Reduction In Natural Frequencies Research Articles

DAMAGE ASSESSMENT BASED ON MODAL ANALYSIS OF PIPE STRUCTURE

The degradation of steel pipe structures, especially thickness reduction due to corrosion is a common problem found in marine structures. This calls for an evaluation of the thin structure’s structural health condition to ensure its safety and reliability. This study investigates the effect of reduction in pipe wall thickness on the pipe structure’s dynamic behaviour using the fundamental modal parameter as the indicator for its health condition. A finite element analysis utilising Abaqus software has been adapted to model the problem. The corrosion problem was simulated by thickness reduction on six damage cases. The influence of the severity and position of damage on the observed mode upon the percentage of natural frequencies’ reduction and localisation of mode shape of the pipe structure are subsequently displayed. Four mode shapes and its corresponding natural frequency were extracted from baseline model to further evaluate the sensitivity of the selected mode towards the presence of damage. The study found that severe damage in pipe structure leads to significant reduction of natural frequency and localisation of mode shape in the vicinity of damaged region. The vibration results summarised in this paper may serve as a benchmark for researchers and as a reference for design engineers.

Effect of Damage on Natural Vibration Characteristics of Large Semi-cushion Spiral Case Structure

A spiral case structure presents various natural vibration characteristics with damage in different degrees in peripheral concrete. The natural vibration characteristics and resonance safety of a large semi-cushion spiral case structure with damage in varying degrees in peripheral concrete are compared and analyzed. For this purpose, a finite element analysis method of vibration characteristics, a four-parameter damage constitutive model of concrete, and spiral case structure of the Ahai Hydropower Station are utilized. Results show that each mode of natural frequency of the spiral case structure declines after the damage in concrete. The extent of the reduction in natural frequency is large when the damage is serious. The mode shapes of the spiral structure are all complex and twisted before and after the damage in concrete. Afterward, however, amplitude increases. After considering the damage, a new possible source of resonance cannot be identified. Resonance checking shows that the damage in concrete cannot seriously harm the Ahai spiral case structure. Therefore, this study provides new reference and support for the production design and safe operation of large semi-cushion spiral case structure.

Vibration-response due to thickness loss on steel plate excited by resonance frequency

The degradation of steel structure due to corrosion is a common problem found especially in the marine structure due to exposure to the harsh marine environment. In order to ensure safety and reliability of marine structure, the damage assessment is an indispensable prerequisite for plan of remedial action on damaged structure. The main goal of this paper is to discuss simple vibration measurement on plated structure to give image on overview condition of the monitored structure. The changes of vibration response when damage was introduced in the plate structure were investigated. The damage on plate was simulated in finite element method as loss of thickness section. The size of damage and depth of loss of thickness were varied for different damage cases. The plate was excited with lower order of resonance frequency in accordance estimate the average remaining thickness based on displacement response obtain in the dynamic analysis. Significant reduction of natural frequency and increasing amplitude of vibration can be observed in the presence of severe damage. The vibration analysis summarized in this study can serve as benchmark and reference for researcher and design engineer.

Determination of Natural Frequencies of Fluid Conveying Pipes using Muller’s Method

In the present paper the natural frequencies of fluid-conveying pipes with guided-clamped, guided-simply supported, guided-free and guided–guided boundary conditions are derived explicitly. Numerical results are presented for four cases, and the effect of fluid flow velocity on the natural frequencies is discussed. Critical velocities are determined to find the point of flutter. The pipe is modelled as an Euler–Bernoulli beam. The differential equation of motion for free vibration is established using Hamilton’s principle. The transcendental frequency equation is derived using Muller’s bisection method. By varying the non-dimensional velocities the frequencies are determined for the said boundary conditions. It is observed that there is a reduction in natural frequency with an increase in velocity, for all the boundary conditions. The influence of fluid velocity on the instability of the pipe is determined. The results obtained analytically were validated by experiment for guided–guided end condition only.

