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Research On Nonlinear Ultrasonic Testing Of The Fatigue Damage In Metallic Components

Posted on:2016-07-19Degree:DoctorType:Dissertation
Country:ChinaCandidate:H J YanFull Text:PDF
GTID:1221330476450735Subject:Mechanical engineering
Abstract/Summary:PDF Full Text Request
The plate or plate-like metallic components are widely used in various equipments. Fatigue is one of main damage mechanisms of components. Fatigue of components makes metallic components stress concentration or micro crack and so on. If measures are not taken, cracks expand under the cyclic loading. It induces components fracture and even causes catastrophic accidents.It is important to study early detection, evaluation and position of fatigue damage of metallic components in theory and practical applications. But the position of early fagitue damage has concealment, which bring hardships to actual testing.The nonlinear ultrasonic theory is that ultrasonic waves interact with micro defects and the higher harmonic waves appear when the ultrasonic waves propagate in materials. The acoustic nonlinear parameters are sensitive to micro defects in the nonlinear materials. They are used to characterize the nonlinear characteristics of materials. It can realize the fatigue status detection of metal components.According to the above problem, the nonlinear ultrasonic theories is studied and the characterization of dislocation, stress and micro cracks in fatigue metal by the nonlinear ultrasonics are studied. The metallic plates are selected as the research object, and the nonlinear ultrasonic testing system is build to detect the received signals which propagate in materials with fatigue damage. The variation trends of the nonlinear parameters is studied in tensile testing and fagitue bending testing. The microstructures evolution in the loading process is studied by microscope and SEM. And the reason of the nonlinear parameters change are analyzed in microscopic view. The main contents can be summarized as follows.(1) Based on the nonlinear elastic wave theory, the acoustic nonlinear phenomenon is studied. According to the constitutive equation of stress and strain, the nonlinear ultrasonic wave equation is resolved by the perturbation method. The relationship of the nonlinear coefficients β、δ and the fundamental and harmonic amplitueds is confirmed. The inner connection of the nonlinear coefficients and fatigue damage in metallic components is discussed. The characterization method of nonlinear ultrasonic and fatigue damage is built.(2) The advantages and disadvantages of the dislocation monopole model and the dislocation dipole model are studied. Based on the dislocation gliding model, the dislocation dipole model is deduced. The dislocation comprehensive model of the second order nonliear coefficient and the dislocation dipole model of the third order nonlinear coefficient are proposed. The relationship of acoustic nonlinear phenomenon – microstructure – mechanical properties(stress or fatigue) is built.(3) The theory of ultrasonic characterization of stress is studied. The sensitivity of the acoustic wave to stress is discussed. The dislocation models are used to analyze the nonlinear coefficients as the function of stress. It is verified by experiments.(4) The effect of micro crack on acoustic wave is studied. The elastic contact model, the nonlinear spring model and FEM method are used to analyze the open and close of micro crack. The effection on the ultrasonic received signal by length and width of crack is derived. They are verified by experiments.(5) The properties of specimens are detected online under the bending load. The variation trends of the relative nonlinear coefficients are researched under different fatigue bending cycles. The micro reason of the relative nonlinear coefficents change is explored. The dislocation model is used to simulate the nonlinear coefficients as a function of fatigue life. The experiments are used to verify the dislocation models.Based on classic fatigue cumulative damage rule, the dispersibility of experimental data is studied. Based on the probabilistic analysis and the nonlinear coefficents, the forecasting method of fatigue life is proposed.
Keywords/Search Tags:Nonlinear ultrasonic, Dislocation model, Tensile stress, Fatigue crack, Fatigue life
PDF Full Text Request
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