阅读说明：本技术 一种椭圆合成孔径聚焦的激光超声信号成像方法 (Laser ultrasonic signal imaging method with elliptical synthetic aperture focusing ) 是由 赵纪元 卢秉恒 王彪 王琛玮 于 2021-01-22 设计创作，主要内容包括：本发明公开了一种椭圆合成孔径聚焦的激光超声信号成像方法,该方法的主要实现步骤是：1、获取原始超声波信号；2、对原始超声波信号进行归一化处理；3、邻域相减,获取缺陷回波信号；4、确定椭圆簇边界；5、划分波场平面成像网格；6、获取实际的成像区域；7、将缺陷回波信号幅值映射至实际的成像区域；8、椭圆簇叠加与聚焦成像。通过该方法成像效果好,分辨率好,算法简单,并且实现了缺陷特征的有效识别。(The invention discloses a laser ultrasonic signal imaging method with elliptic synthetic aperture focusing, which mainly comprises the following implementation steps: 1. acquiring an original ultrasonic signal; 2. carrying out normalization processing on the original ultrasonic signals; 3. performing neighborhood subtraction to obtain a defect echo signal; 4. determining an ellipse cluster boundary; 5. dividing a wave field plane imaging grid; 6. acquiring an actual imaging area; 7. mapping the amplitude of the defect echo signal to an actual imaging area; 8. and (4) ellipse cluster superposition and focusing imaging. The method has the advantages of good imaging effect, good resolution and simple algorithm, and realizes effective identification of defect characteristics.)

1. A laser ultrasonic signal imaging method with an elliptical synthetic aperture focus is characterized by comprising the following steps:

step 1: acquiring original ultrasonic signal B by a laser ultrasonic detection system with an excitation probe and a receiving probe separated_ij；

Step 2: for original ultrasonic signal B_ijCarrying out normalization processing;

and step 3: performing neighborhood subtraction on the ultrasonic signals subjected to normalization processing to obtain a defect echo signal B'_ij；

And 4, step 4: determining an ellipse cluster boundary:

step 4.1, determining an ellipse cluster of any scanning point;

because the excitation point and the receiving point are separated from each other in the laser ultrasonic detection system, any propagation point P on the wave front of the acoustic field_dThe corresponding sound path is that point to the excitation point P_gAnd a reception point P_rAnd satisfies the relation: d_gd+D_dr>D_gr(ii) a Wherein D is_gdIs a point P_dTo the excitation point P_gThe distance of (d); d_drIs a point P_dTo a receiving point P_rThe distance of (d); d_grIs an excitation point P_gTo a receiving point P_rThe distance of (d); the set of isochronous propagation points in the sound field is formed by P_g、P_rIs a focal point, and has a major axis length of (D)_gd+D_dr) The series of ellipses forming an ellipse cluster;

step 4.2, determining the boundary of the ellipse cluster

The semi-focal distance of the oval cluster is c ═ D_gr/2, then oval cluster start boundary k₀Comprises the following steps:

c is the ultrasonic wave speed of the test piece to be detected, f is the sampling frequency delta k which is the allowance, adjustment is carried out according to the actual imaging effect, and the value range of delta k is 10-50;

oval Cluster termination boundary k_nTaking the length L, k, of the original ultrasonic signal_n＝L；

And 5: dividing a wave field plane imaging grid to obtain an imaging area;

calculating the semimajor axis a of the corresponding ellipse in the upper boundary of the ellipse cluster_maxAnd a semi-minor axis b_max：

Setting the imaging range in the x direction to be Deltas N_x+2a_maxAnd the imaging range in the y direction is deltas.N_y+2b_maxAnd the imaging grid size is delta z, the number of imaging points in the X direction and the Y direction is X_n、Y_nRespectively as follows:

where Δ s is the scanning step, N_xScanning the point number in the X direction; n is a radical of_yScanning the point number in the Y direction;

the imaging area can be represented as Z (X)_n，Y_n)；

Step 6: acquiring the relation between any ellipse in the ellipse cluster of any scanning point (i, j) and the imaging area, thereby obtaining the actual imaging area of any scanning point (i, j)