Natural Frequency Reduction of Seismic Isolation Table Using DC Motor and Electrical Circuit

Natural Frequency Reduction of Seismic Isolation Table Using DC Motor and Electrical Circuit

Natural Frequency Reduction of Seismic Isolation Table Using DC Motor and Electrical Circuit

Natural Frequency Reduction of Seismic Isolation Table Using DC Motor and Electrical Circuit

Centrifuge modeling to evaluate natural frequency and seismic behavior of offshore wind turbine consideringSFSI

AbstractUnderstanding of dynamic response of offshore wind turbine is important to reduce vibration of offshore wind turbine induced by structural and environmental loadings. Although dynamic characteristics of the offshore wind turbine such as natural frequency and seismic behavior are affected by foundation and soil conditions, there are little experimental studies about the dynamic behavior of offshore wind turbine with consideration of proper soil–foundation–structure interaction (SFSI). The goal of this research is to evaluate the natural frequency and seismic behavior of offshore wind turbine with a monopod foundation considering SFSI. Scaled model of offshore wind turbine and monopod foundation is produced for this research. Geotechnical centrifuge tests in fixed‐based and SFSI condition were performed to measure natural frequency in each case. Also, a series of seismic loadings with different intensities are applied to observe seismic behaviors of the offshore wind turbine during the earthquake and permanent changes after the earthquake. Experimental results show apparent natural frequency reduction in SFSI condition compared with the fixed‐based condition, non‐linear changes in dynamic response during a series of earthquakes and permanent changes occurred in natural frequency and rotational displacement after earthquakes. Copyright © 2017 John Wiley & Sons, Ltd.

Vibration analysis and control of smart composite plates with delamination and under hygrothermal environment

An analysis of delaminated composite plates with integrated active fibre composite (AFC) actuators and sensor under hygrothermal environment has been undertaken in the present work. The top and bottom plies of the laminate constitute AFC actuators and sensors. In the present investigation, hygrothermal loading is taken into consideration, and the effect of moisture and temperature on the delaminated plate structures is analysed. The minimum total potential energy approach is used for arriving at the governing equation. A finite element model for centrally located delamination is developed on the basis of first order shear deformation theory. An eight noded serendipity element with five degrees of freedom per node is considered for the finite element formulation. A parametric study on the effect of stacking sequence, boundary conditions and environmental conditions in the presence of delamination on the laminated plate is studied. The key observations from the parametric study are: there will be reduction in natural frequencies in the presence of delamination and/or hygrothermal loading, and reduction in structural stiffness due to the presence of delamination can be negated by applying voltage to the active fibre actuator layer.

Abnormal brain oscillations persist after recovery from bipolar depression

When directly perturbed in healthy subjects, premotor cortical areas generate electrical oscillations in the beta range (20–40Hz). In schizophrenia, major depressive disorder and bipolar disorder (BD), these oscillations are markedly reduced, in terms of amplitude and frequency. However, it still remains unclear whether these abnormalities can be modulated over time, or if they can be still observed after treatment. Here, we employed transcranial magnetic stimulation (TMS) combined with EEG to assess the frontal oscillatory activity in eighteen BD patients before/after antidepressant treatments (sleep deprivation and light therapy), relative to nine healthy controls. In order to detect dominant frequencies, event related spectral perturbations (ERSP) were computed for each TMS/EEG session in all participants, using wavelet decomposition. The natural frequency at which the cortical circuit oscillates was calculated as the frequency value with the largest power across 300ms post-stimulus time interval. Severity of depression markedly decreased after treatment with 12 patients achieving response and nine patients achieving remission. TMS/EEG resulted in a significant activation of the beta/gamma band response (21–50Hz) in healthy controls. In patients, the main frequencies of premotor EEG responses to TMS did not significantly change before/after treatment and were always significantly lower than those of controls (11–27Hz) and comparable in patients achieving remission and in those not responding to treatment. These results suggest that the reduction of natural frequencies is a trait marker of BD, independent from the clinical status of the patients. The present findings shed light on the neurobiological underpinning of severe psychiatric disorders and demonstrate that TMS/EEG represents a unique tool to develop biomarkers in psychiatry.

