جستجوی مقالات مرتبط با کلیدواژه « آنالیز مودال » در نشریات گروه « مکانیک »

Characteristics such as excellent mechanical properties, high heat resistance, corrosion resistance and low density have made composites like carbon fiber widely used in various industries including medical, aerospace and automotive. However, due to characteristics such as poor thermal conductivity, low cutting surface quality and high strength, it is difficult to cut it using conventional methods. The aim of this article is to design and build a type of cutting tool that uses ultrasonic waves to reduce the cutting force and improve the cutting surface quality of carbon fiber workpieces. In this research, the design and construction of the horn part of the cutting tool is specifically discussed. First, a stepped horn is designed by performing analytical calculations, and then by adding a conical part with different angles to it, the effect of changing the angle on the amplitude amplification, modal spacing and stress distribution has been investigated by finite element simulation method.The results of numerical simulation show the insignificant effect of changing the angle of the conical part on the result. Finally, the optimized design is built.The results of experimental tests show that the use of ultrasonic waves has a significant effect in reducing the cutting force up to one-sixteenth and significantly improving the quality and straightness of the cutting edge of the carbon fiber workpiece compared to the case without ultrasonic assistance.

In this paper, effect of pitch variation on vibrations of a propeller variable pitch is studied numerically and experimentally. The free vibrations equations of a propeller blade were extracted using the Galerkin method. A three-dimensional scanner was utilized to obtain blade profile and static trust with the aim of determining propeller loading status. To obtain modal analysis, the simulation analysis was done using Abaqus software in rotating condition, and results were validated with experimental modal test in static status. Both of experimental and analytical methods results represent that with increasing pitch, the variations of natural frequency are reduced and increased at odd and even frequency modes, respectively. Considering the campbell diagram, no resonance problem was observed in the case of the propeller blade, at working period.

Flanges are parts that provide the possibility of connection, change of direction, access for repairs and inspection between large and main parts of equipment and structures. In this research, due to the importance of maximum lightening while maintaining the strength of parts in aerospace structures, composite flanges and shells were made, which are used in aeronautical structures such as bossting rockets and satellite carriers. They are responsible for connecting the parts and large shells of the structure. The material of the flanges used in these structures is aluminum in the lightest state, and in this research, with the aim of lightening as much as possible, 4 flanges and 4 shells were made in 2 sets, each set includes 2 flanges and 2 shells. After connecting the flanges and shells to each other, experimental modal analysis and tensile tests were performed. Also numerical simulation of the modal and tensile test was performed with ANSYS software. The results of experimental tests and numerical analysis on the manufactured parts show that the specific frequency of the composite flange is higher than that of the steel and aluminum flange (in low frequency range). Also, the delamination factor in the composite flange blade has more effect on increasing the length of the structure due to the tensile force, rather than the failure of the matrix and fibers.

Optimized structural architect of an Unmanned Lambda Wing Aerial Vehicle (ULWAV) during conceptual design has proven to be a significant challenge due to its unconventional configuration and no historical data for the structural architect. This paper aims to develop a new approach for structural design layout of new generation unconventional aircraft especially for ULWAV. The first step in this approach is to investigate the affecting parameters on structural architecture of these aircrafts then using of modal analysis to identification the behavior of these unconventional geometries for a general methodology development. The result of this modal analysis is apportion aircraft geometry to conventional substructures with already well-established defined architect. ULWAV structure architecture has been done by combining the structural architecture of sub-sections and with respect to requirements for systems installation, stealth design objective and packaging requirements. Here the CATIA software is used to optimize general architect with parametric model. To validate the results, the maximum static stress and maximum displacement of the structure were calculated using MSC / Patran software and compared with the values obtained from the optimization process. Dynamic analysis and buckling stability of structural composite shells are also controlled below. The architect methodology was applied to a case study of an ULWAV. The weight of the optimized structure has also been confirmed by typical case study researches.

In the present paper, the dynamic responses of a variable mass sled sample under the variable forces of propulsion, lift, drag and friction are investigated. First, by presenting a complete model of the various components of the sled system, the governing equations are extracted and the state space method is used. Then the variable forces applied to the designed sled model are extracted. In order to validate the results of the analytical method, the natural frequencies of the sled laboratory model are obtained experimentally. The first and second natural frequencies of the sled were 12.8 and 21.4 Hz, respectively, which were in good agreement with the analytical results. The results show that due to the disturbance applied to the sled, the dampers damped 40% of the vibrations. Also, in all cases, the oscillation rate of the rear sleeper is more than the front sleeper, this is due to the effect of the turbulence of the front sleeper on the rear sleeper. On the other hand, variable forces increase the sled displacement by 10%.

