Ultrasonic Phased Array Testing Of Cracks In Low-pressure Turbine Disc Rim

With increasing needs for electricity, nuclear power industry is developingrapidly in our country, and the number of nuclear units under construction in ourcountry is the highest all around the world. For common straddle mount-typelow-pressure turbine disc, the rim position tends to form stress corrosion cracking dueto long time high speed rotation, meanwhile, safety loophole as a huge problem existsin it. Testing of early cracks in the complex structure has important and real meaning.Recently, ultrasonic phased array technology has been widely used in industrybecause of its’ special advantages, especially in a complicated structure, it is the bestpractice for defect inspection.Ultrasonic phased array inspection system is used to test straddle mount-typelow-pressure turbine disc rim which contains several crack defects in this paper. Afterthe simulation by the CIVA software and the analysis of the simulation results, thedetective area, the models of probe and wedge and the work mode are determined. Onthat basis, simulation result of the defect response gives full evidence to the feasibilityof the design scheme. Finally, the experimental verification is made in the mockup,and the result is uniform to the simulation.After analysis of experimental results, many problems remain to be researchedand solved. The key problems include the research of location, direction andquantification of cracks. Then the paper tries to present some new methods to solvethe problems above, which have been verified by theories and experiments. Themethods include the following aspects. Firstly, the image rectification algorithm ofsector-scan is proposed to the problem of low positioning precision, and experimentalverification is made. This algorithm can also intuitively eliminate the pseudo defects,meanwhile, it is more accurate than beam path computing method and instrument’sown correction method of sector-scan images. Secondly, power spectrum computingand wavelet packet analysis method to recognize the defect direction (the angle ofdefect) is proposed, whose feasibility has been verified by experiments. Meanwhile,the relationship between high-frequency energy and the angle between acoustic beamand defect is regressed as the standard to make an estimate of the angle of defect. Thatresult can provide a basis for reliability detection of low-pressure turbine rotor and quantitative research of linear defects. Finally, according to the special structure oflow-pressure turbine disc rim and test request, the method of echo attenuation todetermine defect size is proposed. It can avoid the complex data handling, whosefeasibility has been verified by experiments.In conclusion, this article has achieved the testing of straddle mount-typelow-pressure turbine disc which contains several crack defects using ultrasonic phasedarray technology. Meanwhile, three methods whose feasibility has been verified areproposed for the research of location, direction and quantification of cracks inexperiments, namely the image rectification algorithm, power spectrum computingand wavelet packet analysis method, echo attenuation method.