| Rayleigh wave is a wave that propagates along the surface of a uniform semi-infinite body and a layered semi-infinite body,which has the following characteristics: its energy is mainly concentrated in the shallow part of the medium,mainly distributed in a wavelength depth range,and concentrated in a half wavelength range.As the depth increases,the energy decays rapidly.In a well-distributed elastomer space,the speed of a Rayleigh wave is independent of frequency and only related to the elastic constant of the medium.When there is a crack in the shallow layer,after the Rayleigh wave propagating outward from the center of the vibration source encounters the crack,part of the energy is converted into bulk waves and scattered outward;part of the reflected Rayleigh wave is formed;part of the Rayleigh wave is diffracted along the edge of the crack.At the bottom edge of the crack,some Rayleigh waves undergo mode conversion again to form diffracting volume waves,some continue to diffract along the edge of the crack,and some Rayleigh waves directly pass through the crack.The existence of cracks and the depth of the cracks play a role of low-pass and high-resistance for Rayleigh waves of different frequencies.The changes in the amplitude and frequency of the Rayleigh waves before and after the cracks can be used to quantitatively determine the depth of the cracks.In this paper,the axisymmetric analysis method is used to study the law of the free surface propagation of Rayleigh waves in a homogeneous medium.Based on numerical simulations,the time-domain and frequency-domain responses to the wave fields of Rayleigh waves at cracks of different depths and the effects of The wave field response was studied and analyzed.The relationship between the crack depth and the amplitude ratio of Rayleigh waves before and after the crack was obtained by the amplitude attenuation method in the time domain,and the relationship between the crack depth and the characteristic wavelength was obtained from the characteristic frequency in the frequency domain.The main research contents and results are as follows:1.In this paper,the Ricker source is used for numerical simulations in the presence of cracks in the homogeneous medium and in the homogeneous medium to verify the rationality of the relevant parameters of the model.Through numerical simulation of wave field response,the effects of cracks on the scattering of Rayleigh waves at the interface between cracks and medium are analyzed,and the scattering mechanism of Rayleigh waves by cracks is studied.When the wavelength of the Rayleigh wave is relatively large compared with the depth of the crack,it can directly transmit through the fracture.When the wavelength of the Rayleigh wave is relatively small,the delayed transmission occurs,which indicates the effect of the crack on the low-pass and high-resistance of Rayleigh waves with different frequencies.2.Through numerical simulation calculations of wave field response in the presence of cracks of different depths in a homogeneous medium,the disturbance of the response of the scattered wave to the surface wave field is analyzed in the time domain.Through the characteristics of the disturbance,the relationship between the effects of crack depth,wavelength and energy amplitude are studied.Because the amplitude of the Rayleigh wave changes at different depths before and after the fracture,the ratio of the amplitude ratio of the incident Rayleigh wave before the fracture to the transmitted wave after the fracture and the amplitude ratio without the fracture are taken as the effective amplitude ratio.That is,the transmission coefficient,and then the transmission coefficient of cracks at different depths.Through the data processing and analysis of the ratio of the crack depth to wavelength and the transmission coefficient in MATLAB,the regression relationship and regression formula of the wavelength and crack depth and the transmission coefficient are obtained.In the study of different main frequency domains,it is found that when the transfer coefficient is between 0.2 and 0.8,the crack depth calculated by the regression formula is close to the crack depth in numerical simulation,and error analysis is performed in other main frequency domains.The results show that the accuracy of the fracture depth calculated by the regression formula is high and can be used to quantitatively determine the fracture depth.3.In the frequency domain,according to the changes in the frequency spectrum,the depression points appearing in the frequency spectrum are used as the characteristic points,and a linear regression relationship between the wavelength corresponding to the characteristic frequency value and the crack depth in the Rayleigh wave spectrum is established,and the cracks can be quantitatively determine by the characteristic frequency depth.Due to the phenomenon that the crack passes through the low-frequency and high-frequency to the Rayleigh wave,when the frequency reaches a certain characteristic frequency,the Rayleigh wave will be transmitted directly.This characteristic frequency can be found by the amplitude change in the frequency spectrum.Based on this characteristic,the spectral changes of Rayleigh waves before and after the crack in the frequency domain from 0.2m to 1.2m were studied.Finally,the linear regression relationship between the wavelength corresponding to the characteristic frequency and the crack depth and the regression formula were obtained.The characteristic frequencies at the same crack depth in different main frequency domains are compared,and it is found that the existence of characteristic frequencies has nothing to do with the frequency domain of the vibration source,but the frequency domain distribution of the vibration source will affect the apparent degree of the characteristic points,so it is necessary to use different The excitation source in the frequency domain makes obvious depressions appear in the frequency spectrum to obtain the characteristic frequency,and then the crack depth is calculated from the linear regression relationship between the wavelength and the crack depth.4.Through numerical simulation calculations of the wave field response of Rayleigh waves in non-penetrating cracks and penetrating cracks,the disturbance of the response of the scattered waves to the surface wave field in non-penetrating cracks and penetrating cracks is analyzed in the time and frequency domains.When there are non-penetrative cracks,the amplitude change of Rayleigh wave before and after the crack is different from that of the penetrating cracks by numerical simulation.The transmission coefficient obtained under non-penetrating cracks will be larger.In the frequency domain,after processing the Rayleigh wave frequency domain after the crack,it is found that the frequency of the Rayleigh wave in the non-penetrating crack is higher in the frequency domain than that in the penetrating fracture,and a depression is formed in the frequency domain,which can distinguish non-penetrating cracks from penetrating ones. |
PDF Full Text Request