1,721,016 research outputs found
Enhanced brush model for the mechanics of power transmission in flat belt drives under steady–state conditions: Effect of belt elasticity
No full textThe present paper is an extension of a previously published paper by the authors, where the “brush” model was adopted for the contact stresses between the belt and pulleys. In this paper, the axial stiffness of the belt is introduced, while in previous work, particularly suited for belt with stiff reinforcement fibers, the belt was assumed to be inextensible. The complete set of equations is derived in steady state conditions and the relationship between belt tension and belt speed is introduced based on the continuity condition. The belt tension can be obtained by solving a second order differential equation, for which a closed form solution is given. A numerical procedure is, however, necessary for determining the solution of a given transmission with assigned rotational speed at the driving pulley and resistant moment at the driven pulley. It is shown how the contact angle at which stick–slip phenomenon may occur is influenced by the belt stiffness and the way the transmission effiency is reduced. Allowing to analyze the mechanics of flat belt transmission, the model can be considered a useful tool for the designer
“Brush model” for the analysis of flat belt transmissions in steady-state conditions
Full text linkIn the present work a novel mathematical model for the analysis of the contact actions between belt and pulleys, particularly suited for flat reinforced rubber belt, is presented. The model considers the tension member, composed of the reinforcement fibers, inextensible, and the rubber matrix, which is subjected to tangential stress, as a continuum bed of elastically deformable bristles, fixed to the tension member on one side and in contact with the pulley on the other side. The deformation of the matrix is inversely proportional to the bending stiffness of the bristles, while friction conditions determine the local adhesion/sliding behavior between belt and pulleys. The proposed model can give a detailed description of the contact conditions along the whole contact arc and is able to describe the stick–slip phenomenon which has been experimentally observed by some authors. The model assesses also the power losses due to the contact stresses and to the elastic deformation of the matrix. The results of the model are discussed in comparison with results from classical models, Grashof and Firbank models, available in the technical literature
Analysis of belt transmissions capabilities using the brush model
Full text linkThe mechanics of power transmission is usually modeled by two different theories: the creep theory and the shear theory. Recently, the authors introduced an alternative theory based on the brush model, which allows to compute the tangential stress distribution along the winding arc of pulleys. The brush model is able to predict the speed loss along the driving and driven pulley as a function of the transmission parameters (e.g. pre-load, friction, pulley radii etc.) and the operating parameters (i.e. angular speed and resistant torque). In addition, the energy efficiency of the system is obtained by knowing the speed loss and the energy dissipation; this contribution can be subdivided into energy loss due to friction and energy loss due to the non-recoverable elastic deformation of the bristle.
In the present paper, using the previously developed model, a sensitivity analysis aimed at mapping the transmission capabilities as a function of geometry and operating parameters is proposed. These results, given as look-up table (or contour plot), are very important in mechanical systems simulation (e.g. real-time systems, hardware in the loop systems) since they allow to introduce the phenomenological behavior of the pulley-belt transmission without introducing complex models in the simulation
Validation of the brush model for the analysis of flat belt transmissions in steady-state conditions by finite element simulation
No full textIn this paper a finite element (FE) model for the analysis of the contact stresses in flat belt transmissions was developed, with the intent of comparing the numerical with the theoretical results of the brush model and those of the classical Euler–Grashof (creep) model. The FE model consists of two pulleys and a belt composed of a thin layer of inextensible reinforcement fibers and a rubber matrix in contact with the pulley. The analysis is performed incrementally, under quasi-static conditions; as a consequence, any inertia effect is not accounted for. In the paper, the capabilities of the analyzed models are discussed. The brush model is generally better correlated with the FE results, both in terms of tangential stress along the winding arc and belt tension and it is capable of estimating the power losses due to friction with low computational and time effort. In addition, the effect of the belt thickness on the tangential stress at the entrance and the exit from the pulley, which are generally neglected by simplified model, are highlighted by the FE analysis
Progettazione di prove sperimentali per la calibrazione del modello di rottura di Johnson-Cook
Full text linkResidual stresses influence on the fatigue strength of structural components
Full text linkSeveral production processes, both conventional and innovative, may result in residual stresses arising in critical areas of a component. The main issues include high distortion, reduced fatigue life, fracturing or delamination. In this context, standard fatigue design codes traditionally consider residual stresses through conservative assumptions, leading to either sub-optimal design or unexpected failures. Recently, innovative computational techniques have been developed to address residual stresses in a more comprehensive way. As a result, a more effective material utilisation and a more accurate fatigue life assessment can be achieved. The present work examines the influence of residual stresses on the fatigue endurance of S355JR structural steel components. Both welded and notched components were analysed, carrying out numerical and experimental analyses. In the case of welded components, residual stresses resulting from the welding process were numerically evaluated by means of an uncoupled thermal-structural simulation, while for notched specimens a preload causing limited yielding was used to induce a local residual stress field comparable to that obtained for welded specimens nearby the critical locations. Even if he work is still in progress, tests carried out with different specimens under different loading conditions allowed to understand the effect of residual stresses on the fatigue life
Experimental Bench for the Analysis of Belt Deformation in Belt–Pulley Systems by Digital Image Correlation
