Strain Frequency Response Research Articles

Observed deviations from isotropic linear viscoelastic behavior of asphalt concrete through modal testing

The complex Young’s moduli, complex shear moduli and complex Poisson’s ratio of a beam shaped asphalt concrete specimen have been characterized through low strain (∼10−7) frequency response function measurements. The assumption of isotropic linear viscoelastic behavior has been applied and investigated. The results indicate that the asphalt concrete specimen agree with the isotropic linear viscoelastic assumption at low temperatures and high frequencies (>10kHz at 0°C), but at higher temperatures and lower frequencies, discrepancies from isotropic linear behavior are shown. The dynamic shear moduli calculated from the estimated Young’s moduli and Poisson’s ratio of the asphalt concrete specimen are overestimated for frequencies and temperatures often applied to pavements.

Research on Crack Damage Identification Based on SFRF and Fiber Bragg Grating Sensors

This paper applies strain frequency response function (SFRF), presenting crack damage identification index based on change amplitude of SFRF. Crack propagation identification of four corners fixed thin plate experiment system based on fiber bragg grating sensors is established, experiments and analysis are carried out. Results show that: this crack damage identification system can identify crack damage occurs and propagation effectively.

유한요소해석을 이용한 TPMS용 압전 발전소자의 동특성 해석

Energy harvesting is a clean technology to obtain energy from the surrounding environment such as wind, sun, vibration and so on. In particular, the current TPMS (Tire Pressure Monitoring Device) is very small and attached to the outside of a vehicle and power supply of the TPMS is limited. Therefore, energy harvesting using vibration energy of piezoelectric materials is important to the TPMS. In this paper, we analyzed several models using ANSYS which is one of the FEA (Finite Element Analysis) package and compared corresponding strain frequency response functions of the TPMS. In addition, we confirmed that dynamic characteristics variations according to geometry changes have effects on the performance of the TPMS.

Damage detection by mixed measurements using accelerometers and strain gages

This work investigates the relationship between the FRFs (frequency response functions) measured by accelerometers and strain gages utilized widely for investigating structural performance. Modifying the GDM (global-deviation method), this study examines the possibility of damage detection in utilizing both sensors together. The experimental results on the mixed utilization of two sensors show that the SFRF (strain frequency response function) data measured by strain gages in the neighborhood of end supports are more practical in establishing the baseline curve than the DFRF (displacement frequency response function) data measured by accelerometers. It is shown that the modified GDM can be effectively utilized in detecting damage based on the mixed measurements of accelerometers and strain gages.

An anisotropic micromechanical-based model for characterizing the magneto-mechanical behavior of NiMnGa alloys

As a typical ferromagnetic shape memory alloy (FSMA), NiMnGa alloy at room temperature is a heterogeneous material with martensitic variants and a high magnetic anisotropy property to produce a giant magnetic-induced strain and high frequency response. A theoretical model based on micromechanical and thermodynamic theory is proposed to describe the magneto-mechanical behavior of single-crystal FSMAs during the reorientation process of martensitic variants. It follows the well-established Eshelby equivalent inclusion method and the Mori–Tanaka scheme, and incorporates the influence of the material anisotropy and the variant inclusion morphology on the reorientation of the martensite variants. The modified micromechanical model is further applied to characterize the stress–strain behavior and magnetic-field-induced strain during the martensite variant rearrangement process in a single-crystal NiMnGa rod under applied magnetic field and/or mechanical loading. The simulation results show good agreement with the experimental data. The effects of the material anisotropy and inclusion morphology on the magnetoelastic constitutive behavior of FSMAs are discussed.

FIBER BRAGG GRATING SENSORS - BASED MECHANICAL STRUCTURE DAMAGE IDENTIFICATION

This paper describes a method for identifying the damage location for two dimensional plate - like structures. This method uses measured strain frequency response functions from intact and damage state to define a damage identification index. The strain frequency response functions are measured by distributed fiber Bragg grating (FBG) sensors. Numerical simulation and experimental tests are carried out. Both results show that this method combining strain frequency response function - based index with FBG distributed sensing can successfully identify the damage location.DOI: http://dx.doi.org/10.5755/j01.mech.18.1.1291

