CN112067697A - A method for locating defects based on laser ultrasound B-scan - Google Patents

Abstract

A defect positioning method based on laser ultrasonic B scanning comprises the steps of B scanning on the surface of a metal material to obtain a B scanning signal; preprocessing the B-scanning signal, decomposing the B-scanning signal into different frequency bands, extracting the peak time of the longitudinal wave defect echo, and obtaining the propagation path of the longitudinal wave of each scanning point; drawing n ellipses by taking the excitation laser point and the receiving laser point as two focuses of the ellipse and the longitudinal wave propagation path as the major axis of the ellipse; and solving the intersection points of the adjacent ellipses, and connecting the intersection points in sequence to obtain the outline of the defect and the position coordinate of the defect. The invention takes a metal material with internal defects as a target verification object, utilizes a laser ultrasonic detection platform, detects the internal defects in a certain longitudinal section by utilizing longitudinal waves in a B scanning mode, and realizes reconstruction of B scanning signals by means of signal noise reduction, characteristic extraction and the like, thereby achieving the purpose of visual quantitative identification of the defects from the information such as the shape, the size, the position and the like of the detected defects.

Description

A method for locating defects based on laser ultrasound B-scan

technical field

The invention relates to the technical field of laser ultrasonic detection, in particular to a method for locating defects based on laser ultrasonic B-scanning.

Background technique

Laser ultrasonic technology uses laser pulses to excite ultrasonic waves in the inspected workpiece, and uses laser beams to detect the propagation of ultrasonic waves to obtain workpiece information, such as workpiece thickness, internal and surface defects, material parameters, and so on. It combines the high precision of ultrasonic testing and the advantages of non-contact optical testing. Compared with traditional ultrasonics, it is more suitable for harsh working environments such as high temperature, high pressure and strong corrosion and automated testing processes. It has become an important method in the field of non-destructive testing. .

Due to the different propagation characteristics and applications of ultrasonic signals excited by different excitation modes during laser ultrasonic inspection of defects, laser ultrasonic inspection of defects can be divided into single point excitation mode (A scan), line scan mode (B scan), and surface scan mode (C scan). Laser-ultrasonic defect detection under scanning). Among them, B scan can detect more information about defects than A scan, and has better real-time performance and higher detection efficiency than C scan, but the result is a stack of multiple A scan signals, which cannot be directly reflected. Information on the location, shape and size of the defect.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method for locating defects based on laser ultrasonic B-scanning, the result of which is more intuitive than the original signal of B-scanning, and can realize defect locating and shaping.

The present invention is achieved through the following technical solutions:

A method for locating defects based on laser ultrasonic B-scanning, comprising the following steps:

1) Perform B-scan on the surface of the metal material to obtain the B-scan signal;

2) Preprocessing the B-scan signal to obtain a preprocessed signal;

3) Decompose the preprocessed signal into different frequency bands, and extract the peak time of the longitudinal wave defect echo;

4) Obtain the propagation distance of the longitudinal wave at each scanning point according to the peak time of the defect echo;

5) Taking the excitation laser point and the receiving laser point as the two focal points of the ellipse, and taking the propagation distance of the longitudinal wave of each scanning point as the long axis of the ellipse, draw n ellipses;

6) Solve the intersection points of adjacent ellipses to obtain the intersection points Qi (x _Qi , y _Qi ), _i =1,2,...,n, and connect the intersection points Qi (x _Qi ,y _Qi ) in the order of _i from small to large, Obtain the outline of the defect and the coordinates of the defect position.

A further improvement of the present invention is that, in step 1), the B-scanning is performed on the surface of the metal material by means of simultaneous movement of the excitation laser and the receiving laser at equal intervals to obtain the B-scan signal.

A further improvement of the present invention is that, in step 2), time domain averaging, bandpass filtering and delay removal preprocessing are performed on the B-scan signal to obtain a preprocessed signal.

