Two-stage spatial wave number filtering damage imaging method based on virtual excitation focusing enhancement

Abstract

The invention discloses a two-stage spatial wave number filtering damage imaging method based on virtual excitation focusing enhancement, and belongs to the technical field of aerospace structure health monitoring, and the method comprises the steps: employing a piezoelectric sensor to carry out the excitation and collection of a guided wave signal of a damaged to-be-detected structure; virtual excitation focusing enhancement is carried out on guided wave signals at all angles through time shifting, and the angle corresponding to the highest pixel value is selected as a damage angle in combination with spatial wave number filtering imaging; virtual excitation focusing enhancement is carried out on the guided wave signals at all distances through the time shifting, filtering imaging is carried out in combination with the spatial wave number, and the distance corresponding to the highest pixel value is selected as the damage distance, so that positioning monitoring of damage is achieved. Two-stage discrimination is carried out through virtual excitation focusing enhancement and spatial wave number filtering imaging, damage imaging positioning on the to-be-detected structure is achieved, and the method has the advantages of being simple in arrangement, low in calculation amount and high in signal-to-noise ratio.

Description

Technical Field

[0001] The present invention relates to the technical field of aerospace structural health monitoring, and particularly to a two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement. Background Art

[0002] The structure of aerospace vehicles plays a crucial role in ensuring their service safety. With the increasing complexity of aircraft structure design, the requirements for structural stability and integrity have also been greatly improved. Therefore, the need for structural health monitoring technology in aircraft structural health assessment is becoming more and more urgent. The guided wave array imaging method based on piezoelectric sensor networks has been widely studied in the field of aircraft structural health monitoring and has broad application prospects.

[0003] However, with the increase in the complexity of aircraft structures, not only is the thickness of the structure significantly increased, but also there are more and more complex multi-layer heterogeneous structure forms, resulting in complex layouts of sensors on the structure surface, and large attenuation and low signal-to-noise ratio of the guided wave signals received by piezoelectric sensors arranged on the inner surface of the structure, making it difficult to achieve accurate imaging monitoring of structural damage. Summary of the Invention

[0004] The present invention aims to overcome the problems in the prior art of complex sensor layout, large attenuation of guided wave signals, and low signal-to-noise ratio in guided wave structural health monitoring technology, and provides a two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement.

[0005] To achieve the above object, the technical solution of the present invention is: providing a two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement, including,

[0006] Signal acquisition: performing damage operations on the structure to be measured, and using piezoelectric sensors to excite and acquire guided wave signals;

[0007] Positioning the damage angle: dividing the angles based on the damage monitoring area on the structure to be measured, directionally enhancing the guided wave signals at each angle through time shift, obtaining the first synthesized signal after virtual focusing enhancement and obtaining the enhanced guided wave damage scattering signal, performing spatial wavenumber filtering imaging, and selecting the angle corresponding to the highest pixel value as the damage angle;

[0008] Positioning the damage distance: dividing the distances at the damage angle, directionally enhancing the guided wave signals at each distance through the time shift, obtaining the second synthesized signal after virtual focusing enhancement and obtaining the enhanced guided wave damage scattering signal, performing spatial wavenumber filtering imaging, and selecting the distance corresponding to the highest pixel value as the damage distance to achieve the positioning monitoring of the damage.

[0009] In one embodiment, in the above-mentioned step of positioning the damage angle, the time shift operation includes calculating the time delay τ of each excitation sensor relative to the reference sensor according to each angle θ m (r), and the expression is as follows:

[0010]

[0011] where m is the number of the excitation sensor, Δr is the spacing between adjacent excitation sensors, and c is the wave speed of the guided wave propagation.

[0012] In one embodiment, in the above-mentioned step of positioning the damage distance, the time shift operation further includes calculating the time delay τ of each excitation sensor relative to the reference sensor according to each distance d m (r), and the expression is as follows:

[0013]

[0014] where m is the number of the excitation sensor, Δr is the spacing between adjacent excitation sensors, and c is the wave speed of the guided wave propagation.

