Study On Nonlinear Ultrasonic Detection And Signal Processing For Early Fatigue Damage Of Turbine Blade Steel

This paper takes the research on “Research on the surface micro-structural damage detection method of gas turbine blades based on nonlinear Rayleigh wave”,a National Natural Science Foundation project undertaken by Henan University of Technology.Nonlinear ultrasonic detection method and nonlinear characteristic of ultrasonic wave for turbine blade fatigue damage during early stage were studied.As an important component of steam turbine in power station,gas turbine blades worked under extremely harsh conditions with high temperature,high pressure and high rotational speed medium.Meanwhile the vibration stress,centrifugal force bending stress and high-speed water droplet erosion in wet steam zone which can easily cause fracture and damage for gas turbine blades.The macroscopic crack,open welding and other damage can be effectively detected by technologies such as Magnetic particle inspection and Ultrasonic echo method.However,the detection method for early fatigue damage of turbine blade remains to be studied.Based on the nonlinear ultrasonic nondestructive testing technology,nonlinear characteristics of turbine blade materials are studied in this paper.Based on lots of experiments,the relationship between tensile stress and ultrasonic nonlinear parameter,fatigue degree and ultrasonic nonlinear parameter are studied.At the same time,some problems in ultrasonic signal processing and non-linear ultrasonic nondestructive testing have been studied.The main research contents are as follows:(1)A nonlinear ultrasonic nondestructive testing system had been established and a suitable fixture had been made to meet experimental requirements.Meanwhile,a software has been programed to connect with oscilloscope and to collect,store and analyze ultrasonic data.(2)Based on the discrete Fourier transform(DFT),the deviation of spectral line in the analysis of experimental data had been studied.An ultrasonic signal truncation rule based on the characteristic of excitation signal was been put forward to solve the above problem.(3)An experiment method based on non-resonant standing was used to solve the problems such as low amplitude of ultrasonic signal received in specimen,useful information swamped by noise.(4)Tensile specimen and three-point bending fatigue specimen of turbine blade materials had been made and used in tensile test,fatigue test and nonlinear ultrasonic nondestructive testing experiment.