Stress Corrosion Damage Evaluation Of 304 Stainless Steel Based On Nonlinear Ultrasonic Signal Analysis And Processing

304 stainless steel is widely used in various industries due to its excellent performance,but it is prone to stress corrosion under the coupling of corrosive medium and constant load.Stress corrosion is easily affected by the surrounding environment and has the characteristics of slow crack formation and fast fracture.It is of great significance to carry out stress corrosion early damage detection for practical application of industrial production.Nonlinear ultrasonic testing technology is very sensitive to early damage,but there are few studies on the influencing factors of stress corrosion,and it is difficult to achieve effective characterization of corrosion damage.In view of this,this paper will carry out a study on the stress corrosion damage evaluation of 304 stainless steel based on nonlinear ultrasonic signal analysis and processing.The research contents are as follows:?.This paper introduces the principle of nonlinear ultrasonic surface wave characterization damage,and introduce Hilbert-Huang and information entropy into nonlinear ultrasonic signal processing,discusses the principle,the basis of their specific implementation algorithm and characteristics,and focused on the limitations of Hilbert-Huang signal processing method corresponding solution measures are put forward.?.Set up a stress corrosion test platform,and conduct corrosion experiments by applying a constant load stress to the stress ring device and applying a self-made corrosion groove to the middle of the sample surface.Three dimensional laser confocal microscopy was used to observe the morphology and measure the three dimensional dimensions of the samples with different stress corrosion damage states,and the feasibility of nonlinear ultrasonic surface wave characterization of 304 stainless steel stress corrosion damage was discussed.The effects of corrosion concentration,stress and corrosion time on stress corrosion damage were compared and analyzed.Finally,fast Fourier transform FFT,Hilbert-Huang transform HHT and information entropy were applied to nonlinear ultrasonic surface wave signals to analyze the sensitivity of these three signal processing methods to stress corrosion damage.The research results are as follows:1.The application of HHT in the simulated signal of pitting damage nonlinear ultrasonic surface wave shows that the nonlinear coefficient extracted by HHT increases with the increase of pitting damage in the early stage,and the quantitative characterization of pitting damage can be realized,which verifies the accuracy of HHT program and the feasibility of applying HHT program in the application of corrosion damage nonlinear ultrasonic surface wave.2.Corrosion concentration,corrosion time and stress accelerated the stress corrosion process,which increased the number and size of corrosion pits on the surface of the sample.When the corrosion lasted for 15 hours,micro-cracks were formed at the edge of the pit,and the surface damage degree of the sample significantly enhanced.The normalized nonlinear coefficient based on HHT increases with the increase of the damage degree of samples,which verifies the feasibility of nonlinear ultrasonic surface wave in stress corrosion damage.3.The effect of corrosion concentration,stress and corrosion time on stress corrosion damage was evaluated by the size of corrosion pit and the change rate of normalized nonlinear coefficient.When the concentration of corrosive solution increased from 6%FeCl₃ to 14%FeCl₃,the change rate of nonlinear coefficient was the highest,and the effect of concentration on stress corrosion damage was the largest,followed by the effect of corrosion time from 5h to 15h.When the stress increased from 0MPa to 248MPa,the change rate of nonlinear coefficient was the small.4.The comparison of characteristic parameters of stress corrosion damage by information entropy,Fourier transform and HHT shows that the nonlinear coefficient extracted by HHT is the most sensitive with the change of damage,the information entropy curve is the gentlest and the sensitivity is the lowest,and the normalized nonlinear coefficient extracted by Fourier transform is in the middle.