Asymmetrical roll bending process study : dynamic finite element modeling and experiments

Roll bending is an efficient metal forming technique, where plates are bent to a desired curvature using forming rolls. This type of sheet forming process is one of the most widely used techniques for manufacturing axisymmetric hollow shapes. Moreover, this process is beginning to be taken into serious consideration by industries for producing large, thick parts such as the conically shaped crown of a Francis turbine runner or of a wind turbine tower. Because of the numerous processing parameters, reducing the bending force and improving the accuracy of the final shape are significant challenges in the roll bending process. Therefore, the primary aim of this research is to find the strategies for reducing forming forces and improving final part quality by employing numerical and experimental methods. In this thesis, a 3D dynamic Finite Element (FE) model of an asymmetrical roll bending process is developed using the Ansys/LS-Dyna software. The simulation results are then compared with experiments performed with instrumented parts and roll bending machine. The parameters that affect the accuracy of the final shape, the bending forces and the residual strain left in the formed plate have been investigated. Applying this 3D dynamic FE model in an industrial context may predict the forming forces or the accuracy of the final shape’s radius and thus will decrease the setup time before manufacturing. The forming forces can be reduced by heating the plate. In this research, the relationships between the heating plate temperature and the output parameters of roll bending process such as applied forces and final shape quality have been studied by performing FE simulation and analytical computations. These results yield to a better understanding of the mechanism of the process and provide an opportunity for the design of an efficient heating system to control the heat energy to be input in the plate during the roll bending process. This research also proposes a new, simple approach for reducing flat areas and forming forces. This approach includes moving the bottom roll slightly along the feeding direction and adjusting the bottom roll location. The FE results indicate that this new approach effectively minimizes the flat area extents and reduces also the forming forces

Finite element modeling and simulation of roll bending process for forming a thick conical hollow shape

The objective of the project presented in this thesis is to model and simulate the dynamic roll-bending process for forming a thick plate to a conical hollow shape. The project is realized using the Finite Element Method (FEM), which is integrated into the ANSYS/LS-DYNA software. In this study, the continuous three-roll pyramidal configuration is applied to form a thick plate to a conical hollow shape. The final goal is to provide a simulation tool that allows the optimization of the parameters of the process, and to demonstrate the feasibility of fulfilling the conical geometry adapted to the manufacture of Francis turbine crowns. The kinematic and geometrie relationships of the process are developed first: they allow sliding to be kept to a minimum between the workpiece and the rolls using conical rolls. Detailed steps for modeling the Finite Elements (FM), including the element type, the meshing, the contact surfaces, the boundary conditions and the constraints, are then presented. This 3D simulation is based on the explicit finite element method, which is able to deal with nonlinear elasto-plastic deformation behavior. Next, a typical example is presented to demonstrate the feasibility of modeling the process using this numerical tool. The results of the dynamic roll-bending process, going from the initial plate plane to its final conical form are examined, that is, the progressive change in the geometry along with the evolution of the residual stress distribution in the workpiece. A convergence test, which varies the meshing size, is also carried out in order to ensure that the proper element sizes are chosen. Furthermore, the reaction forces and the residual stress obtained through the numerical simulation are compared with the results of a simplified analytic calculation. To compare the geometric precision of the shape generated with that of that specified, a dimensional verification algorithm was developed. The algorithm, which is based on the Levenberg-Marquardt nonlinear optimization method, allows the smoothing of the coordinates of the nodes of the final shape obtained, based on a conical surface. Next, the deviations are calculated in order to analyze the effects of the variables of the process and of the simulation parameters on the final forming results. These parameters include the friction coefficient, the plate thickness, the materials properties, the spatial arrangement of the outer rolls and the plate temperature. This sensitivity study evaluates the influence of the parameter on the reaction forces, the residual stresses and the geometric compliance of the final formed piece

A contribution to the investigation of the rolling movement of mobile robots based on tensegrity structures

