to obtain certain workability for all concretes. The range of the factors were selected between 300-400 kg/m3 for binder content, 0.48-0.58 (when silica fume was used) or 0.46-0.60 (when fly ash was used) for water/binder ratio and 0.03-0.12 for silica fume/binder ratio or 0.10-0.40 for fly ash/binder ratio. For each series of concretes, the total number of batches was 20. Lime saturated water curing was applied over the half of the specimens, cast to measure compressive strength and permeability, while the remaining ones were stored in air at laboratory conditions for 90 days. In addition to the compressive strength test, rapid chloride permeability test was conducted in accordance with ASTM C 1202. Test results were analyzed and evaluated by using ANOVA (Analysis of Variance). Statistical models are derived for each measured concrete property in two curing conditions by means of three independent components of concrete. Influence of selected mixture parameters on compressive strength and chloride permeability was evaluated by using the test results. It has been found that total binder content of fly ash added concretes was not influential on the compressive strength and chloride penetration. On the other hand, increasing binder content reduced the overall performance of silica fume added concretes because of the higher porosity of cement paste than that of aggregate. It was also seen that the optimum supplementary cementing material contents determined to obtain the minimum chloride permeability are shifted to higher values for the fly ash added concretes under the proper curing conditions rather than storing in air. However, the optimum percentages of the silica fume added concretes cured under both conditions are not changed. Besides, it is found that RCPT method can be employed to make comparisons for all of the air stored concretes produced with or without supplementary cementing material (SCM). However, for water cured SCM added specimens this method can be used for only internal comparisons, it is not appropriate to compare the chloride permeability of OPC concretes with those of pozzolan added concretes. Keywords: Fly ash, silica fume, curing conditions, statistical analysis, central composite design
The investigation of some natural antimicrobial compounds on activities of S. enteritidis, E. coli O157:H7 and L. monocytogenes in fresh chicken meat systems
Full text linkBu çalışmada doğal antimikrobiyal maddeler (greyfurt çekirdek ekstraktı: GFSE, nisin ) ve modifiye atmosfer paketleme yöntemi kullanarak tavuk kıyması ve formulasyonlarında (GFSE, NaCl ve sarımsak tozu ilavesi) bulunması/çoğalması muhtemel patojen bakterilerin (S. enteritidis, E. coli O157:H7, L. monocytogenes 4b) 4ºC ve 10ºC’de kontrolü amaçlanmıştır. Antimikrobiyal maddelerin etkinliği ilk aşamada in-vitro test edilmiştir. Tryptic Soy Broth’da (TSB) nisin (+ 20 mM EDTA) S. enteritidis ve E. coli O157:H7 üzerinde önemli etki göstermemiştir. Öte yandan nisin (pH: 3.44-5.22, 73-500 ppm’de) tek başına L. monocytogenes üzerinde kuvvetli bakterisit etkilidir. Greyfurt çekirdek ekstraktı (GFSE) (60-300 ppm) TSB’de tüm patojenlere bakterisittir. GFSE (3000 ve 6000 ppm) ilavesi ile modifiye (%70 CO2 + % 30 N2) ve aerobik (%21 O2 + %79 N2) paketlenen tavuk kıymasının 10ºC’ de 6 gün depolanmasında S. enteritidis ve E. coli O157:H7’ nin gelişimi üzerinde GFSE, süre ve paket tipi önemlidir (p<0.05). Formulasyonlarda (6000 ppm GFSE’ nin NaCl ve/veya sarımsak ile bileşimleri) 10ºC’lik depolamada patojen bakterilerin gelişiminin önlenmesinde, S. enteritidis ve E. coli için GFSE, NaCl ve paket tipi, L. monocytogenes için ise bunlara ilaveten sarımsak önemli (p<0.05) etkide bulunmuştur. Soğuk depolamada(4ºC) GFSE ve MAP kullanımı ile S. enteritidis ve E. coli kontrolünde, MAP’ın ilave katkısı belirlenmemiştir. Nisinin (250 ppm) tavuk kıymasında MAP ile kombinasyonu soğuk depolamada L. monocytogenes’in kontrolünü sağlamış, 6 gün sonunda kontrol-aerobik örneklere göre bakteri sayısını 2,5 log10 düşürmüştür. Anahtar Kelimeler: Salmonella, E. coli O157:H7, Listeria, greyfurt çekirdek ekstraktı, nisin, tavuk eti, MAP.In the first part of this study, the effectiveness of natural antimicrobial substances [nisin (0-500 ppm) and/or EDTA (20 mM); grapefruit seed extract; GFSE (0-300 ppm)] on the growth of each pathogenic bacteria (S. enteritidis, E. coli O157:H7, L. monocytogenes 4b) on the level 5 log10/ml were investigated in Tryptic Soy Broth (TSB) media. In the second part of the study, the effects of addition of grapefruit seed extract, nisin, garlic powder and NaCl on the bacteria were investigated in minced chicken meat samples or formulations under different temperature storage (4 and 10ºC) and packaging (aerobic and modified atmosphere) conditions. In microbial culture media, nisin-20 mM EDTA combination did not exhibit an inhibition effect on S. enteritidis and E. coli O157:H7 (p>0.05), whereas 73-500 ppm nisin alone in the pH range of 3.44-5.22 showed strong bactericidal effect (p 0.05) on L. monocytogenes It was also observed that 20 