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Evaluation Of Serving Damage Of Austenite Stainless Steel Using Nonlinear Ultrasonic Wave

Posted on:2015-08-23Degree:DoctorType:Dissertation
Country:ChinaCandidate:J F ZhangFull Text:PDF
GTID:1221330467476693Subject:Power Engineering and Engineering Thermophysics
Abstract/Summary:PDF Full Text Request
Deformation damage impairs significantly the safety service performance of the austenitic stainless steel components and equipment, which are widely used in petroleum, chemical and electric power industries. It is one of the research hotspots and difficulties in the equipment security field to quantificationally identify and detect deformation damage. Traditional nondestructive testing technology such as X-ray, ultrasound, magnetic powder, etc., can only be effectively detected missing material defects such as cracks and inclusions, while it cannot detect micro-nano scale damage such as dislocation and slip. Recent studies show that nonlinear ultrasonic wave is sensitive to the micro-nano scale damage. However, the existing research work is limited to the relatively simple metal materials such as aluminum, copper and so on. Whether nonlinear ultrasonic wave technique can be used to detect micro-nano scale damage of austenitic stainless steel engineering structural materials and equipment still needs further research.Based on the theory of nonlinear ultrasonic wave, mixed dislocation model has been established. Different modes of deformation damage on austenitic stainless steel were inspected using nonlinear ultrasonic wave. The evolving microstructures, mechanical behavior and nonlinear ultrasonic parameter of corresponding damaged specimens were acquired and their triadic relationship was achieved. All this work provides a fundament for detection and evaluation of early serving damage of austenite stainless steel equipment using nonlinear ultrasonic wave technique. The main work and conclusions of this thesis are as follows:(1) The theoretical model of acoustic nonlinearity based on dislocation. It is found that edge dislocation and screw dislocation has different nonlinear ultrasonic response, and nonlinear ultrasonic response of edge dislocation is higher than that of screw dislocation. The existing theoretical models have relatively large error due to neglect the difference nonlinear ultrasonic response of these two dislocation characters. Therefore, a mixed dislocation model of acoustic nonlinearity is proposed. Comparing with Hikata et al.’s model and Cash et al.’s model, the proposed model has better predicting results in the process of cold rolling deformation of304stainless steel.(2) Evaluation of plastic damage of austenite stainless steel under the tensile loading using nonlinear ultrasonic wave. Nonlinear ultrasonic parameters of longitudinal wave and Rayleigh surface wave of specimens with different plastic damage are obtained. A function relationship is developed between plastic strain and nonlinear ultrasonic parameter. According to the data of microstructures, the variation of nonlinear ultrasonic parameter during the plastic damage process of304stainless steel is interpreted successfully using the mixed dislocation model. Results show that the nonlinear ultrasonic parameters of longitudinal wave and Rayleigh surface wave increase with the increasing plastic damage as the increasing dislocation multiplication and martensite phase; the increasing growth rate of nonlinear ultrasonic parameters is associated with the increasing proportion of edge dislocation; the difference in the stress exponent indexes is mainly due to the deformation damage difference between the near surface material and the material far away from the surface.(3) Evaluation of fatigue damage of austenite stainless steel using nonlinear ultrasonic wave. Nonlinear ultrasonic parameters of304stainless steel specimens with different fatigue damage are obtained. A comprehensive model of dislocation monopole and dislocation dipole is proposed, which considers the dependence of acoustic nonlinearity of dipole with the stress. According to the data of microstructures, the proposed model is used to interpret successfully the nonlinear correlation between the nonlinear ultrasonic parameter and the fatigue cycles of304stainless steel. The theory analysis and micro structure analysis reveals that:in stage I, the increasing nonlinear ultrasonic parameter corresponds to the increase of monopole density and dipole density of planar structure; while the decreasing nonlinear ultrasonic parameter is caused by the decrease of both monopole density and dipole height accompanying with the formation and increase of dislocation walls and cells structure at stage Ⅱ.(4) Characterization of cyclic softening/hardening behavior of austenitic stainless steel using nonlinear ultrasonic wave. Ultrasonic nonlinear parameter is associated with strain amplitude of304stainless steel under stress control mode during the low cycle fatigue process. Results reveal that the primary hardening leads to the increase of acoustic nonlinearity, while secondary hardening causes the reverse tendency. The distinct acoustic nonlinear response during cyclic hardening/softening of304stainless steel is governed by two competitive mechanisms:in the primary hardening stage, the ascended acoustic nonlinearity is ascribed to the increase of planar dislocation structures. While in the second hardening stage, the development of cell structures causes the decrease of acoustic nonlinearity. In addition, the deformation induced martensitic transformation also contributes to the increase of acoustic nonlinearity, which depends on the stress levels.(5) Evaluation of ratcheting damage of austenite stainless steel using nonlinear ultrasonic wave. Nonlinear ultrasonic parameters of304stainless steel specimens with different ratcheting damage are obtained. Combining with the change of nonlinear ultrasonic parameter of fatigue damage suffered the same stress amplitude loading cases, the evaluation of acoustic nonlinearity and its internal mechanism during ratcheting damage process is explored. Results show that:the first increase of nonlinear ultrasonic parameter is due to the increasing dislocation density of planar dislocation structures, and then the decrease of nonlinear ultrasonic parameter is owing to the development of dislocation walls and cells, finally the mild increase of nonlinear ultrasonic parameter is related to the formation of martensite phase; comparing with the microstructures of fatigue damage, the evolution of dislocation structures of ratchetting damage is slow, while more dislocation and martensite phase are generated in ratchetting damage process.(6) The influence of damage modes on the nonlinear ultrasonic response of austenite stainless steel. Analysis based on nonlinear ultrasonic parameter and yield stress of304 stainless steel experienced tensile plastic damage, fatigue damage and ratcheting damage manifests that:nonlinear ultrasonic response of strength with different damage modes is distinct; nonlinear ultrasonic response from strong to weak is in turn as fatigue damage, ratcheting damage and tensile damage. It is mainly caused by the different strength response and nonlinear ultrasonic response of microstructures. Yield strength is more sensitive to martensite phase than nonlinear ultrasonic parameter.
Keywords/Search Tags:Nonlinear ultrasonic wave, Austenite steel, Deformation damage, Plastic damage, Nondestructive testing (NDT)
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