Approximating the minimum hub cover problem on planar graphs

We study an approximation algorithm with a performance guarantee to solve a new NP-hard optimization problem on planar graphs. The problem, which is referred to as the minimum hub cover problem, has recently been introduced to the literature to improve query processing over large graph databases. Planar graphs also arise in various graph query processing applications, such as; biometric identification, image classification, object recognition, and so on. Our algorithm is based on a well-known graph decomposition technique that partitions the graph into a set of outerplanar graphs and provides an approximate solution with a proven performance ratio. We conduct a comprehensive computational experiment to investigate the empirical performance of the algorithm. Computational results demonstrate that the empirical performance of the algorithm surpasses its guaranteed performance. We also apply the same decomposition approach to develop a decomposition-based heuristic, which is much more efficient than the approximation algorithm in terms of computation time. Computational results also indicate that the efficacy of the decomposition-based heuristic in terms of solution quality is comparable to that of the approximation algorithm

Approximating the minimum hub cover problem on planar graphs

We study an approximation algorithm with a performance guarantee to solve a new NP-hard optimization problem on planar graphs. The problem, which is referred to as the minimum hub cover problem, has recently been introduced to the literature to improve query processing over large graph databases. Planar graphs also arise in various graph query processing applications, such as; biometric identification, image classification, object recognition, and so on. Our algorithm is based on a well-known graph decomposition technique that partitions the graph into a set of outerplanar graphs and provides an approximate solution with a proven performance ratio. We conduct a comprehensive computational experiment to investigate the empirical performance of the algorithm. Computational results demonstrate that the empirical performance of the algorithm surpasses its guaranteed performance. We also apply the same decomposition approach to develop a decomposition-based heuristic, which is much more efficient than the approximation algorithm in terms of computation time. Computational results also indicate that the efficacy of the decomposition-based heuristic in terms of solution quality is comparable to that of the approximation algorithm

Kaynaklı plakalarda kalıntı gerilimlerin ölçülmesi.

The purpose of this study is to estimate the residual stress state in steel weldments by using Magnetic Barkhausen Noise (MBN) technique. For obtaining accurate, fast and continuous residual stress measurements a set up for single pass welded plates was designed and used. In order to convert the MBN values to residual stress values a calibration set up was also designed and a procedure for obtaining calibration curves was developed. After welding of low-C steel plates, residual stresses on heat affected zone (HAZ) and parent metal were measured by MBN technique. The results were verified by the hole drilling method. Microstructural investigation and hardness measurements were also conducted.M.S. - Master of Scienc

Demiryolu raylarında Kalıntı Gerilimlerin Manyetik Barkhausen Gürültüsü Yöntemi ile Ölçülmesi

Ülkemizde son yıllarda demiryolu yatırımları oldukça artmış ve yeni hızlı tren hatlarının yapımına başlanmıştır. Hızlı tren hatlarında kullanılan rayların taban bölgesinde üretimden kaynaklı gerilimlerin ölçülmesi için tahribatlı testler uygulanmaktadır. Fakat raylarda kaynaklı imalat sonrasında veya kullanım sırasında da kalıntı gerilimler oluşmakta ve çekme kalıntı gerilimleri çatlak oluşumuna ve ilerlemesine neden olabilmektedir. Bu nedenle bu gerilimlerin dağılımının ve büyüklüğünün hızlı bir şekilde belirlenebilmesi beklenmedik kırıkların ve dolayısı ile kazaların önlenebilmesinde kritik bir öneme sahiptir. Bu çalışmanın amacı ray üzerindeki kalıntı gerilim dağılımının ölçülmesi için tahribatsız bir yöntem geliştirmektir

E-learning Options for NDT Training

Proper training is stated as a must for certification of NDT personnel in widely used certification standards. New technologies in internet and computer science bring new learning environments to each people regardless of their ages. Today even engineering degrees can be earned by online training. There are also different approaches and options for online NDT training and certification. NDT community should discuss the positive and negative aspects of online training. This paper discusses the effectiveness of the online learning options for training NDT personnel

Kaynak işlemi nedeniyle oluşan kalıntı gerilmelerin mikro-manyetik teknikle tahribatsız olarak ölçülmesi

Bütün imalat yöntemleri, parçalarda kalıntı gerilmeler oluşturmaktadır. Kalıntı gerilmeler, parçaya etkiyen dış gerilmelerle toplam bir etkiye sebep olduğundan, imalat aşamalarında veya kullanım sırasında çarpılma veya erken hasarlanma riski artmaktadır. Kalıntı gerilmeleri ölçmek için çeşitli tahribatlı ve tahribatsız teknikler geliştirilmiş, imalat işlemlerini modelleyerek kalıntı gerilme dağılımını tahmin eden ticari yazılımlar kullanıma alınmıştır. Son yıllarda Manyetik Barkhausen Gürültüsü metodu yardımıyla ferromanyetik malzemelerin yüzeyinde oluşan kalıntı gerilmelerin tahribatsız olarak ölçülmesi ilgi çekmektedir. Bu bildiride, kaynaklı birleştirme işlemi sonrasında çeliklerde meydana gelen kalıntı gerilmelerin Manyetik Barkhausen Gürültüsü yöntemi ile ölçülmesi üzerine literatürdeki mevcut çalışmalar özetlenmekte ve ODTÜ’de tamamlanan yüksek lisans ve doktora tezlerinde elde edilen sonuçlar örnek olarak sunulmaktadır

New possibilities on training of NDT personnel

Training is the most important step for qualification of certifying NDT personnel. In order to certify a person for any NDT method satisfactory training is a must in all standards. Due to increased utilization of computer and internet, educational principles have being started to change in the way of online-learning. It is an inevitable fact that NDT societies should adopt themselves in the progress of educational concepts. Since NDT training includes both theoretical and practical parts, a blended learning concept will be the solution for NDT training. © 2017 Slovenian Society for Non-Destructive Testing. All rights reserved

Non-destructive determination of residual stress state in steel weldments by Magnetic Barkhausen Noise technique

The purpose of this study is non-destructive determination of residual stresses in the welded steel plates by Magnetic Barkhausen Noise (MBN) technique. A MBN-stress calibration set-up and a residual stress measurement system with scanning ability were developed. To control the accuracy and the effectiveness of the developed system and procedure, various MBN measurements were carried out. The MBN results were verified by the hole-drilling method. Microstructural investigation and hardness measurements were also conducted. It was concluded that if the calibration procedure including the effect of microstructure is appropriate then MBN is a very promising method for non-destructive, fast and accurate prediction of residual stresses in the welded plates

PoD Analysis of Phased Array and Conventional Ultrasonic Techniques

Non-Destructive Testing applications have broad usage area from the medical to heavy industries. Several advanced methods, mainly developed from the conventional methods, are used to render the technique more flexible and easier to interpret. Ultrasonic Phased array technology is one of them which can be employed in almost any test, where conventional Ultrasonic Testing (UT) has traditionally been used. Weld inspection, flaw detection and the wall thickness determination in corrosion surveys are the most important applications. The benefits of Phased Array technology over conventional UT come from its ability to use multiple elements to steer, focus and scan beams with a single transducer assembly. In this study, the advantages and limitations of the Phased Array system were investigated via using special test blocks of St37 and SAE 4140 (42CrMo4) steels having well-defined artificial defects. The reliabilities of the Phased Array and conventional Ultrasonic Testing methods were compared using a statistical approach, the Probability of Detection