Related topics Angle beam probe, ultrasound echography

Angle beam measurement

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Related topics

Angle beam probe, ultrasound echography (ultrasonography), A-mode, reflection, beam entry angle, refraction, longitudinal wave, shear wave (transverse wave), corner echo, and skip distance

Principle

This experiment demonstrates the use of ultrasonic angle beam probes for the non-destructive testing of materials. The echoes of shear and longitudinal waves in an aluminium test block are measured with the aid of three different delay lines. In the case of normal probes, the calibration simply results from the time of flight and the sound velocity, but in the case of angle beam probes, the length of the delay line, sound velocity of the shear wave, beam entry angle of the probe, and sound exit point of the delay line must also be determined. The calculated values are verified via a measurement of the half and full skip distance at a cylindrical discontinuity.

Equipment

1 Basic Set “Ultrasonic Echoscope” consisting of:

1x Ultrasonic echoscope

1x Ultrasonic probe 1 MHz

1x Ultrasonic probe 2 MHz

1x Ultrasonic test block

1x Ultrasonic cylinder set

1x Ultrasonic test plates

1x Ultrasonic gel

1 Extension set : Non destructive testing

1 Vernier calliper

1 Ruler, plastic, 200 mm

Additional equipment

1 PC with a USB port, Windows XP or higher

13921-99

13921-01

03010-00

09937-01

Fig. 1: Equipment for ultrasound angle beam measurements, experimental set-up

P5160400 PHYWE Systeme GmbH & Co. KG © All rights reserved www.phywe.com

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Caution!

Pay close attention to the special operation and safety instructions in the manual of the ultrasonic echoscope.

Tasks

1. Study the half and full skip distance at an aluminium test object with three different delay lines. Determine the probes with which the longitudinal and shear wave echoes can be measured.

2. Measure the times of flight and the positions of the probe when a corner echo occurs at half and full skip distance, first with the 38° delay line and then with the 17° delay line.

3. Calculate the sound exit point, angle of incidence, simple path length, sound velocity, and length of the delay line based on the measurement data.

4. Verify the probe data (calibration) at the cylindrical discontinuity. Measure the depth and the projection distance or the shortened projection distance of the flaw in the test block, and compare the measured values with the drawing.

Set-up and procedure

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-

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Prepare the echoscope (read the manual of the echoscope).

Connect the echoscope to the PC.

Set the selector switch to “Reflection”.

Connect the 2 MHz probe to the “Probe (Reflection)” port.

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-

-

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Couple the delay line to the 2 MHz ultrasonic probe with the aid of some coupling gel. Use the delay lines in the following order: 56°, then 38°, and finally 17°. Apply a pea-sized quantity of gel to the centre of the probe surface and attach the delay line. Move the delay line back and forth in order to distribute the gel evenly on the probe surface. Ensure that no air is entrapped in the coupling layer.

Couple the resulting probe assembly to the front edge of the test object with the aid of some gel so that the delay line protrudes from the edge by approximately 1 cm. Start with a medium setting of the "GAIN" and "OUTPUT controllers.

Move and rotate the angle beam probe on the test object until the first echo near the test object edge shows a maximum. This ensures that the sound cone fulfils the geometrical requirements that are necessary for the calculations.

Adjust the emitting power (“OUTPUT”) and the gain (“GAIN”) at the ultrasonic echoscope so that the echo amplitude reaches approximately 0.5 to 0.8 V. TGC is usually not required.

Fig. 2: Sound paths with different delay lines

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-

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Change the delay line and repeat the measurements. Compare the measurements.

Measure the height of the test block with the vernier calliper.

Calibrate the 38° delay line. Determine the positions of the probe where there is a corner echo at half skip distance (see the description above).

Measure the distance between the probe and the edge of the test object with a vernier calliper or ruler. Determine the time of flight of the ultrasound with the measurement cursor in the software.

Position the cursor at the base of the echo (not in the middle of the peak) and read off the time of flight.

2 PHYWE Systeme GmbH & Co. KG © All rights reserved P5160400

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Move the probe to the maximum of the corner echo at full skip distance. Ensure that the delay line remains coupled to the test object. If necessary, apply some fresh ultrasound gel.

