Experimental Modal Analysis Techniques Research Articles

A sounding rocket payload experiment on zero gravity fuel gauging using modal analysis

For the past three years Carthage Space Sciences faculty and students have been developing Ex- perimental Modal Analysis (EMA) techniques for use in microgravity fuel gauging of spacecraft propellant tanks. Our experiments were initially conducted on parabolic flights in which periods of microgravity of 20 seconds can be obtained. Parabolic flights were necessary to demonstrate the effectiveness of the technique in a relevant environment, but the short period of microgravity available during each parabola is insufficient to probe the resolution of the technique for settled, non-sloshing propellant. Here we report the results of an experiment in which the EMA technique is implemented in a sounding rocket payload developed through participation in the RockSat-C program of the Colorado and Virginia Space Grant Consortia.Â

Finite element modelling of cracked railway pre-stressed concrete sleepers

Loading conditions on railway tracks are the result of the sum of the static axle load of vehicles and augments caused by dynamic effects. Dynamic loads have several causes and can be quasi-static or impacts. Cracking in pre-stressed concrete sleepers is mainly due to these impact loads and vibrations, converting the central and the rail seat sections of the critical sections in sleepers. This paper presents the development of a finite element model of a cracked sleeper that may be adapted to every condition of damage related to cracking. Cracks have been modelled as volumes characterised by a lower stiffness in order to simulate the loss of inertia. The models have been updated in terms of the frequencies of the first three bending modes of vibration of cracked and uncracked sleepers, obtained using experimental modal analysis technique, as presented in previous author’s work. The updating process led to the mechanical properties that define the whole model. Results point out that frequencies obtained with the numerical model agree experimental data obtained in laboratory. These models may be used in further investigations to predict the static influence of cracked sleepers in complete finite element models of ballasted railway tracks.

An impact excitation system for repeatable, high-bandwidth modal testing of miniature structures

Recent advances in various micro-manufacturing techniques have enabled utilization of miniature devices in numerous applications. However, testing, modeling, and predicting performance of these structures still poses various challenges. Experimental modal analysis techniques have been widely used to obtain the dynamic characteristics of structures; however, having very high natural frequencies, small vibration amplitudes, and high compliance, miniature structures require additional care during modal testing. Although recent developments in sensor technology enabled to obtain accurate high frequency vibration measurements during modal testing of miniature structures, excitation of miniature structures without any damage and within high frequency range in a reproducible manner is still being investigated. This paper presents design, development, and performance evaluation of a custom-made impact excitation system that enables repeatable, high bandwidth, single-hit impacts, and controllable impact force for modal testing of miniature structures. The system is equipped with a miniature force sensor attached to a custom designed flexure and an automated release mechanism driven by an electromagnet. The excitation bandwidth and the impact force exerted to the test structure can be controlled through control parameters: initial displacement given to the flexure and gap between the impact tip and the test surface.

Application of Vibration Based Damage Identification Techniques on Metallic Structures

In this work, a vibration based damage identification technique (VBDIT) is used to determine the presence of damages in metallic structures, using the undamaged structure information as reference. A complex structure is used in order to contrast the results obtained with those reported for techniques previously employed specially on single beam simulated structures. A finite element model of the structure is developed and validated as a support tool for the study. Theoretical and experimental modal analysis techniques were used as the main tool for damage identification.

Theoretical and experimental vibration analyses of trapezoidal and sinusoidal corrugated plates

In this study, the dynamic behavior of trapezoidal and sinusoidal corrugated plates, which are widely used in the fields of space, aviation, automotive design, construction, and shipbuilding, was analyzed. A total of 330 different surface models of corrugated plates with varying corrugation heights, widths and numbers of corrugations were created, which have various manufacturing parameters. At this stage, the total number of natural frequency and mode shape analyses conducted for different boundary conditions is 660, where 330 are for four sides free and 330 are for four sides encastre. Mode shapes were also obtained using the finite element method. In addition, changes in the trapezoidal cross-sectional profile were investigated by analyzing 38 different plates with varying cross-sectional profiles. Examining these results, the effects of corrugation height and number on the natural frequencies and mode shapes of the plate were determined. As a result of the study, a total of 368 plate drawings were prepared, and 736 analyses for two types of boundary conditions were performed. Moreover, the theoretical results were verified using the experimental modal analysis technique for some selected models that are being manufactured in the market.