mM EDTA itself at pH 5,22 had a bacteriostatic effect on S. enteritidis and E. coli O157:H7. Grapefruit seed extract (60-300 ppm) was found to have a bactericidal effect at pH 6.0 against all pathogenic microorganisms in the first hour and at the end of the 24-hour incubation period (p>0.05) in TSB. The number (log10 CFU/g) of S. enteritidis and L. monocytogenes in minced chicken meat samples treated with 2000 ppm GFSE was found to be 0.5 log10 and 1 log10 CFU/g less respectively than in control at 4ºC over the 6-day storage period. It was found that all factors (GFSE, period and packaging type) had a significant effect on the growth of S. enteritidis and E. coli O157:H7 over the 6-day storage period in minced chicken meat samples which were treated with 3000 and 6000 ppm of GFSE and packaged under aerobic (%21 O2+%79 N2) and modified atmosphere packaging (MAP) (%70 CO2+% 30N2) conditions at 10ºC (p 0.05). High concentration level of GFSE (6000 ppm) was more effective in inhibitions of S. enteritidis and E. coli O157:H7. The growth of these pathogenic bacteria was also inhibited using modified atmosphere packaging (p 0.05). A statistically significant difference was found between packaging methods for L. monocytogenes, whereas no statistically significant difference was found between the inhibition effects of 3000 and 6000 ppm GFSE. In the formulation study, the effects of eight different combinations of GFSE (6000 ppm) with garlic powder (% 2 w/w) and NaCl (% 2 w/w) on pathogenic microorganisms were investigated under aerobic and modified atmosphere packaging conditions at 10ºC during 6-day storage period. The results showed that growths of S. enteritidis and E. coli O157:H7 were significantly influenced by GFSE, NaCl, time and packaging method at 10oC. In addition to these factors, garlic powder had a significant (inhibitory) effect on L. monocytogenes growth (p=0.018). It was observed that treated samples with GFSE resulted in a reduction of 2 log10 mean for S. enteritidis and E. coli O157:H7 counts compared to controls over the first 3-day storage period. However, no further reduction activity was observed at the end of the 6-day storage period. MAP packaging resulted in 1 log10 mean reduction for S. enteritidis and 2 log10 mean reduction for E. coli O157:H7 compared to aerobic packaging over the 6-day storage period. During the first 3 days of storage the growth of L. monocytogenes was controlled with GFSE. At the end of 6 days storage, the number of L. monocytogenes was lowered by 2.5 log10 mean compared to controls with the addition of GFSE. MAP packaging led to a reduction of 1 log10 mean for L. monocytogenes compared to aerobic packaging. The addition of GFSE (6000 ppm) resulted in approximately 2 log10 reduction of S. enteritidis after 24 h at 4oC in aerobic packages. MAP packaging did not provide any advantage in the reduction of S. enteritidis. It was found that packaging method did not have a significant effect on E. coli O157:H7 (p=0.279). GFSE and MAP packaging was found to be effective in controlling Listerial growth at 4ºC.It was found that nisin had a significant inhibition (p 0.05) effect on L. monocytogenes growth in minced chicken meat at 4ºC. At the end of 6 days, the use of 250 ppm nisin with modified atmosphere packaging lowered Listeria counts by 2.5 log10 mean compared to aerobic-controls. Keywords: Salmonella, E. coli O157:H7, Listeria, grapefruit seed extract, nisin, chicken meat, MAP
Production and development of wrought magnesium alloys
Full text linkTürkiye’nin ilk magnezyum alaşımı levhası, ikiz merdaneli sürekli döküm tekniği ile üretilmiştir. Magnezyum AZ31, AZ61, AZ91, AM50 ve AM60 alaşımı levhalar 4-8 mm kalınlığında, 1500 mm eninde başarılı bir şekilde elde edilmiştir. Levhalar daha sonra homojenleştirme ısıl işlemlerine maruz bırakılmıştır. Levhaların mikroyapıları yüzey, en ve boy yönlerinde optik mikroskop ve Taramalı Elektron Mikroskobu (SEM) ile incelenmiştir. Daha detaylı mikroyapı incelemeleri Geçirimli Elektron Mikroskobu (TEM) ile yapılmıştır. Yarı nicel ve nicel elementel analizler SEM-EDS (Enerji Dağılım Spektrometresi), TEM-EDS ve EPMA (Elektron Prob Mikro Analiz Cihazı)-WDS (Dalgaboyu Dağılım Spektrometresi) sistemleri ile yapılmıştır. X-ışınları difraksiyonu (XRD) teknikleri karakterizasyon ve yönlenme incelemeleri amaçlı olarak kullanılmıştır. Malzemelerin mekanik özellikleri çekme deneyi ve sertlik deneyleri ile ölçülmüştür. Çekme deneyleri hadde yönü, hadde yönüne 45 derece açı ve 90 derece açı olmak üzere üç farklı yönde ekstensometre yardımı ile yapılmıştır. Ayrıca numune yüzeylerinde ve farklı kesitlerde mikro Vickers ve Brinell Sertlik taramaları yapılmıştır. Elde edilen levhalar üzerinde soğuk hadde, sıcak hadde ve ısıl işlem gibi termomekanik işlemler denemeleri de gerçekleştirilmiştir. Levhalar öncelikle laboratuar ölçeğinde sıcak hadde ile inceltilmişlerdir. Daha sonra elde edilen bilgiler ışığında magnezyum AZ31 alaşımı levhalar ısıtıldıktan sonra endüstriyel ölçekli hadde sistemi ile 1 mm kalınlığa kadar inceltilmiştir. Elde edilen