Measure the distance again between the probe and the edge of the test object and determine the

Fig. 3: Measurement of the time of flight of the corner echo at half skip distance

-

-

-

-

- time of flight of the ultrasound.

Repeat these two measurements several times and calculate the mean values.

Repeat the same measurements with the 17° delay line. The probe coupling and the measurements must be performed as described above. With this delay line that has a smaller beam entry angle it must be ensured that the longitudinal waves are not eliminated by total reflection as is the case with the 38° delay line. As a result, two maxima will occur. These two maxima must be examined separately, since different results can be expected for the sound velocity and angle of incidence of the two waves. This means that separate measurements of the times of flight and distances must be performed for the longitudinal and shear waves.

Calculate the various characteristic values for the delay lines of 17° and 38°, i.e. the sound exit point, skip distance, beam entry angle, sound velocity, and delay line length.

In the last part of the experiment, measure the discontinuity in the test object with the 38° delay line.

Attach the 38° delay line to the probe and position it on the test object. Move the probe until the sound is reflected at the discontinuity and the signal amplitude reaches its maximum. The echo has a shorter time of flight than the echoes that were measured beforehand.

Use this time of flight to determine the depth and the projection distance of the discontinuity.


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Software

The “measure Ultra Echo” software records, displays, and evaluates the data that are transferred from the echoscope. After the start of the program, the measuring mode is active and the mai n screen “A-

Scan mode” is displayed. All of the available actions and evaluations can be selected and started in this window.

The upper part of the main screen shows the A-scan signal, the frequency of the connected transducer, and the operating mode (reflection/transmission). The current positions of the cursors (red and green line) are displayed at the bottom of the window. The cursors can be positioned by a mouse click. The time of flight is displayed under the cursor buttons.

Note

The ultrasonic cylinders and the probes should be cleaned immediately after use with water or a standard detergent. Dried residues of ultrasonic gel are difficult to remove. If necessary, use a soft brush.

Never use alcohol or liquids with solvents to clean the cylinders or probes. Deep surface scratches affect the coupling and can induce measurement errors.

Theory and evaluation

While the distance calibration of normal probes simply results from the time of flight and the sound velocity, some additional geometrical factors must be taken into consideration in the case of beam angle probes.

The geometry of the test task and selection of the beam angle probes

When a sound wave propagates from one medium to another, a part of the sound wave is reflected at the interface while the other part is refracted.

The reflection follows the law:

  

L

[1]

The angle of incidence equals the angle of reflection.

The refraction follows: sin sin





L

 c medium 1 c medium 2

This results in:

[2]



L

 arcsin



 sin

 c medium 2 c medium 1



 [3]

From this equation, it can be derived as a boundary condition that at:

Fig. 4: Refraction and reflection of an ultrasound wave sin

 c medium 2 c medium 1 a surface wave occurs at the interface and that at:



1 [4] sin

 c medium 2 c medium 1

 1 [5] the incident wave is totally reflected on the interface, which means that the sound does not penetrate the test object.



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If a sound wave hits a solid object in a nonperpendicular way, a longitudinal wave and a shear wave may propagate through the object. Both waves propagate at a different velocity in the solid object so that – in accordance with the law of refraction [3] – they follow two different directions of propagation.

According to the literature, the propagation velocity of a longitudinal wave and shear wave in aluminium is 6300 and 3100 m/s, respectively, and 2700 and 1100 m/s in plexiglass.

Since, with this experiment set-up, the sound wave hits the delay line perpendicularly, only the longitudinal wave propagates in the delay line. In the test object, it can split up into a longitudinal wave and a shear wave. Figure 6 shows the directions of propagation of these two waves as a function of the angle of incidence.

90

85

80

75

70

65

60

55

50

45

40

35

Refraction

C long.

C shear

The diagram shows that the maximum angle of a plexiglass delay line for studying an aluminium test object must not be greater than

60.5°, since otherwise a total reflection would occur at the interface. If the angle is smaller than 25.4°, a longitudinal wave and a shear wave will result. This means that an echo that is received can be assigned to two different locations. These angles are not permissible for non-destructive material testing.