Experimental investigation of tri-axial self-centering reinforced concrete frame structures through shaking table tests

This paper presents an experimental study on the seismic performance of a tri-axial self-centering reinforced concrete (RC) frame structure. In such a structure, the column-base joints were free to uplift, and the beam-column joints were relaxed to open. Post-tensioned (PT) strands passed through the RC beams and columns to provide re-centering capability. In order to accommodate the gap openings in beam-column joints, rubber pads were placed underneath the floor slabs to provide a smooth sliding motion between the RC beams and the slab. A 1/2.5-scale model of a tri-axial three-story self-centering RC frame structure was designed and tested on a shaking table under a series of earthquake excitations with increasing amplitudes. The results showed that the self-centering RC frame structure had desirable seismic performance with little damage even under extreme earthquakes. The reduction in natural frequencies were quite small after the extreme earthquakes, indicating that the structure had remained almost elastic. The test model showed a desirable tri-axial re-centering capability with negligible residual deformation. The sliding of the slabs was also measured throughout the tests, and the data indicated that the slabs re-centered to their original positions after the tests.

The Effect of Fin Pitch on Fluid Elastic Instability of Tube Arrays Subjected to Cross Flow of Water

Failure of tubes in shell and tube exchangers is attributed to flow induced vibrations of such tubes. There are different excitations mechanisms due to which flow induced vibration occurs and among such mechanisms, fluid elastic instability is the most prominent one as it causes the most violent vibrations and may lead to rapid tube failures within short time. Fluid elastic instability is the fluid–structure interaction phenomenon which occurs when energy input by the fluid force exceeds energy expended in damping. This point is referred as instability threshold and corresponding velocity is referred as critical velocity. Once flow velocity exceeds critical flow velocity, the vibration amplitude increases very rapidly with flow velocity. An experimental program is carried out to determine the critical velocity at instability for plain and finned tube arrays subjected to cross flow of water. The tube array geometry is parallel triangular with cantilever end condition and pitch ratios considered are 2.6 and 2.1. The objective of research is to determine the effect of increase in pitch ratio on instability threshold for plain tube arrays and to assess the effect of addition of fins as well as increase in fin density on instability threshold for finned tube arrays. Plain tube array with two different pitch ratios; 2.1 and 2.6 and finned tube arrays with same pitch ratio; 2.6 but with two different fin pitches; such as fine (10 fpi) and coarse (4 fpi) are considered for the experimentation. Connors’ equation that relates critical velocity at instability to different parameters, on which instability depends, has been used as the basis for analysis and the concept of effective diameter is used for the present investigation. The modal parameters are first suitably modified using natural frequency reduction setup that is already designed and developed to reduce natural frequency and hence to achieve experimental simulation of fluid elastic instability within the limited flow capacity of the pump. The tests are carried out first on plain tube arrays to establish the same as the datum case and results are compared to known results of plain tube arrays and hence the quality of the test rig is also assessed. The fluid elastic vibration tests are then carried out on finned tube arrays with coarse and fine fin pitches and effects of fins and fin pitch on instability threshold are shown. The vibration response of the tube is recorded for each gradually increasing flow rates of water till instability point is reached. The parameters at the instability are then presented in terms of dimensionless parameters to compare them with published results. It is concluded that, arrays with higher pitch ratios are unstable at comparatively higher flow velocities and instability threshold for finned tube arrays is delayed due to addition of the fins. Further, it is concluded that, instability threshold for finned tube arrays with fine fin pitch is delayed compared to coarse fin pitch and hence for increased fin density, instability threshold is delayed. The experimental results in terms of critical velocities obtained for different tube arrays subjected to water cross flow will serve as the base flow rates for air–water cross flow experiments to be conducted in the next phase.