Back Cover: Phys. Status Solidi RRL 3–4/2010

physica status solidi (RRL) – Rapid Research LettersVolume 4, Issue 3-4 Back Cover Back Cover: Phys. Status Solidi RRL 3–4/2010 First published: 07 April 2010 https://doi.org/10.1002/pssr.201090004AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract Bimorph actuators with improved performance have been built by researchers at the Italian Institute of Technology using chemically modified single walled carbon nanotubes with an aromatic diamine (SW-amide). In their Rapid Research Letter on p. 64 the authors show how these actuators significantly outperform those prepared by using the pristine (p-SW) or oxidized carbon nanotubes (SW-COOH). The carbon nanotube specific capacitance doubles, the actuator maximum operating frequency becomes five times higher and for same injected charge the strain reaches a fourfold increase. The back cover figure shows the last step of the chemical reaction on carbon nanotubes that enhances the binding between nanotubes, the structure of the bimorph actuator and a comparison of the strain frequency response for actuators built using pristine, oxidized and amino-modified materials. Volume4, Issue3-4April 2010 RelatedInformation

Hypersensitivity of strain-based indicators for structural damage identification: A review

Because of the sensitive characteristic to local defects, strain has been widely utilized to identify the presence, location, and severity of damage in structures, and this paper reviews recent publications regarding strain-related techniques on structural damage identification. A brief description on the strain–displacement relationship at defective areas is first presented. Then, damage identification using various strain-related parameters, such as strain (curvature) mode shape, strain energy, strain frequency response function, etc., is explored and the hypersensitivity of corresponding damage indicators will be examined systematically. Finally, some generalized criteria on the selection of strain-based parameters for damage identification are concluded and suggestions for future work are discussed.

Structural Damage Detection in the Frequency Domain using Neural Networks

A bi-level damage detection algorithm that utilizes dynamic responses of the structure as input and neural network (NN) as a pattern classifier is presented. The signal anomaly index (SAI) is proposed to express the amount of changes in the shape of frequency response functions (FRFs) or strain frequency response function (SFRF). SAI is calculated by using the acceleration and dynamic strain responses acquired from intact and damaged states of the structure. In a bi-level damage identification algorithm, first the presence of damage is identified from the magnitude of the SAI value. Then the location of the damage is identified using the pattern recognition capability of the NN. The proposed algorithm is applied to an experimental model bridge to demonstrate the feasibility of the algorithm. Numerically simulated signals are used for training the NN, and experimentally acquired signals are used to test the NN. The results of this example application suggest that the SAI based pattern recognition approach may be applied to the structural health monitoring system for a real bridge.

Dynamic measurements on a star tracker prototype of AMS using fiber opticsensors

An aluminum prototype of the AMICA (astro-mapper for instrument check of attitude)star tracker support (ASTS) of the AMS_02 (alpha magnetic spectrometer) spaceexperiment has been instrumented with fiber Bragg gratings (FBGs) for the acquisition ofstrain fields during dynamic tests. The excitation has been provided by an instrumentedimpact hammer; the mechanical response of the structure was provided by surface bondedFBGs and accelerometers nominally in the same location. All time histories have beenrecorded, transformed into the frequency domain to retrieve frequency response functions(FRFs)—from accelerometers—and strain frequency response functions (SFRFs)—fromFBGs, both providing resonant frequencies and displacement (strain) shapes of theASTS. Numerical simulations of this structure have been performed to predict theaforementioned dynamic features. Good agreement was found for the two firstbending modes between the two approaches and in turn with the numerical analysis.

Investigation on ferromagnetic shape memory alloys

Ferromagnetic shape memory alloys, exhibiting large recoverable strain and rapid frequency response, appear to be promising shape memory actuator material. These materials exhibit large shape memory effect associating with martensitic transformation, and magnetic-field-induced strain in the martensite state. The recent development in researches on NiMnGa, NiFeGa, and CoNiGa in our group is briefly reviewed. The perspectives of the ferromagnetic shape memory alloy are also described.

Sensitivity studies of parameters for damage detection of plate-like structures using static and dynamic approaches

In this paper, the sensitivities of static and dynamic parameters to damage occurring in plate-like structures are investigated systematically, and corresponding damage indices are proposed to analyse their identification capabilities. For static analyses, damage indices are formulated using the out-of-plane deflection, and its slope and curvature based on a finite element model; for dynamic analyses, two damage indices related to the curvature mode shape (CMS) and the strain frequency response function (SFRF) are proposed. Compared with the existing results, the effects of the numbers of the selected modes and defective areas on the CMS index and the influence of the “natural frequency shift” on the SFRF index are investigated. Numerical simulations and experimental tests are performed to verify the identification capability of the proposed indices, and guidance on selecting the proper parameters for damage detection is given.

