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Model Updating and Damage Identification for Frame Structure

Author: LiangWenGuang
Tutor: YangYouFa
School: Chongqing University
Course: Disaster Prevention and Reduction Engineering and Protective Engineering
Keywords: model updating Genetic Algorithm damage identification direct analyzing method mixed iteration method
CLC: TU375.4
Type: Master's thesis
Year: 2009
Downloads: 170
Quote: 7
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Abstract

The vibration-based method is an easy and quick method to damage identification. In the filed,highly effective studies have been done by domestic or overseas scientists, and a good many methods which detect damage through changes in modal parameters were advanced, but those methods are difficult in application, because they are grave impacted by measure noise, incompletely measure, model errors etc. Two main parts are included in application of damage identification and on-line monitoring. The detials are as follows: firstly, the accurate finite element model is built by model updating method using measured data; secondly, the location and size of damages are detected by damage identification technique, which is based on the finite element model and modal parameters when damage occurred in structure. In order to practice, these two main parts of frame structural damage diagnosis are researched in this paper, and its theory mathematic models, solving methods and application were studied in each part. The main achievements and conclusions of this paper are summarized below.①The main achievements and conclusions, which are achieved during building the accurate finite element model by model updating method, are summarized below.1) The practical frame structure is simulated by ANSYS9.0 using solid45 finite element model. Plane beam finite element theoretical model, which is neede by damage identification technique, is set up by MATLAB7.1. According to the modal parameters obtained by solid model, the theoretical model is updated.2) The mathematical principle of model modification is a nonlinear constrained optimization problem.3) The choice of reference basis for model updating is explored. The conclutions are as follows: reference basis should choose modal parameters of solid model which has the similer mode shape to the theoretical model; if choose the mode with different shape, correction precision will decline rapidly.4) Taking effective length factor of frame beam and column as the modified variables, theoretical model is updated with Genetic Algorithm. The objective function of GA is improved by single objective instead of multi-objective. The indexes of modal assurance criterion (MAC) and frequency relative error are used to compare the modal parameters before and after updating. It is testified that the theoretical model modified with improved single objective function of GA can reflect the dynamical characters of solid model in low order mode.②The main achievements and conclusions, which are achieved during damage identification based on the finite element model and modal parameters when damage occurred in structure, are summarized below.1) A direct analyzing method of structural identification is derived by sensitivity theory and iteration theory. Its mathematical principle is an inconsistent overdetermined set of linearequations.2) The least modal number based on the direct analyzing method is studied. It is considered that the modal number should satisfy two conditions bellow: first, the rank of sensitivity matrix should be equal to the numbers of damage variables; second, considering the modal truncation error, iteration convergence is also should be satisfied.3) Starting from the engineering practice, two aspects have been improved in damage identification with direct analyzing method. They are summarized below: first, the model reduction techniques is introducted, with which damage values can be sloved by the basic equation only needed mode displacement of X direction (not needed the angular displacement), From the results of caculation examples, it is testified that the improved method is effective; second, the modal truncation error is improved, the least modal number is 23 using unimproved method, but it is only 15 by improved method.4) 3 optimization algorithms based on the mathematics equations of direct analyzing method for damage identification were summed up, and the mixed iteration with least square method for direct analyzing method were presented according to the characteristics of the basic equation of direct analyzing method. The precision to locate and to quantify for the mixed iteration method is higher than the precision of the others.5) The anti-noise ability of improved direct analyzing method is studied. The results by 20 kinds of random white noise showed that: with the white noise level of 0.1%, the guarantee rate of correct damage results can be more than 80%; as the white noise level is higher, error identification or spurious identification would be shown. So the anti-noise ability of direct analyzing method for damage identification still has some needs to continue thorough research.

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