Modal Testing Technique Research Articles

Modal parameters and damage evaluation of steel fiber reinforced concrete under flexural cyclic loadings

Structures may be subjected to static and dynamic loads during their service life. When it comes to the dynamic spectrum, wind action, ocean waves and seismic loads are key examples and are responsible for cracking evolution and material degradation along their lifespan. Therefore, the fatigue behavior of cementitious materials has been receiving great attention due to cyclic loading that can be of natural occurrence or induced by human activity lately. The present research aims to investigate the evolution of the modal parameters of steel fiber reinforced concretes under flexural cyclic load and to evaluate how the mechanical degradation can be monitored from a non-destructive impact modal testing over the cycles. The dynamic properties of the material, including natural frequencies, vibration modes and damping ratios, were tracked to assess the level of fatigue damage using modal testing techniques. Hence, it was possible to evaluate the material mechanical degradation as a function of the dynamic parameters. From the dynamic test, it was possible to evaluate the presence of damage, which was corroborated by the observed stiffness degradation. The first flexural mode and the following second, third and sixth vibration modes were more affected along the cycles. Finally, the proposed non-destructive methodology was effective in assessing the mechanical decay of cementitious composites.

Structural Integrity Assessment of Concrete Sleepers by Modal Test Technique.

Concrete sleepers used in railway engineering are subject to damage, such as cracks and breakage. Damaged concrete sleepers undergo changes to their material and structural properties, including response, mode shape, and natural frequency. Therefore, we have proposed modal testing in this study to quantitatively evaluate the structural integrity of concrete sleepers. The results of modal testing were compared with those of numerical analysis and visual inspection. In addition, an impact hammer test was conducted to evaluate the structural performance of damaged concrete sleepers. The results show that natural-frequency analysis using the modal-testing technique can usefully complement visual inspection for structural performance evaluation in the field.

Modal testing of masonry constructions by ground-based radar interferometry for structural health monitoring: A mini review

Modal testing is one of the most effective experimental techniques for the structural health monitoring of masonry constructions, as it provides useful information for the calibration of structural models and for the assessment of structural damage. However, the application of modal testing to masonry constructions is sometimes hindered by the complexity of the conventional experimental set-up, which is generally based on contact sensors. In order to overcome this issue, several researchers are exploring the application of the ground-based radar interferometry, which is an increasingly popular measurement technique for remotely monitoring displacement and vibration of structures. Given the recently increasing number of articles on this subject, here we propose a mini review on the most significant works dealing with the application of ground-based radar interferometry for modal testing of masonry constructions. In particular, we show the current state of the art and highlight the main research gaps with the purpose of assessing the effectiveness of ground-based radar interferometry for the structural health monitoring of these constructions. Our mini review is primarily aimed at engineers and scientists who already know about modal testing and radar interferometry technique and are interested in the specific application to masonry constructions.

A Full-Field Non-Contact Thermal Modal Testing Technique Under Ambient Excitation

Temperature has significant effects on modal parameters of a structure. However, it is difficult to do a full-field dynamic measurement and identify modal parameters of the structure under high temperature and working conditions. In this work, three-dimensional digital image correlation combined with Bayesian operational modal analysis is used to yield a full-field non-contact thermal modal testing technique under ambient excitation. Thermal modal verification experiments are carried out on a titanium plate with free boundary conditions under ambient excitation by a low-cost high-temperature dynamic measurement system. Natural frequencies and mode shapes of the plate under different average high temperatures are studied in detail. From the experimental process, it is believed that the thermal modal testing technique proposed in this work is an effective alternative testing technique to traditional contact testing techniques, and after some improvements, one can achieve thermal modal analysis of a structure under high temperature and working conditions.