[0015] In one embodiment, in the above-mentioned steps of positioning the damage angle and positioning the damage distance, according to the time delay τ m (r), the excitation guided wave signal is directionally enhanced to obtain the first synthesized signal or the second synthesized signal after virtual focusing enhancement, and the expression is as follows:

[0016]

[0017] M is the total number of excitation sensors in the array, f m (t) is the guided wave signal of the m-th excitation sensor, and F(r, t) is the first synthesized signal or the second synthesized signal.

[0018] In one embodiment, in the above-mentioned steps of positioning the damage angle and positioning the damage distance, the receiving sensor collects the enhanced guided wave damage scattering signal, as follows:

[0019] s(r, t) = [s(r -M , t), L, s(r m , t), L, s(r M , t)] (5)

[0020] where s(r m , t) is the amplitude of the guided wave damage scattering signal collected on the m-th receiving sensor at time t, and can be expressed as follows:

[0021]

[0022] where, gm (t) is the guided wave damage scattering signal collected by the m-th receiving sensor, ω c and k c are the frequency and wave number of the guided wave signal, and d and θ are the distance and angle of the damage relative to the excitation sensor array.

[0023] In one embodiment, in the above steps of positioning the damage angle and positioning the damage distance, the discrete Fourier transform is performed on the guided wave damage scattering signal after virtual excitation enhancement to obtain the wave number spectrum as follows:

[0024]

[0025] where t is time, ω c is the signal frequency, k c is the wave number of the signal, d and θ are the distance and angle of the damage relative to the reference excitation sensor array, k is the wave number, s(t) is the spatial sampling signal of the collected guided wave signal at time t, and k c cosθ is the wave number projection of the guided wave signal on the piezoelectric sensor array.

[0026] where

[0027]

[0028] The above formula is the Dirac function. When and only when k = k c cosθ, the spatial sampling signal passes through the filter.

[0029] In one embodiment, in the above steps of positioning the damage angle and positioning the damage distance, a spatial wave number filter is used to perform spatial wave number filtering imaging on the guided wave damage scattering signal after virtual excitation enhancement, and the pixel value is calculated by the following formula:

[0030]

[0031]

[0032] where k i is the central wave number of the spatial wave number filter, t is time, s(r,t) is the spatial sampling signal collected by all receiving sensors at time t, is the element of the spatial wave number filter with the central wave number k i .

[0033] In one embodiment, in the above steps of positioning the damage angle and positioning the damage distance, the maximum central wave number value is calculated as follows:

[0034]

[0035] The sampling rate of the spatial signal is 2π / Δr, and k i ranges from -k max , k max . Define a wavenumber resolution of Δk, and k i is calculated by the following formula:

[0036] k i = -k max + (i - 1)Δk, i = 1, 2, …, I (12)

[0037] where I is the maximum number of filtering times and is calculated by the following formula:

[0038]

[0039] In one embodiment, in the above signal acquisition step, at least two groups of linearly arranged piezoelectric sensors are arranged on the structure to be measured. The piezoelectric sensors are installed on both sides of the damage monitoring area, and the piezoelectric sensors include an excitation sensor and a receiving sensor; the excitation sensor includes a reference excitation sensor, and the reference excitation sensor is used to provide a reference signal.

[0040] In one embodiment, in the above step of positioning the damage angle, with the reference sensor as the origin and the linear excitation sensor array as the polar axis, the angle is divided.

[0041] In summary, the present invention provides a two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement. By combining virtual excitation focusing enhancement and spatial wavenumber filtering imaging, and combining angle and distance for two-stage discrimination, damage imaging and positioning on the structure to be measured are realized. The present invention provides a method for arranging a simple piezoelectric sensor array on the structure to be measured with high guided wave propagation attenuation for damage monitoring. The method has the characteristics of low computational complexity and high signal-to-noise ratio, and has good prospects in the field of aerospace structural health monitoring.

[0042] To make the above features and advantages of the invention more obvious and understandable, specific embodiments are given below and detailed descriptions are made in conjunction with the accompanying drawings as follows. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 is a flowchart of the present invention.

[0044] Figure 2 is a schematic diagram of a thermal protection structure with piezoelectric sensors installed from the first perspective in the present invention.

[0045] Figure 3 is Figure 2 the corresponding physical diagram.

[0046] Figure 4 Schematic diagram of the thermal protection structure with piezoelectric sensors installed from the second perspective in the present invention.