A further improvement of the present invention is that, in step 4), the peak time of the longitudinal wave defect echo is multiplied by the longitudinal wave velocity in the metal material to obtain the propagation distance of the longitudinal wave at each scanning point.

A further improvement of the present invention is that, in step 5), the expression of the ellipse is as follows:

In the formula, f is the laser transceiver distance, s is the scanning step length, i is the scanning point serial number (the number of scanning points is n), a _i is the semi-major axis, and b _i is the semi-minor axis.

Compared with the prior art, the present invention has the beneficial effects: the present invention takes the metal material with internal defects as the target verification object, and with the aid of the laser ultrasonic testing platform, the longitudinal wave is used to detect the longitudinal wave in a certain longitudinal section by means of B scanning. Internal defects, supplemented by means of signal noise reduction and feature extraction, realize the reconstruction of the B-scan signal, and achieve the purpose of visual and quantitative identification of defects from the shape, size and position of the detected defects. The specific advantages are as follows:

1. The present invention adopts the defect information automatic extraction method to automatically extract the peak moment of the longitudinal wave defect echo of each scanning point in the B-scan signal, avoiding the tediousness and error of manual selection;

2. In the present invention, the idea of replacing the defect reflection points with adjacent elliptical intersections approximately, overcomes the problem that the defect echo reflection points cannot be solved when the defect is unknown.

Further, the present invention adopts the method of moving the excitation source and the receiver at equal intervals at the same time, which is more suitable for on-line detection integrated in the complex shape metal manufacturing process compared to the method of fixing one end and moving the other.

Further, the B-scan signal method adopted in the present invention can solve the problem of asynchronous signal reception and laser emission, and time-domain averaging and band-pass filtering can improve the signal-to-noise ratio and highlight defect signal characteristics.

Further, the present invention uses the ultrasonic propagation time and wave speed in the B-scan signal to obtain its propagation path, and finds the position of the echo reflection point on the defect, thereby drawing the defect outline, so that the B-scan signal has a clearer and more intuitive expression, and the defect position, The shape and size of the part can be seen at a glance, enabling visual and quantitative detection of defects.

Description of drawings

Figure 1 is a general flow chart of the detection method.

Figure 2 is a schematic diagram of a laser ultrasonic testing platform.

FIG. 3 is a schematic diagram of a defect location method for laser ultrasonic B-scanning.

FIG. 4 is a schematic diagram of the principle of ellipse positioning.

FIG. 5 is a schematic diagram of an ellipse set.

FIG. 6 is a schematic diagram of an elliptical intersection point approximating instead of an echo reflection point.

FIG. 7 is a schematic diagram of an ellipse positioning result.

Detailed ways

The present invention will be described in detail below with reference to the accompanying drawings.

In the defect location method based on laser ultrasonic B-scan inspection of the present invention, the B-scan signal is reconstructed into an image, and the internal defects of the material are detected by longitudinal waves through the B-scan mode. As shown in Figure 1, it includes seven parts, including B scanning, signal processing, extraction of echo peak time, calculation of echo propagation distance, ellipse drawing, calculation of adjacent ellipse intersection and output image, as follows.

1) B scan

B-scanning is performed on the upper surface of the aluminum material by moving the excitation laser and the receiving laser at equal intervals at the same time to obtain the B-scan signal, and each scanning point is excited and received multiple times.

在工控机上获取B扫信号{x_i,i＝1,2,…,n}。Specifically, with the help of the existing laser ultrasonic testing platform (as shown in Figure 2, the laser ultrasonic testing platform includes the excitation laser, the receiving laser, the lens, the signal acquisition system, the industrial computer, the electric guide rail and other main key components.) B-scanning is carried out at multiple scanning points for aluminum materials. As shown in Figure 3, the fixed excitation laser and receiving laser are irradiated on the surface of the tested aluminum material, the laser sending and receiving distance is f=5mm, and the electric guide rail is moved at the same time, the scanning step size is s=0.1mm, and the scanning distance L=15mm , scan the points

Obtain the B-scan signal { _xi ,i=1,2,...,n} on the industrial computer.

