Efficient ultrasonic phased array phase shift migration imaging method for defects in multi-layer structure

1 . A method for detecting a defect in a multi-layer structure based on efficient ultrasonic phased array phase shift migration imaging, comprising the following steps:

(1) acquiring, by an ultrasonic phased-array sensor, full matrix capture data D (x r , x s , t) of a multi-layer structure, wherein x s represents a position of an s th transmitting element; x r represents a position of an r th receiving element, D (x r , x s , t) represents a signal transmitted from the s th transmitting element and received by the r th receiving element at a time t;

(2) transforming, by a processor, the full matrix capture data D (x r , x s , t) to a frequency-wavenumber domain wavefield data D (k rx , k sx , ω) by means of three-dimensional fast Fourier transform, wherein k rx represents a horizontal wavenumber of a receiving element, k sx represents a horizontal wavenumber of a transmitting element, and ω represents a frequency;

(3) for a non-traversed layer in the multi-layer structure, extrapolating, by the processor, a surface wave field of the multi-layer structure to an upper interface of the non-traversed layer, to obtain wave field information of the non-traversed layer;

(4) according to the obtained wave field information, performing, by the processor, focus imaging on the non-traversed layer in the frequency-wavenumber domain;

(5) repeating, by the processor, the steps (3) and (4), until all layers are traversed, to obtain an imaging result of the multi-layer structure; and

(6) determining a defect in a multi-layer structure based on the imaging result.

2 . The method according to claim 1 , wherein in the step (3), the multi-layer structure is traversed in sequence from top to bottom.

3 . The method according to claim 1 , wherein in the step (3), the surface wave field of the multi-layer structure is extrapolated with a double-square-root vertical wavenumber.

4 . The method according to claim 1 , wherein in the step (4), the non-traversed layer is subjected to focus imaging in the frequency-wavenumber domain with a time t=0 as an imaging condition.

5 . The method according to claim 4 , wherein an expression of an imaging result I(x, z) with the time t=0 as the imaging condition is as follows:

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wherein k rx is the horizontal wavenumber of a receiving element; i is an imaginary unit; k sx is the horizontal wavenumber of a transmitting element; x represents an arrangement direction of an array of phased array sensors; ω represents a frequency; k z,n represents the wavenumber of an n th layer in a z direction; z represents an extrapolation depth; z n-1 represents a depth of a lower interface of an (n−1) th layer; n ranges from 1 to N, N representing the number of layers of the multi-layer structure; k z,m represents the wavenumber of an m th layer in the z direction; and d m represents a thickness of the m th layer.

6 . The method according to claim 1 , wherein in the step (4), specific operation of performing focus imaging on the non-traversed layer in the frequency-wavenumber domain according to the obtained wave field information comprises the following steps:

1) Performing layer-by-layer recursion of the obtained wave field information in an imaging region according to a resolution of a depth direction, to obtain discrete wave fields of the frequency-wavenumber domain;

2) Superposing the obtained discrete wave fields in frequency dimension, to obtain a wave field at a time 0;

3) Transforming the wave field at the time 0 from the wavenumber domain to a spatial domain by means of two-dimensional fast Fourier transform; and

4) Extracting a wave field corresponding to x r =x s =x from the spatial domain as an imaging result of the non-traversed layer, wherein x represents an arrangement direction of an array of phased array sensors, x s represents a position of a transmitting element, and x r represents a position of a receiving element.