Evaluation Of Fatigue Damage In High-Strength Steel(FV520B) Using Nonlinear Ultrasonic Testing

High-strength FV520 B is often used to make centrifugal compressor impellers and it often suffers fatigue failure.If the fatigue failure of engineering components is not found in time,it will often break suddenly and cause heavy losses.At present,although there are many kinds of fatigue detection methods,most of them are not sensitive to the early fatigue damage of materials,and the detection methods are complicated.In contrast,the nonlinear ultrasonic detection technology has higher sensitivity to early fatigue damage,and it is relatively simple.The nonlinear ultrasonic testing and evaluation method of High-strength FV520 B is studied by using nonlinear ultrasonic testing technology,which is helpful to effectively evaluate the fatigue damage degree of materials and reduce the accident rate.The main research contents of this article are as follows:(1)The overall scheme of nonlinear ultrasonic testing experiment for fatigue damage of High-strength FV520 B was designed.The fatigue test scheme was established and the specimens with different fatigue damage degrees were made for the nonlinear ultrasonic testing experiment.The detection mode of ultrasonic nonlinear system and the parameters of each module are determined.(2)Due to the strong penetrability of ultrasonic Lamb wave,it is suitable for nonlinear ultrasonic detection of fatigue damage of FV520 B plate.The propagation characteristics of Lamb wave in the FV520 B specimen were analyzed and discussed,and the phase velocity dispersion curve and group velocity dispersion curve of Lamb wave in FV520 B fatigue specimen were drawn.The conditions of Lamb wave in the experiment were determined.(3)The effects of the parameters in dislocation model and dislocation dipole model on ultrasonic nonlinear parameters were analyzed.The influence of cracks on ultrasonic nonlinearity was analyzed from the perspective of contact nonlinear acoustics,and the finite element software ABAQUS was used to simulate it,which proved that the crack can produce nonlinear effects.The relationship between nonlinear parameters and internal crack morphology was analyzed.(4)A series of nonlinear tests of FV520 B fatigue specimens were carried out by Lamb wave.The results show that the material has good ultrasonic nonlinear cumulative effect and the ultrasonic nonlinear parameters have high sensitivity to the early fatigue damage of FV520 B.(5)In order to study the microscopic mechanism of the relationship between ultrasonic nonlinear parameters and fatigue damage,the microstructure of fatigue specimens with different cycles was observed.It was found that as the number of cycles increased,the microstructure of the fatigue specimen continued to deteriorate,and the nonlinear parameters increased accordingly.These degraded microstructures,such as micropores and cracks,are the source of the second harmonic generation.There is a good mapping relationship between the ultrasonic nonlinear parameters and the fatigue damage degree of FV520 B.The relationship between the ultrasonic nonlinear parameters and the length of the main crack with the equivalent length of the micro crack was analyzed.It is concluded that compared with main crack length,equivalent length of the microcrack ultrasonic and nonlinear parameters maintain a better consistency,indicating that the generation of ultrasonic nonlinear parameters is mainly caused by the appearance of internal microcracks.Ultrasonic nonlinear parameters can effectively characterize the fatigue damage state of High-strength FV520B.