Journal of Vibroengineering
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Crack breathing behavior of unbalanced rotor system: A Quasi-static numerical analysis
Get PDFCrack opening and closing during shaft rotation of a cracked rotor system have long been a focus of many previous studies. Previously published modeling work in the literature uses weight-governed crack breathing model for very large rotor systems. However, for lightweight or vertical or lightly damped rotors the opening and closing statuses of a crack are not always weight dominated as there is significant influence from dynamic loads. Further, the dependence of the breathing mechanism on the crack location has not been investigated yet. In this paper, the crack breathing behavior of an unbalanced shaft at the different crack location of a rotating shaft is investigated. A three-dimensional finite element model, consisting of a two-disk rotor with a transverse crack, is used. Finite element model is simulated using ABAQUS/Standard. Crack breathing behavior is found to strongly depend on its axial position, angular position, depth ratio, unbalanced force ratio and angular position. Two different crack breathing regions along the shaft length are identified, where unbalanced shaft stiffness may be larger or smaller than the balanced shaft, depending on the unbalance force orientation, magnitude and crack location. Further, four specific crack locations along the shaft length have been identified, where the crack remains fully closed or open or just behaves like in the balanced shaft. The results suggest that more accurate prediction of the dynamic response of cracked rotors can be expected when the effects of unbalance force and individual rotor physical properties on the crack breathing have been taken into account
Calculation of earth pressure on rigid retaining walls with considerations to the seismic load and soil stress-deflection
Get PDFAt present, the calculation of active earth pressure behind retaining walls is mainly based on the hypothesis that the fracture surface of rolling earth behind retaining walls is straight-running through wall heels. However, most experiments have proven that this hypothesis is false. In this study, active earth pressure behind retaining walls under seismic loading was discussed from the perspective of stress deflection. Stress on soil layer behind the vertical retaining wall was analyzed by quasi-static method. Then, the expression of seismic angle of rupture was proposed by referring to the balance of horizontal forces and changes with wall height. On this basis, the calculation formulas of active earth pressure, seismic active earth force, total moment at the wall and the point of application of active thrust from the base of wall were acquired by solving this balance equation. Calculated results were compared with test data and results of other methods. The rationality of the proposed method was verified. Thus, the proposed method is applicable to multi-layered filling behind the retaining wall
Voltage enhancing using multi-magnetic arrangement for low frequency vibrational energy harvesting
Get PDFPower extraction from ambient vibrations is currently needed or the technology, and the amount of energy in ambient vibration is low and is sufficient to operate small devices. In the present article a methodology is proposed to extract the maximum amount of power from these sources. This paper includes the method to increase the output voltage in a vibration energy harvesting device. The vibration amplitude depends on stiffness, mass damping, vibrating source amplitude and frequency. Various methods are developed to increase the voltage output from these devices. An induced voltage can be further increased by changing the initial relative position between the magnet and coil. Magnetic flux density is found to be maximum in the vicinity of the coil. When the top magnet side matches with the coil centre, then the induced voltage is found to be maximum as compared to other relative positions between the coil and magnet. The multi-magnet arrangement has been used to enhance the total output of the device by keeping appropriate relative position of magnet with respect to the coil. A theoretical and experiment investigation is conducted on these methods, and a theoretical simulation is carried out using a FEA tool, and the experimental results closely match with the theoretical results. Four magnet coils have been considered in this study which gives the maximum output of 1.937 V, 1.426 V, 2.01 V and 2.27 V at 14 Hz frequency. With the help of a multi-magnet arrangement, maximum 7.64 V and 11.44 mW is reached. It observed that the multi-magnet arrangement is the best and gives the maximum voltage output as compared to other methods
Variational mode decomposition: mode determination method for rotating machinery diagnosis
Get PDFVariational mode decomposition (VMD) is a modern decomposition method used for many engineering monitoring and diagnosis recently, which replaced traditional empirical mode decomposition (EMD) method. However, the performance of VMD method specifically depends on the parameter that need to pre-determine for VMD method especially the mode number. This paper proposed a mode determination method using signal difference average (SDA) to determine the mode number for the VMD method by taking the advantages of similarities concept between sum of variational mode functions (VMFs) and the input signals. Online high-speed gear and bearing fault data were used to validate the performance of the proposed method. The diagnosis result using frequency spectrum has been compared with traditional EMD method and the proposed method has been proved to be able to provide an accurate number of mode for the VMD method effectively for rotating machinery applications
Active control of a nonlinear suspension with output constraints and variable-adaptive-law control
Get PDFA variable-adaptive-law control algorithm for application to common problems like multi-objective control, actuator output constraints, and suboptimal adaptive laws is proposed in this paper. The multi-objective control problem of a nonlinear suspension is converted to the constrained stability problem of a sprung mass using a quarter nonlinear-suspension model. A variable-adaptive-law controller is then used, along with feedback from the output error, and considering the constraints of the actuator output. The controller modifies the adaptive law to reduce the active control force and restores it to the unsaturated zone. This ensures that the suspension system is always in a controlled state when the output saturation occurs. The controller was simulated for the following two cases: (i) a bump road and (ii) a C-grade road. The analysis is verified by experiments in the end
Vibration characteristic analysis of a circular thin plate with complex pre-stress distribution
Get PDFThe influence of complex pre-stress on the circular thin plate is investigated in this study to solve the non-uniform stress distribution problem. The differential equation of the circular plate with complex pre-stress distribution is derived based on the strain-displacement equation. The analytical method of free vibration for the circular plate with complex pre-stress distribution is proposed, in which the complex pre-stress and displacement function of the circular plate are expanded into the cosine trigonometric series. The influence of the different types of welding residual stress distribution on the natural frequency and the mode shape of the circular plate structure is compared. Finally, the effectiveness of the proposed model is verified through finite element method