Die vorliegende Arbeit hat ihren Ursprung in den Algorithmen und Ansätzen zur Analyse eines Roboters mit variabler Geometrie, der an der TU Ilmenau entwickelt wurde. Ein solcher Roboter hat einen anfangs zylindrischen Roboter in eine kegelstumpfförmige Geometrie umgewandelt, um um die vertikale Achse drehen zu können, die sich am Erzeugungspunkt eines solchen Kegels befindet. Die vorliegende Arbeit beginnt mit einer Einführung der Grundlagen für das Studium von Tensegrity- Strukturen; Später werden aus den variablen Parametern der geometrischen Beschreibung des Roboters die kinematischen Gleichungen von Position, Geschwindigkeit und Beschleunigung für jeden Punkt der Geometrie des Roboters ausgewertet. In demselben Kapitel wird eine Alternative zum Vorhersagen der Bahn des Roboters unter Verwendung einer Klothoide vorgeschlagen. Diese Kurve hat den Zweck, Gleichungen der Bewegungsbahn des geometrischen Mittelpunkts des Roboters zu geben. In einem folgenden Kapitel werden die Steuerungsalgorithmen für die Abrollbewegung des Roboters sowie eine mögliche konstruktive Form vorgestellt, die zuvor für eine ähnliche Anwendung verwendet wurde. Im letzten Kapitel werden verschiedene Alternative von Materialien für die Herstellung des Roboterkörpers sowie die Grundlagen eines Ansatzes zur Bewertung gekrümmter Zugstrukturen vorgestellt. Darüber hinaus wird eine Berechnungsform für die spätere Entwicklung der endgültigen Geometrie des Roboters vorgeschlagen.The present work has its origin in the algorithms and approaches for the analysis of a robot of variable geometry developed in the TU Ilmenau. Such robot hat an initially cylindrical robot changes to a truncated conical geometry, in order to be able to rotate around the vertical axis located at the generating point of such a cone. The present thesis begins with a basic explanation of the foundations for the study of tensegrity structures; later, from the variable parameters of the geometric description of the robot, the kinematic equations of position, velocity and acceleration are evaluated for every point of the geometry of the robot. In the same chapter an alternative of predicting the path of the robot using a clothoid curve is proposed, this curve has the purpose of giving equations of the trajectory of the geometric center of the robot. In a following chapter, the control algorithms for the rolling movement of the robot are presented, as well as a possible constructive form previously used for a similar application. Finally, the last chapter presents alternative materials for manufacturing the body of the robot, as well as the bases of an approach for the evaluation of curved tensile structures; additionally, it proposes a form of calculation for later development of the final geometry of the robot.Tesi

Multi-body dynamics: historical evolution and application

The historical developments in the discipline of engineering dynamics are briefly reviewed, with attention paid to the formulation and solution of the dynamic behaviour of multi-body systems. It is shown that the dynamic characteristics of practical multi-body systems are dependent upon the interactions of many physical phenomena that can induce, restrain or constrain motion of parts. The long process of understanding and formulating the physics of multi-body motions, in some cases with pioneering contributions centuries old, together with continual refinements in numerical techniques and enhanced computing power has resulted in the solution of quite complex and practical engineering problems. Linking the historical developments to the fundamental physical phenomena and their interactions, the paper presents solutions to two complex multi-body dynamic problems. The practical implications of the approach in design of these systems are highlighted

A review of process advancement of novel metal spinning

Metal spinning technology has seen a rapid development in recent years. Novel spinning processes, such as non-axisymmetrical spinning, non-circular cross-section spinning and tooth-shaped spinning, are being developed. This has challenged the limitation of traditional spinning technology being used for manufacturing axisymmetrical, circular cross-section, and uniform wall-thickness parts. In this paper, the classification of the traditional spinning processes is proposed based on the material deformation characteristics, the relative position between roller and blank, mandrel spinning and mandrel-free spinning, and temperature of the blank during spinning. The advancement of recently developed novel spinning processes and corresponding tool design and equipment development are reviewed. The classification of the novel spinning processes is proposed based on the relative position between the rotating axes, the geometry of cross-section and the variation of wall-thickness of the spun parts. The material deformation mechanism, processing failures and spun part defects of the aforementioned three groups of novel spinning processes are discussed by analyzing four representative spinning processes of industrial applications. Furthermore, other novel spinning processes and their classification as reported in the literature are summarized

Three-Dimensional Finite Element Analysis of Conventional and Ultrasonic Vibration Assisted Micro-Drilling on PCB