sonuçlar ışığında otomotiv, savunma ve elektronik endüstrileri için uygun magnezyum levha alaşımlarının üretimi olası gözükmektedir. Anahtar Kelimeler: Magnezyum alaşımları, levha, ikiz merdaneli sürekli döküm.Magnesium alloy sheet has been produced by twin roll strip casting first time in Turkey. Magnesium AZ31, AZ61, AZ91, AM50 and AM60 alloy sheets of 4-8 mm thick, 1500 mm wide were successfully achieved. Afterwards, homogenization heat treatments were applied on the sheets. Microstructures of the sheets have been analysed by optical microscope and scanning electron microscope, SEM by plan, longitudinal and transverse views. More detailed microstructure investigation was analysed by transmission electron microscope, TEM. Semi-quantitative and quantitative elemental analyses has been performed by SEM-EDS (Energy Dispersive Spectrometer), TEM-EDS and EPMA (Electron Probe Micro Analyser)-WDS (Wavelength Dispersive Spectrometer) systems. XRD (X-ray Diffraction) techniques are used for both characterization and also texture purposes. Mechanical properties have been investigated by tensile tests and also hardness measurements. Tensile tests have been performed at three different directions: rolling direction, 45 degrees to rolling direction and transverse direction by using an extensometer. Micro Vickers and Brinell Hardness test measurements were done on plan view and different cross-section directions. In addition, produced sheets were investigated by cold rolling, hot rolling and annealing tests. From the results of this study production of wrought magnesium alloys suitable for automotive, military and electronics industries seems possible. Magnesium is the lightest of all structural metals with a density of 1.74 g/cm3. Aluminum is 1.5 times, titanium is 3 times and iron is 4 times of magnesium in density. Magnesium alloys have high specific strength, high specific stiffness, good castability and machinability, low heat content per unit of volume, high damping capacity, and good electro-magnetic (EMI) shielding. Magnesium is dimensionally stable, it welds easily, and it has impact and dent resistant. It is the sixth most abundant metal and eighth element on the earth's surface. Furthermore, magnesium is readily recyclable. Magnesium alloys also have effective heat dissipation. Due to these properties, there is increasing interest in using magnesium alloys especially in electronics and transportation industries. Almost 30% of the applications are structural applications (portable electronic equipment, such as laptop computers, cellular phones and video cameras; military equipment; aircraft parts; sporting goods and hand tools. Recently, using magnesium alloys that are lighter than aluminum alloys are being investigated for the automotive industry. Magnesium alloys are already used within the automotive interior as instrumental panel substrate, seat frame, seat riser, seat pan, console bracket, steering wheel, steering column parts; in the power train as valve cover, transmission cases; in the body as door and roof frames, sunroof panel, mirror bracket, tailgate; in the chassis as wheel, brake pedal brackets. High flexural/buckling stiffness and bending strength are needed for automotive body components such as doors, boot, and bonnet. VW 3L Lupo bonnet is a good prototype example for future application of magnesium sheets. Other possible wrought alloy automotive applications are extruded profiles such as window frames, seat and supporting structures. Magnesium components are usually produced by the die casting process. In spite of cost effective, the die casting process is not suitable for manufacturing large flat parts, such as hood, door, and lift-gate substrates. Also, mechanical properties of the cast parts, particularly fatigue resistance, can be substandard. Parts requiring good mechanical properties and fatigue endurance strength are best produced from wrought alloys. Replacement of conventional sheet metals with magnesium can reduce the vehicle mass, thus promote energy efficient transportation. By using wrought magnesium alloys, a decrease in vehicle weight up to 100 kg and a reduction of 5 % fuel consumption can be realized. Application of wrought magnesium alloys especially in the form of sheet is limited due to the price of conventional rolling product. However, the demand for decreasing the magnesium sheet prices is high and can be met through twin-roll casting. Keywords: Magnesium alloys, sheet, twin roll casting
Synthesis of B4C thin films by plasma-enhanced magnetron sputtering