[0047] Figure 5 is Figure 4 corresponding physical diagram.

[0048] Figure 6 Waveguide damage scattering signal spectrum enhanced by virtual excitation at 90° in the present invention.

[0049] Figure 7 Waveguide damage scattering signal spectra before and after virtual excitation enhancement at 90° in the present invention.

[0050] Figure 8 Schematic diagram of the spatial wavenumber filtering imaging process in the present invention.

[0051] Figure 9 Variation diagram of the maximum pixel value at each angle and distance in the present invention. Detailed implementation manners

[0052] To make the objectives and technical solutions of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0053] The present invention provides a two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement. Figure 1 is the flowchart of the present invention. As Figure 1 shown, it includes the following steps:

[0054] S1, Signal acquisition: Perform damage operations on the structure to be measured, and use piezoelectric sensors to excite and collect guided wave signals.

[0055] S2, Locate the damage angle: Divide the angle based on the damage monitoring area on the structure to be measured, directionally enhance the guided wave signals at each angle through time shift, obtain the first synthesized signal after virtual focusing enhancement and the enhanced guided wave damage scattering signal, perform spatial wavenumber filtering imaging, and select the angle corresponding to the highest pixel value as the damage angle.

[0056] S3. Locate the damage distance: Divide the distance on the damage angle, directionally enhance the guided wave signals at each distance through the time shift, obtain the second synthesized signal after virtual focusing enhancement and the enhanced guided wave damage scattering signal, perform spatial wavenumber filtering imaging, and select the distance corresponding to the highest pixel value as the damage distance to achieve the positioning and monitoring of the damage.

[0057] In the following, the present application will be described by taking the thermal protection structure with ablation damage as an example. The method in the present application can also be used to monitor fatigue cracks, corrosion, impact damage, etc., and can also be used for other structures and materials except the thermal protection structure.

[0058] In the step of "signal acquisition", the structure to be measured is a thermal protection structure, and the damage is ablation damage. The material of the thermal protection structure can be selected from, but not limited to, ceramics. The size of the thermal protection structure is 250 mm × 250 mm, and the thickness is 30 mm. A piezoelectric sensor array is installed on the surface of the thermal protection structure. As Figure 2 and Figure 3 shown, the piezoelectric sensor array is linear and at least two groups are provided, including an excitation sensor and a receiving sensor. The two groups of piezoelectric sensors are respectively installed on both sides of the damage monitoring area. The excitation sensor is used to excite the guided wave signal, and 9 piezoelectric elements can be set, denoted as A -4 ~A 4 ; The receiving sensor is used to receive the guided wave signal, and 9 piezoelectric elements can be set correspondingly, denoted as S -4 ~S 4 . Among them, A 0 is a reference excitation sensor, and the reference excitation sensor is used to provide a reference signal, and the remaining excitation sensors are used to virtually focus and enhance the guided wave signal. The number of piezoelectric elements can be adjusted according to the actual situation. Preferably, the number of excitation sensors and the number of receiving sensors are the same. The distance between the two groups of piezoelectric sensors can be set to 200 mm. The frequency at which the excitation sensor excites the guided wave signal can be a single frequency of 60 kHz.

[0059] Perform ablation treatment on the opposite side of the piezoelectric sensor installation surface. Oxygen-acetylene can be selected, but not limited to, to create ablation damage. As seen in Figure 4 and Figure 5 shown, the initial setting position of the ablation damage in the coordinate system with the A 0 excitation sensor as the origin and the connection line of A 0 -A -4 as the polar axis is (90°, 100 mm).

[0060] At the same time, use the guided wave structural health monitoring system for monitoring, and use the piezoelectric sensor array for individual excitation sensing to collect and obtain the guided wave signal spectrum.

[0061] In the step of "positioning the damage angle", taking the reference excitation sensor as the origin and using the linear excitation sensor array as the polar axis for angle division. Preferably, the angles are equally divided. Calculate the time delay τ m (r) of each excitation sensor in the excitation sensor array relative to the reference sensor, and perform virtual focusing enhancement on the guided wave signal according to the time delay τ m (r), so that the excitation sensor undergoes virtual displacement, ensuring that the guided wave signal is directionally focused at a certain angle simultaneously to obtain the enhanced first synthesized signal. Among them, calculate the time delay τ m (r) of each excitation sensor relative to the reference excitation sensor according to the preset angle or distance. The expression is as follows:

[0062]

[0063] where m is the excitation sensor number, r d is the distance from the reference excitation sensor to the ablation damage, r m is the distance from the excitation sensor numbered m to the damage, and c is the wave speed of the guided wave propagation.

