Facta Universitatis, Series: Mechanical Engineering

3-10

Authors: Roman Pohrt, Valentin L. Popov

Using the concept of stress intensity factors, we suggest a way to include adhesion into boundary elements simulation of contacts. A local criterion concerning the maximum admissible surface stresses decides whether the adhesive bonds in particular grid points fail or not. By taking into account the grid spacing, a robust methodology is found. Validation is done using the theoretically derived cases of JKR adhesion.

21-25

Authors: Mikhail Popov

Structural damping is discussed for the contact of two fibers in a woven material. In the presence of both normal and tangential oscillations, structural (relaxation) damping takes place even with perfect sticking in the contact, where slip-related frictional damping disappears. For the case of an infinite coefficient of friction and small amplitudes a closed-form solution for energy lost during one oscillation cycle is obtained.

33-38

Authors: Lars Voll

Adhesion between an elastomer and a steel indenter was studied experimentally and described with an analytical model. Cylindrical indenters having different roughness were brought into contact with an elastomer with various normal forces. After a “holding time”, the indenter was pulled with a constant velocity, which was the same in all experiments. We have studied the regime of relatively small initial normal loadings, large holding times and relatively large pulling velocities, so that the adhesive force did not depend on the holding time but did depend on the initially applied normal force and was approximately proportional to the pulling velocity. Under these conditions, we found that the adhesive force is inversely proportional to the roughness and proportional to the normal force. For the theoretical analysis, we used a previously published MDR-based model.

47-65

Authors: Rainer Heise

In this study, we elaborate on the friction between a one-dimensional elastomer and a one-dimensional rigid randomly rough surface. Special emphasis is laid on the energy dissipation in the elastomer. Its subsequent temperature change is under inspection. The elastomer is modeled as a spring and a damper in parallel (Kelvin model) in a one-dimensional substitute model according to the concept of the method of dimensionality reduction (MDR). The randomly rough surface is a self-affine one-dimensional fractal whose Hurst exponent H is varied in an extended range between -1 and 3. In full contact, the temperature shift is dominated by ratio between the typical power dissipated in the elastomer to the power that is led away. It is independent of the normal force and proportional to the sliding speed squared. The flash temperature behavior is discussed for different Hurst exponents.

195 - 207

Authors: Hajar Razi, Annette I. Birkhold, Manfred Zehn, Georg N. Duda, Bettina M. Willie, Sara Checa

Though bone is known to adapt to its mechanical challenges, the relationship between the local mechanical stimuli and the adaptive tissue response seems so far unclear. A major challenge appears to be a proper characterization of the local mechanical stimuli of the bones (e.g. strains). The finite element modeling is a powerful tool to characterize these mechanical stimuli not only on the bone surface but across the tissue. However, generating a predictive finite element model of biological tissue strains (e.g., physiological-like loading) encounters aspects that are inevitably unclear or vague and thus might significantly influence the predicted findings. We aimed at investigating the influence of variations in bone alignment, joint contact surfaces and displacement constraints on the predicted strains in an in vivo mouse tibial compression experiment. We found that the general strain state within the mouse tibia under compressive loading was not affected by these uncertain factors. However, strain magnitudes at various tibial regions were highly influenced by specific modeling assumptions. The displacement constraints to control the joint contact sites appeared to be the most influential factor on the predicted strains in the mouse tibia. Strains could vary up to 150% by modifying the displacement constraints. To a lesser degree, bone misalignment (from 0 to 20°) also resulted in a change of strain (+300 με = 40%). The definition of joint contact surfaces could lead to up to 6% variation. Our findings demonstrate the relevance of the specific boundary conditions in the in vivo mouse tibia loading experiment for the prediction of local mechanical strain values using finite element modeling.

223 - 234

Authors: Maria Kyrarini, Siniša Slavnić, Danijela Ristić-Durrant

In this paper, an inverse kinematics based control algorithm for the joystick control of the mobile platform of the novel mobile robot-assisted gait rehabilitation system CORBYS is presented. The mobile platform has four independently steered and driven wheels. Given the linear and angular velocities of the mobile platform, the inverse kinematics algorithm gives as its output the steering angle and the driving angular velocity of each of the four wheels. The paper is focused on the steering control of the platform for which a fuzzy logic controller is developed and implemented. The experimental results of the real-world steering of the platform are presented in the paper.