Full text linkBor karbür, yüksek sertliği, mekanik, tribolojik, elektronik, optik özelliklerinin yanı sıra yüksek nötron absorblama özelliği ile de dikkat çeken bir malzemedir. Düşük yoğunluğu, yüksek Young modülü, çok yüksek termal ve kimyasal kararlılığı vb. özellikleri olan B4C, elmas ve c-BN’den sonra bilinen en sert malzeme olmasına karşın bu durum oda sıcaklığında geçerlidir. Elmas ve c-BN’ün sertliği artan sıcaklıkla kademeli olarak düşerken B4C termal kararlılığı sayesinde yüksek sıcaklıklarda sertliğini muhafaza etmektedir ve özellikle 1100 °C’nin üzerindeki sıcaklıklarda bilinen en sert malzemedir. Bu özellikleriyle B4C ince filmler, kesici takımların, fren balatalarının, sabit disklerin ve çeşitli makine parçalarının kaplanması gibi mekanik, tribolojik uygulamaların yanı sıra, yüksek sıcaklık ortamları gibi zorlayıcı şartlarda çalışacak transistörler vb. elektronik ve optik uygulamalarda da kullanılmaktadır. Bu çalışmada, bor karbür tozlarının sıcak preslenmesiyle elde edilmiş olan bor karbür hedef malzeme kullanılarak, plazma-destekli doğru akım manyetik alan sıçratma tekniğiyle 350–400 nm kalınlığında homojen ve taban malzemeye iyi yapışan bor karbür ince filmler üretilmiştir. Biriktirme sıcaklığı tüm kaplamalar için 250 °C olarak sabitlenmiş ve 0–250 V arasında uygulanan taban malzeme voltajının kaplama yapısına olan etkileri incelenmiştir. Elektron sondası mikro analizleri (EPMA), kaplamaların elementel bileşimlerinin üretim şartlarından bağımsız olduğunu ortaya koymuştur. Kesitten gerçekleştirilen taramalı elektron mikroskobu (SEM) incelemeleri sonucunda, bu çalışma şartlarında bor karbür filmlerin kolonsuz yapıda biriktiği tespit edilmiştir. Nanosertlik testleri neticesinde, taban malzeme voltajının arttırılmasıyla sertlik ve Young modülü değerlerinde önemli bir artışın meydana geldiği tespit edilmiştir. Anahtar Kelimeler: Bor karbür, ince film, sıçratma, iyon bombardımanı, nanomekanik.Boron carbide (B4C) is the third hardest material at room temperature, which has many other attractive properties such as good wear resistance, high modulus, high chemical and thermal stability. These properties make boron carbide a promising candidate as hard and protective coating for cutting tools, automobile parts, hard disk drives and other wear-resistant applications. Within the range of 8-20% of carbon, boron carbide is ideally described by a rhombohedral unit cell with icosahedral arrangement of 12 atoms, in addition to a three-atom chain along the crystallographic c-axis that interconnect the icosahedra. For amorphous boron carbide films deposited by sputtering, it is believed that the structure is still based on a random icosahedral network at a carbon content less than 50%. Several techniques have been used to synthesize boron carbide thin films, including chemical vapor deposition, plasma-enhanced chemical vapor deposition, cathodic arc, atmospheric plasma spraying, electromagnetically accelerated plasma spraying, RF magnetron sputtering, and DC magnetron sputtering. Among these, magnetron-sputtering techniques have been successfully commercialized in a large scale because of their high film-deposition rate and low-temperature features. In this study, boron carbide powders were obtained from the carbothermal reduction of boric acid in a graphite resistance furnace at 2000 ºC. The powder thus obtained was hot pressed in pure nitrogen atmosphere with 100 MPa applied force at 2100 °C for 15 minutes to obtain boron carbide target used in this study. The sputtering target was 15 cm in diameter and 7 mm in thickness. Boron carbide thin films were deposited by plasma-enhanced DC magnetron sputtering of hot-pressed boron carbide target. AISI M2 steel and Si (100) wafers were used as substrates in each deposition. High-purity (99.999%) Ar was used as precursor and was introduced into the vacuum chamber through a mass flow controller to establish the desired working pressure, which was 0.3 Pa. The cathode power was fixed at 500 W for all the experiments. Microstructural examinations revealed the presence of continuous and homogeneous B4C films with 350-400 nm thickness. Elemental composition of the films was measured by EPMA. Grazing-angle XRD of the coatings over showed no characteristic peaks for boron carbide, indicating that the coatings were amorphous. Infrared spectra presented two broad bands, one centered at ~1100 cm-1 and the other at ~1570 cm-1. The band at 1100 cm-1 is attributed to B?C bonds in the icosahedra and is characteristic of B4C thin film structure. The band at 1570 cm-1 has been attributed either to the presence of graphite or free carbon in boron carbide structure or to the stretching in the linear chains that interconnect the icosahedra. Nanoindentation results demonstrated that boron carbide films deposited are remarkably hard and the increase in the negative bias voltage led to an increase in the measured hardness from 32 GPa for the film deposited at floating potential to a maximum value of ~40 GPa for the coating deposited at 100 V bias voltage. Further increase in the bias voltage to 250 V resulted with a decrease in the hardness to 32 GPa. The same tendency of increase was observed for the Young's modulus, from 270 GPa for the films deposited at floating potential, it reached its maximum value for 300 GPa at 100 V bias voltage and then decreased thereafter to 265 GPa for 250 V bias voltage. Modified sputtering configuration led non-columnar, featureless microstructures with smooth surface morphologies. There was no significant effect of the deposition parameters on the thickness and elemental composition of the films deposited. Films were amorphous and exhibited remarkably high hardness and Young's modulus values with high elasticity. With the increase in the bias voltage, increases in the hardness and Young's modulus of boron carbide films were observed. Keywords: Boron carbide, thin film, sputtering, ion bombardment, nanomechanics