Developments In Paint Based Piezoelectric Thick-film Strain Sensors

This paper describes developments in the manufacture of thick-film dynamic strain sensors based on piezoelectric paint for structural vibration monitoring. A new generation of paints have been developed as the vehicle for depositing piezoelectric composites as thick films. These are water based to be compatible with likely future environmental legislation and are suitable for use with conventional spraying equipment. Results of tests on strain amplitude response and frequency response are presented, showing that sensors made using these materials have a wide bandwidth and good dynamic range making them suitable for use in structural dynamics applications.

Nondestructive Detection of Cracks in Tubular T-Joints Using Vibration Characteristics

This paper presents the results of an experimental investigation of the nondestructive inspection (NDI) of cracks in an intermediate scale tubular T-joint similar to those used in offshore platforms. To simulate the real situation, cracks were generated under fatigue loading. Modal testing and analysis were then carried out to study the vibration characteristics of the tubular T-joints. Strain gages were used as the main transducers to acquire data because strains are more sensitive to the presence of cracks. Strain frequency response functions were particularly analyzed. Three salient phenomena were observed: (a) antiresonance shifts, (b) quasi-static phenomenon, and (c) nonlinearity, all of which are indicative of the presence of cracks, even at a normalized crack size (the ratio of cracking area to load-bearing area) as small as 0.07. Therefore, the method developed in this study renders a more promising NDI technique than the frequency monitoring technique. Finite element analyses were also conducted to confirm the experimental results and to depict the mode shapes.

THEORETICAL AND EXPERIMENTAL STUDY OF MODAL STRAIN ANALYSIS

The relationship between the strain mode and the displacement mode for vibrating elastic structures is derived. It is based on the idea that when a structure is subjected to dynamic loading, its strain response can be expressed by the superposition of contributions from the “natural strain modes ”. Both the natural strain and natural displacement modes represent the dynamic characteristics of a vibrating structure. By using the finite element method, it has been possible to relate the Strain Frequency Response Function (SFRF) to the Displacement Frequency Response Function (DFRF). The technique used in the modal strain test and the method used for parameter identification are described. An experimental study of the modal strain analysis of cantilever rectangular thin plates has been performed. This shows that the strain mode is more sensitive to local changes of the structure than the displacement mode.

Monitoring crack growth through change of modal parameters

Modal analysis was performed on a cantilever plate with a small crack, using finite element analysis. Modal parameters such as natural frequencies, magnitude of frequency response functions, displacement mode shapes and strain mode shapes were calculated against cracking in the plate. It was found that the surface crack in the structure will affect most of the modal parameters, such as the natural frequencies of the structure, amplitudes of the response and mode shapes. Some of the most sensitive parameters are the difference of the strain mode shapes and the local strain frequency response functions. By monitoring the changes in the local strain frequency response functions and the difference between the strain mode shapes, the location and severity of the crack that occurs in the structure can be determined. It was also observed from other parallel studies that the use of a proper and careful experimental procedure would eliminate the noise problems always present in strain gauge instrumentation.

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

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.

Thin-film head domain structures versus Permalloy composition: strain determination and frequency response

The influence of the composition of plated Permalloy on the resulting magnetic domain structure in the upper yoke of inductive thin-film magnetic recording heads has been investigated. The response of the magnetization from four otherwise similar thin-film heads, ranging in iron content from 18 to 21% by weight, has been imaged by means of a scanning magnetooptic photometer. Radially compressive (and/or circumferentially tensile) strain is shown to exist in each of these heads about the rear magnetic closure, yielding a transition from radial to circumferential domain structures across the range of compositions. Mechanisms for the conduction of magnetic flux in each of these structures are demonstrated at 1 and 20 MHz. >

Experimentally Determined Dynamic Strains in an Elastic Mechanism

Measured dynamic bending strains in the links of a model four bar mechanism are presented. Cyclic strain variation and frequency response data are given, and comparison is made between calculated and measured values for one of the mechanism configurations.