Research Of Nonlinear Ultrasonic Detection Of Microcrack Damage On The Surface Of Stainless Steel Under Loading

Austenitic stainless steel has good using performance,and is often used in the manufacture of important equipments in the petrochemical,aerospace,and nuclear industry.However,the harsh service environment may cause early damage such as micro-cracks on the surface of the component,which seriously affects the normal use of the equipment,and even lead to catastrophic fracture accidents.Therefore,Timely detection of early damage such as microcracks is particularly important.The method for detecting cracks is conventional ultrasonic detection,but it is insensitive to early damage such as microcracks which are much smaller than the wavelength of ultrasonic,therefore,nonlinear ultrasonic detection method with higher detection sensitivity is proposed.Based on this,this subject adopts nonlinear ultrasonic testing technology combined with COMSOL multiphysics simulation software,with the combination of theoretical research,simulation,and experimental verification to carry out the research on the microcrack damage of austenitic stainless steel under loading based on nonlinear surface wave detection technology.The effect of stress on the propagation behavior of surface waves in materials containing microcracks is revealed and characterized.Based on the analysis of the subject background,research significance and research status,the basic principles of nonlinear ultrasonic testing is introduced,the nonlinear theory of contact acoustics is explained,that is,the phenomenon that ultrasonic waves encounter microcracks to produce high-order harmonics,nonlinear coefficients are derived which used as the basis for defect characterization.Then,the COMSOL multiphysics simulation software was used to create a nonlinear surface wave detection model of austenitic stainless steel microcracks under loading,which reproduced the propagation process of surface waves in the structure.The influence of the parameter setting of the finite element software on the simulation effect is analyzed and compared,the coupling problem of the stress field and the ultrasonic field is solved,and the advantages and feasibility of the model are verified.On this basis,the influence of stress on the propagation behavior of surface waves in materials containing microcracks was discussed,and the numerical simulation results found that:(1)Under constant stress,for the microcracks with surface depth of 100～300 μm and width of 10～50 nm,the nonlinear coefficient decreases with the increase of the microcrack width,and the two are negatively correlated;the nonlinear coefficient increases with the increase of depth,the two are positively correlated;When under tensile stress,The opening of the microcrack becomes larger,the contact state of the crack boundary changes,the effect of opening/closing is weakened,the nonlinear response is lower than the unstressed state,and the situation is the opposite when it is in the compressive stress state;(2)With the increase of tensile alternating stress from 1 MPa to 5 MPa,the contact effect of microcrack weakens and the nonlinear coefficient of surface waves decreases significantly.With the increase of cycle times from 5 to 200,the microcrack crack gradually,the contact area of crack interface increases,and the nonlinear coefficient increases gradually.(3)Under the condition of a certain excitation signal,as the microcrack density increases from 0.1 to 0.3,the nonlinear coefficient increases nearly four times.Microcrack density c can be used as a quantitative index for ultrasonic nonlinear detection of the damage state of metal components.Finally,the nonlinear surface wave detection experiment of prefabricated microcracks under stress corrosion and alternating stress was carried out to realize the characterization of microcrack damage on the surface of stainless steel under loading.The results showed that:with the stress corrosion time or the alternating stress cycles increases,the nonlinear response of the system will be enhanced.The two kinds of experimental data and the numerical simulation results are mutually verified,and the results confirm that the simulation results and the experimental results maintain good consistency.