Analysis of micro gas flows with DSMC (Direct Simulation Monte Carlo) method
Full text linkDoğrudan Benzetim Monte Carlo (DSMC) seyreltik ve mikro boyutlu gaz akışlarını çözümlemek için kullanılan molekül esaslı bir yöntemdir. DSMC hesaplama süresi olarak pahalı bir yöntem olduğundan genelde Euler ve Navier-Stokes gibi geleneksel sürekli ortam denklemlerinin kullanılmasının uygun olmadığı durumlarda kullanılmaktadır. Ortamın geleneksel sürekli ortam denklemlerine uygun olup olmadığına ise boyutsuz Knudsen sayısına bakılarak karar verilmektedir. Knudsen sayısı yükseldikçe önce sınırlarda hız kayması ve sıcaklık atlaması oluşur. Takiben ısı akı vektörünün sıcaklık gradyeni, gerilme tensörünün ise gaz akış hız gradyeni ile olan doğrusal ilişkisi sona erer. Bu şartlar oluştuğunda hesaplamalar için deneylere uygun sonuçlar veren DSMC yöntemine ihtiyaç duyulmaktadır. DSMC yönteminde DSMC molekülleri her bir zaman adımında sahip oldukları hız nedeni ile konum değiştirirler. Bu konum değişikliği nedeni ile içinde yer aldıkları hücrelerde değişebilmektedirler. DSMC molekül çarpışmaları ve makro değer hesaplamaları hücre temelinde modellendiğinden her zaman adımı sonunda DSMC moleküllerinin yeniden konuşlandıkları hücrelere ait bilgilerin güncellenmesine ihtiyaç vardır. Geleneksel DSMC yönteminde moleküller hücreden hücreye takip edilerek son gittikleri hücre bilgisine ulaşılır. Eğer gaz akış bölgesi dikdörtgen bir geometriye dönüştürülebilir ise DSMC molekülünün gittiği son hücre yapısal ağ bilgisinden yararlanılarak basit bir aritmetik işlem ile bulunabilmektedir. Bu çalışmada DSMC hesaplama verimliliğini arttırmak için mikro ölçekli yakınsak-ıraksak bir soğuk gaz lülesi için koordinat dönüşümü ile kare şeklinde bir hesaplama bölgesi oluşturulmuş ve hücre bilgisi hesaplama süresi kısaltılmıştır. İlaveten her zaman adımı sonunda moleküllerin yeniden hücre numarasına göre sıraya dizilmesi işlemine olan ihtiyaca yeni bir yaklaşımla son verilmiştir. Anahtar Kelimeler: DSMC, mikro gaz akışları, mikro-lüle, genelleştirilmiş koordinatlar.In the last 25 years a number of Micro Electro Mechanical System (MEMS) were developed. These MEMS devices not only include the mechanical systems but also the fluids. Knowledge about fluid flows in this scale is not as mature as the mechanical properties of the MEMS. As their dimensions are between 1 mm and 1 micron, gas flows related with the MEMS devices have high Knudsen numbers similar to high atmosphere flights. If is higher than 0.1, instead of the classical continuum based Euler or Navier-Stokes (N-S) equations, higher order continuum based equations like Burnett equations or molecular models like DSMC should be used. This is due to the departure from local thermodynamic equilibrium with increasing number. First velocity slip and temperature jump are formed on the boundaries. Next, low order constitutive equations are lost their validity because relations both between shear stress and velocity gradient and heat conduction and temperature gradient are not linear any more. Additionally the ratio of flow surface area to flow volume is dramatically increased in micro gas flow conditions. So surface forces dominate the volume forces. Consequently, compressibility and viscous heating (dissipation) effects become more important in micro gas flows in addition to rarefaction effects. Even in low Mach numbers, large density and temperature gradients prevail. It is found that a micro scale gas flow can behave differently from the large-scale one, which is generally studied with hydrodynamic models. Our application of DSMC starts with the division of the computational domain into smaller cells. Linear dimensions of these cells are of the same order as the mean-free-path ( ) of the gas. A group of physical gas molecules are represented by one representative molecule that called DSMC molecule in this study. Every DSMC molecule carries position, velocity, cell number and if applicable internal energy information on it. In DSMC method molecule movements and collisions are separated from each other. As a first step, molecules move according to their velocities and initial conditions. Their velocities, positions and cell numbers are updated. In the collision step, stochastic approach is used and molecule velocities are updated according to the collision model chosen. Next step is the calculation of the macroscopic gas flow properties for each cell from the microscopic molecule information. For steady flows, time averaging is used for the calculation of macroscopic properties. DSMC method is computationally expensive. To shorten the computation time new approaches are needed.. Generally molecule movements are traced