Vibration analysis of a rectangular-hollow-sectional beam with an external crack

The vibration property of a rectangular-hollow-sectional beam with an external crack were theoretically and experimentally studied in this paper. The external part-through crack and through-thickness crack were considered. Linear elastic fracture mechanics were employed to establish the theoretical formula for computing the equivalent torsion spring stiffness of the two external cracks. Then, the experimental method of obtaining the equivalent torsion spring stiffness was developed. It is seen that the test data match fairly well with the analytical data. On this basis, the transfer matrix method was introduced to obtain the frequency equations of these cracked beams under three classic boundary conditions, and followed by the numerical method for calculating natural frequencies. Experimental tests on clamped–free beams were used to verify the accuracy. It should be noted that a part-through crack may lead to a more distinct natural frequency reduction than the through-thickness crack.

Hygrothermal effect on natural frequency and damping characteristics of basalt/epoxy composites

This research focuses on the experimental and analytical study of hygrothermal effects on natural frequency of fibre reinforced plastic composites. Fiber reinforced composites are widely used in structural applications like aero industry, marine industry, civil industry ext; due to its good strength and stiffness. But these composites absorb moisture or water when it is used in the moisture environment and the composites will be swollen and which reduces the strength and stiffness of the material. This moisture may lead to reduction of strength and stiffness Natural frequency of materials is connected to stiffness of the material. This work is concentrated on how the natural frequency of Basalt Fiber reinforced composites (BFRP) are affected due to weave pattern, thickness of the material, water absorption at various immersion conditions. Three types of Basalt fabrics are selected (Plain weave, twill weave and qudraxial) BFRP composites with two different thickness(3mm, 5mm)are fabricated using hand lay-up process..Fabrication of composite materials using basalt fibres has been done. We have chosen materials with two different thickness and also basalt fibre with different weave patterns. The experimental setup for beams has been done. Moisture absorption co-efficient is determined by immersing the material in mineral water at various time durations. Modal analysis have been carried out to determine the natural frequency and damping coefficient for BFRP composites with three different weave patterns and two different thickness. Results shows that damping coefficient varies with weave pattern and moisture conditions.

Investigation of vibration and stability of cracked columns under axial load

In this paper, an analytical method is proposed to study the effect of crack and axial load on vibration behavior and stability of the cracked columns. Using the local flexibility model, the crack has been simulated by a torsional spring with connecting two segments of column in crack location. By solving governing eigenvalue equation, the effects of crack parameters and axial load on the natural frequencies and buckling load as well as buckling load are investigated. The results show that the presents of crack cause to reduction in natural frequencies and buckling load whereas this reduction is affected by the location and depth of the crack. Furthermore, the tensile and compressive axial load increase and decrease the natural frequencies, respectively. In addition, as the compression load approaches to certain value, the fundamental natural frequency reaches zero and instability occurs. The accuracy of the model is validated through the experimental data reported in the literature.

P.1.b.027 Shared reduction of oscillatory natural frequencies in bipolar disorder, major depressive disorder and schizophrenia

P.1.b.027 Shared reduction of oscillatory natural frequencies in bipolar disorder, major depressive disorder and schizophrenia

Vibration-based non-destructive technique to detect crack in multi-span beam

This article presents the study on identification of a single open crack in a straight multi-span beam using natural frequency-based non-destructive technique. The crack is assumed to be transverse and one-dimensional partial cut of a beam, and is modelled as equivalent elastic rotational spring. The effects of crack location and depth on the natural frequency for multi-span uniform beam are demonstrated. The reduction in natural frequency due to presence of crack is utilised to detect crack location and its severity. The formulation is validated experimentally.

