Sharif Digital Repository

The powder metallurgy process is ideally suited to the mass production of structural parts of complicated shape and with tight tolerances without appreciable loss of material. Because of its unique attributes such as reducing weight and production cost, powder metallurgy has made very significant in roads as a substitution technology in automotive parts, such as cams and gears in this process, we can improve the mechanical properties such as fatigue resistance of PM materials by changing of manufacturing parameters, for example, density, sintering temperature, sintering time, etc. In this research, the fatigue behaviour of sintered steel (Fe - 1.5 Mo - 3 Mn - 0.7 C) was investigated with... 

Semi-active devices are low-cost as well as small and by changing the properties of low-power intensive structures, the aims of control are accomplished. On the other hand, the limited control force which can be applied to the structure for each damper causes more dampers to be used in the structures compared to the larger and stronger control devices which are more costly. These dampers coupled with sensors and the structure themselves make a complex dynamic system which is best controlled by a decentralized method such as Market-Based Control (MBC). In MBC, the actuators and the supply energy are modeled as buyer and seller respectively in the market place. To define the demand function of... 

In recent years, PM steel parts have been used due to materials saving and cost. Mechanical properties of PM parts made of Ultrapac-LA (Fe-.5Mo-1.5Cu-1. 75Ni) with 0.2 and 0.5% graphite addition have been studied. The chemical composition of Ultrapac-LA is similar to Distaloy AB. Specimens with dimensions of 89×6×5.5 mm were prepared. The green compacts were sintered at 1120°C in a 75 H2 -25 N2 atmosphere for 40 minutes to densities in the range of 6.7g/cm3 to 7 g/cm3. They were tested in static and cyclic loading, and their microstructures were studied by optical microscopy. Results show that porosity has a definite effect on fatigue resistance of this alloy. The formation of bainite and... 

PM steel parts have become more widely used in recent years by reason of materials and cost savings. This paper deals with the mechanical properties of PM parts made of ULTRAPAC-LA (Fe-0.5Mo-1.5Cu-1.75Ni) with 0.2 and 0.5% graphite addition. ULTRAPAC-LA having the same chemical composition as Distaloy AB was investigated. Specimens with dimensions of 89×6×5.5 mm3 were prepared. The green compacts were sintered at 1120°C in a 75 H2-25 N2 atmosphere for 40 minutes to densities in the range of 6.7g/cm3 to 7g/cm3. They were tested in static and cyclic loading, and their microstructures were studied by optical microscope. The results show that porosity has a definite effect on fatigue resistance... 

Developing a computational model for studying tribological performance is essential for computing accurate life cycle of various materials. Caused by the existence of complicated and nonlinear interactions between material surfaces, exact modeling of wear behavior is very difficult. Artificial intelligence (AI) can be used in distinguishing similar patterns in experimental data and predictive modeling of a certain material’s wear behavior. In this paper, artificial neural networks (ANNs) approach, adaptive neural-based fuzzy inference system (ANFIS) technique, and fuzzy clustering method (FCM) are used to develop a simple, accurate, and applicable model for predicting the wear behavior of... 

The influence of manufacturing parameters such as compacting pressure, heat treatment and microstructure on the static and dynamic properties of Fe-1.75 Ni-1.5 Cu-0.5 Mo-0.6 C was investigated. Wear and fatigue tests were carried out on reciprocate wearing and tension compression mode, respectively. The results show that delamination wear mechanism of sintered steel is similar to that of conventional wrought or cast materials. But, in sintered material, open porosity acts as sites of generation and collection of wear debris or formation of subsurface cracks. Wear rate is decreased and fatigue strength is increased by heat treatment. © 2002 Elsevier Science Ltd. All rights reserved  

User-generated data in TripAdvisor.com consists of considerable amount of useful information that can help managers to provide better services to their customers. This study aims to forecast a new user's rate to a hotel based on information of the hotel and user. To do so, all reviews on all hotels of Tehran on TripAdvisor.com as real data are selected and 8 different supervised machine learning models are applied to the data to select the best method including K-nearest neighbors (KNN), Naïve Bayes, decision tree, logistic regression, support vector machine, neural network, random forest, and gradient boosting. KNN algorithm which uses similarity and distance measures for classification is... 