No full textBelt–pulley transmissions are a classical topic in mechanical engineering, usually studied following two approaches: the creep theory (Euler or Grashof model) and the shear theory. Recently, the authors introduced a new theory to study the belt–pulley contact mechanics, which is inspired to the brush model used for pneumatic tires. Basing on this theory, the belt is considered as an almost axially rigid tension member connected to a series of bristles, which are, at the other end, in contact with the pulley. In this paper, a test bench is presented and designed to experimentally validate the brush model. The bench is made up of two pulleys connected to two shafts driven by independently controlled motors; a belt is installed between the pulleys, and the shafts are equipped with sensors measuring the angular velocity and the transmitted torque. The belt preload, which is measured by a load cell, can be varied by changing the distance between the two shafts. The belt was painted creating a suitable texture (random speckle pattern) to be interpreted using the Digital Image Correlation (DIC) technique. The first results obtained by carrying out tests at low speed with different transmitted torque values are discussed, appreciating the variation in the tension of the belt along the winding arc and the dependence of the radial compression of the belt from the transmitted torque. The tangential deformation of the belt under the action of different torque values and direction of rotation of the pulleys is also presented, which is consistent with that foreseen by the brush model
An efficient algorithm for critical plane factors evaluation
Full text linkFatigue of structural components is a widely discussed subject on which extensive research is still being carried out, both in the scientific and industrial communities. Fatigue damage still represents a major issue for both metallic and non-metallic components, sometimes leading to unforeseen failures for in-service parts. Among all the assessment methodologies, critical plane methods gained a lot of relevance, as they allow the identification of the component’s critical location and the direction of early crack propagation. However, the standard method employed for calculating critical plane factors is very time-consuming as it makes use of nested for/end loops and, for that reason, it is usually applied in a research context, or when the critical areas of the component are known. Very often, however, the critical regions cannot be identified, due to complex geometries, loads or constraints, or the fatigue assessment has to be carried out with tight time scheduling, which is typical of the industry. In this work, an efficient algorithm for calculating critical plane factors, useful to speed up the fatigue assessment process, is presented. The algorithm applies to all critical plane factors that require the maximization of a specific parameter based on stress and strain components or a combination of them. The methodology maximizes the parameter utilizing tensor invariants and coordinates transformation law. In order to validate the proposed methodology, without loosing generality, the Fatemi-Socie critical plane factor was considered. The new algorithm was tested on different geometries (i.e. hourglass, notched and welded joint geometries) under different loading conditions (i.e. proportional/non-proportional, uniaxial and multiaxial loading) and showed a significant reduction in computation time respect the standard plane scanning method, without any loss of solution accuracy
Influence of the stress history and of the Lode angle on the determination of the ductile fracture locus for two steel alloys
No full textThe fracture locus of ductile materials is primarily related to the stress triaxiality. Recently, the Lode angle was also found to be relevant in the fracture locus determination. In order to characterize a material in terms of fracture locus, usually several tests, spanning a wide range of stress triaxiality and Lode angle values, have to be performed using different specimen geometries, obtained both from round bars and sheets. In this paper the fracture loci of two steel alloys (S500MC and 22MnB4) are obtained using a simplified procedure, based on the Bai–Wierzbicki model. The procedure considers, for experimental tests, only flat specimens, avoiding the use of round specimen of the same material which were not available. Experimental tests were performed on differently shaped specimens and the results were used to train finite element (FE) models. The triaxiality and Lode angle histories, computed for each test by the FE model, were used to obtain the fracture loci following two different approaches: the proportional loading approach and the non-proportional loading approach. The former does not consider how the stress triaxiality and Lode angle vary during the test, while the latter considers their history in the computation of the fracture locus. The results show how the Lode angle influences the fracture locus, especially for 22MnB4, and how non-proportional loading approach is more accurate to compute the damage for the different specimen geometries
How many critical planes? A perspective insight into structural integrity
No full textThe topic of material fatigue is a subject extensively investigated within both scientific and industrial worlds. Fatigue-induced damage remains a critical concern for a variety of components, encompassing both metallic and non-metallic materials, often leading to unexpected failures during their operational lifecycle. In cases necessitating the assessment of multiaxial fatigue, critical plane methodologies have emerged as a valuable approach. These methodologies offer the means to pinpoint the component's critical regions and anticipate early-stage crack propagation. Nevertheless, the conventional technique (i.e., plane scanning method) for computing critical plane factors is a time-intensive process, reliant on nested iterations, predominantly suited for research purposes. In numerous cases, where the critical area within a component is unknown in advance (i.e., primarily due to complex geometries and loading conditions) the method proves impractical. Furthermore, the plane scanning method does not provide a deep comprehension of the critical plane concept; indeed, it is just a numerical artifice for calculating stress and strain quantities on different planes. Recently, the authors introduced an efficient algorithm for evaluating critical plane factors. This algorithm is based on a closed form solution and is applicable to all instances where the maximization of a specific parameter, based on stress or strain components, is required. The methodology relies on tensor invariants and coordinates transformation principles thus enhancing the investigation of various critical plane methods. The paper addresses two formulations of the Fatemi-Socie critical plane factor and discusses how the number of critical planes depend on the loading conditions the component is subjected to. By the use of a closed form solution a deep insight of critical planes orientation can be achieved
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