Experimental modal analysis and dynamic strain fiber Bragg gratings for structural health monitoring of composite antenna sub-reflector

The objective of this work is to develop a new structural health monitoring system for composite aerospace structures based on dynamic response strain measurements and experimental modal analysis techniques. Fiber Bragg grating (FBG) optical sensors were used for monitoring the dynamic response of the composite structure. The structural dynamic behavior has been numerically simulated and experimentally verified by means of vibration testing. The hypothesis of all vibration tests was that actual damage in composites reduces their stiffness and produces an eigenfrequency shifting to lower values in the same sense as mass increase produces. Thus, damage was simulated by slightly varying locally the mass of the structure at different zones. The correlation between the simulated damage and the loss of stiffness was analytically defined. Experimental modal analysis based on the strain responses was conducted and the extracted strain mode shapes were the input for the damage detection expert system. A feed-forward back propagation neural network was the core of the damage detection system. The features-input to the neural network consisted of the strain mode shapes, extracted from the experimental modal analysis. Dedicated training and validation activities were carried out based on the experimental results. The system showed high reliability, confirmed by the ability of the neural network to recognize the size and the position of damage on the structure. The experiments were performed on a real structure i.e. a lightweight antenna sub-reflector, manufactured and tested at EADS CASA ESPACIO. An integrated FBG sensor network, based on the advantage of multiplexing, was mounted on the structure with optimum topology. Numerical simulation was used as a support tool at all the steps of the work. Potential applications for the proposed system are during ground qualification extensive tests of space structures and during the mission as modal analysis tool on board, being able via the FBG responses to identify a potential failure.

Planetary gear modal vibration experiments and correlation against lumped-parameter and finite element models

Experimental modal analysis techniques are applied to characterize the planar dynamic behavior of two spur planetary gears. Rotational and translational vibrations of the sun gear, carrier, and planet gears are measured. Experimentally obtained natural frequencies, mode shapes, and dynamic response are compared to the results from lumped-parameter and finite element models. Two qualitatively different classes of mode shapes in distinct frequency ranges are observed in the experiments and confirmed by the lumped-parameter model, which considers the accessory shafts and fixtures in the system to capture all of the natural frequencies and modes. The finite element model estimates the high-frequency modes that have significant tooth mesh deflection without considering the shafts and fixtures. The lumped-parameter and finite element models accurately predict the natural frequencies and modal properties established by experimentation. Rotational, translational, and planet mode types presented in published mathematical studies are confirmed experimentally. The number and types of modes in the low-frequency and high-frequency bands depend on the degrees of freedom in the central members and planet gears, respectively. The accuracy of natural frequency prediction is improved when the planet bearings have differing stiffnesses in the tangential and radial directions, consistent with the bearing load direction.

Experimental Evaluation of the Influence of Human-Structure Interaction for Vibration Serviceability

AbstractThe effects of human-structure interaction on the dynamic performance of occupied structures have long been observed. The inclusion of the effects of human-structure interaction is important to ensure that the dynamic response of a structure is not overestimated. Previous observations, both in service and in the laboratory, have yielded results indicating that the effects are dependent on the natural frequency of the structure, the posture of the occupants, and the mass ratio of the occupants to the structure. These results are noteworthy, but are limited in their application,because the data are sparse and are only pertinent to a specific set of characteristics identified in a given study. To examine these characteristics simultaneously and consistently, an experimental test structure was designed with variable properties to replicate a variety of configurations within a controlled setting focusing on the effects of passive occupants. Experimental modal analysis techniques were employed to both t...