In performing vibration analysis of the car engine on the engine support, it is necessary to know the mass properties of the engine including mass, position of the center of mass and mass inertia moments. Because the car engine has a relatively complex geometric shape and is composed of different components with different densities, determining its mass properties faces many challenges. There are various methods to determine the mass properties of a car engine, one of the most common of which is the use of modal analysis (mass line method). In this paper, the mass line method is presented in order to find the mass properties of rigid bodies. In order to evaluate the efficiency and accuracy of the method, dynamic modeling of a rigid body with known mass properties is performed in MSC Adams software. So according to the theory of mass line, the rigid body is hung from a soft suspension and a force is applied to it in different locations. Then, by performing Adams simulations, the corresponding acceleration values ​​are extracted at various points. After that, the mass line method is coded in MATLAB software and the applied forces and accelerations obtained from Adams in the time domain are entered as input to the MATLAB code and by executing the MATLAB code, the mass properties of the rigid body are calculated. Finally, the values ​​of the mass characteristics obtained from the MATLAB code are compared with the actual values ​​of the rigid body simulated in Adams, and the accuracy of the mass line method in finding the mass properties is evaluated.

In this paper, the vibrations on the sample of a single-rail sled tester system with dampers are investigated. First, by extracting the vibration equations governing the problem coded in MATLAB software and by drawing the FFT diagram, the natural frequencies of the system are obtained. The first and second frequencies of the system are related to torsional displacement and transverse sled displacement, respectively. Due to the structural damping of the sled system, to extract the equivalent stiffness and equivalent damping values, the experimental test is used by the harmonic dynamic test device. Then, by simulating the sled model in ABAQUS software, modal analysis is performed and natural frequencies and mode shapes are extracted. Finally, by constructing a designed sled model and performing experimental modal analysis, the natural frequencies of the system are compared with the previous two methods and confirmed with an error of less than 9%.KeywordsSled tester system, natural frequency, modal analysis, equivalent stiffness, equivalent damping.

With the increasing utilization of polymer electrolyte membrane (PEM) fuel cells in cars, ships and airplanes, the study of vibrational behavior of fuel cells has gained particular importance.In this paper, modal analysis of 400-watt 4-cells fuel cell with an active surface area of 225 〖cm〗^2 has been performed numerically and experimentally. Time domain method has been used to extract the global fuel cell frequencies. This method has found widespread applications for models that do not have finite element simulation before the modal test.The considered fuel cell model has been excited from several directions and the output data of the sensors were recorded at several points. Then, by interpreting the results in the time domain and using the phase response angle, two natural frequencies of the model were extracted. The results of the test showed that the first transverse and longitudinal frequency of model is 500 Hz and about 2500 Hz, respectively.Then, simulation of finite elemental fuel cell components was studied in detail.Comparison of the frequencies obtained from the test and numerical analysis showed that maximum difference is about 8%. Therefore, numerical analysis of the model with sufficient detail can adequately cover the vibrational properties of the real model. Also, the results showed that by changing the geometrical and mechanical properties of the membrane by 45%, the natural frequency of fuel cell changes through 4%. Furthermore, Removing the membrane plates, in addition to reducing the number of model elements, reduces the contact constraints.

Stewart robot falls into the category of parallel robots, consisting of two upper and lower platforms. The top platform, the moving platform and the bottom platform, are fixed, with the two arms connected by six spheres with spherical joints. In this study, we present a novel method for controlling robots and a comprehensive model for vibration detection. After developing the governing equations of the robot arms, we performed a modal analysis to control the robot position. The results of rigid and flexible arms in different parts of the robot arm using the aforementioned controller were studied. Comparing the results, it can be said that in the robot model with elastic joints, due to the increase in degrees of freedom, the level of vibrations increased, thus, controlling the robots with elastic joints is more demanding than the that in the corresponding robot with rigid joints.. Also, by applying perturbations to the system, it was found that the performance of the controller was affected by disturbances to the system and the location error was increased. Finally, it can be stated that in the control system, determining the optimal coefficients in the relationships plays a major role in the performance of the controller. Keywords

