جستجوی مقالات مرتبط با کلیدواژه "آنالیز مودال" در نشریات گروه "فنی و مهندسی"

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.

One of the most important issues in the design of a vessel structure is the vibrations of the structure and its effect on the comfort of the crew and the life of the equipment. The most important factor about the comfort of the crew in a vessel is the range of free and forced vibrations under various internal and external factors in the structure of the vessel. One of the serious factors in stimulation of vibrations in the hull vessel is the propeller. Excessive vibrations as well as the being the structure in the frequency range of propeller excitation, cause fatigue, components exhaustion, and also the resonance phenomenon. Resonance and vibration of the components are the structural design challenges, so in this paper, vibrations caused by a five-bladed propeller KP505 excitation were investigated using numerical simulation on a container vessel (KCS). First, the free vibrations of the hull vessel in wet mode were investigated. Then, to investigate the forced vibrations caused by the propeller excitation, the pressure distribution on the hull in self-propulsion mode was obtained from the numerical solution of the fluid flow by computational fluid dynamics. To validate the results, the natural frequencies obtained in free vibrations were compared with empirical formulas. Comparing the values of the first, second and, third bending frequencies with the empirical values, showed that the analysis error was 5.5, 26, and 26.6, respectively, which explain the accuracy of the analysis. Comparing the results of forced vibrations with the standard allowable range, it was shown that the vibrations are within the allowable range. Also, the structure has not being within the range of the excitation frequency of the propeller. As a result, the resonance phenomenon has not occurred.

Operational modal analysis typically requires the physical attachment of a sensor to measure the structure vibration. As a non-contact method, digital cameras are relatively inexpensive, fast, and providing high spatial resolution results. The aim of this study is to identify the dynamic parameters of structures (frequency and damping ratio) by using video measurement (output only) based on the phase. For this purpose, it was only used mobile smartphones to identify modal parameters. The accelerometer sensor in the 3 attached smartphones was used to extract vibrational information. Video recorded by another mobile smartphone simultaneously identifies the dynamic parameters. In video measurement, the steerable pyramid transform is applied to frames that were extracted from the video to identify the local phases encoded in the image. After that, blind source separation (complexity pursuit) and single value decomposition techniques were applied to the image phase. Frequencies and damping ratios are extracted by the Fourier transform and logarithmic reduction technique. Thus, to validate the proposed method, two samples of 3D wall sandwiches panel, including a concrete sample and a straw sample that were already loaded and their stiffness was reduced by an average of 90%, were used. The results showed that the proposed method was able to extract the frequencies and damping ratios of the panels with an error of less than 2%.

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.

This paper investigates the output-only methods in recognizing system identification and modal parameters of infrastructures, such as bridges and offshore platforms, etc., as an alternative to traditional methods such as impact testing and visual inspection. In engineering sciences, system identification means identifying a structural system's dynamic characteristics, i.e., stiffness, mass, damping or, Modal shapes, natural frequencies, and damping ratios, called structural identification. In this article, after defining the concepts of system identification and output-only, time domain, frequency domain, and time-frequency domain methods are reviewed. In the end, the latest research on this subject is pointed out. Finally, a simply supported beam has been modeled in finite element software, and the frequencies and shapes of the modes are compared to those coded in MATLAB program. The output algorithms' results (SSI method) showed good agreement with the finite element model, which can be considered a reliable method for health monitoring and damage detecting structures.

Damage to structures is inevitable. Besides, the accumulation and spread of local damage can be a source of global damage; hence, the significance and necessity of monitoring the health of the structure. Wavelet analysis of structural responses is a method of damage detection with optimal performance in the time-frequency domain, which yields more information from the analyzed response of the structure in both time and frequency domains. As beams and columns are among the most fundamental structural elements and, expectably, the last elements to be damaged, it bears greater importance to identify damage in them than in other structural elements. This paper draws on modal analysis data of steel beams modeled in Abacus and a proposal of the wavelet analytical method to track different positions of damage along the beam, where the damages are defined as a decrease in the elastic modal. A decreasing shift in frequency values occurred in all modes due to damage. To detect the damage, the mode shape signal was defined as the damage mode shape and the differentiation between the healthy and the damage mode shapes as the wavelet transform input signal. The output signals from the wavelet analysis of the input signal indicated maximum and minimum relative fluctuations in different situations of damage so that the centers of damage were identified with zero error. The results also revealed that with increasing the severity of the damage, only the irregularity of the wavelet coefficients of that damage position increases.

Nowadays, maintenance and health monitoring of structures and infrastructures of the country are the top priority of engineers. Therefore, acceleration sensors, which are installed on the bridge are used to extract the bridge's modal response, and even the latest studies have shown that sensors that have been attached to the car's chassis can record the bridge's response to vibration. Thus, it is important to use an accurate method, which is based on the output-only, to measure frequency, damping ratio, and mode shape. As a result, practical modal analysis is used for that purpose. In this paper, a new method of blind source separation, called complexity pursuit (CP), is investigated which is able to measure and extract dynamic characteristics in the time domain. Complexity pursuit is a well-known method, that is able to recover hidden sources (independent responses of every degree of freedom) and it can calculate the mixing matrix from measured mixed output data only. Some various conditions are considered in this paper to verify complexity pursuit (3 degrees of freedom mass-spring with various damping ratios, in white Gaussian noise conditions, in non-diagonal damping matrix, close modes, and for 12 degrees of freedom mass-spring system). Comparison of the CP results and modal analysis under all the conditions showed that the proposed method is able to detect, measure and extract the modal characteristics with an error of less than 3% and in most cases less than 1%.

