Wavelet packet energy analysis of laser ultrasonic

Abstract: The blades on the plane are one of the most important parts of the engine, in the course of service, due tohigh temperature, strong vibration and great centrifugal force and so on. The using environment is very bad, so it iseasy to produce fatigue cracks in the welding site and the near surface of the root, which will seriously affect the bladeof the work intensity and fatigue life, and even the safety of aircraft structure, causing a huge security risk. Therefore, itmust be tested. In order to solve the problem of the rapid detection of aircraft engine in situ cracks, and gett the relationship between feature information and detect depth, the laboratory experimental platform was built, laser was usedto excite laser ultrasonic signals on a range of aviation aluminum plates with different depth defects, the collected signal was processed by wavelet de-noising, and the band energy distribution of the reflected echo signal was studied byusing wavelet packet. The results show that the energy of reflected echo signal is mainly concentrated in the S₈₀～S₈₇band. When the depth of defect is 0.2 mm to 0.4 mm, the energy is mainly concentrated in the adjacent bands. Whenthe depth of defect is 0.5 mm to 0.7 mm, the energy is mainly concentrated in the two bands. This method provides away to quantify surface micro-defects by ultrasonic signals, which will lay a foundation for the future analysis of crackdepth from band energy.

In order to avoid the interference of other irregular cracks, the cracks of the aviation aluminum parts are used as artificial way for producing. The overall size of the specimen is 200 mm×80 mm×100 mm, the width of the defect is 0.15mm, the range of the defect depth is 0.2 mm~0.7 mm, step size is 0.1 mm, and the total number of the specimen is six.After the experimental data is proposed, choosing the reflected echo signal for analysis, performing wavelet packettransform, the decomposition layer is 8. The percentage in the S₈₀~S₈₇ band is 89.77%、 91.82%、 91.41%、 90.94%、90.19%、 and 87.86%. The result shows that most of the energy is concentrated in the first eight bands. Therefore, thepaper selects the first eight bands for analysis.

In order to analyze the distribution characteristics of the different depth defect and the band energy, the energy distribution of the first four bands of the defect depth of 0.2 mm to 0.4 mm is plotted in Fig, according to the spectrum,getting the center frequency were 3.14 MHz, 2.58 MHz, 2.17 MHz. These frequencies are located in the S₈₃, S₈₂, S₈₂band, respectively, which are the largest energy band, but the energy distribution in the adjacent segment S₈₂ also accounts for a larger proportion. When the depth of the defect increases from 0.2 mm to 0.4 mm, the center frequencydecreases gradually, and the sum of the energy of the center frequency band and the adjacent higher energy band increases gradually.