251 - 260

Authors: Atta Badii, Ali Khan, Rajkumar Raval, Hamid Oudi, Ricardo Ayora, Wasiq Khan, Amine Jaidi, Nagarajan Viswanathan

Awareness of emerging situations in a dynamic operational environment of a robotic assistive device is an essential capability of such a cognitive system, based on its effective and efficient assessment of the prevailing situation. This allows the system to interact with the environment in a sensible (semi)autonomous / pro-active manner without the need for frequent interventions from a supervisor. In this paper, we report a novel generic Situation Assessment Architecture for robotic systems directly assisting humans as developed in the CORBYS project. This paper presents the overall architecture for situation assessment and its application in proof-of-concept Demonstrators as developed and validated within the CORBYS project. These include a robotic human follower and a mobile gait rehabilitation robotic system. We present an overview of the structure and functionality of the Situation Assessment Architecture for robotic systems with results and observations as collected from initial validation on the two CORBYS Demonstrators.

277 - 288

Authors: Dhanushkodi Saravanan, Vincent H. Wilson, Sudhakar Kumarasamy

Drying of Cashew nut to remove testa is one of the most energy-intensive processes of cashew nut process industry. For this reason a hybrid dryer consisting of a solar flat plate collector, a biomass heater and a drying chamber is designed and fabricated. 40 kg of Cashew nut with initial moisture of 9 % is used in the experiment. The performance test of the dryer is carried out in two modes of operation: hybrid-forced convection and hybrid-natural convection. Drying time and drying efficiency during these two modes of operation are estimated and compared with the sun drying. The system is capable of attaining drying temperature between 50º and 70ºC. In the hybrid forced drying, the required moisture content of 3% is achieved within 7 hours and the average system efficiency is estimated as 5.08%. In the hybrid natural drying, the required moisture content is obtained in 9 hours and the average system efficiency is 3.17%. The fuel consumption during the drying process is 0.5 kg/hr and 0.75 kg/hr for forced mode and natural mode, respectively. The drying process in the hybrid forced mode of operation is twice faster than the sun drying. The dryer can be operated in any climatic conditions: as a solar dryer on normal sunny days, as a biomass dryer at night time and as a hybrid dryer on cloudy days. Based on the experimental study, it is concluded that the developed hybrid dryer is suitable for small scale cashew nut farmers in rural areas of developing countries.

305 - 323

Authors: Jimit R. Patel, Gunamani Deheri

Efforts have been made to analyze the Shliomis model based ferrofluid lubrication of a squeeze film between rotating rough curved circular plates where the upper plate has a porous facing. Different models of porosity are treated. The stochastic modeling of Christensen and Tonder has been employed to evaluate the effect of surface roughness. The related stochastically averaged Reynolds type equation is numerically solved to obtain the pressure distribution, leading to the calculation of load carrying capacity. The results presented in graphical form establish that the Kozeny-Carman model is more favorable as compared to the Irmay one from the design point of view. It is observed that the Shliomis model based ferrofluid lubrication performs relatively better than the Neuringer-Rosensweig one. Although the bearing suffers due to transverse surface roughness, with a suitable choice of curvature parameters and rotational ratio, the negative effect of porosity and standard deviation can be minimized by the ferrofluid lubrication at least in the case of negatively skewed roughness.

95-106

Authors: Gennadi I. Mikhasev, Ihnat R. Mlechka

Buckling of a thin cylindrical sandwich shell composed of elastic isotropic layers with different elastic properties under normal external pressure is the subject of this investigation. Differential equations based on the assumptions of the generalized kinematic hypothesis for the whole sandwich are used as the governing ones. Two variants of the joint support conditions are considered at the shell edges: a) there are the infinite rigidity diaphragms inhibiting relative shears of layers along the shell edges, b) the diaphragms are absent. Using the asymptotic approach, the critical pressure and buckling modes are constructed in the form of the superposition of functions corresponding to the main stress-strain state and the edges integrals. As an example, a three-layered cylinder with the magnetorheological elastomer (MRE) embedded between elastic layers under different levels of magnetic field is studied. Physical properties of the magnetorheological (MR) layer are assumed to be functions of the magnetic field induction. Dependencies of the buckling pressure on the variant of boundary conditions and the intensity of applied magnetic field are analyzed.