cell-by-cell in DSMC solvers both in structural and unstructured meshes. At each time step DSMC molecules move to a new position. Then each DSMC molecule is checked whether they left the cell or not. If it is determined that DSMC molecules left the cell then which cell they stopped is calculated. To do this either all the neighbor cells should be searched or which edge molecule left the origin cell should be determined. And then new cell is found. This procedure requires many mathematical calculations and time. If non-rectangular physical flow geometry can be converted a rectangular computational domain, then it is possible to calculate the DSMC molecule cell information in a very short time with a very simple mathematical operation. Additionally current DSMC solvers use complex and time consuming indexing mechanism to realize molecule collisions. All the molecules put in order 1-Dimensional arrays at the end of each time step. Molecule collision partners are selected from this array, which are required to be in the same cell. In this study using a new data structure which consist of a cell number and a molecule number in that cell. Each molecule completing its movement is renumbered according to its new cell information and what number in this cell. This new data structure is copied onto old data structure at the end of each time step. Consequently complex indexing mechanism is found to be obsolete now. A cold gas micro-nozzle test problem is chosen from the literature. In this problem working medium is Argon. A test study is performed with Argon gas flow through a convergent-divergent micro-nozzle to determine the efficiency of the new method. Both macro properties of Argon gas flow through the micro-nozzle and solution times of each method is reported. Cold gas flow through a micro-nozzle is chosen because they are thought important for micro-propulsion systems. Keywords: DSMC, micro gas flows, micro-nozzle, generalized coordinates
New design approach for high-resolution current-steering DACs
Full text linkGünümüzde Sayısal-Analog Dönüştürücüler (SAD) birçok elektronik sistemlerin kilit elemanları olmaktadır. Veri dönüştürücüler analog ve sayısal dünyalar arasında bir köprü oluşturdukları için hızlı ve verimli bir şekilde gerçekleştirilmeleri yüksek derecede arzu edilmektedir. Özellikle haberleşme uygulamalarında yüksek hızlı (birkaç 100 MHz) ve yüksek çözünürlüklü (10-bitten fazla) SADlar için artan rağbet, akım yönlendirmeli SAD’ların kullanımını mecbur kılmaktadır. Yayınların çoğunda yüksek performanslı akım yönlendirmeli SAD’ların tasarımında ve gerçekleştirmesinde kesimleme (segmentation) yöntemi kullanılmaktadır. Bu yöntem, yüksek hız ve yüksek çözünürlük gerektiren uygulamaların çoğunda avantajlı olmasına rağmen uzun süren tasarım zamanı, karmaşıklık ve yüksek maliyet yüzünden değer kaybetmektedir. Böylece, bazı uygulamalar için zaman ve maliyet açısından bu yöntemin kullanılması hızlı ve verimli olmayabilir. Bu problemlerin üstesinden gelmek için yüksek çözünürlüklü SADların hızlı tasarımını sağlayan verimli yöntemler dikkate alınmaktadır. Akım yönlendirmeli SADlar gibi karmaşık karma yapılı sistemlerin tasarımı için davranışsal modelin oluşturulması zorunlu olmaktadır. Bu amaçla yapılan modellerin çoğu (matematiksel veya devre tabanlı), sistemin davranışı hakkında istenilen eksiksiz manzarayı vermemektedirler. Bu yüzden tasarımı hızlandırabilen ve sistemin davranışını doğru bir şekilde yansıtabilen modeller geliştirilmektedir. Sonuç olarak uygulanan yöntemin verimliliğini ve davranışsal modelin doğruluğunu sınamak için AMS 0.35µm CMOS proses teknolojisi için tasarlanan bir 12 bitlik melez akım yönlendirmeli SAD kullanılmaktadır. Yapılan iyileştirmeler ve kullanılan farklı yöntemler gerçekleştirilen SAD’ın serimindeki ilgili kısımlarda yer almaktadırlar. CADENCE Geleneksel Tümleşik Devre Tasarım Araçları kullanılarak serim sonrası benzetimleri yapılmaktadır ve SAD’ın performans karakteristikleri incelenmektedir. Anahtar Kelimeler: Sayısal-analog dönüştürücü, akım yönlendirmeli, davranışsal model, melez SAD.Nowadays DACs have become key elements in many electronic systems. In order to interface electronic systems with the real world, digital signals have to be translated into physical signals, which require a conversion into analog signals that is performed by DACs. Since data converters form the bridge between the analog and digital world their efficient implementation is highly desirable. The increase in demand for high speed (several 100 MHz) and high resolution (higher than 10-bit) DACs, especially in communication applications forces the use of current-steering based DACs. The current-steering DACs are widely used, since they satisfy the requirements of high-speed and high-resolution necessary in communication applications. However, due to their numerous features and wide range of application uses, it is very difficult to define and to follow a single way in which current-steering DACs can be specified and designed. A similar systematic design methodology can be considered during design