Use of experimental dynamic substructuring to predict the low frequency structural dynamics under different boundary conditions

2017 ◽  
Vol 23 (11) ◽  
pp. 1444-1455
Author(s):  
Walter D’Ambrogio ◽  
Annalisa Fregolent
Keyword(s):  
Boundary Conditions ◽  
Rigid Body ◽  
Frequency Response ◽  
Response Function ◽  
Frequency Response Function ◽  
Low Frequency ◽  
Response Functions ◽  
Frequency Response Functions ◽  
Different Boundary Conditions ◽  
Rigid Body Modes

Flexible structural components can be attached to the rest of the structure using different types of joints. For instance, this is the case of solar panels or array antennas for space applications that are joined to the body of the satellite. To predict the dynamic behaviour of such structures under different boundary conditions, such as additional constraints or appended structures, it is possible to start from the frequency response functions in free-free conditions. In this situation, any structure exhibits rigid body modes at zero frequency. To experimentally simulate free-free boundary conditions, flexible supports such as soft springs are typically used: with such arrangement, rigid body modes occur at low non-zero frequencies. Since a flexible structure exhibits the first flexible modes at very low frequencies, rigid body modes and flexible modes become coupled: therefore, experimental frequency response function measurements provide incorrect information about the low frequency dynamics of the free-free structure. To overcome this problem, substructure decoupling can be used, that allows us to identify the dynamics of a substructure (i.e. the free-free structure) after measuring the frequency response functions on the complete structure (i.e. the structure plus the supports) and from a dynamic model of the residual substructure (i.e. the supporting structure). Subsequently, the effect of additional boundary conditions can be predicted using a frequency response function condensation technique. The procedure is tested on a reduced scale model of a space solar panel.

Change of modal parameters due to crack growth in a tripod tower platform

1993 ◽  
Vol 20 (5) ◽  
pp. 801-813 ◽  
Author(s):  
Yin Chen ◽  
A. S. J. Swamidas
Keyword(s):  
Finite Element ◽  
Frequency Response ◽  
Response Function ◽  
Frequency Response Function ◽  
Experimental Results ◽  
Modal Testing ◽  
Modal Parameters ◽  
Response Functions ◽  
Frequency Response Functions ◽  
Strain Frequency

Strain gauges, along with an accelerometer and a linear variable displacement transducer, were used in the modal testing to detect a crack in a tripod tower platform structure model. The experimental results showed that the frequency response function of the strain gauge located near the crack had the most sensitivity to cracking. It was observed that the amplitude of the strain frequency response function at resonant points had large changes (around 60% when the crack became a through-thickness crack) when the crack grew in size. By monitoring the change of modal parameters, especially the amplitude of the strain frequency response function near the critical area, it would be very easy to detect the damage that occurs in offshore structures. A numerical computation of the frequency response functions using finite element method was also performed and compared with the experimental results. A good consistency between these two sets of results has been found. All the calculations required for the experimental modal parameters and the finite element analysis were carried out using the computer program SDRC-IDEAS. Key words: modal testing, cracking, strain–displacement–acceleration frequency response functions, frequency–damping–amplitude changes.

scholarly journals Damage detection based on output-only measurements using cepstrum analysis and a baseline-free frequency response function curvature method

2022 ◽  
Vol 105 (1) ◽  
pp. 003685042110644
Author(s):  
Ayisha Nayyar ◽  
Ummul Baneen ◽  
Muhammad Ahsan ◽  
Syed A Zilqurnain Naqvi ◽  
Asif Israr
Keyword(s):  
Damage Detection ◽  
Frequency Response ◽  
Response Function ◽  
Frequency Response Function ◽  
Damage Index ◽  
Real Life ◽  
System Response ◽  
Response Functions ◽  
Frequency Response Functions ◽  
Output Only

Low-severity multiple damage detection relies on sensing minute deviations in the vibrational or dynamical characteristics of the structure. The problem becomes complicated when the reference vibrational profile of the healthy structure and corresponding input excitation, is unavailable as frequently experienced in real-life scenarios. Detection methods that require neither undamaged vibrational profile (baseline-free) nor excitation information (output-only) constitute state-of-art in structural health monitoring. Unfortunately, their efficacy is ultimately limited by non-ideal input excitation masking crucial attributes of system response such as resonant frequency peaks beyond first (few) natural frequency(ies) which can better resolve the issue of multiple damage detection. This study presents an improved frequency response function curvature method which is both baseline-free and output-only. It employs the cepstrum technique to eliminate [Formula: see text] decay of higher resonance peaks caused by the temporal spread of real impulse excitation. Long-pass liftering screens out the bulk of low-frequency sensor noise along with the excitation. With more visible resonant peaks, the cepstrum purified frequency response functions (regenerated frequency response functions) register finer deviation from an estimated baseline frequency response function and yield an accurate damage index profile. The simulation and experimental results on the beam show that the proposed method can successfully locate multiple damages of severity as low as 5%.

