Study On Propagating Characteristics Of Acoustic Emission In Structural Material

In locating source using traditional acoustic emission, its accuracy depends on the following factors: threshold setup of sensor passages, different sensitivities among various sensors and static defects etc. In order to resolve this problem, new location approaches were presented based on the research of propagating characteristics of acoustic emission.A propagating model of acoustic emission was established in one-dimension structure material by simplifing the propagating mode of elastic wave. Propagating characteristics of one-dimension wave and the influence on acoustic emission by static defects were both discussed. A new source location method or transmission wave method was presented to resolve the damage-location problem of structure material which had dynamic damage and static defects simultaneously. Its availability was demonstrated by experiments based on the research of damage location with the waveform acoustic emission technique and Gabor wavelet time-frequency analysis. Compared with locating source using one-dimension location method of traditional acoustic emission, it only used one sensor and can avoid the influence of threshold setup and different sensitivities of sensors on damage location. Furthermore, it can give qualitative analysis for the isotropic elastic rod with change section.With Rayleigh-Lamb wave function deduced and dispersive characteristics of Lamb wave propagating in thin plates discussed, the function's numerical value answer was worked out and commonly there only exist basic symmetrical mode and basic antisymmetrical mode over the frequency range 0-0.4MHz in a thin plate. In order to solve the problem that different sensitivities among a number of sensors had a great influence on the accuracy of arrival time-difference location in traditional source location of acoustic emission, two new approaches of planar location were put forword based on the research of dispersive characteristics of Lamb wave, arrival time of different modes determined by using the peak of the magnitude of Gabor wavelet time-frequency analysis on the view of modal acoustic emission and introduced velocity factor. One method of location was based on the same frequency and different modes and the other was based on the same mode and different frequency. These two methods whose accuracy was demonstrated by experiments can solve the proposed problem completely, enlarge the monitoring area, reduce the number of sensors in planar location, avoid false location in traditional triangular location and beused to monitor structures which were not feasible for the traditional method of source location. In addition, for the purpose of solving the problem that it was difficult to locate defects in structural materials in service as the static defect and the dynamic damage existed in the same structure, a new and applicable approach of planar location was brought forward based on using the characteristics of waveform acoustic emission technique recording real-time waveforms of signals in time domain, the research of the dispersive characteristics of Lamb wave and arrival time of different modes determined by using the peak of the magnitude of Gabor wavelet time-frequency analysis. This approach can locate the static defect and the dynamic damage in sequence by using triangular location and elliptical location. Furthermore, utilizing the wave speed without dispersive characteristics at certain frequency of basic symmetrical mode and relative time difference of the same signal at some frequency in time domain can reduce the error of elliptical location which was easily affected by wave speed change and sensitivity differences between sensors. According to the new location method based on the same frequency and different modes, we can estimate whether the signal received by the sensor had the information about static defects. Gabor wavelet time-frequency analysis can improve the plannar location accuracy of traditional triangular location. The locating result of defect position by above method is close to the position of real defect demonstrated by experiments.The research of location methods in this paper was all carried out in lab condition. These methods still need make improvement for engineering practice. Combined with other sophisticated signal processing method or theory, location accuracy can be enhanced further.