flow of a predetermined current-steering DAC architecture. Such a methodology is concerned with the entire mixed signal system and requires a top-down design flow starting with DAC's specifications, architectural decisions, cell-level circuit decisions and ending with DAC layout issues both for digital and analog parts of the system.Most publications made use of the segmentation method for the design and the implementation of high performance current-steering DACs. Although this methodology is advantageous in most of the applications requiring high-speed and high-resolution, it suffers from the prolonged design time, complexity and high cost. Thus, the use of this methodology for some applications is not efficient concerning the time and the cost. To overcome these problems efficient methodologies for the high speed design of high-resolution DACs are considered. Behavioral modeling is necessary for the design of complex mixed-mode systems like current-steering DACs. Most of the models constructed (mathematical or circuit based) can not give a complete view of the system's behavior. For this reason, models that speed up the design and reflect accurately the behavior of the system prior to transistor level implementation are developed. The use of DAC modeling tool is helpful to obtain converters having the best performance in terms of speed and accuracy. Through these models (SPICE, Simulink etc. based models), the mixed signal designs even with large complexity can be easily evaluated. To conclude, the efficiency of the applied methodology and the accuracy of the behavioral model are validated through the implementation of a 12-bit hybrid current-steering based DAC in a relatively cheap 0.35µm CMOS process technology. The proposed hybrid DAC consists of four 3-bit parallel matched current-steering subDACs and resistive networks that properly weight the current output of each subDAC to obtain the overall voltage-mode output of the 12-bit hybrid DAC. The improvements in the building blocks and the different approaches used are reflected in the respective parts of the layout of the implemented DAC. Post-layout simulations are obtained using CADENCE Custom IC Design Tools and the performance characteristics of the DAC are investigated. The performance of the implemented DAC is compared with recently introduced DACs designed for different applications, through Figure of Merit (FoM). The architecture and design methodology used for the implementation of the DAC offer advantages like design speed up and a small active area. The performance of the hybrid DAC is validated through static and dynamic performance metrics. All simulations are performed with a 3.3V power supply. Simulations indicate that the DAC has an accuracy of 12-bit and a SFDR higher than 66 dB in whole Nyquist frequency band. The simulated INL is better than 1LSB, while simulated DNL is better than 0.25LSB. At an update rate of 200MSample/s the SFDR for signals up to 1MHz is higher than 70 dB. Similarly at an update rate of 100MS/s the SFDR is higher than 65 dB for signals up to 5 MHz. Even for update rates like 500MSample/s or 1GSample/s the SFDR is higher than 60dB for sinusoidal input signals up to 1MHz. The FoM of the implemented hybrid DAC is better than recently presented DACs with different resolutions and implemented using various process technologies. The proposed hybrid DAC supporting high update rates with good dynamic performance can be used as an alternative in various applications in industry including video, digital TV, cable modems etc. Keywords: Digital-to-analog converter, current-steering, behavioral model, hybrid DAC
A second order Newton method for reconstruction of perfectly electric conducting objects
Full text linkZamanda harmonik dalgaların ters saçılma problemi radar, sonar, tahribatsız değerlendirme, geofizik, tıbbi görüntüleme gibi uygulamalar için temel öneme sahiptir. Temel olarak, bu uygulamalarda bilinmeyen cisimden saçılan dalganın ayrık noktalarda ölçülmesiyle elde edilen veri kullanılarak saçıcının şekil, konum, elektromagnetik parametreleri gibi özellikleri bulunur. Bu çalışmada, düzlemsel bir dalga ile aydınlatılmış rastgele kesitli silindirik mükemmel iletken bir cisimden saçılan alanların uzak alan ölçümlerinden şeklinin bulunması yeni bir yöntem olan ikinci dereceden Newton metoduyla ilk kez bu çalışmada incelenmiştir. Bu yöntem Newton iterasyonu ve dekompozisyon methodunun daha gelişmiş bir şeklidir. Buradaki iteratif yöntemin ana fikri Huygen prensibini kullanmaktır yani saçılan alanı tek tabakalı bir potansiyelle ifade etmektir. Saçıcının alınan bir yaklaşıklığı için bu Tikhonov regülarizasyonuyla çözülebilen birinci dereceden ill-posed bir integral denklem elde edilir. Daha sonra, ikinci dereceden Taylor açılımıyla mükemmel iletken sınır koşulu sağlanacak şekilde cismin şekli değiştirilir. Iterasyon yönteminde bu iki adım belirlenmiş bir durma koşulu sağlanana kadar devam ettirilir. Bu yöntemin temel avantajları her bir iterasyonda düz probleminin çözümünün gerekmemesi ve birinci dereceden