Support Flexibility and Natural Frequencies of Pipe Mounted Thermowells

A simplified model of a pipe mounted thermowell provides a measure of insight into the design and application of intrusive pipe fittings. A combination of Fourier and Green’s function methods together with a distributed load model of the thermowell/pipe wall interface are used to calculate the support compliance and subsequently the natural frequencies of the thermowell. These are compared with limited though independent calculations. This comparison confirms a profound reduction in natural frequencies for commonly encountered thermowell installations, reductions that should not be ignored where the risk of flow-induced resonance is high.

Shared reduction of oscillatory natural frequencies in bipolar disorder, major depressive disorder and schizophrenia

IntroductionRecent studies have demonstrated that cortical brain areas tend to oscillate at a specific natural frequency when directly perturbed by transcranial magnetic stimulation (TMS). Fast electroencephalographic (EEG) oscillations, which typically originate from frontal regions, have been reported to be markedly reduced in schizophrenia. MethodsHere we employed TMS/EEG to assess the natural frequency of the premotor area in a sample of 48 age-matched participants (12 each in major depression disorder (MDD)), bipolar disorder (BPD), schizophrenia (SCZ) and healthy controls. Event related spectral perturbations (ERSP) were obtained for each study participant using wavelet decomposition. ResultsTMS resulted in a significant activation of the beta/gamma band response (21–50Hz) to frontal cortical perturbation in healthy control subjects. By contrast, the main frequencies of frontal EEG responses to TMS were significantly reduced in patients with BPD, MDD and SCZ (11–27Hz) relative to healthy subjects. ConclusionsPatients with bipolar disorder, major depression and schizophrenia showed a significantly lower natural frequency of frontal cortico-thalamocortical circuits compared to healthy controls. These results suggest a common neurobiological mechanism of corticothalamic impairment. The most likely candidates include dysfunction of GABAergic circuits. LimitationsFurther studies are needed to consider other biological markers, gene variants, and their interaction with clinical variables.

Analysis of Crack Detection of A Cantilever Beam using Finite Element Analysis

Abstract- This paper describes finite elemental analysis of a cracked cantilever beam and analyzes the relation between the modal natural frequencies with crack depth, modal natural frequency with crack location. Also the relation among the crack depth, crack location and natural frequency has been analyzed. Only single crack at different depth and at different location are evaluated and the analysis revels relationship between crack depth and modal natural frequency. As we know when a structure suffers from damage its dynamic property can change and it was observed that crack caused a stiffness reduction with an inherent reduction in modal natural frequencies. Consequently it leads to change in dynamic response of the beam. The analysis was performed using ANSYS software. The material of the beam is taken as aluminum. The proposed technique represents actually a modal analysis having great benefits for health monitoring of structures. For this 3D model of cantilever beam with single crack is created in ANSYS. Total 49 model of crack cantilever beam has been analyzed. Thus result obtained from ANSYS software we can draw the graph of modal natural frequency Vs crack depth for constant crack location and modal natural frequency Vs crack location keeping crack depth constant. And finally the value obtained from ANSYS is checked with result obtained from analytical method.

An investigation into the effect of scour on the natural frequency of an offshore wind turbine

Rapid expansion of the offshore wind industry has stimulated a renewed interest in the behaviour of offshore piles. There is widespread acceptance in practice that pile design methods developed for the offshore oil and gas industry may not be appropriate for designing wind turbine foundations. To date, the majority of offshore wind turbines are supported by large diameter monopiles. These foundations are sensitive to scour which can reduce their ultimate capacity and alter their dynamic response. In this paper, the use of a vibration-based method to monitor scour is investigated. The effect of scour on the natural frequency of a model monopile was measured in a scale model test. A spring–beam finite element numerical model was developed to examine the foundation response. The model, which used springs tuned to the small-strain stiffness of the sand, was shown to be capable of capturing the change in frequency observed in the scale test. This numerical procedure was extended to investigate the response of a full-scale wind turbine over a range of soil densities, which might be experienced at offshore development sites. Results suggest that wind turbines founded in loose sand would exhibit the largest relative reductions in natural frequency resulting from scour.