Other metal materials such as steel or titanium alloys are also possible, and are not limited to the upper surface.

2) Signal processing

The time-domain averaging, band-pass filtering and delay removal are performed on the signal _xi of each scanning point. The specific process is as follows:

① Perform time-domain averaging processing on the signal _xi , perform preliminary noise reduction, and improve the signal ratio;

2) Band-pass filtering the signal after averaging in the time domain for further noise reduction and highlighting the characteristics of the defect signal;

③Delay the signal processed by band-pass filtering, remove the blank signal at the beginning of the signal acquisition, and reduce the error.

3) Extract the peak moment of the longitudinal wave echo

The wavelet packet analysis is performed on the signal after de-delay processing, the de-delayed signal is decomposed into different frequency bands, and the peak time t _i of the longitudinal wave defect echo is extracted from it.

4) Calculate the longitudinal wave propagation distance

By multiplying the propagation time t _i of the longitudinal wave defect echo at each scanning point by the longitudinal wave velocity v _L in the tested aluminum material, the propagation distance of each scanning point longitudinal wave S _i =v _L ×t _i is obtained.

5) Draw the ellipse

As shown in Figure 4, it is known that the sum of the distances (long axis) from any point on the ellipse to the two focal points is constant, then the excitation laser point and the receiving laser point hit on the surface of the metal material to be measured are the two focal points of the ellipse, and the The propagation distance S _i of the longitudinal wave at each scanning point is the long axis of the ellipse. When an ellipse is drawn, as shown in Figure 5, a certain point on the ellipse is the position of the echo reflection point on the defect.

The expression for an ellipse is

f为激光收发距离，s为扫查步长，半长轴

半短轴

i为扫查点数量，i＝1,2,…,n表示第i个扫查点或第i个椭圆，x、y是曲线上任意点的横、纵坐标，坐标系的原点为第一个椭圆的左焦点(激励或接收的起点)。Among them, the half focal length

f is the laser transceiver distance, s is the scanning step, and the semi-major axis

semi-minor axis

i is the number of scan points, i=1,2,...,n represents the i-th scan point or the i-th ellipse, x and y are the horizontal and vertical coordinates of any point on the curve, and the origin of the coordinate system is the first The left focus of an ellipse (the origin of excitation or reception).

When i<n, go to step 2), when i≮n, go to step 6).

6) Solve the intersection of adjacent ellipses

The two adjacent ellipse expressions are

It is difficult to write the expression of the solution because there are many parameters involved and the order of the two equations is relatively high. Considering the translation coordinate system here, the problem of the intersection of any two adjacent ellipses is transformed into the problem of finding the intersection of the first ellipse and the second ellipse.

When the origin of the coordinate system is taken as the center of the first ellipse, the expressions of the first two ellipses are respectively

Convert it to the general form as

It can be noticed that there is no first-order term of y in the two equations, so the equation (6) is substituted into equation (7) to eliminate y ² , and the quadratic equation of one variable about x is obtained (eq. 8).

The solution of the intersection point is changed from solving two-variable quadratic equations to one-variable quadratic equations.

by the root formula

Bring equation (9) into equation (6) to get

To make the equations and solutions look more concise, let

Then equations (8) and (9) become

Ax ² +Bx+C=0 (12)

What follows is a discussion of the solution case:

(1) Δ≥0, the equation has a solution

①When a ₁ =a ₂ , A = 0, the root formula is not available, but the equation becomes a one-dimensional linear equation (the situation at this time is that the two adjacent ellipses are the same size).

②When a ₁ ≠a ₂ , the formula for finding the root

Note: Negative values of x and positive values of y have been omitted according to actual conditions.

(2) Δ<0, the equation has no solution, and the ellipse has no intersection.

y_Qi＝y_i得到原始坐标系下的交点Q_i(x_Qi,y_Qi)。The intersection coordinates (x _i , y _i ) after solving the translation coordinate system, let

y _Qi =y _i obtains the intersection point Qi ₍ x _Qi ,y _Qi ) in the original coordinate system.