Step 6.1: establishing parameter equation of any ellipse in ellipse cluster

Let i equal 1, 2, … …, N_x，j＝1，2，……，N_yThen, the calculation process of the ellipse cluster of any scanning point (i, j) is as follows;

let k equal to k₀,k₀+1,……,k_nThe semimajor axis a of the kth ellipse_kAnd semi-minor axis b_kRespectively as follows:

the parametric equation for the kth ellipse can be expressed as:

wherein the content of the first and second substances,the centrifugal angle of the ellipse parameter equation is taken as the value range of 0-360 degrees;

step 6.2: establishing the relation between the k-th ellipse and the imaging grid according to the parameter equation of the k-th ellipse to obtain the actual imaging area of any scanning point (i, j)The specific relational expression is as follows:

and 7: mapping the defect echo signal amplitude of any scanning point (i, j) to the actual imaging areaPerforming the following steps;

traversing all ellipses in the ellipse cluster of any scanning point to obtain:

and 8: ellipse cluster superposition and focusing imaging:

traversing all the scanning points, and finally superposing the actual imaging areas of all the scanning points to realize focusing imaging, wherein the final imaging result is as follows:

Z＝∑_i∑_jZ_i,j。

2. the elliptical synthetic aperture focused laser ultrasound signal imaging method of claim 1, wherein:

the specific process of the step 2 is as follows:

step 2.1: for each original ultrasonic signal amplitude value is marked as B_ij(k) Calculating the root mean square value of the amplitude of the original ultrasonic signal

Step 2.2: dividing the amplitude of each original ultrasonic signal by the root mean square value X corresponding to the signal_rmsAnd realizing signal normalization.

3. The elliptical synthetic aperture focused laser ultrasound signal imaging method of claim 1, wherein: the specific process of the step 3 is as follows:

subtracting the original laser ultrasonic signal of the last scanning point after normalization processing from the original laser ultrasonic signal of the current scanning point after normalization processing to obtain a defect echo signal B'_ij。

Technical Field

The invention belongs to the field of laser ultrasonic detection, and particularly relates to a laser ultrasonic signal imaging method with an elliptic synthetic aperture focusing function.

Background

The electric arc welding additive manufacturing technology realizes the low-cost and high-efficiency manufacturing of large aerospace structural parts, and plays an important role in the development of the aerospace field. The control of the quality of a formed part plays a decisive factor for the development of an electric arc welding additive manufacturing technology, and the service performance of a product is influenced by severe temperature change, fast solidification of a molten pool, external environment influence, air holes, cracks, slag inclusion and other defects which are easily generated in the electric arc welding additive manufacturing process. Therefore, a defect detection technology is urgently needed to realize online detection in the process of manufacturing the arc welding additive, and realize quality control in the process of manufacturing the arc welding additive.

The laser ultrasonic detection technology is used as a non-contact detection mode, a receiving and transmitting separation mode is adopted, a laser ultrasonic exciter emits an excitation signal, a laser interferometer receives the laser ultrasonic signal, and due to the fact that powder splashes and environmental noise is large in the arc welding manufacturing process, the laser ultrasonic signal noise signal is obvious, the imaging effect is poor, and defects are difficult to identify.

Chinese patent, publication No. CN111751448A discloses a leaky surface wave ultrasonic synthetic aperture focusing imaging method, which realizes imaging detection of surface or near surface defects of a part and provides an effective nondestructive detection method for evaluating the surface quality of metal construction. Although the method can realize the imaging of the surface or near-surface defects of the part, the method has the defects of poor imaging effect, low resolution, incapability of carrying out synthetic aperture focusing imaging on the defect wave signals of longitudinal wave components during internal defect detection and the like.

Disclosure of Invention

The invention provides a laser ultrasonic signal imaging method with elliptical synthetic aperture focusing, aiming at solving the problems that the existing ultrasonic synthetic aperture focusing imaging method has low imaging efficiency, and the method can not carry out synthetic aperture focusing imaging on the defect wave signals of longitudinal wave components during internal defect detection.