[0064] Based on the far-field assumption, the spacing of the excitation sensors is much smaller than the distance from the excitation sensor array to the ablation damage. Consider the angle of the ablation damage relative to each excitation sensor as θ, as shown in the following formula:

[0065]

[0066] where m is the excitation sensor number, Δr is the spacing between adjacent excitation sensors, c is the wave speed of the guided wave propagation, M is the total number of excitation sensors in the array, f m (t) is the excitation guided wave signal of the m-th excitation sensor, and F(r,t) is the first synthesized signal.

[0067] The first synthesized signal is equivalent to amplifying the excitation guided wave signal of the reference excitation sensor by N times and can be expressed by the following formula:

[0068] F(t) = N·f m (t - τ m (r d )) (4)

[0069] After obtaining the synthesized first synthesized signal, the guided wave damage scattering signal collected by the receiving sensor can be regarded as enhanced together with the excited guided wave signal. Assuming there are 2M + 1 receiving sensors, there are 2M + 1 sampling points for the spatial sampling signal at each time point t. The guided wave damage scattering signal is as follows:

[0070] s(r,t) = [s(r -M ,t), L, s(r m ,t), L, s(r M ,t)] (5)

[0071] where s(r m ,t) is the amplitude of the guided wave damage scattering signal collected by the m-th receiving sensor at time t, and can be expressed as follows:

[0072]

[0073] where, g m (t) is the guided wave damage scattering signal collected by the m-th receiving sensor, ω c and k c are the frequency and wave number of the guided wave signal, and d and θ are the distance and angle of the ablation damage relative to the excitation sensor array.

[0074] Specifically, in the coordinate system with the reference excitation sensor A 0 as the origin and the line connecting A 0 -A -4 as the polar axis, the area within the range of 30° - 150° is monitored, and the angle interval can be selected as 1°. Use the excitation sensors A -4 ~A 4 to perform excitation one by one, and sense by the receiving sensor S -4 to obtain 9 guided wave signals after the ablation damage. Select 90° as the signal enhancement angle, combine Equation (2) and Equation (3) for virtual excitation focusing to obtain the corresponding first synthesized signal, and receive the enhanced guided wave damage scattering signal by the receiving sensor (as shown by the 9th guided wave damage scattering signal in Figure 6 ). The enhanced guided wave damage scattering signal can be represented by Equation (5). Successively use the remaining receiving sensors for sensing and perform virtual excitation focusing to obtain the remaining guided wave damage scattering signals in Figure 6 .

[0075] As shown in Figure 7 (a), it is the guided wave damage scattering signal collected by the receiving sensor S 0 at 90°. The receiving sensor S 0The enhanced guided-wave damage scattering signal received at the [specific location] is as follows Figure 7 as shown in (b). This operation can enhance the characteristic signal from the ablation damage site, while relatively weakening other irrelevant signals, so as to improve the quality and recognizability of the signal, reduce the interference of noise on the signal, and increase the amplitude and signal-to-noise ratio.

[0076] Furthermore, the enhanced guided-wave damage scattering signals at various angles are subjected to discrete Fourier transform (DFT) to obtain the wavenumber spectrum of the guided-wave damage scattering signal, as shown in Equations (7) and (8):

[0077]

[0078] where t is time, ω c is the signal frequency, k c is the wavenumber of the signal, d and θ are the distance and angle of the ablation damage relative to the reference excitation sensor array. k is the wavenumber, s(t) is the spatial sampling signal of the collected guided-wave signal at time t, and k c cosθ is the wavenumber projection of the guided-wave signal on the piezoelectric sensor array.

[0079] where

[0080]

[0081] The above formula is the Dirac function. When and only when k = k c cosθ, the spatial sampling signal can pass through the filter. Thus, k c cosθ can be obtained by searching for the matching wavenumber k value.

