| Abstract:Traditional structural damage identification based on the structural dynamics theory has been widely used in engineering, and it is becoming the main method of structural damage identification. This method is effective but also has many disadvantages. First the modal data is obviously affected by the error of extract and measurement, secondly the theory of the modal vibration based on the conventional structural dynamics relies too much on a few of the whole structure of low frequency vibration. Experience shows that structural damage can not be identified by low-frequency modal. Only when the damage is large enough, it has obvious influence on the low-order modal, nevertheless the structure is already in danger. In terms of these reasons above, this paper discusses a kind of structural damage detection method based on the migration imaging principle and the theory of elastic wave transmission. This approach is one of the local damage detection technology. It has been widely used in the complex geological structure and various kinds of mineral identification and detection of the geophysical field since the last1980s which achieves a good recognition effect. If the earth’s interior space is regarded as the engineering structure while the various kinds of the bad geological structure is regarded as the internal damage of structure, this method can be applied to the engineering structure damage identification similarly. Main research results are as follows:1The eternal velocity medium imaging formula of the analytic form and the finite difference form are deduced based on the pseudo spectrum method of zero offset imaging damage records from the two-dimensional scalar wave equation;According to the theory of explosion reflection interface, the analytic and the numerical forms of the imaging formula of upward wave equation are deduced by the hybrid method. The finite difference formula of the two-dimensional homogeneous isotropic medium of the second order elastic wave equation are discussed and the numerical characteristics such as the stability, compatibility, convergence, numerical dispersion, numerical singularity and the anisotropy of finite difference grid are also analyzed; The staggered grid finite difference of the first-order wave equation is analyzed; As a numerical example, a plate structure with a single crack was imaged in two ways and the wave field snapshot of different time and zero offset records are obtained, the amplitude, energy, displacement, frequency, speed, phase and the stress of the wave field with the variation of damage are analyzed; the variation of dynamic stress concentration factor around the damage are analyzed; Finally the post-stack imaging method of zero offset record is applied to detect the different diseases of dam in southern Shanxi province.2The post-stack imaging methods which apply to the relatively complex damage are discussed. The post-stack depth migration imaging includes the post stack reverse-time depth migration and a new hybrid method of the depth migration. The analytical and discrete expressions and stability conditions of the the post-stack reverse-time depth migration imaging are discussed by the example of the second order and the three-dimensional wave equation. The irregular grid difference discrete method of the first-order and the three dimensional wave equation expressed by the velocity and stress were derived, the irregular grid difference operator of elastic wave equation with the fourth order spatial accuracy and the second-order time accuracy were deduced, the numerical form of wave field extrapolation of two-dimensional one-way wave equation in ω-x field is solved by the mixed method, the numerical dispersion relations, stability, velocity and the source absorbing boundary processing are analyzed. Take a plate structure with a single crack as an example, the wave field snapshot is acquired by using the post-stack depth imaging method.The amplitude, energy, displacement, frequency, speed, phase and stress of wave field with the variation of damage are analyzed; the change of dynamic stress concentration factor around the damage are analyzed; Finally Rayleigh wave post-stack depth imaging methods is applied to test the project to find out the hidden damage of the seawalls.3Taking the plate structure with a hole as an example, the shot gather of prestack depth migration and reflectivity imaging expression were deduced; Taking the division method of the five node quadrilateral mesh as an example, the one-dimensional and second-order wave equation of the finite difference grid is discreted; The numerical frequency scattering and stability of the finite difference method are analyzed; the first-order wave equation imaging and extrapolation formula were deduced by double square root operator in the frequency-space domain and frequency-wave number domain. The stability and numerical frequency scattering of finite difference are analyzed; The extrapolation and imaging formula of the first-order two-dimension upward wave equation were deduced by the split step Hartley transform. The amplitude, energy, displacement, frequency, speed, phase and stress of the wave field with the change of damage are analyzed; The change of dynamic stress concentration factor around the damage are analyzed; The snapshot imaging of wave field are acquired. Finally the bad geological phenomenon of the hotel foundation can be detected and identified by the prestack depth migration.4Firstly three kinds of prestack time migration expression and imaging conditions were deduced. The implementation steps, key parameters and the influence factors to realize prestack time migration technology are analyzed. The wave field snapshots and migration imaging figure of different time are obtained. The amplitude, energy, displacement, frequency, speed, phase and stress of the wave field with the change of damage are analyzed; the bad geological structure of tunnel are detected and identified by the prestack reverse time migration method. |
PDF Full Text Request