Bearing failure prediction using Wigner-Ville distribution, modified Poincare mapping and fast Fourier transform
Get PDFThis study outlines the experimental investigation methods of condition monitoring to predict bearing failures using the experimental vibration signatures. The purpose of condition monitoring is to maximize the machine availability and utility of the machine components. Bearings being one of the most common component in any rotating machinery, it is vital to study the health of the bearing and can predict bearing failure, its location and severity. This prevents machine downtime, monetary loss and unfortunate accidents. A test rig was fabricated to get the vibration signatures of bearings. Prediction of bearing failure relies on the presence of the bearing characteristic frequencies – inner race frequency, outer race frequency, ball pass frequency and fundamental train frequency – and its harmonics in the vibration signal acquired. These frequencies are present in the vibration signature due to the interaction of surfaces of different bearing components that have defects in them. Both time and frequency domain numerical signature analysis were performed on the vibration signatures acquired. Simple frequency domain method like Fast Fourier Transform (FFT), chaotic vibration method like modified Poincare mapping and time-frequency domain Wigner-Ville distribution (WVD) were used in detecting bearing failure. Using the FFT analysis method, it is hard to predict the failures, hence better signal processing methods like modified Poincare mapping and WVD are used. Also, it is observed that the chaotic vibration signatures found in the lower-order mechanical systems like bearings. With the chaotic analysis methods like, Poincare Mapping and Wigner-Ville Distribution, the location and the severity of the bearing failure can be predicted
Wake and structure model for simulation of cross-flow/in-line vortex induced vibration of marine risers
Get PDFThree dimensional responses of riser subjected to Vortex Induced Vibration (VIV) are investigated. Proportionality relations of stress and fatigue damage are mentioned. A computer code has been developed for time domain modeling of VIV of riser accounting for both Cross-Flow (CF) and In-Line (IL) vibration. The wake oscillator model is used to calculate the VIV of each strip. The wake oscillators are coupled to the dynamics of the long riser, while the Newmark-beta method is used for evaluating the structural dynamics of riser. The wake dynamics, including IL and CF vibrations, is represented using a pair of non-linear Van der Pol equations that solved using modified Euler method. The existing experimental and numerical results for stepped and sheared current are used to validate the proposed model and the results show reasonable agreement. The proposed model was implemented on Amir-Kabir semi-submersible riser deployed at the water depth of 713 meters of Caspian Sea. CF/IL VIV of this riser is simulated for various current velocities. The results show that although displacement amplitude of IL direction is lower than CF direction but because of higher curvature, stress values of IL direction for some cases can be higher than CF direction. Also because of higher frequency of IL direction, fatigue damage of this direction can be higher than CF one in some cases. It is shown that with increasing of current velocity; however, variation of displacement amplitude of two directions is low but stress increased and fatigue damage also increased with higher rate. For lower velocities which the modes are controlled with tension, stress and fatigue damage of IL direction is higher than CF direction
Structural health monitoring of 3D frame structures using finite element modal analysis and genetic algorithm
Get PDFIn this paper, we present a new application based on Genetic Algorithm (GA) to detect damage in 3D frame structures. Finite Element Method (FEM) is used to build models for intact and damaged structures. The identification of damage is formulated as an optimization problem using GA and the changes in natural frequencies. A 3D frame structure with two floor is used, as a numerical example, for damage identification. The proposed method is then applied to identify some removed elements. The results obtained using FEM is validated using experimental benchmark test of a 3D frame structure with eight floors. The results show that the proposed technique gives good damage identification compared with literature [1]. Furthermore, there is no error recorded in the prediction of location, however a small error is recorded in detecting damage severity. It is concluded that GA is an efficient tool to quantify single and multiple damages with high precision in 3D frame structures
Characteristic comparison of wound and wrapped composite tubes made from carbon prepreg using modal analysis
Get PDFThe lightweight constructions are nowadays often based on composite materials. The best benefits are possible to obtain especially with a dispersion in the form of long fibres. In this study, the main properties of two almost identical composite rods manufactured from preimpregnated carbon fibres using two different methods will be compared. The first one is called “wrapping” which is helical layering of one wide tape in each ply. The main benefit of this method is good quality of final parts, but we are limited only by straight shapes. The second way “winding”, it is simultaneous deposition of several thinner fiber filaments. This method could handle the problem of curved shapes and fluently variable cross-sections. The disadvantage of this method is imperfect alignment of the placed fibers. This could be caused by a significant tortuosity and stickiness of the individual fiber strands. The goal of the carried work was to experimentally compare the vibrational transfer of these two almost identical rods with the same structure of layers, weight and curing parameters. Based on the certified properties of the base fibres and known boundary conditions, the material model and simulation of the vibrational response was created. The results of the empirical model compared with the experimental on the wrapped part were in good agreement. However, the use of this model also for the wound tubes is questionable, because of the found differences in natural frequencies and a shape of the transfer function. As part of the analytical study also the sensitivity analysis on the mesh quality has been done. As the conclusion is possible to say, that for determination of the main modes the mesh in a solved model could be just standard – it means without visible defects. Changes in the results achieved by improving the mesh quality are just really small. The more significant problem is that with using the contemporary knowledge and methods, it is not possible to simply create analytical model of the wound parts. The only option is trying to adjust the mechanical properties of base materials according to the empirical results