Newton yöntemine göre arzulanan bir doğruluğa ulaşmak için daha az iterasyon gerektirmesidir. Önerilen yöntem yakın alan ve sınırlı alan ölçümlerinin saçılan alan olarak kullanılması durumuna kolayca geliştirilebilir. Anahtar Kelimeler: Newton iterasyonu, ters saçılma, integral denklemler. Inverse scattering problems for time harmonic waves are of fundamental importance in applications such as radar and sonar, nondestructive evaluation, geophysical exploration, medical imaging and others. In principle, in these applications the wave scattered by an unknown object is measured at a number of discrete locations and information such as shape parameters, location parameters and electromagnetic parameters of the scatterer are extracted from these data. In this study, a new second order Newton method for reconstructing the shape of a arbitrary cylindrical perfectly electrical conducting (PEC) scatterer from the measured far-field pattern for scattering of time harmonic plane waves is presented the first time in this thesis. This method extends a hybrid between regularized Newton iterations and decomposition methods. The main idea of our iterative method is to use Huygen's principle, i.e., represent the scattered field as a single-layer potential. Given an approximation for the boundary of the scatterer, this leads to an ill-posed integral equation of the first kind that is solved via Tikhonov regularization. Then, in a second order Taylor expansion, the PEC boundary condition is employed to update the boundary approximation. In an iterative procedure, these two steps are alternated until some stopping criterium is satisfied. Main advantages of method is that method does not need forward solver in each iteration step and needs less iteration than first order Newton method in order to obtain desired accuracy. Although there are a few results available on the convergence of regularized Newton iterations for inverse obstacle scattering problems, this issue is not satisfactorily resolved. Despite the progress made with this espect, so far it has not been clarified whether the general results on the solution of ill-posed nonlinear equations in a Hilbert space setting are applicable to inverse obstacle scattering or, in general, to inverse boundary value problems in the frame work of solving the operator equation. This remark also implies to the convergence results of researches on the second order method with respect to its applicability for the inverse obstacle scattering problem. The more problem oriented approaches for a convergence analysis suffers from the restrictive assumption of a non vanishing normal derivative of the total field on the boundary in the case of exact data. Furthermore, in the analysis for noisy data, convergence for the noise level tending to zero, as usual, requires a stopping rule and with this particular rule the method has not yet been numerically implemented. These comments also apply to the case of the first order method of Kress and Serranho. At present only convergence results in the spirit of Potthast are available in literature. Therefore, we view it as legitimate to present our second order variant of this approach without a detailed convergence analysis and confine ourselves to some heuristic considerations. Of course, as in all of the iterative methods for the inverse obstacle scattering problem, the ill-posedness corrupts the high order convergence. Here, the ill-posedness enters through the integral equation of the first kind leading to inevitable errors occurring in its regularized solution. It is to be expected that a convergence analysis with respect to the noise level can be carried out analogous to the first order method. We refrain from working out the details since the result would be of a qualitative character only and would not lead to the possibility for a quantitative comparison on the convergence for the first and second order method. However, from the better convergence order for the exact data case one might expect some advantages in the numerical performance. Indeed, our numerical examples in the next section illustrate an improvement in the quality of the reconstructions and the speed of convergence connected with an increase in the stability with respect to noisy data. Proposed Newton reconstruction methods can be extended for reconstruction of perfectly electric conducting (PEC) objects located on known domain by using the fundamental solution of Helmholtz equation on known domain and its far field pattern and using the incident field as field in the absence of the PEC object. Fundamental solution of Helmoltz equation on known domain and Field in the absence of the PEC object are well known direct scattering problem. Proposed method can be easily extended for limited angular and near field measurements of scattered fields. Proposed method is described in detail and illustrated its feasibility through examples with exact and noisy data. Keywords: Newton method, inverse scattering, integral equations
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