123 - 136

Authors: Boban R. Anđelković, Biljana R. Đorđević, Nataša R. Jovanović

In this paper a contactless method for the detection of an object’s defect by applying the thermovision camera is proposed and implemented. The thermal image processing is realised in two perpendicular directions and the analysis of the results is presented. An advantage of the proposed method is a simple and efficient application of the standard techniques, which are not specially designed for thermal image processing. By comparing the analysis of results obtained by this method with those obtained by already known methods, the applicability of this method could be confirmed. Using the powerful software packages and applications the thermovision images are being transformed into digital records that can be processed on personal computers. The conducted experiments show that this contactless method is very efficient in the detection of the defects in forms of cavities and damages caused by overload or by material fatigue.

149 - 155

Authors: Žarko Ćojbašić, Vlastimir Nikolić, Emina Petrović, Vukašin Pavlović, Miša Tomić, Ivan Pavlović, Ivan Ćirić

In recent years, the finite element method has been widely used as a powerful tool in the analysis of engineering problems. In the simulation of deformable objects using the finite element method, a complex system of nodes which make a mesh grid is used. The FEM model includes material and structural properties, which altogether determine the model’s response to certain loading conditions. A reliable simulation is supposed to provide for an easier, faster and less expensive development of structures. The real-time simulation of shell-type deformable objects using the finite element method for a non-linear analysis is a challenging task because of the need for fast systems that do not demand high computational cost. In this paper, we present an efficient method based on neural networks for simulating the real-time behavior of a thin walled structure modeled by the finite element method in the commercial FE software. Using the finite element method, the structures displacements are computed offline, by applying forces in the specified range. In the online application mode, a trained neural network is used for obtaining required results for specified loads.

171 - 181

Authors: Miloš Milovančević, Aleksandar Veg, Aleksandar Makedonski, Jelena Stefanović Marinović

The purpose of the research presented in this paper is the development of the optimal micro configuration for vibration monitoring of pumping aggregate, based on Microchip’s microcontroller (MC). Hardware used is 10-bit MC, upgraded with 12/bit A/D converter. Software for acquisition and data analysis is optimized for testing turbo pumps with rotation speed up to 2000 rpm. This software limitation is set for automatic diagnostics and for individual and manual vibro-diagnostic; the only limitation is set by accelerometer performance. The authors have performed numerous measurements on a wide range of turbo aggregates for establishing the operational condition of pumping aggregates.

1-14

Authors: Valentin L. Popov, Markus Hess

The Method of Dimensionality Reduction (MDR) is a method of calculation and simulation of contacts of elastic and viscoelastic bodies. It consists essentially of two simple steps: (a) substitution of the three-dimensional continuum by a uniquely defined one-dimensional linearly elastic or viscoelastic foundation (Winkler foundation) and (b) transformation of the three-dimensional profile of the contacting bodies by means of the MDR-transformation. As soon as these two steps are completed, the contact problem can be considered to be solved. For axial symmetric contacts, only a small calculation by hand is required which does not exceed elementary calculus and will not be a barrier for any practically-oriented engineer. Alternatively, the MDR can be implemented numerically, which is almost trivial due to the independence of the foundation elements. In spite of their simplicity, all the results are exact. The present paper is a short practical guide to the MDR.

27-36

Authors: Miloš Madić, Miroslav Radovanović

Companies operating in today's machining environment are focused on improving their product quality and decreasing manufacturing cost and time. In their attempts to meet these objectives, the machining processes optimization is of prime importance. Among the traditional optimization methods, in recent years, modern meta-heuristic algorithms are being increasingly applied to solving machining optimization problems. Regardless of numerous capabilities of the Monte Carlo method, its application for solving machining optimization problems has been given less attention by researchers and practitioners. The aim of this paper is to investigate the Monte Carlo method applicability for solving single-objective machining optimization problems and to analyze its efficiency by comparing the optimization solutions to those obtained by the past researchers using meta-heuristic algorithms. For this purpose, five machining optimization case studies taken from the literature are considered and discussed.