THE TRANSFER MATRIX METHOD FOR MODAL ANALYSIS OF CRACKED MULTISTEP BEAM

The present study addresses the modal analysis of multistep beam with arbitrary number of cracks by using the transfer matrix method and modal testing technique. First, there is conducted general solution of free vibration problem for uniform beam element with arbitrary number of cracks that allows one to simplify the transfer matrix for cracked multistep beam. The transferring beam state needs to undertake only at the steps of beam but not through crack positions. Such simplified the transfer matrix method makes straightforward to investigate effect of cracks mutually with cross-section step in beam on natural frequencies. It is revealed that step-down and step-up in beam could modify notably sensitivity of natural frequencies to crack so that the analysis provides useful indication for crack detection in multistep beam. The proposed theory was validated by an experimental case study

Experimental analysis of pre-compressed circular cylindrical shell under axial harmonic load

In this paper the nonlinear dynamics of circular cylindrical shells under axial static (compressive) and periodic resonant loads have been experimentally investigated, the goal is to study the dynamic scenario and to analyze nonlinear regimes. A special test rig has been developed for the experiment in order to apply a static axial load combined with a dynamic axial load. The setup allows for investigating the linear behavior under static preload by means of the usual modal testing techniques; moreover, it allows for analyzing the nonlinear response which occurs when the dynamic axial load is periodic and gives rise to complex resonances. The complex dynamics, arising when a periodic axial load excites the asymmetric (shell like) modes, are analyzed by means of amplitude frequency diagrams, waterfall spectrum diagrams, bifurcation diagrams of Poincaré maps; a deep analysis of time histories, spectra, phase portraits and Poincaré maps completes the study of the complex dynamic scenario.

Static and dynamic behaviours of helical spring in MR fluid

MR fluid has been used in automobile industry for vibration suppression device. However its dynamic interaction between structural spring and electro-magnetised MR fluid has not been thoroughly investigated. As a result, this paper highlights static and dynamic behaviours of helical spring interacting with MR fluid magnetised at various levels. Static hysteresis behaviours have been evaluated altogether with the dynamic modal properties of the system. Modal impact hammer testing technique was used to investigate the modal parameters. It is found that MR fluid improves the hysteresis capacity and dynamic properties of the systems when it is electro-magnetised. The outcome of this study will lead to a new development of new spring-dashpot system using MR fluid for better control in adaptive tuneable vibration damping and stiffness suppressing real-time dynamic motions such as the train body, passenger seats, train door, etc.

Dynamic testing of free field response in stratified granular deposits

The dynamic free field response of two stratified deposits with different stiffness ratios between the top and the bottom layer was analysed by shaking table testing. The granular deposits were contained in a laminar shear box and subjected to a wide set of dynamic inputs with different frequency content. Two exploratory modal testing techniques were employed to measure the natural frequency of the individual layers and the results were employed in the calculation of the fundamental period of the overall stratified profile by an extended variant of the Madera procedure [1]. The dynamic response was investigated in relation to the frequency content of the dynamic excitation, the granular material properties and the stiffness characteristics of the enclosing container. The measured dynamic stiffness for the mono-layered and the bi-layered sand deposits compare well with previous empirical curves for sands increasing the confidence in the shaking table and shear stack testing as tools of dynamic investigation of granular media.

Investigation of warping effect on coupled torsional-axial vibration of drilling tool

Coupled torsional-axial vibration of the drilling tool plays a significant role in the machining dynamics of the drilling process. This paper models the torsional-axial vibration of the drilling tool due to the warping deformation of the pretwisted flute. The warping-dependent cross-sectional properties of the drill flute are obtained from 2D finite element method. Natural frequency and mode shape corresponding to the torsional-axial vibration of the drill are predicted. The effects of helix angle, web thickness, and aspect ratio on the coupled torsional-axial dynamics are analyzed for conventional and micro drilling. The simulations are validated by experimental results using modal testing technique. The proposed model is able to provide guidance in selecting geometric configuration of the drilling tool for required torsional-axial dynamic behavior with high computational efficiency and guide the drilling process in order to reach desired hole quality.