The specific technical scheme of the invention is as follows:

the laser ultrasonic signal imaging method with the elliptic synthetic aperture focusing comprises the following steps:

step 1: acquiring original ultrasonic signal B by a laser ultrasonic detection system with an excitation probe and a receiving probe separated_ij；

Step 2: for original ultrasonic signal B_ijCarrying out normalization processing;

and step 3: performing neighborhood subtraction on the ultrasonic signals subjected to normalization processing to obtain defect echoesSignal B'_ij；

And 4, step 4: determining an ellipse cluster boundary:

step 4.1, determining an ellipse cluster of any scanning point;

because the excitation point and the receiving point are separated from each other in the laser ultrasonic detection system, any propagation point P on the wave front of the acoustic field_dThe corresponding sound path is that point to the excitation point P_gAnd a reception point P_rAnd satisfies the relation: d_gd+D_dr>D_gr(ii) a Wherein D is_gdIs a point P_dTo the excitation point P_gThe distance of (d); d_drIs a point P_dTo a receiving point P_rThe distance of (d); d_grIs an excitation point P_gTo a receiving point P_rThe distance of (d); the set of isochronous propagation points in the sound field is formed by P_g、P_rIs a focal point, and has a major axis length of (D)_gd+D_dr) The series of ellipses forming an ellipse cluster;

step 4.2, determining the boundary of the ellipse cluster

The semi-focal distance of the oval cluster is c ═ D_gr/2, then oval cluster start boundary k₀Comprises the following steps:

c is the ultrasonic wave speed of a test piece to be detected, f is sampling frequency, delta k is allowance, adjustment is carried out according to the actual imaging effect, and the value range of delta k is 10-50;

oval Cluster termination boundary k_nTaking the length L, k, of the original ultrasonic signal_n＝L；

And 5: dividing a wave field plane imaging grid to obtain an imaging area;

calculating the semimajor axis a of the corresponding ellipse in the upper boundary of the ellipse cluster_maxAnd a semi-minor axis b_max：

Setting the imaging range in the x direction to be Deltas N_x+2a_maxAnd the imaging range in the y direction is deltas.N_y+2b_maxAnd the imaging grid size is delta z, the number of imaging points in the X direction and the Y direction is X_n、Y_nRespectively as follows:

where Δ s is the scanning step, N_xScanning the point number in the X direction; n is a radical of_yScanning the point number in the Y direction;

the imaging area can be represented as Z (X)_n，Y_n)；

Step 6: acquiring the relation between any ellipse in the ellipse cluster of any scanning point (i, j) and the imaging area, thereby obtaining the actual imaging area of any scanning point (i, j)

Step 6.1: establishing parameter equation of any ellipse in ellipse cluster

Let i equal 1, 2, … …, N_x，j＝1，2，……，N_yThen, the calculation process of the ellipse cluster of any scanning point (i, j) is as follows;

let k equal to k₀,k₀+1,……,k_nThe semimajor axis a of the kth ellipse_kAnd semi-minor axis b_kRespectively as follows:

the parametric equation for the kth ellipse can be expressed as:

wherein the content of the first and second substances,the centrifugal angle of the ellipse parameter equation is taken as the value range of 0-360 degrees;

step 6.2: establishing the relation between the k-th ellipse and the imaging grid according to the parameter equation of the k-th ellipse to obtain the actual imaging area of any scanning point (i, j)The specific relational expression is as follows:

and 7: mapping the defect echo signal amplitude of any scanning point (i, j) to the actual imaging areaPerforming the following steps;

traversing all ellipses in the ellipse cluster of any scanning point to obtain:

and 8: ellipse cluster superposition and focusing imaging:

traversing all the scanning points, and finally superposing the actual imaging areas of all the scanning points to realize focusing imaging, wherein the final imaging result is as follows:

Z＝∑_i∑_jZ_i,j。

further, the specific process of the step 2 is as follows:

step 2.1: for each original ultrasonic signal amplitude value is marked as B_ij(k) Calculating the root mean square value of the amplitude of the original ultrasonic signal

Step 2.2: dividing the amplitude of each original ultrasonic signal by the root mean square value X corresponding to the signal_rmsAnd realizing signal normalization.

