accuracy in sizing discontinuities with phased array ... · accuracy in sizing discontinuities with...

19th World Conference on Non-Destructive Testing 2016

1 License: http://creativecommons.org/licenses/by/3.0/

Accuracy in sizing discontinuities with Phased array ultrasonic technique

Giuseppe NARDONI1, Mario CERTO2, Pietro NARDONI3, Marco FEROLDI4 1, 2, 3 I&T Nardoni Institute, Folzano, Brescia, Italy

Contact e-mail: nardoni@numerica.it, nardoni.campus@gmail.com

Abstract. In fracture mechanics criteria applied by ASME Code sec. VIII Div.2, when ultrasonic is used in lieu of radiography, the height of the indication has to be expressed in linear dimension and not in amplitude volume.

In TOFD Technique the answer is easy; using the cursor and making reference to positive and negative phase the height can be estimated with an accuracy of less than 0,5 mm.

In Phased Array technique, because it is a pulse echo technique with multiple angles, the estimation of height has to be made making reference to the echo dynamic curve.

The echo dynamic curve in Phased Array is represented in sectorial scanning by the colored curved strings.

The fundamental colors are red, yellow, green , blue (noise level). The paper presents the results of experimental test in height estimation from

planar defects with height 3 mm and 6 mm. The results are very promising.

1. Scope

The scope of our research has been to identify a criteria to measure, into reasonable tolerance the height of indications detected with Phased Array technique.

For TOFD to measure the height there is a dedicated software through which this parameter can be measured with tolerance less then 0.5 mm.

TOFD is based on diffraction technique that allows this procedure. Phased Array is basically a pulse echo technique with multiple crystals activated

according to specific sequence of time (Phase).

Fig. 1: Phased Array image of a 1st

degree crack Fig. 2: Phased Array image of a 3rd

degree crack 6 mm height Fig. 3: Phased Array image of a

3rd degree crack 3 mm height

More info about this article: http://ndt.net/?id=19658

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2. Research plan

The research plan has been based on artificial discontinuities representing volumetric (side drilled holes) and planar indication (EDM slits).

The height of the slits in the test block were 3 and 6 mm.

Fig. 4: 1st degree crack Fig. 5: 2nd degree crack Fig. 6: 3rd degree crack

Fig. 7: 1st – 2nd degree crack – x 25 Fig. 8: 3rd degree crack – x 25 Fig. 9: 3rd degree crack – x 25

Fig. 10: 3rd degree crack in CrNMoV welds

These slits simulate small planar crack (3rd degree cracks) in the weld bead or lack of

fusion at the side of the weld pass. These type of cracks or lack of fusion have smooth surfaces compared with wave length and height (a; 2a) in the range of 3- 6 mm.

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3. Test block

The test block realized for the tests is indicated in Fig. 11. The test block has side drilled holes of 3 and 6 mm diameter and two planar slits of 3

and 6 mm height; the width is ≤ 0,3 mm, the length is 25 mm, the depth is 30 mm. The first part of the research has been limited to a depth of 30 mm.

Fig. 11: Test block used for experimental tests

30mm

30mm

300mm

100mm

Ø3

Ø6

3 6

45° 60° 70°

45° 60° 70°

30mm

30mm

300mm

100mm

Ø3

Ø6

3 6

45° 60° 70°

45° 60° 70°

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4. Experimental tests

Four different experimental tests have been carried out. 4.1 Experimental test N° 1 Evaluation of the heights of slits and SDH measuring the length of the sectorial scanning signals (see Fig. 12- 19).

Fig. 12: Side drilled hole ø 3 mm Fig. 13: Side drilled hole ø 6 mm

Fig. 14: Slit 3 mm 50° Fig. 15: Slit 6mm 50°

Weak Diffracted satellite signal

Weak Diffracted satellite signal

Relevant amplitude diffracted echo signal

Relevant amplitude diffracted echo signal

Relevant amplitude reflected signal

Relevant amplitude reflected signal

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Fig. 16: Slit 3 mm 60° Fig. 17: Slit 6mm 60°

Fig. 18: Slit 3 mm 70° Fig. 19: Slit 6mm 70°

Fig. 20: Diagram showing measured values of the heights of the slits with respect to nominal values (3 mm and 6 mm)

Relevant reflected signal

Weak diffracted signal

Relevant reflected signal Weak diffracted signal

High amplitude reflected signal Weak diffracted signal

High amplitude reflected signal Weak diffracted signal

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4.2 Experimental test N° 2

Amplitude comparison between side drilled holes and planar slits with three different angle beam 45°, 60°, 70°.

