Study on Lamb wave-based fatigue crack detection using seam assigned model

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Study on Lamb wave-based fatigue crack detection using seam assigned model

 

Author: Jiang, Zheng
Title: Study on Lamb wave-based fatigue crack detection using seam assigned model
Degree: M.Sc.
Year: 2012
Subject: Lamb waves
Nondestructive testing.
Structural health monitoring.
Structural analysis (Engineering)
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: xv, 122 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2507573
URI: http://theses.lib.polyu.edu.hk/handle/200/6681
Abstract: Material fatigue is a serious threat to the proper operation of machines. Early detection of the fatigue cracks is a key factor to prevent failure and extend structure service life. Structural health monitoring (SHM) technology is an increasing employed approach to monitor the damage with high performance and low cost. And Lamb wave based damage identification is a typical SHM approach for structural interrogation with considerable properties, which was employed in this thesis to do the fatigue damage identification. Although substantial development achieved by recently intensive researches, a proper way to actually model the fatigue crack in FEM is still requested to realized the transmission of the Lamb waves through the crack line. Besides, further investigation of the lamb wave propagation in complicated structures need to be conducted to extend the applicability of Lamb wave based damage identification. Bearing theses in mind, substantial works have been done in this thesis.
Firstly, a steel plate with PZT sensing network mounted on its surface was chosen to be modeled and simulated in ABAQUS/Explicit. A novel approach was developed by assigning seam on the partitioned crack faces was employed to simulate the fatigue crack case. Compared with the usually applied Element Removed approach, the Seam Assigned method was proven with better applicability to model the fatigue crack in FEM. Besides, by contrasting the fatigued model with the intact, the propagation characteristics of Lamb waves on the fatigued plate were also investigated. Secondly, a steel angle structure (SAS) with more complicated geometry than a single steel plate was applied for FEM simulation to further study the interaction of Lamb waves with a through-thickness fatigue crack. It can be concluded that the Lamb wave propagation characteristics in the SAS are similar to those of Lamb waves in a flat steel. Then the transmission coefficient, obtained by dividing the amplitude of the corresponding energy peak by the incident energy peak in the time domain, was introduced to assess the fatigue crack size. Conclusion was given that less Lamb waves could penetrate through the fatigue crack with the growth of the fatigue crack. At last, another steel plate with a series of through-thickness fatigue crack lengths was inspected by FEM and validated in experiment. Good correlation between the FEM analysis and experiment was observed. Further prove the efficiency of the Seam Assigned approach to simulate the fatigue crack. Moreover, the effects of the actuator/sensor locations and fatigue crack length on the transmission coefficients were discussed in detail. Same tendency and good agreement of the extracted transmission coefficient in both FEM and experiment were learnt. Finally, speculations about the effects of the horizontal distance between the actuator/sensor and the fatigue crack on the transmission coefficient were proposed, which need further investigation.

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