Further, the specific process of step 3 is as follows: subtracting the original laser ultrasonic signal of the last scanning point after normalization processing from the original laser ultrasonic signal of the current scanning point after normalization processing to obtain a laser ultrasonic signal B 'mainly containing a defect echo signal'_ij。

The invention has the beneficial effects that:

1. based on the characteristic that the sound field isochronous propagation line of the laser ultrasonic detection system with the excitation probe and the receiving probe separated is elliptical, the method realizes focusing imaging by drawing the isochronous propagation elliptical clusters of a single scanning point in an imaging area and superposing all the elliptical clusters, greatly reduces the complexity of the algorithm and effectively improves the calculation efficiency compared with the traditional synthetic aperture imaging algorithm focusing point by point.

2. The method utilizes the characteristic that the non-defect waveforms in the ultrasonic signals of adjacent scanning points are basically consistent in appearance time, and adopts a mode of subtracting adjacent original signals to carry out data preprocessing, thereby effectively inhibiting artifacts formed in focusing imaging by the non-defect waveforms in the original signals, including but not limited to swept-surface longitudinal waves, surface direct waves, multiple bottom waves, mode conversion waves and boundary echoes, avoiding the defect imaging characteristics from being submerged by the artifacts, and further realizing the effective identification of the defect characteristics.

3. According to the method, before focusing imaging, the signal is normalized by using the root mean square value of the original signal, so that the problem of false defects in the focusing imaging process caused by probe coupling effect, test block reflectivity (aiming at laser ultrasound), roughness and environmental factors is solved, and the condition of misjudgment of the defects in the imaging result is reduced.

Drawings

FIG. 1 is a flow chart of the implementation of the present embodiment;

FIG. 2 is a schematic diagram of the principle of ellipse cluster stacking;

FIG. 3 is a graph of raw ultrasonic signals;

fig. 4 is a signal diagram after processing by the method of the present invention.

Detailed Description

The method of the present invention is further described below with reference to the accompanying drawings and examples.

The embodiment provides a laser ultrasonic signal imaging method with an elliptical synthetic aperture focus, and the specific implementation process is as shown in fig. 1:

1. obtaining raw ultrasonic signals

A laser ultrasonic detection system is adopted to carry out a detection experiment of the additive manufacturing titanium alloy prefabricated defect test block (wherein the size of the test block is 100 multiplied by 24 multiplied by 3, three defects are buried in the test block, the aperture of each defect is 0.8mm, 0.8mm and 0.8mm respectively, the distance between the defect 1 and the defect 2 is 20mm, the distance between the defect 2 and the defect 3 is 30mm, and the buried depth is 1mm), and the distance D between an excitation probe and a receiving probe in the laser ultrasonic detection system is determined_gr8.2mm, scan step Δ s 0.2mm, sampling frequency: f is 125MHz, and the wave speed C of the surface wave is 2.94 mm/mus; number of X-direction scanning points N_XScanning point number N in Y direction as 400_y1 is ═ 1; the signal length of each scanning point is L1250, and the laser ultrasonic detection system collects the original ultrasonic signal B according to the parameters_ijWhere i 1250, j 400, as shown in fig. 3;

2. normalization processing of ultrasonic signals

For each original ultrasonic signal amplitude value is marked as B_ij(k) Calculating the root mean square value of the original ultrasonic signalThen dividing the amplitude of each original ultrasonic signal by the corresponding root mean square value X of the signal_rmsThe method and the device realize signal normalization and prevent false defects caused by signal amplitude fluctuation caused by test block reflectivity, roughness, probe coupling effect and environmental factors in the superposition imaging process.

3. Obtaining defect echo signal by neighborhood subtraction

Because the direct wave and the bottom wave in the a-scan signal generate larger interference and artifacts in the focusing imaging process, especially for the laser ultrasonic signal, the signal has various waveforms including but not limited to swept longitudinal wave, surface direct wave, multiple bottom waves, mode converted wave, boundary echo, and other waveforms, and the direct imaging will submerge the defect features.