Simultaneous measurement of elastic constants of full-size engineered wood-based panels by modal testing

Abstract Engineered wood-based panels are widely used in structural applications. Accurate measurement of their elastic properties is of great importance for predicting their mechanical behavior during structural design. In this study, an efficient non-destructive test method for measurement of effective elastic constants of orthotropic wood-based panels is proposed based on a modal testing technique. An algorithm was developed based on an improved approximate frequency equation of transverse vibration of orthotropic plates under the boundary condition, in which two opposite sides are simply supported and the other two are free (SFSF). The method is able to predict the frequency ranges and mode indices as well as corresponding normalized sensitivity to elastic constants based on initial estimates of orthotropic ratios with uncertainties and measured fundamental natural frequency. Full-size engineered wood-based panels including cross laminated timber (CLT), oriented strand board (OSB), and medium density fiberboard (MDF) were tested with the proposed method. In general, the measured elastic constants of the three types of panel based on modal test agreed well with those corresponding values measured by static tests. More tests are needed with a range of panel sizes and types for further validation of the proposed test method.

Crack detection in pile by measurements of frequency response function

Explicit expressions of either frequency equation or frequency response function (FRF) have been derived for multiple cracked bar. The expressions allow to thoroughly investigate the effect of cracks on the modal parameters such as natural frequencies and FRF of bar with different boundary conditions. The exact expression of FRF provides a basis to propose an efficient procedure for multi-crack detection in pile by modal testing technique. Numerous examples are numerically examined for illustrating the developed theory, and an experimental study is accomplished for validating the proposed crack detection procedure.

Development of a finite element model updating technique for estimation of constituent level elastic parameters of FRP plates

A methodology has been developed combining the experimental modal testing and finite element technique to determine the constituent level fiber and matrix elastic properties of fiber reinforced plastics (FRP) composite plates. The methodology implements an inverse eigensensitivity algorithm based on the minimization of error function defined by weighted difference of eigenvalues and mode-shapes. The method is robust against modal and coordinate sparsity, even under random noise. The proposed model updating technique can be deployed for rapid assessment of degrading elastic properties at the constituent level for existing FRP structures in-situ; thus is suitable for health monitoring exercise.

A modified complex modal testing technique for a rotating tire with a flexible ring model

Natural frequencies, mode shapes and modal damping values are the most important parameters to describe the noise and vibration behavior of a mechanical system. For rotating machinery, however, the directivity of the propagation wave of each mode should also be taken into account. For rotating systems, this directivity can be determined by complex modal testing. In this paper, a rolling tire is represented as a flexible ring model. The limitation of application of the complex modal testing which requires two directional measurements at a certain point, which is difficult to measure in practice, has been overcome through a modified complex modal testing which requires only one directional measurements at any two points. The technique is described in detail and applied to both a numerical example and to an experimental data set of a real rotating tire.

English

Brake squeal is largely accepted by Scientists and technologists as a major problem which is induced by friction induced vibrations. It is well accepting to be frequently occurring at a frequency above 1 KHz. This report is interested in the finite element analysis (FEA) and experimental modal analysis (EMA) of a commercial disc of the brake system. The modal analysis has been used in this work to determine the natural frequency and mode shape pattern. The destination is to predict the squeal at the former phase of invention using a more realistic model. Firstly, the FE models of the disc brake parts were produced using 3D solid element. The experimental modal testing technique known as an impact hammer test has been held away to obtain modal parameters of the disc brake structure. The FE model is validated by comparing experimental results of the brake components. It is ground that good correlation is achieved in the dynamic properties of the brake parts.

Operational Modal Analysis of Torsional Modes in Rotating Machinery

Traditional experimental modal testing techniques rely on controlled and measured excitation together with measured responses in order to identify the mode shape, natural frequency, and damping factor of each mode. Applying a controlled and measured excitation to a rotor train when in operation is logistically difficult and especially challenging in the field. Operational modal analysis (OMA) identifies the modal parameters of a system from measurement of response due to some (unknown) excitation. OMA has proven successful over the past several decades on nonrotating structures but has relatively rarely been applied to rotating machinery. Case studies are presented demonstrating the use of OMA in identifying torsional modes on an electric motor driven reciprocating compressor, on a diesel engine driven fire water pump, and on a marine propulsion system. In contrast to lateral modes, torsional modes of rotor trains are typically not speed dependent. However, phenomena exist whereby the torsional modes may be different at stand still, off-load and at different loads. The case studies provide examples of such phenomena and also of significant differences between predicted and measured behavior which suggests that improvements in industrial practice would be beneficial. Such improvements should be based on reconciliation of measured and predicted behavior and OMA offers a valuable tool to facilitate this. OMA provides a significant benefit in investigating and understanding torsional behavior in operation.