Modal evaluation of fluid volume in spacecraft propellant tanks

Propellant mass-gauging in unsettled (sloshing) fluids is an important and unsolved problem in spacecraft operations and mission design. In the present work, we demonstrate the efficacy of the experimental modal analysis technique in determining the volume of fluid present in model spacecraft propellant tanks undergoing significant sloshing. Using data acquired over approximately 37 minutes of time in zero-gravity conditions provided over two years of parabolic flights, we estimate the resolution of the technique at low tank fill-fractions where other mass-gauging techniques are known to fail.Â

실험 모드해석을 이용한 균열 적층복합판의 손상평가

본 연구에서는 실험적 모드해석 기법을 이용하여 캔틸레버 및 양단고정 직사각형 적층복합판의 동적 테스트가 수행되었다. 균열 성장으로 인한 손상평가를 위하여 적층복합판에 인위적인 단계별 손상(균열)을 가하였으며, 충격해머 모드실험에 의해 얻어진 주파수응답함수(FRF), MAC(Modal Assurance Criterion) 값 및 모드매개변수(진동수, 모드형상, 감쇠비)의 변화를 분석하였다. 각 단계별 손상에 대한 적층복합판의 실험적 모드매개변수를 검증하기 위하여 유한요소해석으로부터 구한 고유진동수와 모드형상을 비교하였다. 손상은 벤치마크로서의 유한요소모델을 보정하는 과정으로부터 얻게 되는 적층복합판의 기하학적 특성 및 구조적 거동의 변화를 통하여 평가될 수 있음을 보여주었다. In this study, vibration tests are performed on cantilevered and clamped-clamped laminated composite rectangular plates using experimental modal analysis technique. The damages are simulated by applying progressive line cracks to the laminated composite plates for damage evaluations due to crack growth. The changes of frequency response functions(FRFs), MAC values, and modal parameters (frequency, mode shape and damping ratio) of the damaged composite plates, which are obtained by the modal testing of impact hammer, are investigated. Each experimental modal parameter of the progressively damaged composite plates is compared with natural frequencies and mode shapes obtained by finite element analysis. It is seen that the damage can be evaluated from the changes in the geometric properties and structural behaviors of the laminated composite plates resulting from the model updating process of the finite element model as a benchmark.

Study of Flow-Induced Vibration for the High-Head and Large Dimension Gate

In this paper study findings of flow-induced vibration of gate in the emptying culvert of Tishengqiao power station are presented. At first stage structure’s dynamic characteristics are investigated carefully using the experimental modal analysis technique, and entire structure modal information is obtained. In addition, flow pulsating pressure properties acting on the gate are investigated. Various physical values including vibration acceleration, dynamic displacement and dynamic stress caused by high velocity flow are studied. Finally gate structure’s dynamic reliability is analyzed and safety operation principle are presented.

Full-scale 실험 모드해석을 이용한 노후화된 철도판형교의 진동특성

본 연구에서는 실험적 모드해석 기법을 이용하여 외부환경에 직접 노출되어 있는 실제 철도판형교의 full-scale 동적 테스트가 수행되었다. 충격해머 모드실험에 의해 얻어진 철도판형교의 모드 매개변수를 유한요소해석으로부터 구한 고유진동수와 모드형상과 비교, 분석하였다. 실험적 모드해석에 의해 측정된 실험 데이터와 해석적 진동분석에서 얻어지는 출력만의 데이터를 교량 부재의 기하학적 특성 및 재료적 특성을 다양하게 고려하여 모델보정 테크닉에 적용하였다. 철도판형교의 실험적 모드해석 결과를 검증하기 위한 유한요소모델이 모드인식 기법을 이용하여 보정되었다. 실험 데이터와 유한요소해석 기준모델의 모델보정과정의 결과와 함께 부재특성의 변화를 통하여 이루질 수 있는 손상평가에 대한 기초적 데이터베이스가 제공된다. In this study, experimental vibration tests are performed on a real full-scale railway steel plate girder bridge, which resides in open-space environments. Using experimental modal analysis techniques, the modal parameters of the railway steel plate girder bridge yielded by the modal testing of the impact hammer are compared and investigated with the natural frequencies and mode shapes obtained by finite element analysis. This work focuses on the application of model updating techniques to measured experimental data and output-only data from an analytical vibration study that takes into account various geometric and material properties of the bridge members. A finite element model of the railway bridge structure is used to verify the modal experimental results. It is subsequently updated using the corresponding modal identification technique. The basic database is provided to evaluate damage, which can be determined based on the changes in the element properties, resulting from the process of updating the finite element model benchmark and experimental data.

