Research On Real-time Structural Health Monitoring For Crack Growth

Structure health monitoring technology is very important for increasing the security of structure and reducing their maintenance cost. But till today, it is still difficult to monitor small size damage in structure, for example fatigue crack, on line and in real time. In this paper, in order to monitor the small size damage expansion such as crack growth, in real-time, the online health monitoring methods are researched.The main researches in this thesis are list as followed:1) The thesis researches the on-line damage expansion monitoring methods, such as the diagnostic Lamb wave based structural health monitoring method, the electromechanical impedance technology base method and the acoustic emission technology. Additionally, the theory of the wavelet transform and its application in Lamb wave base method are also researched.2) Using Lamb wave technology, this thesis develops an experimental monitoring system to monitor the fatigue crack growth in an aluminum alloy specimen. Based on Labview software, the experimental software is built. Based on the analysis of Lamb wave signal, the valuable wavelet is found and extracted using the wavelet transform. Using the method of numerical integral, energy of the wavelet is calculated and the relationship between damage index and the relative crack size is constructed.3) Using electromechanical impedance technology, the fatigue crack growth monitoring technology is also researched using similar aluminum alloy specimen. The Euclidean norm is adopted to calculate the damage index. The relationship between damage index and relative crack size is built. The experiment results using the Lamb wave based technology and the electromechanical impedance based technology are discussed and compared.4) Using the acoustic emission technology, the damage expansion on a carbon-fiber composite specimen during a material performance experiment is monitored. In the experiment, the relationship between the time and the number of acoustic emission events and also the relationship between the time and the acoustic emission energy are presented. The thesis also uses the concept of the sum of acoustic emission energy to represent the damage degree in the material. At last the relationships between the experimental loads and the cumulation energy, the stress and the cumulation energy, the strain and the cumulation energy, the distortion and the cumulation energy are presented.