The Use of Modal Testing within Nuclear Weapon Dismantlement Verification

The technical verification of a possible future nuclear arms control agreement is a complex challenge for technology developers. The focus of this article is on the use of modal testing techniques as a method for maintaining a chain of custody over containerized treaty accountable items (TAI) and monitoring equipment. Modal testing is a specialized form of resonant vibration analysis often used for the purpose of structural identification, condition monitoring, and damage detection. From a chain of custody perspective, it was postulated that a modal vibration signature might be used to identify a particular treaty accountable container or container/object system, or provide evidence of tampering. This article considers the advantages and disadvantages of modal testing as a potential chain of custody tool. Experimental results are discussed relating to deployment, tamper indication, unique identification and data analysis methodology.

Finite element model calibration of steel frame buildings with and without brace

This paper focuses on both the monitoring of the dynamic response of steel buildings with and without brace elements and the importance of model calibration on the steel buildings. The study involves the application of modal testing techniques to collect data from a three-story steel frame building model tested at the Civil Engineering Department of Karadeniz Technical University. The experimental measurements are performed under randomly generated loads. Dynamic characteristics (natural frequencies, mode shapes and modal damping ratios) obtained from bare and braced steel frame models are compared with each other. The initial analytical models of the steel model for bare and braced cases are developed and calibrated according to the experimental measurement results. The calibration process aims to minimize the differences between experimental and analytical natural frequencies. The connection rigidities of the beam-to-column are selected as a calibrating parameter in the model calibration process. It is observed that the brace elements cause an increase in the natural frequencies due to the increasing stiffness as well as the changes in the modal behavior. Static analyses of the steel frame model for bare and braced cases are carried out to emphasize the importance of the model calibration by comparing maximum lateral displacements. The calibrated analytical models produce larger lateral displacements than the initial models. The results reveal that the dynamic behavior of steel structures should be evaluated considering the calibrated models for safety of these structures.

Experimental Modal Testing of the Elastic Tube Bundle

The elastic tube bundle is a new-style heat exchange element that can enhance heat transfer efficiency and reduce energy consumption. It is difficult to obtain exact analytical solutions because of the complex shape and constraint condition. An experimental modal testing technique is applied to get the dynamic properties. Some issues to which one should pay attention during the experiment are emphasized. Natural frequencies and mode shapes are identified from the test and compared to numerical results. Agreement is found for most frequencies of interest. But, some discrepancies exist for the vibration in-plane due to the inevitable operation error.

Dynamic Performance Test and Analysis of Spindle System of High Speed Grinding Machine Tools

The high speed machine tool spindle plays an important role in machining operations. Based on modal test techniques and rotating machinery signature techniques, the research focused on the test and analysis of the dynamic characteristics of spindle system of high speed grinder. First, an experimental modal analysis (EMA) is performed in the spindle system; then, the frequency response characteristics and the FRFs of the system should be considered. Furthermore, rotating machinery signature techniques are used to analyze the dynamic behavior of the spindle system in process due to the limitation of modal test techniques, and the theoretical proof is given to explain the spindle system order. The results show that it requires the machine tool to avoid the use of vibration-sensitive speed in the production process and the machine production process, especially bearing manufacturing to improve its accuracy.

항공기 구조 동특성 해석을 위한 외부 장착 포드의 단순화 유한요소 모델 구축

In this study, the natural frequencies and mode shape of an external mounting pod were verified using the modal analysis and modal testing technique for a pod mounted on an aircraft. The procedure associated with the FE model building of an external mounted pod to predict the dynamic behavior of aircraft structures is described. The simplified FE model reflecting the results of the modal testing of a pod is built through the optimization and will be applied to the structural dynamic model of an aircraft which is used to verified the stability of vibration and flutter of an aircraft.