This is why only angle beam probes with beam entry angles within a limited interval are used for non-destructive material testing (green area). In this interval, only shear waves can propagate.

30

25

20

15

10

5

0

0 10

17

20 30

38

40

Incident angle

50

56

60 70

Fig. 6: Direction of propagation of a shear wave and a longitudinal wave from a plexiglass delay line into an aluminium test object as a function of the beam entry angle.

Please note that, in combination with angle beam probes, only the sound velocity of the shear wave is relevant. For industrial applications, delay lines that are made of Perspex are often used. For the combination of Perspex and steel, the permissible beam entry angles are in the range o f 27.5° to 57°. This corresponds to a direction of propagation in the steel between 33.3° and 90°. The angles that are stated on the industrial angle beam probes correspond to the direction of propagation in steel, and the angles that are used most frequen tly are 35°, 45°, 60°, and 70°. The sound velocity in steel is 5900 m/s for longitudinal waves and 3500 m/s for shear waves.

Fig. 5: Beam e ntry angle smaller than 25.4°


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Calibration of the angle beam probes

In order to determine the distance based on the time of flight and sound velocity, angle beam probes also require the beam entry angle, delay line length, and sound exit point to be determined.

The calibration is achieved with different reference objects or via a distance calibration based on projection distances on a perpendicular edge of two plane-parallel surfaces.

1. The reference objects have a circular-arc-shaped reflector whose echoes always come from the same depth regardless of the beam entry angle



. A saw cut that is conveniently applied in turn leads to the generation of multiple echoes. The time-of-flight intervals between the multiple echoes are measured. As a result, the influence of the delay line can be eliminated. With maximum amplitude, the sound exit point is located exactly at the centre of the circle. This point is usually stated as the distance from the probe edge. The sound wave propagation angle can be read off a scale on a cylindrical reflector (drilled hole).

2. In the case of a calibration based on projection distances, one measures the times of flight of the maximum corner echoes at the perpendicular edge of two plane-parallel surfaces with a known distance as well as the projection distances at half and full skip distance. Then, all of the required values are calcula-

Fig. 7: Standard test object for the calibration of angle beam probes

Fig. 8: Projection distances at half and full skip distance ted based on these measurements.

The projection distance p’ usually refers to the sound exit point. Since the sound exit point is usually unknown, in most cases the shortened projection distance of the probe is used instead. It refers to the edge of the angle beam probe.

(shortened) projection distance: p

 p '

 x v

[6]

In the following sections, the shortened projection distances are used as the projection distances p.

The sound exit point of the probe is calculated as the difference between the projection distances at half



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Sound exit point: x v

 p

1



2

 p

0

[7]

The simple path length between the upper and lower surfaces of the test object is called the skip distance. Based on the geometry of the sound paths at the test block, the skip distance can be calculated with the aid of the following equation:

Skip distance: x a

2  a

2 

 p

1

 p

0



2

[8]

From this, the propagation angle of the sound wave for the corresponding material results as follows:

Propagation angle: tan(



)

 p

1

 a p

0 [9]

The sound velocity is calculated based on the difference between the measured times of flight for the half and full skip distance.

With the double path lengths for the echo and the measured times of flight t0 and t1, the sound velocity in the test block material is:

Sound velocity: c



4 x t

1 a





2 t

0 x a

 t

1

2 x a

 t

0

[10]

With the determined sound velocity of the material, one can then determine the time of flight of the delay line:

Delay line: t v

 t

0



2 x a c

[11]

When locating discontinuities, the time of flight of the delay line must be subtracted from the determined times of flight of the echoes. The depth T and the projection distance p of the discontinuity are calculated as follows:

Depth: T

Projection distance:

 c p





 t

 t v



 cos(



) [12]

2 c



 t

 t v



 sin(

2



)

 x v

[13]

Fig. 9:

Echo of the longitudinal wave with a 17° delay line


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Results

The following screenshots show the A-scans of various ultrasound waves at the aluminium test block with delay lines of 17°, 38°, and 56°.

Fig. 10: Echo of the shear wave with a 17° delay line

Fig. 11:

Echo of the shear wave with a 38° delay line. With this angle of incidence, the longitudinal wave has already exceeded the angle of total reflection.