[0082] Furthermore, a spatial wavenumber filter is used to perform spatial wavenumber filtering imaging on the guided-wave damage scattering signal. Wavenumber filtering is performed using the wavenumber k i at all times t, and a wavenumber-time graph (see i ) with the wavenumber k Figure 8 as the abscissa and time t as the ordinate is obtained. The pixel value in the wavenumber-time graph can be calculated by the following formula:

[0083]

[0084] where k i is the central wavenumber of the spatial wavenumber filter, t is time, s(r,t) is the spatial sampling signal collected by all receiving sensors at time t, is the element of the spatial wavenumber filter with the central wavenumber k i .

[0085] Considering the Nyquist sampling theorem, the maximum central wavenumber needs to be less than half of the sampling rate. The sampling rate of the spatial signal is 2π / Δr. Therefore, the maximum central wavenumber is calculated as follows:

[0086]

[0087] k i ranges from [-k max , k max , and a wavenumber resolution of Δk is defined. k i can be calculated by the following formula:

[0088] k i = -k max + (i - 1)Δk, i = 1, 2, …, I (12)

[0089] where I is the maximum number of filtering times and can be calculated by the following formula:

[0090]

[0091] According to Equation (8), the maximum pixel value at each angle can be obtained, and the angle corresponding to the maximum pixel value is selected as the damage angle. As Figure 9 (a) shows, the angle corresponding to the maximum pixel value is 93°, that is, the damage angle is obtained as 93°.

[0092] In the step of "locating the damage distance", the time delay at each distance is calculated at the selected damage angle through Equation (14), and the guided wave signal is virtually focused and enhanced at each distance in combination with Equation (2) to obtain the enhanced second synthesized signal. According to the second synthesized signal, the enhanced guided wave damage scattering signal is obtained at the receiving sensor. Further, the guided wave damage scattering signal is subjected to discrete Fourier transform and spatial wavenumber filtering imaging using the spatial wavenumber filter, and the maximum pixel value of the imaging diagram at each distance is obtained by combining Equations (9) and (10).

[0093]

[0094] As Figure 9 (b) shows, for the guided wave damage scattering signal obtained in the range of 50 mm - 150 mm, spatial wavenumber filtering imaging is performed, the maximum pixel value at each distance calculated is obtained, and the distance corresponding to the maximum pixel value is selected as the damage distance, which is 91 mm. That is, the monitoring result is (93°, 91 mm). Comparing the detection result with the initial set position (90°, 100 mm) shows the feasibility of the method.

[0095] In summary, the present invention provides a two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement. By combining virtual excitation focusing enhancement and spatial wavenumber filtering imaging, and performing two-stage discrimination in combination with angle and distance, damage imaging and positioning on the structure to be measured are achieved. The present invention provides a method for arranging a simple piezoelectric sensor array on the structure to be measured with high guided wave propagation attenuation for damage monitoring, which has the characteristics of low computational complexity of the method and high signal-to-noise ratio, and has good prospects in the field of aerospace structural health monitoring. The above method is not limited by the specific type of the structure and the type of damage, and can be widely applied to the structural health monitoring of various aerospace structures and even other fields, providing a highly universal, efficient and reliable solution for structural damage detection.

[0096] Although the present invention has been disclosed above by way of examples, it is not intended to limit the present invention. Any person with ordinary knowledge in the technical field to which the present invention pertains may make some modifications and refinements without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be determined by the scope of the appended patent application.

Claims

1. A two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement, characterized in that: include, Signal acquisition: Perform damage operations on the structure to be tested, and use piezoelectric sensors to excite and collect guided wave signals; Positioning the damage angle: dividing the damage monitoring area on the structure to be tested into angles, directionally enhancing the waveguide signal at each angle through time shift, obtaining a first synthetic signal after virtual focus enhancement and an enhanced waveguide damage scattering signal, performing spatial wavenumber filtering imaging and selecting the angle corresponding to the highest pixel value as the damage angle; Positioning the damage distance: performing distance division at the damage angle, directionally enhancing the waveguide signal at each distance through the time shift, obtaining a second synthetic signal after virtual focusing enhancement and obtaining an enhanced waveguide damage scattering signal, performing spatial wavenumber filtering imaging and selecting the distance corresponding to the highest pixel value as the damage distance, so as to realize positioning monitoring of the damage.

