Nonlinear Ultrasonic Lamb Wave Model Selection And Characterization Of Creep Damage Of P92 Steel

High-temperature creep damage is a potential threat on the safety operation of key equipment applied in ultra-supercritical power unit, so there is a significant engineering value to establish an effective method for testing and evaluation of high-temperature creep damage. Recent studies have shown that the damage state of material has a close relationship with the nonlinear effect of ultrasonic waves propagating in material. Compared with the bulk acoustic waves, ultrasonic guided waves such as Lamb waves have obvious advantages, such as one-side access of pitch-catch configuration and high efficiency in long distance propagation, so it has the potential engineering application value. So far the mode selection of nonlinear ultrasonic Lamb wave has been lack of basis, due to the multi-mode and dispersive properties of ultrasonic Lamb waves, so the research on nonlinear ultrasonic Lamb wave in engineering application is relatively backward.In view of the above problems, the present thesis has conducted continuing creep tests for P92 steel plates in laboratory and obtained diffident damage state of creep-induced plates, in which performed the creep damaged of P92 steel using ultrasonic nonlinear Lamb waves to test and evaluate was proposed, the main researches were as followings:(1) The thesis studied second harmonic excitation efficiency of different primary Lamb wave modes, to choose and optimize primary modes. Theoretical excitation efficiency parameters corresponding to the disperse curves of Lamb wave are calculated through theoretical analysis. Two types of primary Lamb modes with different excitation efficiency parameters are selected for experimental comparison, which are called longitudinal velocity type S1 mode and intersection point type A2/S2 modes. Then, the second-harmonics generations of the selected two different primary Lamb waves are measured. It was found that the longitudinal velocity type S1 mode with a higher excitation efficiency parameter corresponds to the higher second harmonics of Lamb wave. Therefore, excitation efficiency parameter developed in this contribution could be an effective indicator for the mode selection of efficient second harmonic generation of Lamb waves.(2) In this thesis, we analyzed the response of different Lamb wave primary modes to the creep damage, and verified the reliability by using the excitation efficiency parameter choosing primary modes the reliability of the model. We studied longitudinal velocity type S1 mode and intersection point type A2/S2 modes responsed to a creep damage specimen through experiment. The results showed that the excitation efficiency parameter of the longitudinal velocity type S1 mode is higher in the original specimen, and it is larger in the creep damage sample, so excitation efficiency parameters are feasible to select the primary Lamb modes.(3) The thesis used nonlinear ultrasonic Lamb wave to measure creep damage material, and analyzed the relationship between creep damage and nonlinear Lamb wave from the microstructure evolution of the material. The results showed that when the creep strain was larger than 2%, nonlinear parameter was monotonically increasing with the creep strain increased, it may be because precipitations growth and coarsening resulted in the increased material nonlinear.