Fig. 12:

Echo of the shear wave with a 56° delay line



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The calibration of the 38° delay line at the test block at half and full skip distance. The values shown in table 1 are the measured time of flight and the distance between the edge of the test block and the front edge of the angle beam probe, determined at half and full skip distance. The measurement was repeated several times in order to determine a mean value.

Table 1: Measure ment with the 38° delay line at the test block at half and full skip distance

Distance p0 [mm] Time of flight t0 [µs]

Half skip distance

Mean value

Full skip distance

Mean value

18.5

19.0

16.0

17.0

17.0

17.5

56.0

57.0

51.0

51.5

51.5

51.8

51.0

51.5

49.9

50.4

50.1

50.6

83.9

82.6

81.5

81.7

81.5

82.24

Fig. 13: Sound path of the shear wave echo at half skip distance

Fig. 14: Sound path of the longitudinal wave echo at half skip distance

Fig. 15: Sound path of the shear wave echo at full skip distance

Fig. 16: Sound path of the longitudinal wave echo at full skip distance

As the second probe, the 17° delay line was used.

With this delay line with a smaller beam entry angle it must be ensured that the longitudinal waves are not eliminated by total refl ection as is the case with the 38° delay line. The two resulting maxima must be examined separately since the sound velocities and angles of incidence of the two waves differ from each other. The measurements of the longitudinal wave and of the shear wave are performed separately. Locating these signals can be rather complicated, since the correct assignment is difficult with numerous amplitudes. The following illustrations show the various sound paths at half and full skip distance.


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Fig. 17 : Shear wav e echo of the angle beam probe with a 17° delay line at half skip distance

The first peak that appears at the edge of the test object belongs to the shear wave. When the probe is moved, another peak appears before this peak, which is due to the reflection of the longitudinal wave at the front face and bottom of the test object. The fact that the longitudinal wave hits the front face probably leads to the generation of a surface wave. The structural interference of both wave parts leads to a higher signal amplitude. The different time of flight compared to the corner echo of the longitudinal wave at half skip distance results from the different sound velocities of the two waves. This peak is not a corner echo and, therefore, it cannot be used for the evaluation.

When the probe is moved even further, the maximum of the corner echo of the significantly quicker longitudinal wave appears after a short time of flight.

Fig. 18 : Longitudinal wave echo of the angle beam probe with a 17° delay line at half skip distance

After the measurement of these first two values at half skip distance, the corresponding values are determined at full skip distance. The maximum of the shear wave already appears when the first echo of the longitudinal wave is still visible, but only after a considerably longer time of flight.



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The next peak with the shortest time of flight belongs to the second echo of the longitudinal wave.

The repetition of the measurements and the calculation of mean values are dispensed with for reasons of time.

Fig. 20: Lon gitudinal wave echoes of the angle beam probe with a 17° delay line at full skip distance

Longitudinal

Shear

Table 2: Measurement with the 17° delay line on a test block at half and full skip distance

Half skip

Distance [mm] Time of flight [µs]

Full skip

Distance [mm] Time of flight [µs]

14

-3

27.3

36.7

44

8.7

41.6

60.5

The height of the test block is: h = 34.9 mm.

The calculation of the values for the delay lines of 17° and 38° leads to the following results:

Table 3: Characteristi c values of the 17° and 38° delay lines

38° 17° 17°

Sound exit point

Shear

16.8

Longitudinal

16.0

Shear

14.7

Unit mm

Skip distance

Beam entry angle

Sound velocity

Delay line

48.9

44.5

3091.2

18.9

46.0

40.7

6436.6

13.0

36.8

18.5

3093.2

12.9 mm

° m/s mm


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The following diagram shows the echo of the discontinuity in the test ob ject measured with the 38° delay line 38°. The echo has a shorter time of flight than the other, previously measured echoes.

This time of flight and the known formulae can now be used in order to calculate the depth and the projection distance of the discontinuity.

Fig. 21

: Locating a discontinuity with an angle beam probe and a 38° delay line

Table 3: Depth and projection distance of the discontinuity

Time of flight of the echo 39.1 µs

Depth 22.2 mm

Projection distance 5.1 mm


Related topics Angle beam probe, ultrasound echography

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