The Research Of Damage Detection Vibration-based For Beam Structure

Engineering structures in their lifespan serving in complicated environment, for example, environment erosion, material aging, fatigue effect and loading effect etc., often suffer different kinds of damages, accumulation of such damages easily results in catastrophes, so it is very important for safety of engineering structures to detect their early damages. The purpose of structural damage identification is to detect the existence, the location and the degree of structural damages based on the characteristics represented by structures. The identification of location and grade of structural damages is involved in sensors optimization placement techniques and quantitative analysis of structural damages. Most of engineering structures is made of all kinds of beam structure element, or, they can be simplified to beam structure, so the main contents of this paper are as following:1. Modal analysis of damaged beam considering boundary conditions.Theoretical calculation of frequency for beam structure is pointed out. Mode function and frequency equation of simple beam are deduced and transcendental equations of frequency for beam with uniform section under all kinds of boundary conditions are presented. And then frequency equation of beam with non-uniform section (here is stepped beam) is deduced, it is the basement on calculating vibration characteristics of analogous structures. An approximate method to calculate frequency of beam with uniform section under different boundary conditions is studied.2. Structural damage identification based on frequency changes.Two damage identification methods regarding frequency as damage index are presented. They are global damage identification method based on frequency change and the square ratio of frequency change method. The methods are verified by damage identification of a cantilever beam. The results show that the global frequency is not sensitive to damage and can not identify the damage position, thus the square ratio of frequency change method can identify the damage position not identify the damage degree. Combining these two methods the damage position and degree can be identified at the same time.3. Structural damage identification based on strain and curvature mode changes.From modal analysis results of simple beam with single and multiple damage position under diverse damage degrees, it can be concluded that the small damage is not sensitive to frequency and displacement mode but sensitive to strain and curvature mode. It is a differential process to calculate strain with displacement and the differential process amplify the small change of displacement, so, the curvature mode has an abrupt shift at the damage position. If damage is located at node of some order curvature mode the damage location can not be identified only by this order curvature mode.