Currently, composite structures have many applications in various industries including aerospace, automotive, marine, and petrochemicals. In most of these applications, the structure is under dynamic and static loads and it can cause buckling, vibration, and fatigue. Therefore, the static and dynamic analysis of these structures is essential in order to understand their characteristics, including buckling, natural frequency, and the shape of vibrating modes. One of the most important non-destructive methods for predicting the buckling load of the structure is the vibrational correlation technique (VCT), which is based on frequency variations with the axial load. In this study, an experimental study of the buckling load of composite sandwich plates with lozenge core has been investigated. The hand lay-up method has been used for fabrication of the composite sandwich plates. One of the specimens was used for the modal test. In order to verify the results of the VCT, the buckling load of four specimens was calculated by the experimental buckling test. The error of VCT was 2.1 %. Hence, the efficiency of the VCT for composite sandwich plates with lattice core was confirmed. Also, by investigating the effect of applied load percentage on the accuracy of the VCT, it was found that for the applied load of more than 63% of the buckling load, the accuracy of prediction of the vibrational correlation technique is acceptable.

Since the invention of ultrasonic vibration assisted turning, this process has been widely considered and investigated. The reason for this consideration is the unique features of this process, which include reducing machining forces, reducing wear, and friction, increasing the tool life, creating periodic cutting conditions, increasing the machinability of difficult-to-cut material, increasing the surface quality, creating a hierarchical structure (micro-nano textures) on the surface and so on. Different methods have hitherto been used to apply ultrasonic vibration to the tip of the tool during the turning process. In this research, a unique horn has been designed and constructed to convert linear vibrations of piezoelectrics to three-dimensional vibrations (longitudinal vibrations along the z axis, bending vibrations around the x axis, and bending vibrations around the y axis). The advantage of this ultrasonic machining tool compared with other similar tools is that in most other tools, it is only possible to apply one-dimensional (linear) and two-dimensional (elliptical) vibrations, while this tool can create three-dimensional vibrations. Additionally, since the nature of the designed horn can lead to the creation of three-dimensional vibrations, there is no need for piezoelectric half-rings (which are stimulated by 180 phase difference) to create bending vibrations around the x and y axes. The reduction of costs as well as simplicity of applying three-dimensional vibrations in this new method can play an important role in industrializing the process of three-dimensional ultrasonic vibration assisted turning.

The structure of marine vessels is usually being subjected to the dynamic loading conditions of sea-surface waves. The dynamics and vibrational characteristics of such structures is expected to be affected by sea environmental loadings. In this paper, a rectangular plate, that was immersed with water at different immersion depths and different boundary conditions was numerically analyzed by the Abaqus software, and the results have been presented and discussed in details. For validation the numerical method, the effects of immersion depth on the natural frequency of circular plate, which is parallel to the free surface of water, have been investigated experimentally and numerically. By evaluating 81 extracted vibrational modes, it became clear that a natural drop in natural frequencies begins with a rise in the fluid's height from a specific position, which is directly related to the type of boundary conditions. In the modal analysis of structures, it can be admitted that the major decreasing frequency due to increased mass is caused by one-way contact of the fluid and the structure and then, if the other side of the structure is in contact with the fluid, it will have a very small effect on the natural frequency reduction, which is clearly apparent in this study.

In the present paper the forced convection heat transfer of non-Newtonian Power-law fluid through an isothermal tube is studied analytically. Exact solutions for thermally developing and fully developed Power-law flows are presented. In the case of thermally developing flow, the energy equation is solved by applying the Separation Method resulting in a homogeneous system of differential equations including Sturm-Liouville equation. Temperature distribution of Power law flow through tube is presented for two states of shear thinning n=0.2 and Newtonian n=1in closed form solutions. The diagram resulted of plotting the local Nusselt number versus the dimensionless tube length for both of two considered states, represents this fact that increasing of the Power law index the Nusselt number is decreased. For the fully developed Power law flow after solving the energy equation finally an ordinary second order differential equation is derived in which the flow temperature distribution is determined by Modal analysis. For different values of Power-law indices firstly, the Nussselt number is determined by wall thermal boundary condition and then the flow temperature distribution is presented by Frobenius method. The results show that the Nusselt number and maximum temperature are decreased by increasing the Power-law index meaning a decrease in the tube heat transfer.