2. The two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement as claimed in claim 1, characterized in that: In the above-mentioned damage angle positioning step, the time shift operation includes calculating the time delay τ of each excitation sensor relative to the reference sensor according to each angle θ. m (r), the expression is as follows: Wherein, m is the number of the excitation sensor, Δr is the distance between adjacent excitation sensors, and c is the wave velocity of guided wave propagation.

3. The two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement as claimed in claim 1, characterized in that: In the above-mentioned step of locating the damage distance, the time shift operation also includes calculating the time delay τ of each excitation sensor relative to the reference sensor according to each distance d. m (r), the expression is as follows: Wherein, m is the number of the excitation sensor, Δr is the distance between adjacent excitation sensors, and c is the wave velocity of guided wave propagation.

4. The two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement as claimed in claim 1, characterized in that: In the above steps of locating the damage angle and locating the damage distance, according to the time delay τ m (r) directionally enhancing the excitation waveguide signal to obtain the first synthetic signal or the second synthetic signal after virtual focusing enhancement, the expression is as follows: M is the total number of excitation sensors in the array, f m (t) is the waveguide signal of the mth excitation sensor, and F(r, t) is the first synthetic signal or the second synthetic signal.

5. The two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement as claimed in claim 4, characterized in that: In the above steps of locating the damage angle and locating the damage distance, the receiving sensor collects the enhanced guided wave damage scattering signal, as shown in the following formula: s(r,t)=[s(r -M ,t),L,s(r m ,t),L,s(r M ,t)] (5) Where s(r m ,t) is the amplitude of the guided wave damage scattering signal collected by the mth receiving sensor at time t, which can be expressed as follows: Among them, g m (t) is the waveguide damage scattering signal collected by the mth receiving sensor, ω c and k c are the frequency and wave number of the waveguide signal, d and θ are the distance and angle of the damage relative to the excitation sensor array.

6. The two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement as claimed in claim 5, characterized in that: In the above steps of locating the damage angle and locating the damage distance, the waveguide damage scattering signal after virtual excitation enhancement is subjected to discrete Fourier transform to obtain a wave number spectrum, as follows: Where t is time, ω c is the signal frequency, k c is the wave number of the signal, d and θ are the distance and angle of the damage relative to the reference excitation sensor array, k is the wave number, s(t) is the spatial sampling signal of the waveguide signal collected at time t, k c cosθ is the wave number projection of the waveguide signal on the piezoelectric sensor array, in, The above formula is a Dirac function if and only if k = k c When cosθ, the spatially sampled signal passes through the filter.

7. The two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement as claimed in claim 6, characterized in that: In the above steps of locating the damage angle and locating the damage distance, a spatial wavenumber filter is used to perform spatial wavenumber filtering imaging on the waveguide damage scattering signal after virtual excitation enhancement, and the pixel value is calculated by the following formula: Among them, k i is the central wavenumber of the spatial wavenumber filter, t is time, s(r,t) is the spatial sampling signal collected by all receiving sensors at time t, The center wave number of the spatial wave number filter is k i elements.

8. The two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement as claimed in claim 7, characterized in that: In the above steps of locating the damage angle and locating the damage distance, the maximum central wave value is calculated as follows: The sampling rate of the spatial signal is 2π / Δr, k i The range is [-k max ,k max ], define a wave number resolution of Δk, k i Calculated by the following formula: k i =-k max +(i-1)Δk,i=1,2,L,I (12) Where I is the maximum number of filters, calculated as follows:

9. The two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement as claimed in claim 1, characterized in that: In the above-mentioned signal acquisition step, at least two groups of linearly arranged piezoelectric sensors are arranged on the structure to be tested, and the piezoelectric sensors are installed on both sides of the damage monitoring area. The piezoelectric sensors include excitation sensors and receiving sensors; the excitation sensors include reference excitation sensors, and the reference excitation sensors are used to provide a reference signal.

10. The two-stage spatial wavenumber filtering damage imaging method based on virtual excitation focusing enhancement as claimed in claim 9, characterized in that: In the above-mentioned step of locating the damage angle, the reference sensor is used as the origin and the linear excitation sensor array is used as the polar axis for angle division.

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