Considering that the positions of waveforms such as direct waves and bottom waves in adjacent A-scan signals are basically fixed in the scanning detection process, subtracting the original laser ultrasonic signal of the previous scanning point from the original laser ultrasonic signal of the current scanning point, suppressing the interference of the direct waves and the bottom waves on focusing imaging, and highlighting a defect echo signal B'_ijThe focusing imaging effect is improved;

4. determining an ellipse cluster boundary:

4.1, determining an ellipse cluster of any scanning point;

as shown in FIG. 2, since the excitation point is not coincident with the receiving point in the laser ultrasonic detection system, any point P on the wave front of the acoustic field_dThe corresponding sound path is that point to the excitation point P_gAnd a reception point P_rAnd satisfies the relation: d_gd+D_dr>D_gr(ii) a Wherein D is_gdIs a point P_dTo the excitation point P_gThe distance of (d); d_drIs a point P_dTo a receiving point P_rThe distance of (d); d_grIs an excitation point P_gTo a receiving point P_rThe distance of (d); the set of isochronous propagation points in the sound field is formed by P_g、P_rIs a focal point, and has a major axis length of (D)_gd+D_dr) The series of ellipses forming an ellipse cluster;

based on this principle, we consider the sound field of a single scan point to be P_g、P_rThe method is characterized in that the method is an ellipse cluster of a focus, and when scanning points perform line scanning or surface scanning, a point-by-point focusing process of a synthetic aperture focusing algorithm can be converted into a superposition process of the ellipse cluster of each scanning point;

4.2 determining oval Cluster boundaries

The semi-focal distance of the oval cluster is c ═ D_gr4.1 mm/2, in order to ensure that the ellipse can be formed effectively (the major axis is larger than the focal length), the starting boundary k of the ellipse cluster₀Comprises the following steps:

wherein, the delta k is the allowance and is adjusted according to the actual imaging effect, and the value range is generally 10-50;

oval Cluster termination boundary k_nTaking the length of the signal, i.e. k_n＝L＝1250；

5. Dividing a wave field plane imaging grid to obtain an imaging area;

calculating the semimajor axis a of the corresponding ellipse in the upper boundary of the ellipse cluster_maxAnd a semi-minor axis b_max：

Setting the imaging range in the x direction to be Deltas N_x+2a_maxAnd the imaging range in the y direction is deltas.N_y+2b_maxThe size of the imaging grid is delta z, and the delta z is excellent according to the imaging effect and the calculation efficiencySelecting 0.1, the number of imaging points in the x direction and the y direction is respectively as follows:

the imaging area can be represented as Z (X)_n，Y_n)；

6. Acquiring the relation between any ellipse in the ellipse cluster of any scanning point and an imaging area:

6.1, establishing a parameter equation of any ellipse in the ellipse cluster

Let i equal 1, 2, … …, N_x，j＝1，2，……，N_yThen, the calculation process of the ellipse cluster of any scanning point (i, j) is as follows;

let k equal to k₀，k₀+1,……,k_nThen, the semimajor axis and semiminor axis of the kth ellipse are respectively:

the parametric equation for the kth ellipse can be expressed as:

wherein the content of the first and second substances,the centrifugal angle of the ellipse parameter equation is taken as the value range of 0-360 degrees;

6.2, establishing a kth ellipse according to a parameter equation of the kth ellipseThe relation between the circle and the imaging grid obtains the actual imaging area of any scanning point (i, j)The specific relational expression is as follows:

7. mapping the defect echo signal amplitude of any scanning point to the actual imaging areaPerforming the following steps;

traversing all ellipses in the ellipse cluster of any scanning point to obtain:

8. ellipse cluster superposition and focusing imaging:

traversing all the scanning points, and finally superposing the actual imaging areas of all the scanning points to realize focusing imaging, wherein the final imaging result is shown in fig. 4, and the expression is as follows:

through focusing imaging, noise in a defect echo signal of a single scanning point is effectively suppressed through multiple times of superposition, and the defect echo characteristics are enhanced through superposition, so that high-quality imaging of defects can be realized.