Strain Frequency Response Function Research Articles

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.

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.

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.