Evaluation Of Early Damage In High-temperature Components Based On Nonlinear Ultrasonic Guided Waves

High-temperature induced damage is a potential threat on the safety operation of key equipments applied in some fields such as petroleum industry, chemical industry and fossil power plants. It has an important engineering value to establish an effective method for testing and evaluation of high-temperature induced damge at early-term. Recent studies have shown that the damage state of material has a close relationship with the nonlinear effect of ultrasonic waves propagating in material, and it would cause a clear ultrasonic harmonics, which has recently attracted much more research interests. Compared with the nonlinear ultrasonic techniques that use bulk acoustic waves, ultrasonic guided waves such as Lamb wave have obvious advantages, such as high efficiency in the long distance propagation and one-side access of pitch-catch configuration. So far the theoretical studies on the nonlinear guided waves have got a big progress, however the experimental studies and applications of them have still lagged behind due to the dispersive and multi-mode nature of guided waves. Therefore, it is important to pay a further attention to the theoretical relationship and experimental measurement method between the nonlinear guided waves and thermal damages of components/materials, which is hopful to provide a new nondestructive evaluation (NDE) method for micro-damage detection and early precaution of large-sized components.The present paper has performed an in-depth analysis on the evaluation of the thermal damaged specimens with plate-like shape using nonlinear guided waves, in which the generation and propagation of the nonlinear guided waves in the complex alloy materials was theoretically analyzed, and the relationship between them in view of the microstructure evolution during ageing in specimens was also studied, and then experimental measurements and residual life prediction method were established. The main researches and results were as followings:(1) The present work analyzed the complex second-harmonic generation of ultrasonic guided wave (Lamb wave) in a two-layered solid waveguide (such as functionally graded material) by combining the modal analysis method and the nonlinear reflection of acoustic waves at interfaces and obtained an analytical solution of the second-harmonic field. Numerical computation results show that the analytical solution obtained by the combined method is convenient for numerical analysis and mode selection of the cumulative second-harmonic field of Lamb waves in the waveguide, and can provide some helpful data for experimental measurements such as the maximum propagation distance of the double frequency Lamb waves (DFLWs) and the influence on the amplitudes of the DFLW modes with different deviations of phase velocity between the primary Lamb wave and the DFLW mode. Experimental measurements of the amplitudes of nonlinear Lamb waves on the surface of HP40Nb specimens were performed to verify the influence of the relative deviations of phase velocity on the generation of the second-harmonic of Lamb waves.(2) An analytical model of the effect of microstructure evolution on the nonlinear ultrasonic Lamb wave was derived by the general solution of the second-harmonic of Lamb waves obtained by the combined method, and the dislocation-string model. In this analytical model, the parameters relating to the microstructure evolution in thermal damaged specimens was creatively introduced into the general solution of the nonlinear Lamb wave field, which could pave a foundation for the theoretical computation and analysis on the amplitudes of the nonlinear Lamb waves in the specimens with different thermal damage levels.(3) The thermal degradation in ferritic Cr-Ni alloy steel plates (HP40Nb material) was measured using the nonlinear effect of Lamb wave propagation. A "mountain-shape" change in the normalized acoustic nonlinearity of Lamb wave versus the level of thermal degradation in the specimens has been observed. The variation in the measured acoustic nonlinearity reveals, based on metallographic studies and microstructure evolution, that the normalized acoustic nonlinearity increases due to the second phase precipitates in the early stage and it decreases as a combined result of dislocation change and micro-void initiation in the material. The hardness of the damaged HP40Nb specimens was also measured which exhibited the same change trend as the variation of the normalized acoustic nonlinearity of Lamb wave. It can be inferred that, to a certain extent, the results of nonlinear Lamb waves could be used to characterize the change state of the mechanical property in material.(4) A continues creep test was conducted for titanium alloy plates Ti60 material in laboratory, and then a relationship between the normalized acoustic nonlinearity of Lamb wave and the creep damage was determined by measuring the nonlinear Lamb wave propagation in the damaged specimens. It is shown that the normalized acoustic nonlinearity of Lamb wave exhibites a variation of "increase-decrease" trendency as a function of creep damage, where the maximum level of the normalized acoustic nonlinearity is located at about 63%creep fraction life. The microstructures of the Ti60 specimens were analyzed based on metallographic studies. The results show that the rising of the normalized acoustic nonlinearity is due to the precipitate and coalescence of the second phase in the matrix when Ti60 specimens experienced an early-term creep loading, which would cause an increase of material nonlinearity. Moreover, the dislocation density in the matrix of specimens has a gradual accretion and the loop length of dislocation also has a corresponding change under the tensile stress of creep test. All these changes of microstructures in specimens would at last have an integrative effect on the variation of the normalized acoustic nonlinearity of Lamb wave which displays a monotonic rising. With the creep loading continuing, the precipitates in matrix keep gradually stable, and there begins to produce micro-pores at the grain boundary between the matrix and the precipitated phases and then to cause a corresponding variation in the dislocation density and loop length. All these changes of microstructures in specimens would make the normalized acoustic nonlinearity of Lamb wave monotonically drop from 63%creep life fraction to the fracture of specimen. A theoretical computation was performed by the analytical model relating to the interaction between the microstructure evolution and the nonlinear ultrasonic Lamb wave, the results show that the computation results from the analytical model displays a good agreement with the experimental measurements.(5) A method relating to nondestructive evaluation of residual life of the creep-induced damaged material based on the nonlinear Lamb waves was proposed. The relationship curve of the normalized acoustic parameter of Lamb wave as a function of the creep life was determined by an interrupted creep test of the specimen and then made the relationship curve to be a benchmark one for the residual life evaluation of the in-service components. Two set of creep damage examples have proved that the proposed method is feasible and effective to perform a quantitatively nondestructive evaluation of the creep residual life of material by using the benchmark curve and its experimental errors can satisfy the demands of engineering practice.