Airframe vibration and damping characteristics of flight and laboratory test structures

Vibration damping levels over a wide frequency band for complex built-up structures are extremely difficult to predict. In this work, rotorcraft airframe structures were targeted for vibration based Structural Health Monitoring (SHM) technology development and implementation. Damping is a critical parameter for understanding the vibration characteristics of a structure and realistic damping levels must be established for the application of relevant vibration based damage detection simulations and laboratory testbed based assessments to be made. The goal of this investigation was to quantify the vibra¬tion and damping characteristics of an actual flight system structure, compare and contrast the results to laboratory development test structures, and identify any important trends and discrepancies. Experimental modal analysis and decay techniques were applied to a UH-60 Blackhawk airframe in a �flight� conflguration. Structural mobility, resonance frequencies, modal damping, and high frequency one third octave band decay based damping levels were mea¬sured and quantified. Similar measurements were also made on laboratory test structures used for technology development and verification testing. The mea¬sured (flight configuration) airframe damping loss factors varied from approx¬imately 0.01 to 0.10 over the frequency range 2 to 10,000 Hz. The flight vehicle damping characteristics were compared and contrasted with laboratory test structures, quantifying the damping effects due to built-up structure complexity. Methods for enhancing the laboratory test structures were also investigated showing positive results.

804 双安定性を有する非対称積層複合材シェルの振動特性評価

Asymmetric laminates of fibrous composites have strong elastic and thermal anisotropic properties, resulting in the curved surfaces caused by residual thermal strain during curing process. Specific asymmetric laminates indicate bi-stable states in terms of surface shapes, and these bi-stable states enable large deformation with relatively small energy input. This becomes an advantage when the asymmetric laminate is applied to the aerospace structures such as a morphing airfoil. The present paper first reveals occurrence criteria of bi-stable states for asymmetric laminates. In-plane buckling strengths are employed as an index of occurrence and calculated for plates with different width-to-thickness ratio and lay-up configurations. Then, vibration properties of bi-stable composite shells are evaluated by using both numerical study with the Ritz method based on Donnel-Mushtari theory and the experimental modal analysis technique. The numerical and experimental results show that there are some relations between the buckling strength and the occurrence of bi-stable states within plates having specific lay-up configurations, and asymmetric laminates with bi-stable states indicate different vibration characteristics before and after snap-thorough deformation.

Modal Survey Test and Model Correlation of the CASSIOPE Spacecraft

A comprehensive modal survey test based on multi-input multi-output experimental modal analysis techniques was conducted on the CASSIOPE spacecraft. This paper describes the details of the methodology used to perform the successful experimental modal test to efficiently extract the critical modes of the spacecraft. Results from the modal test have been used to validate the analytical finite element model and to provide confidence in the structural integrity of the spacecraft design. The test was performed on the flight model of the CASSIOPE spacecraft in the final stages of integration, which included all of the payload and bus instruments and electronics boxes. The multiple-input excitation for the spacecraft was generated using two portable electrodynamic modal shakers installed on the top and bottom of the spacecraft to distribute the excitation energy and the response was measured using 81 miniature accelerometers. A digital multi-channel data acquisition system was used to record the time domain data and calculate the frequency domain spectra. Advanced modal analysis software was used to extract modal parameters from the measured data and critical modes were compared with predictions from the finite element model. Most modes identified through the experimental data compared favorably with the predictions. Nevertheless, some differences were large enough to require iterative update of the finite elelement model. The structural dynamics information from the updated finite element model was used to plan the mechanical vibration qualification test and predict the response of the spacecraft to the launch vehicle environmental loads through coupled loads analysis.

Computer aided analysis of mild steel and FRP composite foundations

This paper outlines the development of a marine foundation structure using lightweight composite materials to replace the existing steel structure material. The research is conducted using deterministic and experimental modal analysis techniques. Structural analyses such as static, modal and frequency response analysis are carried out in finite element analysis package ANSYS. Experimental modal analyses of mild steel and composite foundations are carried out to validate the results of computational analysis and finite element model of both the foundations. Further validation of software Msc NASTRAN is used to compare the results of modal analysis of composite foundation in ANSYS. Experimental shock test is also carried out to composite foundation. The results show that the composite marine foundation structure is able to reduce the weight, provide better vibration damping characteristics and achieve the safety as compared with that of mild steel foundation.

