Research On Acoustic Emission Source Location And Preliminary Study On The Crack Propagation

Cracks are the source of accident, especially in large-scale complex structural parts such as Francis turbine etc. AE (Acoustic Emission) technique is applied widely as a new dynamic detecting way in equipment of nondestructive examination and on-line monitoring. Ascertaining AE source location for the crack is a critical step. It is important to evaluate accurately the longevity of the part using AE method on detecting crack initiation and growth. Based on sufficient learning about foreign and domestic research results, for successfully applying AE on detecting crack in Francis turbine runner, the methods of AE source location and forecasting propagation of the crack have been deeply investigated in this thesis, which is the project 'Research of Theory and Method on Monitoring Crack of Francis Turbine Runner Blade' supported by national fund of natural science. (50465002)Firstly, dislocation movement and crack initiation and propagation of AE mechanics were discussed. The model, velocity and decay of AE wave's physical foundation were introduced. The wave velocity and decay measurement of AE signal in the Francis turbine runner were done.Faced to the structure of Francis turbine, in this paper, three methods for locating AE sources in complex structure including three-dimension time arriving location, zone location, intelligent location, were proposed. Computer-aided system for AE location was developed. The adaptive analyses of these location methods were compared.After analyzing the configuration and the crack of the runner of Francis turbine, the intelligent location empirical scheme can be obtained. The intelligent location models of the Francis turbine runner's crack monitoring system were established by use of BP,RBF neural networks with the collecting AE signal. Then, the characteristics were indicated respectively, and the valid of the method was demonstrated by the experiments.According to the experimental results of three-point bending specimen, AE techniques monitoring fatigue crack propagation were certificated not only to resemble variable rule of fatigue crack propagation, but also to catch generation of fatigue crack in real time. The equations are cited to set up the correlation between AE parameters and crack propagation velocity. Furthermore, the solution of crack propagation velocity was proposed. The forecasting method of crack propagation in the runner of Francis turbine was preliminarily studied.