A central problem in motor control is to understand how the many biomechanical degrees of freedom are coordinated to achieve a goal. A common assumption is that Central Nervous System (CNS) would minimize a performance index to achieve this goal which is called objective function. In this paper, two popular objective functions are utilized to design the optimal trajectory of trunk movements. A 3D computational method incorporated with 18 anatomically oriented muscles is used to simulate human trunk system. Inverse dynamics allows us to compute torque which is generated around Lumbosacral joint. This torque is divided among muscles by static stability-based optimization. Trunk movement from... 

A central problem in motor control is to understand how the many biomechanical degrees of freedom are coordinated to achieve a goal. A common assumption is that Central Nervous System (CNS) will plan tasks based on open-loop optimal control theory which simultaneously predicts state variables and motor commands based on a compound objective function. A 3D computational method incorporated with 18 anatomically oriented muscles is used to simulate human trunk system. Direct collocation method allows us to convert a constrained optimal control problem to a common nonlinear programming problem to assume the spinal stability condition. Trunk movement from the upright standing to 60 degrees of... 

In this study, microstructural aspects that control fracture and deformation resistance of P/M materials are evaluated. Several low-alloy steels were produced under both experimental and commercial conditions for achieving different matrix phases and porosity levels with varying shape factors. A 'porosity map' was constructed and used for a quantified study of the dominant mechanism controlling mechanical behaviors. The role of inclusion gathering and secondary pores, as well as sintering mechanism and alloying method is considered and discussed. The mechanism assessment was performed by using microstructure examination through optical and scanning electron microscopy. The results... 

Learning a foreign language encompasses a process of trial and error that can lead to the progression of the learner's interlanguage, meaning that the learners make use of their language knowledge that receives feedback and brings about progress through diagnosis of the problems and being corrected by the teacher. Therefore, there exists a need to analyze the underlying reasons causing the students to remain taciturn with a low willingness to communicate (WTC) during a language class. There has been much work done investigating the role of the learner's stable personal characteristics (traits) along with their situational changes at a specific time (states) that influence the student's... 

Accurate system identification (SI) is essential for flight controller design of unmanned aerial vehicles and microaerial vehicles (MAVs). SI can be performed either in frequency or time domain using flight test data. As flight testing is often costly and may even lead to catastrophic events before some flight stability analysis is at hand, a low-cost 5 degree-of-freedom free motion platform (FMP) is designed and used for initial identification of MAVs via wind tunnel testing. A typical MAV is constructed as a test case and equipped with inertial sensors for testing and validation purposes. The proposed FMP allows for both rotational as well as translational motions that in turn provide for... 

Using 3-D scanned data to analyze and extract pore network plays a vital role in investigation of porous media's characteristics. In this paper, a new simple method is developed to detect pores and throats for analyzing the connectivity and permeability of the network. This automated method utilizes some of the common and well-known image processing functions which are widely accessible by researchers and this has led to an easy algorithm implementation. In this method, after polishing and quality control of images, using city-block distance function and watershed segmentation algorithm, pores and throats are detected and 3-D network is produced. This method can also be applied on 2-D images... 

In this paper, we try to recognize and distinguish different groups of arrhythmia using novel features which have been obtained from the heart rate's phase space in recent years. For this purpose, we used Triangular Phase Space Mapping (TPSM) and Parabolic Phase Space Mapping (PPSM). For recognition, we used three groups of 15 subjects using the Physionet database (Arrhythmia, Congestive Heart Failure (CHF), and Atrial Fibrillation (AF)) with Normal Sinus Rhythm (NSR). The obtained features discriminate arrhythmia from NSR by p