Recent advancement in society’s demands has forced industries to produce more and more precise micro parts. With an advancement in engineering sciences, current manufacturers in various fields such as aerospace, medical, electronics, automobile, biotechnology, etc. have achieved the potential to fabricate miniaturized products, but with numerous technical challenges. Dimensional accuracy and surface integrity of the machined components are the key challenges and at the same time, cost minimization is strongly desired. To meet these challenges and demands, improvements in machining regarding new procedures, tooling, tool materials and modern machine tools are highly essential. Micromachining has shown potential to achieve the fast-growing needs of the present micro manufacturing sector. Additionally, new machining techniques like ultrasonic machining, laser drilling, etc. have been developed as an alternative source to reduce obstructions caused during macro/micro machining. The present research aims to perform three-dimensional (3D) finite element dynamic analysis for micro-drilling of multi-layer printed circuit boards (PCBs). Both conventional and ultrasonic vibration assisted micro-drilling (UVAMD) FE simulations have been compared to predict and evaluate the effect of process parameters on the output responses like stress generation and reaction forces and burr formation on the workpiece surfaces. The Lagrangian based approach is followed for the FE simulation including the mass and inertial properties of the proposed FE model. The predicted FE results are compared with the past experimental work for thrust force evaluation and burr formation on workpiece surfaces. The present work is supported with modal and harmonic analysis of stepped and conical horns along with micro drill bit. Here, horns made up of Aluminum 6061-T6, Titanium and Mild steel are chosen with micro drill bit of 0.3 mm diameter with varying tool materials (Tungsten carbide and High speed steel). The effects of natural frequencies with different mode shapes within the range of 15-30 kHz are shown. The frequency responses of micro drill with displacement conditions have been presented for longitudinal modes. The present simulation results will be helpful to conduct proper experimentation in order to achieve efficient machining and surface finish. The results enumerate that the drilling parameters have a strong influence on thrust forces and stresses occurring in micro-drilling. Ultrasonic assisted micro-drilling has a good potential in reduction of forces generated by vii selecting proper machining parameters. The FE simulation of UVA micro machining can further be enhanced and extended to various materials like plastics, sheet metal, other PCBs, etc. to predict the performance with varying machining and geometrical parameters

Tube and Sheet Metal Forming Processes and Applications

At present, the manufacturing industry is focused on the production of lighter weight components with better mechanical properties and always fulfilling all the environmental requirements. These challenges have caused a need for developing manufacturing processes in general, including obviously those devoted in particular to the development of thin-walled metallic shapes, as is the case with tubular and sheet metal parts and devices.This Special Issue is thus devoted to research in the fields of sheet metal forming and tube forming, and their applications, including both experimental and numerical approaches and using a variety of scientific and technological tools, such as forming limit diagrams (FLDs), analysis on formability and failure, strain analysis based on circle grids or digital image correlation (DIC), and finite element analysis (FEA), among others.In this context, we are pleased to present this Special Issue dealing with recent studies in the field of tube and sheet metal forming processes and their main applications within different high-tech industries, such as the aerospace, automotive, or medical sectors, among others

Advanced damage modelling of free machining steels

The current available damage models do not accurately predict effective plastic strain to failure in low triaxiality stress states. A damage model was developed for low triaxiality that is appropriate to hot rolling of steel. This work focuses on nucleation and growth of damage as well as the effect of the strain and stress path. The latter is especially important for the rolling of bar and other complex cross-section products. A study of damage mechanisms and methods to model them has been undertaken. It is pointed out that the many models are only useful under certain conditions but can be used when the expected damage mechanisms are active. Several test types were evaluated to assess their ability to simulate stress state in rolling. A program has been written to evaluate the stress state for plane and axisymmetric tests, which allows one to choose the most appropriate test-piece geometry. A test has been designed and implemented. Thermal and mechanical data was gathered, which has been used to relate the stress triaxiality to damage growth and identify appropriate damage growth models. The size and spacing distributions of inclusions in free cutting steels have been measured. The different distributions have an effect on the ductility of the different steels. This effect has been found to change at different strain rates and temperatures. By better accounting for the effect of inclusions on damage growth under a range of test conditions, the damage model can be significantly improved. Free cutting steels that contained different additions of heavy metals were tested. The ductility and damage mechanisms were compared in each of the steels. The effect of the precipitation of the different heavy metals at the inclusion to matrix boundary was highlighted. The same damage mechanisms were observed in each steel but the ability to accommodate damage varied between the steels. Ex-situ synchrotron x-ray micro-tomography was used to better measure and quantify the distribution of inclusions and damage evolution in a free cutting steel. Localised damage coalescence away from the centre of the uniaxial tensile test-piece was attributed to the effect of inclusion clustering. This research was used to develop a realistic damage model, which can predict damage growth and coalescence for a range of forming parameters and different stress-state conditions related to hot rolling applications. The micro-mechanics based model includes the effects of inclusion distribution on damage. The model is calibrated using twenty six temperature based material constants