Method and device for testing defect based on ultrasonic lamb wave tomography

1. A method for testing a defect based on an ultrasonic Lamb wave tomography, comprising following acts performed by a computer:

S 1 , selecting an imaging area on a material to be tested, and partitioning the imaging area into N 1 ×N 2 grids, wherein M electromagnetic acoustic transducers for emitting are set on a first side of the imaging area, M electromagnetic acoustic transducers for receiving are set respectively opposite to the M electromagnetic acoustic transducers for emitting on a second side of the imaging area, and N 1 , N 2 , M are positive integers;

S 2 , exciting the M electromagnetic acoustic transducers for emitting to emit Lamb waves with a A 0 mode in all directions and the M electromagnetic acoustic transducers for receiving to receive the Lamb waves successively, such that M×M testing waves corresponding to M×M Lamb waves are obtained;

S 3 , performing a time-frequency analysis and a mode recognition on the M×M testing waves, so as to obtain time-of-flights of the M×M Lamb waves;

S 4 , recording the time-of-flights of the M×M Lamb waves;

S 5 , determining a first slowness of each grid according to the time-of-flights and a size of each grid to obtain a first defect area;

S 6 , establishing a three-dimensional Cartesian coordinate system in the imaging area, and defining an original emission angle;

S 7 , for a path of a Lamb wave through the defect, obtaining a phase velocity c p at a point P(x,y) on a forward direction of the Lamb wave through the defect and calculating ∂c p /∂x and ∂c p /∂y according to the phase velocity c p , where x is a coordinate value of the point P(x,y)in a x-direction, y is a coordinate value of the point P(x,y)in a y-direction;

S 8 , calculating an extrapolation point of the Lamb wave through the defect by introducing c p , ∂c p /∂x and ∂c p /∂y into an extrapolation formula;

S 9 , judging whether a coordinate value of the extrapolation point reaches or exceeds boundary coordinates of a corresponding grid, if no, using the extrapolation point of the Lamb wave through the defect as a new point on the forward direction of the Lamb wave through the defect and executing steps S 6 -S 9 , if yes, recording the coordinate value of the extrapolation point as a coordinate value of the end point of the Lamb wave through the defect;

S 10 , judging whether the coordinate value of the end point of the Lamb wave through the defect reaches or is close to the electromagnetic acoustic transducers for receiving, if yes, obtaining a path with a shortest time-of-flight of the Lamb wave through the defect, if no, changing the original emission angle and executing steps S 6 -S 10 until the path with the shortest time-of-flight of the Lamb wave through the first defect area is found;

S 11 , obtaining a second slowness of each grid according to the time-frequency analysis result and the shortest time-of-flight and a change of a slowness curve in the defect to obtain a second defect area and to determine a size and a distribution of the second defect area.

2. The method according to claim 1 , wherein a diameter of the electromagnetic acoustic transducer is within a range of 20 mm to 80 mm, a distance between centers of each two adjacent electromagnetic acoustic transducers at the same side is within a range of 20 mm to 100 mm.

3. The method according to claim 1 , wherein the M electromagnetic acoustic transducers for emitting are excited by a radio frequency power amplifier, in which an excitation frequency of the radio frequency power amplifier is within a range of 50 kHz to 500 kHz.

4. The method according to claim 1 , wherein the Lamb wave emitted by the electromagnetic acoustic transducer for emitting is a Lamb wave with a single A 0 mode.

5. The method according to claim 1 , wherein the first or the second slowness of each grid is determined according to formula (1):

T

i

=

∑

j

=

1

n

⁢

L

ij

*

S

j

,

⁢

(

i

=

1

,

2

,

…

⁢

,

m

)

,

(

1

)

where S j is a slowness of a j th grid, L ij is a length of a i th Lamb wave in the j th grid, T i is a time-of-flight of the i th Lamb wave, n =N 1 ×N 2 , and m =M×M .

6. The method according to claim 1 , wherein the original emission angle is within a range of 0˜180°.

7. The method according to claim 1 , wherein obtaining a phase velocity c p at a point P(x,y) on a forward direction of the Lamb wave through the defect and calculating ∂c p /∂x and ∂c p /∂y according to the phase velocity c p comprises:

obtaining 16 pixels with the extradition point used as a center;

obtaining 16 phase velocities corresponding to the 16 pixels;

introducing the 16 phase velocities respectively into a formula (2) to perform a two-dimensional surface fitting on a distribution of the phase velocities:

c

p

⁡

(

x

p

,

y

p

)

=

∑

k

=

0

3

⁢

∑

l

=

0

3

⁢

C

kl

⁢

x

p

k

⁢

y

p

l

,

(

2

)

where x p is a coordinate value of a p th pixel in the x-direction, y p is a coordinate value of the p th pixel in the y-direction, c p (x p ,y p ) is a phase velocity corresponding to the p th pixel, C kl are coefficients to be solved, k=0, 1, 2, 3, l=0, 1, 2, 3, p is a positive integer and 1 ≤p≤16, such that C kl , are obtained;

calculating ∂c p /∂x and ∂c p /∂y according to C kl .

8. The method according to claim 1 , wherein the extrapolation formula is expressed as formula (3):

y

k

+

1

=

2

⁢

y

k

-

y

k

-

1

+

1

c

p

⁢

(

∂

c

p

∂

x

⁢

(

y

k

-

y

k

-

1

)

Δ

⁢

⁢

x

-

∂

c

p

∂

y

)

⁡

[

(

y

k

-

y

k

-

1

)

2

+

(

Δ

⁢

⁢

x

)

2

]

(

3

)

where y k is a coordinate value of a first discrete point on the Lamb wave through the defect in a y-direction at a k th step of extrapolation, y k+1 is a coordinate value of a second discrete point succeeding the first discrete point on the Lamb wave through the defect in the y-direction at the k th step of extrapolation, y k+1 is a coordinate value of a third discrete point preceding the first discrete point on the Lamb wave through the defect in the y-direction at the k th step of extrapolation, Δx is a step size, and ∂y/∂x =(y k −y k−1 )/Δx.

9. The method according to claim 1 , wherein changing the original emission angle comprises:

increasing or decreasing the original emission angle by 2˜6°.

10. The method according to claim 1 , further comprising:

S 12 , judging whether an imaging precision satisfies a predetermined requirement, and if no, repeating steps S 2 -S 11 .