Identification of Rotating Asymmetry in Rotating Machines by Using Reverse dFRF

1999 ◽  
Author(s):  
Chong-Won Lee ◽  
Kye-Si Kwon
Keyword(s):  
Frequency Response ◽  
Response Function ◽  
Frequency Response Function ◽  
Modal Testing ◽  
Vibration Sensor ◽  
Response Functions ◽  
Physical Realization ◽  
Simple Method ◽  
Testing Method ◽  
Asymmetric Rotor

Abstract A quick and easy but comprehensive identification method for asymmetry in an asymmetric rotor is proposed based on complex modal testing method. In this work, it is shown that the reverse directional frequency response function (reverse dFRF), which indicates the degree of asymmetry, can be identified with a simple method requiring only one vibration sensor and one exciter. To clarify physical realization associated with estimation of the reverse dFRF, its relation to the conventional frequency response functions, which are defined by the real input (exciter) and output (vibration sensor), are extensively discussed.

Model updating–based damage detection of a concrete beam utilizing experimental damped frequency response functions

2018 ◽  
Vol 22 (4) ◽  
pp. 935-947 ◽  
Author(s):  
Qianhui Pu ◽  
Yu Hong ◽  
Liangjun Chen ◽  
Shili Yang ◽  
Xikun Xu
Keyword(s):  
Damage Detection ◽  
Frequency Response ◽  
Frequency Response Function ◽  
Concrete Beam ◽  
Model Updating ◽  
Response Functions ◽  
Experimental Frequency ◽  
Frequency Response Functions ◽  
Function Formulation ◽  
Analytical Frequency

This article evaluates the use of experimental frequency response functions for damage detection and quantification of a concrete beam with the help of model updating theory. The approach is formulated as an optimization problem that intends to adjust the analytical frequency response functions from a benchmark finite element model to match with the experimental frequency response functions from the damaged structure. Neither model expansion nor reduction is needed because the individual analytical frequency response function formulation is derived. Unlike the commonly used approaches that assume zero damping or viscous damping for simplicity, a more realistic hysteretic damping model is considered in the analytical frequency response function formulation. The accuracy and anti-noise ability of the proposed approach are first verified by the numerical simulations. Next, a laboratory reinforced concrete beam with different levels of damage is utilized to investigate the applicability in an actual test. The results show successful damage quantification and damping updating of the beam by matching the analytical frequency response functions with the experimental frequency response functions in each damage scenario.

A Practical Estimation of Frequency Response Functions for System Decoupling Indirectly Using a Variable Cross Section Rod

2018 ◽  
Vol 140 (5) ◽  
Author(s):  
Jun Wang ◽  
Tian-Ya Meng ◽  
Ming-Yu Li ◽  
Teik C. Lim ◽  
Wen-Xuan Kuang
Keyword(s):  
Frequency Response ◽  
Cross Section ◽  
Low Frequency ◽  
Coupled System ◽  
Variable Cross Section ◽  
Probe Method ◽  
Variable Cross ◽  
Response Functions ◽  
Frequency Response Functions ◽  
Spatial Limitation

It is of high importance to be able to decouple a system to obtain the dynamic characteristics of its substructures; however, the necessary frequency response functions (FRFs) of the coupling interface are usually challenging to measure due to the limited accessible space and complex geometries. In this paper, a measurement technique in the decoupling process of a coupled system is proposed in order to obtain the FRFs at coupling interface. Specifically, a variable cross section rod is adopted to transmit the dynamic behavior of coupling interface. The proposed technique has three advantages: (a) the thick end with large cross section can provide enough area for applying excitation force like using impact hammer and/or setting up sensors; (b) the slender end with small cross section can break through the spatial limitation more easily; and (c) the convenience that no additional experimental setup is required but just using an available variable cross section rod. Vibrational equation of the variable cross section probe method is derived and then combined with the existing decoupling theories. Finally, the proposed probe method and the new decoupling theory combining probe theory are validated through numerical simulations (FEM) and laboratory experiments, respectively. The results show its great practicability in decoupling process especially in low frequency range.

