Forming Non-Diffracting Beams Using a 2D Matrix Phased Array Probe

J. Menges, J. Bamberg, MTU Aero Engines, Munich, Germany

03.09.2012 2

Introduction

Diffracting beams:

- diverging sound field

- amplitude drops rapidly out of the focal zone

Non-diffracting beams:

- Bessel beam

- Focus a parallel beam to great depths

03.09.2012 3

Phased Array Approach to form bessel beams

• Simulation of an physical axicon lense with adjusted time delays for a 2D matrix array

Time delays to form bessel beamfor the matrix array

Probe with axiconlense

Time delays of bessel beamfocussing compared to regular

focussing

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9 10 11

Probe elements along middle axis

Dela

yti

me

inn

s

bessel beam slope8ns/mm

focussed beam,focal depth 70mm

03.09.2012 4

Equipment

Phased Array Equipment

Olympus Focus LT 64/128

Olympus Tomoview Software

Phased Array Probe

11 x 11 elements

10 MHz

Size of aperture: 23.1mm x 23.1mm

Element size: 2mm x 2mm

Interelement spazing: 0.1mm

Simulation setting for beam computation

Software: CIVA 10

Simulation-Setting:

03.09.2012 5

Material Titanium

(c_Long = 6100m/s, c_Trans = 3120m/s,

density = 4,53 g/cm^3 )

Material geometry 100mm x 100mm x 250mm

Coupling Water

Water distance 50mm

Beam computation 0-250mm in Titanium

03.09.2012 6

Simulation of different slopes

The slope of the time delays can be varied

4 ns/mm 6.5 ns/mm 8 ns/mm

Bessel beams Compared to

13 ns/mm 17 ns/mm Focus 70mm

0

2

4

6

8

10

12

14

16

18

20

1 2 3 4 5 6 7 8 9 10 11

Element on diagonal axis

Tim

ed

ela

yin

µs

Slope 17ns/mm

Slope 13ns/mm

Slope 8ns/mm

Slope 6.5ns/mm

Slope 4ns/mm

03.09.2012 7

Acoustic pressure

Bessel beams have about 4 dB less acoustic pressure compared to the focal spot of afocussed transducer

-20

-15

-10

-5

0

5

10

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Depth in titanium in mm

Aco

usti

cp

ressu

rein

dB

unfocussed beam

-20

-15

-10

-5

0

5

10

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Depth in titanium in mm

Aco

us

tic

pre

ssu

rein

dB

focussed on 70mm

unfocussed beam

-20

-15

-10

-5

0

5

10

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Depth in titanium in mm

Aco

us

tic

pre

ssu

rein

dB

bessel slope 13

focussed on 70mm

unfocussed beam

-20

-15

-10

-5

0

5

10

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Depth in titanium in mm

Aco

usti

cp

res

su

rein

dB

bessel slope 8

bessel slope 13

focussed on 70mm

unfocussed beam

Main lobe to side lobe ratio

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Depth in titanium in mm

Main

lob

eto

sid

elo

be

rati

oin

dB

focussed on 70mm

no side lobes

The effect of the side lobes is only a little worse than for the focussed beam. The beam with aslope of 8ns/mm is for greater depth a lot better than the beam with a slope of 13ns/mm

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Depth in titanium in mm

Main

lob

eto

sid

elo

be

rati

oin

dB

bessel slope 13

focussed on 70mm

no side lobes

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Depth in titanium in mm

Main

lob

eto

sid

elo

be

rati

oin

dB

bessel slope 8

bessel slope 13

focussed on 70mm

no side lobes

no side lobes

Beam width

Bessel beams have a small beam width in great material depth

0

5

10

15

20

25

30

35

40

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Depth in titanium in mm

Beam

wid

thin

mm

unfocussed beam

0

5

10

15

20

25

30

35

40

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Depth in titanium in mm

Beam

wid

thin

mm

focussed on 70mm

unfocussed beam

0

5

10

15

20

25

30

35

40

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Depth in titanium in mm

Beam

wid

thin

mm

bessel slope 13

focussed on 70mm

unfocussed beam

0

5

10

15

20

25

30

35

40

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Depth in titanium in mm

Beam

wid

thin

mm

bessel slope 8

bessel slope 13

focussed on 70mm

unfocussed beam

Varying the water path

By increasing the water path,the effect of the side lobes canbe decreased and a nearsurface inspection is possible

Varying the aperture

With fewer elements, the length of the high acoustic pressure zone is decreased

Varying the frequency

With higher frequencies the beam width is decreased and the effect of the side lobes isincreased

Simulation setting for defect response

Simulation setting:

Material

Material geometry

Titanium

(c_Long = 6100m/s, c_Trans = 3120m/s,

density = 4,53 g/cm^3 )

100mm x 100mm x 150mm

Noise

Density

Amplitude

Structural Noise

0,3 points/mm^3 measured

1,1 S.I.

Coupling

Water path

Water

50mm

Flaws

Depth from surface

Flat bottom hole, Ø 1mm

70mm, 95mm, 120mm

Simulation Defect Response

Bessel beam Focussed beam (70mm)60mm

125mm

depth intitanium

60mm

125mm

depth intitanium

Experimental results

Experimental setup:

#2 FBHs in Nickel base alloys

Depth of FBHs:

38mm, 63mm

Water path: 75mm

Beam 1: Bessel beam

Beam 2: Focus 40mm

38mm

63mm 0

10

20

30

40

50

60

70

80

90

100

38 63

Depth of #2 FBH in Nickel base

alloy in mm

Fu

llscre

en

heig

ht

in%

bessel beam

focussed beam

Conclusion

• Fast and sensitive inspection of great material depths, even if the material is noisy

• As the non-diffracting beam is formed by phased array, the inspection can be changedto high resolution in short time. With the same equipment a focussed beam can be

produced easily.

Prospect:

• Bessel beams have a self-reconstructing ability, especially if they are produced with

such a big aperture as with the matrix array.

• With the self-reconstructing ability flaws which are hidden by other flaws could be

detected.