See diagram in Fig. 21.

Table 1: Values of different angle beam shear waves on reflectors type SDH and slit; Diameter and height 3 mm

Reflectors Angle beam

ΔdB Depth

[mm] Uncertainty

45°

12 dB

30 ± 1,5 ÷2 dB

45 30 ± 1,5 ÷2 dB

60°

2 dB

30 ± 1,5 ÷2 dB

60° 30 ± 1,5 ÷2 dB

70°

1 dB

30 ± 1,5 ÷2 dB

70° 30 ± 1,5 ÷2 dB

Fig. 21: Diagram representing the difference in amplitude (ΔdB) between planar slit and side drilled holes of same diameter and height with different angle beam.

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4.3 Experimental test N° 3

Comparison between 3 and 6 mm planar slits with 45°, 60° and 70° angle beam. See diagram in Fig. 22.

Table 2: Values of different angle beam shear waves on reflectors type slit; Height 3 mm

Reflectors Angle

beam ΔdB

Depth

[mm] Uncertainty

45°

1 dB

30 ± 1,5 ÷2 dB

45 30 ± 1,5 ÷2 dB

60°

5 dB

30 ± 1,5 ÷2 dB

60° 30 ± 1,5 ÷2 dB

70°

4 dB

30 ± 1,5 ÷2 dB

70° 30 ± 1,5 ÷2 dB

Fig. 22: Diagram representing the difference in amplitude (ΔdB) between 3 and 6 mm planar slits with different angle beam.

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4.4 Experimental test N° 4

Amplitude Comparison between side drilled holes of 3 and 6 mm with 45°, 60° and 70° angle beam.

See diagram in Fig. 23.

Table 3: Values of different angle beam shear waves on reflectors type SDH; Diameter 3 mm and 6 mm

Reflectors Angle beam

ΔdB Depth

[mm] Uncertainty

45°

6 dB

30 ± 1,5 ÷ 2 dB

45 30 ± 1,5 ÷ 2 dB

60°

5 dB

30 ± 1,5 ÷ 2 dB

60° 30 ± 1,5 ÷ 2 dB

70°

5 dB

30 ± 1,5 ÷ 2 dB

70° 30 ± 1,5 ÷ 2 dB

Fig. 23: Diagram representing the difference in amplitude (ΔdB) between cylindrical holes 3 and 6 mm diameter with different angle beam.

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5. Comments to the results

5.1 Comment A

The length of the sectorial scan signal cannot be considered proportional to the height of the indication.

5.2 Comment B

When hitting planar slits for angle of 60° and 70° the relevant signal is determined by the reflection from the surface and there is a good correlation of 6 dB between 3 and 6 mm slit.

The height in this case can be determined following the amplitude (DAC line); the amplitude is proportional to the height.

For angles in the range of 45° the reflection is negligible and the signal is only due to the diffraction from the tips.

This is why there is no difference in terms of amplitude between 6 and 3 mm slit; the difference is very expressive if we consider the size through the diffracted signal.

5.3 Comment C

The comparison between SDH of 3 mm and slit of 3 mm confirm comment B. For 45° angle beam the difference between hole and slit is 12 dB; for 60° and 70° the

difference is only 2 dB. This means that with 45° angle beam on slit we have only diffraction and negligible

reflection; for 60° and 70° reflection is the primary and DAC reference for height estimation can be done.

5.4 Comment D

On volumetric indications as for hole of 3 and 6 mm the amplitude is fully corresponding to the increased height of the indication.

Between 3 mm and 6 mm SDH the difference is in the range of 6dB as it has to be for doubling the area if the reflectors.

6. Conclusions

The experimental tests confirmed that the amplitude of the reflected or diffracted signals are related to the type of reflectors, planar or volumetric, and to the angle beam that hits the discontinuity.

This two factors, angle beam and volumetric planar type defect, have to be considered primary parameters when sizing the height in PA for a correct measurement of the height. PA is a pulse echo technique with multiple crystals.

For 45° – 50° angle beam hitting planar defects only diffracted echoes from upper and lower tip are reference for detection and sizing.

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