51-60

Authors: Florian Ion Petrescu, Relly Victoria Petrescu

The paper presents an original method for determining gear efficiency, gearing forces, velocities and powers. It analyzes the way in which certain parameters affect gear efficiency. Furthermore, an original method for determining geared transmissions efficiency as a function of the contact ratio is concisely presented. With the presented relations, one can make a dynamic synthesis of geared transmissions with the aim of increasing gearing mechanisms efficiency.

73-84

Authors: Jasmina Bogdanović-Jovanović, Božidar Bogdanović, Ivan Božić

Regarding the present state of knowledge in the field of the turbomachinery design, the method for designing small bulb turbines with unequal specific work distribution of the turbine runner's elementary stages near the hub is presented in the paper. The distribution function of specific work of all the elementary stages is obtained, according to which the averaged axisymmetric flow surfaces of the turbine runner have a negligibly small deviation from the cylindrical flow surfaces. The specific work of the near-the-hub elementary stages, in the given distribution function, can be reduced up to 60% of the required (design) specific work, still achieving nearly cylindrical flow surfaces.

123-131

Authors: Qiang Li, Roman Pohrt

A new model of mixed lubrication is proposed in the frame of the Method of Dimensionality Reduction (MDR). In this model the dynamic lubricated rolling contact between rough surfaces is simulated based on the results from elastohydrodynamic lubrication (EHL). In order to account for the break-up of the additional boundary layer on a local micro contact area, a supplemental criterion is imposed. For comparison, a twin-disc test rig is set up to measure the electrical resistance between two lubricated rolling surfaces under different normal forces, rotation speeds and temperatures. We investigated the probability of boundary layer breakthrough for both experiment and simulation and found good agreement.

141-151

Authors: Dragan Jovanović

Results of reconstruction of potential strain energy surfaces at the region of crack and basic procedures of the reconstruction method are presented. Pertinent limitations and specifications of the method are pointed out in evaluating the best fitting curves and the best fitting three-dimensional surfaces of stress components and potential strain energy. Examples of cracks named in the paper as PR-Z-16 and PR-P-22, with exactly the same geometry of crack and plate are analyzed. The same finite element lattice for calculation in both cases is selected. The first example of a plate is loaded with in y - direction on the outer border of the plate, perpendicularly on the plane of crack, while in the example PR-P-22 the plate is loaded with continually and perpendicularly on the crack contour surface. The reconstruction procedure of potential strain energy surfaces for two sections of plates y=0 and z=0, and graphical presentation of these surfaces, are carried out. Based on reconstructed potential strain energy surfaces, isoenergy lines at the region in front of the crack tip for both analyzed examples, are obtained. It is observed that the distribution of stresses as well as the distribution of potential strain energy are very similar in both cases, which indicates a significant influence of geometric shape of contour surfaces of the crack. For comparison, stress state and potential strain energy state, which are obtained by using relations from literature, are shown.

169-180

Authors: Mića V. Vukić, Goran Vučković, Predrag Živković, Žarko Stevanović, Mladen Tomić

A shell and tube heat exchanger (STHE) is one of the most often used apparatuses in chemical industry. One of the main goals of the STHE manufacturers is to improve their exploitation reliability and efficiency. Two approaches to the STHE design improvement are possible: experimental investigation, which is very expensive and time-consuming because of the shell side complex geometry, and numerical investigations. Numerical simulations can be used to check the old and to develop new and more efficient STHE designs. In this paper, the results of the numerical investigations of fluid flow and heat transfer in the laboratory experimental STHE are presented. Numerical simulation has been performed by using the PHOENICS code. The tube bundle has been modeled by using the concept of porous media. Standard k-e turbulence model is used.

193-202

Authors: Predrag Milić, Dragan Marinković

Finite element method (FEM) is a tool that is mostly used for the structural analysis. The method is based upon approximation of the actual geometry and displacement field of the structure over the finite element domain. In practice, Lagrange polynomials are mostly used as shape functions (approximate functions). They provide only the C0-continuity at the element boundaries. This paper elucidates a possible approach based on the so-called isogeometric analysis to improve this property. The isogeometric FEM analysis applies the same functions to describe the CAD model, the element geometry and the element displacements. The paper represents developed algorithm and the obtained results for a relatively simple test case.