7) Ellipse intersection proof and result output

Each scanning point can draw an ellipse that passes through the defect (that is, is tangent to the contour of the defect), and the echo reflection point on the defect is the tangent point _Pi (x _Pi , y _Pi ) between the ellipse and the defect.

In order to obtain the tangent point Pi (x _Pi , y _Pi ) of the defect and the ellipse, the adjacent ellipse intersection Q _i (x _Qi , y _{Qi )} obtained above can be used as an approximate replacement. As shown in Figure 6, when the ellipse is sufficiently dense, that is, the scanning step s is sufficiently small, the intersection point Qi (x _Qi , y _Qi ) will gradually approach the tangent point P _{i (} x _Pi , y _Pi ).

来表示二者得接近程度；The average distance between the intersection point Q _i (x _Qi , y _Qi ) and the tangent point P _i (x _Pi , y _Pi ) is as follows

to indicate the closeness of the two;

相比较缺陷的直径D＝3mm，

与D的比值仅为0.00173。因此，交点与切点的距离可以忽略不计，表示可以用相邻椭圆的交点来近似代替缺陷上的回波反射点。From the experimental data, when the scanning step length s=0.1mm,

Compared with the diameter of the defect D=3mm,

The ratio to D is only 0.00173. Therefore, the distance between the intersection point and the tangent point is negligible, indicating that the echo reflection point on the defect can be approximated by the intersection point of the adjacent ellipses.

Adjacent the adjacent elliptical intersections _Qi (x _Qi , y _Qi ) in sequence, the upper edge contour of the defect and its position coordinates can be obtained, as shown in FIG. 7 , it can be seen that the method for locating defects of the present invention can detect The shape, size and location of defects are used to achieve the purpose of visual and quantitative identification of defects.

Claims (5)

1. a method for locating defects based on laser ultrasonic B-scanning, is characterized in that, comprises the following steps: 1) Perform B-scan on the surface of the metal material to obtain the B-scan signal; 2) Preprocessing the B-scan signal to obtain a preprocessed signal; 3) Decompose the preprocessed signal into different frequency bands, and extract the peak time of the longitudinal wave defect echo; 4) Obtain the propagation distance of the longitudinal wave at each scanning point according to the peak time of the defect echo; 5) Taking the excitation laser point and the receiving laser point as the two focal points of the ellipse, and taking the propagation distance of the longitudinal wave of each scanning point as the long axis of the ellipse, draw n ellipses; 6) Solve the intersection points of adjacent ellipses to obtain the intersection points Qi (x _Qi , y _Qi ), _i =1,2,...,n, and connect the intersection points Qi (x _Qi ,y _Qi ) in the order of _i from small to large, Obtain the outline of the defect and the coordinates of the defect position. 2. a kind of method for locating defects based on laser ultrasonic B-scanning according to claim 1, is characterized in that, in step 1), adopts the mode that the excitation laser and the receiving laser move at the same interval at the same interval to carry out the B-scanning on the metal material surface Check to obtain the B-scan signal. 3. a kind of locating defect method based on laser ultrasonic B-scanning according to claim 1, is characterized in that, in step 2), carry out time-domain averaging, band-pass filtering and removing delay preprocessing to B-scanning signal, obtain preprocessed signal. 4. a kind of locating defect method based on laser ultrasonic B scanning according to claim 1, is characterized in that, in step 4), multiply the wave crest time of longitudinal wave defect echo by longitudinal wave velocity in metal material, obtain each. Scan the propagation path of the longitudinal wave. 5. a kind of locating defect method based on laser ultrasonic B scanning according to claim 1, is characterized in that, in step 5), the expression of ellipse is as follows:

In the formula, f is the laser transceiver distance, s is the scanning step length, i is the number of scanning points, a _i is the semi-major axis, and b _i is the semi-minor axis.

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