| With the rapid development of economy in China,the use of sheet metal material is more and more widely in the field of construction.They play an important role in steel structure,super high-rise structure and long-span structure and so on.However,sheet metal materials are prone to fatigue cracks when subjected to reciprocating loading,and the losses are very heavy.Therefore,the early detection of fatigue cracks in metal plates is of great significance.The traditional method for detecting fatigue cracks is linear ultrasonic technique,but this method is not sensitive to the early stage of fatigue cracks.In order to make up for the shortcomings of this method,scholars have proposed nonlinear ultrasonic testing methods,the most promising of which is the nonlinear wave modulation spectroscopy.At present,the nonlinear wave modulation spectroscopy is based on dual frequency excitation,that is,the input signal is a low frequency vibration signal and a high frequency ultrasonic signal.There are many disadvantages to this method: To produce an obvious modulation phenomenon,it is relatively difficult to select the frequency of excitation;Modulation signals are easily affected by environmental factors and operational conditions;Low frequency vibration signals will have some influence on the structure itself.In order to solve the above shortcomings,this dissertation puts forward the nonlinear wave modulation spectroscopy based on a broadband signal excitation.It is instead of dual frequency signals using a broadband signal.The damage factors are defined and used to characterize the microcrack damage of metal plates.Based on this method,the corresponding theoretical and numerical investigations are carried out.The solution of nonlinear ultrasonic wave equation excited by a broadband signal in solid medium is studied,and then the phenomenon of nonlinear wave modulation spectroscopy is explained by this theory.In the case of neglecting the higher order nonlinearity and the hysteretic nonlinearity,the two-order nonlinear coefficient agrees with the numerical analysis in the latter.Because the nonlinear wave modulation spectroscopy based on a broadband signal excitation is very complex.Therefore,the finite element analysis software ABAQUS is first used to study the nonlinear wave modulation spectroscopy excited by dual frequency signals.The first and two order modulation frequency signals can be extracted from the analysis results.The modulation phenomenon of different directions of micro cracks is also analyzed,the results show that this method is also effective for the detection of micro cracks with different orientations.Aiming at the complex of response signals of the nonlinear wave modulation spectroscopy based on a broadband signal,the damage factors is used to characterize the microcracks damage of metal plates.By studying the modulation frequency signals,it is found that the damage factors can include both the amplitude characteristic of the modulation sidebands and the order or quantity characteristics of the modulation sidebands.On this basis,models of metal plates with different sizes and different exciting positions are set up.It is found that the magnitudes of damage factors increase with the length growing of microcracks and becomes smaller as the width increases of microcracks;When the distance in the horizontal between receiving point and the microcracks becomes larger,the amplitude of the damage factors will become smaller.The relation between the magnitude of damage factors and the location of the stimulated receiving path is not obvious in the vertical direction.The above phenomena are analyzed in detail in this dissertation.In order to further study the positioning method of micro cracks,change the broadband excitation signal energy,the nonlinear of different excitation receiving path in metal plates excited by the broadband signal is different.Finally,the amplitudes of the damage factors in different paths are obviously different,the location of microcracks is thus determined. |
PDF Full Text Request