Ultrasonic Surface Wave-mixing Nonlinear Detection Technology Of Metal Shallow Micro-cracks

Early-state damages,such as fatigue,creep and thermal aging,will degrade the mechanical properties and endanger the health of in-service materials and structures.Therefore,it is important to develop a nondestructive method for testing and evaluating the early-state damage of materials to ensure the service safety.The traditional linear ultrasonic technology is difficult to detect the early fatigue damage,and the nonlinear ultrasonic wave is very sensitive to the changes of the microstructure and mechanical properties of the material,and provides a new research method for the non-destructive detection and evaluation of the early damage of the metal material.The nonlinear acoustics are shown very sensitive to the changes of materials' micro structures and mechanical properties,and have been widely used in nondestructive test and evaluation.Based on the early damage of materials,the thesis carried out theoretical and simulation studies on nonlinear mixing surface waves under the factors of propagation distance and micro-crack size.The main work is as follows:Aiming at the linear ultrasound technology being insensitive to early damage,the solution of the nonlinear ultrasonic dynamic equation excited by wave-mixing in solid medium is studied,and the nonlinear acoustic field modulation is explained.Under the condition of ignoring the high-order nonlinearity and attenuating diffraction,the connection between the nonlinear coefficient and the excitation signal and the received signal is obtained,which lays the foundation for the detection of micro-cracks on the metal surface using the ultrasonic nonlinear coefficient.Aiming at the complex problem of nonlinear sound field response signals under mixing excitation,a method to characterize the micro-crack damage of metal plates using nonlinear coefficients was proposed.Using COMSOL Multiphysics was used to study the nonlinear surface acoustic wave field excited by mixed frequency signals.The second harmonic and sum and difference frequency signals that generate nonlinear phenomena are extracted in the frequency domain of received signals.The establishment of the relationship between the frequency component amplitude and the nonlinear coefficient to achieve effective monitoring of shallow micro defects on the surface of metal components provides a basis for the correspondence between the nonlinear coefficient and the micro-crack size and excitation receiving position.Subsequently,the influence of different factors on the nonlinear coefficient in single layer and bonded metal components was studied.A micro-crack model with different sizes and different excitation receiving position models were set up for single-layer metal components.Models with different excitation receiving positions were set for the debonding phenomenon in the bonding structure,and micro-crack and propagation distances for ultrasonic echo signals of different sizes were studied.With the influence of the value,the variation trend of the ultrasonic nonlinear coefficient caused by different size micro-crack and propagation distance is obtained.Studies have shown that the nonlinear coefficient amplitude decreases with the increase of propagation distance;fixed propagation distance,keeping the width of micro-cracks constant,gradually increase the length of micro-cracks,the amplitude of the nonlinear coefficient gradually becomes larger;to maintain micro cracks When the length does not change but the width gradually increases,the nonlinear coefficient amplitude decreases.In summary,research on the propagation properties of ultrasonic surface waves under wave-mixing excitation and the modulation phenomenon of receive signals in nonlinear acoustic fields in this paper,the influence of micro-crack size and propagation distance on the nonlinear coefficient is studied by establishing a two-dimensional metal surface micro-crack and debonding model,and the influence of the length,width and propagation distance of the micro-crack on the nonlinear coefficient is obtained.Provides theoretical basis for the early detection of micro-cracks in metallic materials.