The precise finite element model is an efficient tool for vibrational analysis. It should be mentioned that, in structural dynamic analysis finite element models of system should be able to accurately predict system characteristics such as natural frequencies. Contrary to static analysis, in structural dynamic analysis, it is not possible to overestimate system characteristics or apply safety factor for predicted characteristics; that means that the exact values of system characteristics such as natural frequencies, should be derived in structural dynamic. According to this, constructing a reliable model in the structure dynamic always has great degree of importance in vibrational analysis. In this study, it has been tried to extract a reliable finite element model for a row of a sample turbine of RollsRoyce brand using empirical results. So, the material properties of the disk and the connection between the disk and the blade are corrected and updated using experimental modal analysis results. Also, it has been tried to propose new method to model and update the disk and blade joint. Finally, reliable finite element model could be used for more analysis such as derivation of Campbell diagrams of system.

Friction stir welding has been categorized amongst permanent joining methods. Since its thermomechanical nature, solid-state joining and high welding loads, Friction stir welding process needs sturdy tools, fixture, and machine tool. Combination of this process along with ultrasonic vibrations decreases welding loads and improves the weld quality. In this study, the idea of comparing superimposed axial and bending ultrasonic vibrational modes on friction stir welding process led to producing a unique set of friction stir welding tool along with an ultrasonic transducer. In fact, one of the aims of this study concerns designing a single ultrasonic assisted friction stir welding tool set which will be able to vibrate in both axial and bending modes separately in different frequencies. In addition, the impressive effects of mass and inertia variations on the weld, due to the usage of the distinct tool set for each modal frequency, will be removed. The final modal analyzed model was manufactured and the amplitude of vibrations in both mentioned modes was measured experimentally. Results indicated that the use of bending vibrations leads to the formation of joints with much more quality than implementing the axial vibrations. Furthermore, doubled amplitudes from 8 μm to 16 μm in bending mode, unlike axial mode, improved the weld ultimate tensile strength.

Flutter is an unintended phenomenon due to interactions between aerodynamic forces, elasticity and inertia, which leads to unstable structural fluctuations, often leading to structural failure. Most high speed and/or flexible aircraft are highly sensitive to flutter. In this research, extended methods in the last few decades in flutter flight testing are reviewed. Similarly, structural excitation systems, measurement systems, digital processing of data and methods for extracting frequency and damping parameters are considered. Also, the practical experience in this context is mentioned and the current state of this research area is explored.

In this study, the effect of lattice geometry on the modal response of cylindrical carbon/epoxy lattice structures have been investigated both experimentally and numerically. Two structural geometries of triangular and hexagonal were chosen. Accurate flexible molds were used to manufacture the carbon/epoxy composite samples using polar-winding process. Experimental and numerical investigation of the vibration response of the manufactured samples were analyzed in order to determine the natural frequency and modal response for both triangular and hexagonal lattice structures for simply supported beam conditions. The results obtained from the Numerical model shows a good agreement with experimental results. after validating the numerical model, other supporting conditions like simply -fixed and fully fixed support was investigated Numerically. Results shows that the triangular structure lattice has higher natural frequency than hexagonal structure for all different support conditions.

Modal analysis is done periodically to ensure blades performance for non-occurrence of resonance obtained under various operating conditions. Since metal or composite helicopter blades have a certain flexibility at different speeds in different planes can lead to deformation of the structure that can have a great effect on the aerodynamic performance of the helicopter. In this paper, modal analysis of helicopter blade has been carried out by the lumped mass methods (myklestad and finite element method) and modal test on the rotor structure. The finite element analysis has been developed by two approach of analytical and software method. Modal testing was also performed in both static and dynamic states, with constant speed, to validate the lumped mass methods. Finally, the results of both methods are compared with each other. After ensuring there correspondence between lumped mass methods and experimental results, an analysis was done according to the operating conditions of the rotor and finally Campbell diagram has been generated. According to this diagram became clear there was not any problem with the resonance of the rotor blades, while the numerical result of case has an acceptable match with the experimental results.

MAFVRO is one of the out-put only modal analysis methods which is able to determine the modal parameters of a vibration system through the free vibration responses. It is worth to note that most of the vibration systems are subjected to random excitations. Therefore, in such cases, the traditional MAFVRO cannot extract the modal parameters. In this study, the mentioned method is developed to randomly excited vibration systems. Then, the modified MAFVRO is applied to a randomly excited vibration system. The random excitation has been considered as a set of white noises which are applied to all masses of a discrete system. Then by employing the developed method, the estimated results have been compared with those obtained through the structural eigenvalue problem. Comparing the results reveals that the developed method can give the natural frequencies and mode shapes of a randomly excited system with a good accuracy, but it estimates the damping ratios lower than their exact values. In addition, the effect of the noise on the accuracy of the estimated modal parameters have been investigated.