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.

Regarding the importance of bridges as one of the most critical infrastructures, their maintenance, and health monitoring is of high priority. Interaction between the moving vehicles and bridges is amongst the fields of study that have been investigated in depth by numerous researchers in the field of bridge engineering. Among different proposed methods of structural health monitoring of bridges, the indirect methods that do not need the healthy structure response are of high interest because of their ease and low maintenance costs.
The response of a moving mass passing through a bridge can be analyzed for the indirect prediction of the beamchr(chr('39')39chr('39'))s mechanical properties. This can lead to the detection of possible damages or degradations in the structure. By mounting high precision accelerometers on the moving vehicle and recording the corresponding signals, it is possible to capture the sudden change of mechanical properties pertaining to the existence of damage in the bridge.
In the current study, an FE code is developed in order to analyze the moving vehicle response. In this code, the bridge is modeled as an Euler-Bernoulli beam, and a complete model comprising stiffness and damping of the suspension system of moving vehicle is built. In order to verify the results of the code, comparisons are made with the outcomes of modal analysis. The sensitivity of the FE results with respect to the number of elements is examined. These comparisons clearly show that both methods reach the same values for a sufficiently high number of elements for the moving vehicle response.
Following verification of the code, a brief review of the concepts underlying the variational mode decomposition (VMD) method is given for a self-contained representation. The VMD can be used to decompose a signal into a number of signals with limited bandwidth. Although it has found many applications in different signal processing cases (e.g. in the field of electronics, mechanical vibrations of machines, or even in the analysis of economic and financial time series), extending its application to the field of structural health monitoring is entirely a recent and ongoing topic of research.
After the introduction of the VMD, damage in the beams is implemented by using fracture mechanics concepts. Different damage scenarios are applied in order to check the reliability and robustness of using VMD as a damage detection method. These include different damage locations (single, dual) and damage severity represented in terms of crack depth. By having a reliable means for the analysis, the novel variational mode decomposition (VMD) is applied to analyze the signals recorded from the vehiclechr(chr('39')39chr('39'))s back axel in search of any possible irregularity in the signal properties. By monitoring results attained for several damage cases, the following conclusions can be given:• The variational mode decomposition (VMD) can highlight the presence of irregularities in mechanical properties that can be reached directly from decomposed signals.
• The location of these signal irregularities coincides with the presumed location(s) of the crack(s).
• The severity of the signal irregularity and corresponding instantaneous energy is proportional to the degree of damage imposed on the beam. 
• The moving vehiclechr(chr('39')39chr('39'))s natural frequency plays an essential role in the bridgeschr(chr('39')39chr('39')) structural health monitoring. The signal processing results exhibit amplified abrupt changes for the vehicles with the natural frequencies close to the beamchr(chr('39')39chr('39'))s fundamental frequencies.  
Regarding the above conclusions, analyzing moving mass response with the VMD can be a reliable damage detection technique.

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

In this study, to investigate and compare the performance of the single mass damper in the maximum modal displacement (roof) and multiple mass dampers vertically distributed in the height of the structure, based on the modal analysis, two linear and nonlinear models of a 40-story structure were selected. The structure has been modeled in OpenSees software using seven acceleration time histories. The analysis results for applied earthquakes under the maximum acceleration of 1.0g show that the control of the linear structure by multiple tuned mass dampers (MTMDs) tuned to the first and second modes have more appropriate behavior than others, and the average reduction of the maximum displacement of the Roof applying this type of dampers is 14.5%, which is about 2 times more than reduction of the STMD tuned to the first mode and the MTMDs tuned to the first or second modes, systems. However, due to the assumption of tuning the design parameters of the dampers corresponding to their elastic behavior, the performance of single and multiple mass dampers slightly decreases in a nonlinear model of the structure while structural responses are still controlled. Also, for the 10% error caused by misadjusting of the dampers, the behavior of MTMDs is more appropriate.

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.

Localized singularities caused by changes in the stiffness or mass of the damaged region cannot be simply visible through smooth mode shape curves. However, the wavelet transform of input signal can identify the location of defects by sudden changes in the spatial variation of transformed response. The aim of this study is to present a new method for damage detection in a squarely damaged plate with element of reduced thickness with damage ratio 3% and to decrease the possible random noises that are come from different sources such as numerical solutions or physical situation. In this approach, the value of each point of mode shape data is subtracted from or is added with its symmetric point with respect to the plate’s point of symmetry (point O). The obtained value is located at its first point, again. The result of this process is a symmetrical mode shape matrix of the damaged plate, which is utilized as the input signal function in wavelet analysis.The results demonstrated that using reconstructed modal data is superior to the original modal data and they provide a better crack indicator than the result of the DWT of the original mode shape data. The effects of crack location and sampling interval are examined. The simulated results show that this approach is more effective in damage detection of plate-like structures, as the method requires the knowledge of only the response of the damaged plate as a baseline for crack detection.

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.