357 充填物を有するフレキシブルニ重配管の振動減衰性能評価

It is well known in the fields of nuclear and chemical plants that various pipes conveying liquids and gasses should be safe and reliable against excitation by earthquakes. This study proposes an effective use of the flexible pipe system, consisting of a double-walled pipes and infilling materials, to increase the bending rigidities and robustness to excitation. In particular, the vibration properties of such double-walled flexible pipe are examined in terms of changes of infilling rates. Aluminum pipes and glass beads are employed in the experiment to assemble the flexible pipe, and the natural frequencies and damping ratios are measured by using an experimental modal analysis technique with an impulse hammer. The present double-walled pipe system with beads successfully results in higher damping ratios than the pipe without beads. To reveal further relationship between infilling rates and damping ratios, experiments for the single-walled pipe with different filling rates are also conducted. It becomes then clear that there is a moderate filling rate to improve the damping properties of the flexible pipe.

Dynamic characterization and modification of dynamic properties of a micro scanner

Micro electro mechanical systems (MEMS) are used in many application areas in different disciplines and took their place among the most promising technologies. The performance of such systems is primarily related to their dynamical characteristics. This study presents the dynamic characterization techniques that are used to identify the modal parameters of a MEMS device and the methods that can be implemented to change its dynamic response. An electrostatic scanner is chosen as the case study to demonstrate the developed methodologies. Initially, the micro scanner is characterized using experimental modal analysis techniques to obtain frequency response function, modal damping, resonance frequencies, and mode shapes. Then, velocity and position feedback control loops are implemented to the scanner system to alter the damping and stiffness characteristics. A closed-loop Simulink model of the scanner is developed to verify the experimental measurements. Several curve fitting methods are used in order to have an accurate representation of the scanner system. Using the model, the influence of both position and velocity feedback on the effective damping, resonance frequency and the transient behavior of the scanner is investigated. The stability limits of the scanner under velocity feedback are also studied via numerical simulations. Based on the experimental and simulation results, the methodology developed in this study proves itself to be very efficient to alter the dynamical characteristics of the MEMS structures and it can be easily adapted to other MEMS applications.

Structural performance assessment and fatigue analysis of a railway bridge

Increased economic activity in South Africa has put pressure on both rail and road transportation systems. In particular, there is a need to increase loading on the rail systems. The railway network consists of a number of steel bridges approaching the end of their design life. The fitness of these bridges to carry higher loads needs to be assessed. The present paper describes a systematic procedure for the condition assessment of a 100 year old wrought iron railway bridge. Experimental modal analysis techniques were used to calibrate the finite-element model. The calibrated model was used to predict the behaviour of the bridge under proposed loading regimes. Strain measurements were taken at selected locations and were used to assess the fatigue life of the bridge. This work demonstrates the effectiveness of combining experimental modal analysis with actual structural response from live loading for condition assessment and performance prediction.

A Comparison of the Vibration Characteristics of Carbon Fiber Reinforced Plastic Plates with those of Magnesium Plates

An experimental and numerical investigation is conducted to evaluate vibration characteristics of an advanced composite material system, namely carbon fiber reinforced plastic (CFRP), relative to a magnesium alloy currently used in the vibration testing industry. Experimental test specimens for both materials, with varying thicknesses, are tested with two boundary conditions — the free condition, to evaluate the natural frequencies and damping of the two materials and, secondly, a general constrained condition, typical of vibration testing. Experimental modal analysis techniques are used to measure the vibration characteristics including natural frequencies, damping and mode shapes. The results from these tests show that the natural frequency and mode shape for relatively thin CFRP plates were comparable with those of magnesium plate. Although the mode shapes also compare well for thick CFRP and magnesium specimens, the natural frequencies were found to have significant differences between the two material systems. The largest difference between the two material systems, present for all thicknesses, is found to be the damping values for the respective vibration modes. This unique characteristic of the CFRP material presents an opportunity for a performance increase in the vibration testing system’s community.