Characterization of Electro-Active Paper Vibration Sensor by Impact Testing and Random Excitation

2015 ◽  
Vol 07 (04) ◽  
pp. 1550065 ◽  
Author(s):  
Zafar Abas ◽  
Dong Ho Yang ◽  
Heung Soo Kim ◽  
Moon Kyu Kwak ◽  
Jaehwan Kim
Keyword(s):  
Frequency Response ◽  
Response Function ◽  
Polyvinylidene Fluoride ◽  
Frequency Response Function ◽  
Low Frequency ◽  
Random Excitation ◽  
Impact Testing ◽  
Vibration Sensor ◽  
Dynamic Strain ◽  
The Time Domain

We characterized a vibration sensor made of piezoelectric paper by measuring the frequency response function of an aluminum cantilever that was subjected to impulse loading and random excitation. The dynamic characteristics of the device were measured by recording the transient response of the smart cantilever beam with a pair of electro-active paper (EAPap) and polyvinylidene fluoride (PVDF) sensors located at a 5 mm distance from the clamped end as well as from a second pair of piezoelectric sensors located at a distance of 140 mm. The responses were measured by impacting the cantilever at its tip and at its mid-point. A fast Fourier transform was applied on the time domain data to measure the resonant frequencies of the vibrating structure. Both the EAPap and the PVDF sensors were observed to be very sensitive to varying levels of dynamic strain. The EAPap sensor showed a low strain sensitivity that was found to be desirable due to the inherent piezoelectricity and eco-friendly behavior of the material. The results revealed that the dynamic sensing ability of the EAPap at a low frequency range was quite comparable to that of PVDF when monitoring structural vibrations. The frequency response function was also measured via random excitation, piezoelectricity of the EAPap sensor shows potential for sensing vibrations with a dynamic response.

scholarly journals Cepstrum-based damage identification in structures with progressive damage

2018 ◽  
Vol 18 (1) ◽  
pp. 87-102 ◽  
Author(s):  
Ulrike Dackermann ◽  
Wade A Smith ◽  
Mehrisadat Makki Alamdari ◽  
Jianchun Li ◽  
Robert B Randall
Keyword(s):  
Frequency Response ◽  
Modal Analysis ◽  
Response Function ◽  
Principle Component Analysis ◽  
Frequency Response Function ◽  
Component Analysis ◽  
Operational Modal Analysis ◽  
Progressive Damage ◽  
Response Functions ◽  
Principle Component

This article aims at developing a new framework to identify and assess progressive structural damage. The method relies solely on output measurements to establish the frequency response functions of a structure using cepstrum-based operational modal analysis. Two different damage indicative features are constructed using the established frequency response functions. The first damage feature takes the residual frequency response function, defined as the difference in frequency response function between evolving states of the structure, and then reduces its dimension using principle component analysis; while in the second damage indicator, a new feature based on the area under the residual frequency response function curve is proposed. The rationale behind this feature lies in the fact that damage often affects a number of modes of the system, that is, it affects the frequency response function over a wide range of frequencies; as a result, this quantity has higher sensitivity to any structural change by combining all contributions from different frequencies. The obtained feature vectors serve as inputs to a novel multi-stage neural network ensemble designed to assess the severity of damage in the structure. The proposed method is validated using extensive experimental data from a laboratory four-girder timber bridge structure subjected to gradually progressing damage at various locations with different severities. In total, 13 different states of the structure are considered, and it is demonstrated that the new damage feature outperforms the conventional principle component analysis–based feature. The contribution of the work is threefold: first, the application of cepstrum-based operational modal analysis in structural health monitoring is further validated, which has potential for real-life applications where only limited knowledge of the input is available; second, a new damage feature is proposed and its superior performance is demonstrated; and finally, the comprehensive test framework including extensive progressive damage cases validates the proposed technique.

Improving Traditional Balancing Methods for High-Speed Rotors

1996 ◽  
Vol 118 (1) ◽  
pp. 95-99 ◽  
Author(s):  
J. Ling ◽  
Y. Cao
Keyword(s):  
Experimental Data ◽  
Frequency Response ◽  
Frequency Response Function ◽  
High Speed ◽  
Rotating Machinery ◽  
Response Functions ◽  
Frequency Response Functions ◽  
Influence Coefficients ◽  
Mathematical Equations ◽  
Rotor Balancing

This paper introduces frequency response functions, analyzes the relationships between the frequency response functions and influence coefficients theoretically, and derives corresponding mathematical equations for high-speed rotor balancing. The relationships between the imbalance masses on the rotor and frequency response functions are also analyzed based upon the modal balancing method, and the equations related to the static and dynamic imbalance masses and the frequency response function are obtained. Experiments on a high-speed rotor balancing rig were performed to verify the theory, and the experimental data agree satisfactorily with the analytical solutions. The improvement on the traditional balancing method proposed in this paper will substantially reduce the number of rotor startups required during the balancing process of rotating machinery.

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