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Side Drilled Holes at different depths

In this part we consider echoes from Ø2mm Side Drilled Holes (SDH) at different depths with different probes:

• Mono-element Immersion probes
• Mono-element Contact probes
• Phased-Array Contact probes

The results show a very good agreement. It can be noticed that Civa generally underestimates the amplitude of the echoes in the very near field (less than 4dB discrepancy).


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Side Drilled Holes at different depths and Immersion probes

Global overview:

Configuration

This validation experiment deals with Ø 2mm SDH at different depths. The measurements are performed upon a planar steel block containing Ø2mm SDH from 4 to 60mm depth with 4mm steps. As a reminder, the steel parameters are: density 7.9, P waves velocity: 5900m/s and SV waves velocity: 3230m/s. Since the SDH are inspected perpendicularly to their axis, the SOV interaction model is considered.


The following picture presents the mock-up that is used.

After scanning along the surface of the block, the following B-scan is displayed:

In this configuration, 6 different immersion probes have been used in Pulse Echo mode. All the probes are circular and have a flat surface (none focused):

The results corresponding to each probe are available by clicking on the size of the probe.


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Results

For each probe, the simulated P or/and SV beam radiated in the specimen and in the incidence plane is displayed for at least one configuration.
The superimposition of measured and simulated curves of maximal relative amplitude of the P and SV specular echoes of the SDH versus the SDH depths are presented in the next figures.

Mono-element immersion probe 2.0MHz, Ø19mm

For the Ø19mm circular immersion probe at 2MHz with 50mm water path, the P0° mode is used for inspection. The input signal frequency is 2.0MHz, with 60% bandwidth and 0° phase.

The acoustic focusing depth is 19mm, deduced from the simulated beam as illustrated below.

The results are calibrated versus the Ø2mm SDH at 20mm depth.

There is a good agreement between the results from the measurements and the results from CIVA software. The discrepancy is always less than 1dB.

Mono-element immersion probe 2.25MHz, Ø12.7mm

For the Ø12.7mm circular immersion probe at 2.25MHz with 50mm water path, the P0° mode is used for inspection. The input signal frequency is 2.25MHz, with 60% bandwidth and 0° phase.

The acoustic simulated beam is illustrated below.

The results are calibrated versus the Ø2mm SDH at 12mm depth.

There is a good agreement between the results from the measurements and the results from CIVA software.

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Mono-element immersion probe 2.25MHz, Ø6.35mm

For the Ø6.35mm circular immersion probe at 2.25MHz, the P0° mode is used for inspection with 20mm water path, and the SV45° with20mm water path. An experimental input signal (the SV45° specular echo of a Ø3mm and 45° tilted Flat Bottom Hole at 10 mm depth) is used for P0° waves. The input signal frequency is 2.25MHz, with 64% bandwidth and 290° phase for the SV45° wave.
The acoustic simulated beam is illustrated below for the P0° mode.

The results are calibrated versus the Ø2mm SDH at 8mm depth.

There is a good agreement between the results from the measurements and the results from CIVA software. The maximum discrepancy is 1dB for P0° and 2dB for SV45°.

Mono-element immersion probe 2.4MHz, Ø20mm

For the Ø20mm circular immersion probe at 2.4MHz with 20mm water path, experiments are carried out with SV45°, SV60°, P45°, P60° and P0° modes. It can be noticed that for P45° and P60°, an associated SV beam radiates at 22° and 26° respectively. The input signal frequency is 2.4MHz, with 53% bandwidth and 170° phase.
The acoustic focusing depth is 32mm for the SV45° mode, deduced from the simulated beam as illustrated below.

The results are calibrated versus the Ø2mm SDH at 32mm depth inspected in SV45° mode.

From the 5 previous figures, there is a good agreement between the results from the measurements and the results from CIVA software. It appears that the amplitude of the main echo is well estimated with less than 2dB in the worst case for nearly all the studied configurations.

Just one case shows more discrepancy, it correponds to the SV26° mode associated with the P60° mode, which shows a discrepancy up to 4dB in near field (less than 4dB difference). It is due to a splitting in the SV beam at the water/steel interface, which is not observed experimentally. The SV beam splitting is due to limitations of the model. The strong variation of the transmission coefficient close to the critical angle splits the SV incident beam. Some waves are not considered, which reduces the amplitude of the SV simulated echo.

Split of the SV beam for a Ø12.7mm circular contact probe generating P45° waves.

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Mono-element immersion probe 4.5MHz, Ø12.7mm

For the Ø12.7mm circular immersion probe at 4.5MHz with 20mm water path, the SV45° to SV60° waves are used for inspection. The input signal frequency is 4.5MHz, with 73% bandwidth and 270° phase.

The acoustic focusing depth is 26mm for the SV45° mode, deduced from the simulated beam as illustrated below.

The results are calibrated versus the Ø2mm SDH at 32mm depth inspected in SV45° mode.

There is a good agreement between the results from the measurements and the results from CIVA software. It can just be noticed that CIVA underestimates the echo in the very near field of around 2dB.

Mono-element immersion probe 4.7MHz, Ø6.35mm

For the Ø6.35mm circular immersion probe at 4.7MHz, different waves are used for inspection (P and SV modes, from 45° to 60°). The water path is 25mm for all cases except one where it is 10mm. The input signal frequency is 4.7MHz, with 56% bandwidth and 255° phase.
The acoustic simulated beam is illustrated below for the P45° mode with 25mm water path.

The results are calibrated versus the Ø2mm SDH at 4mm depth inspected in P45° mode with 25mm water path.

Whatever the incidence angle and the water path, the simulated results fit measurements with less than 2dB discrepancy.
It can just be noticed with this crystal size that for the SV45° mode, the discrepancy tends to increase at larger depths, as it was already observed with the same crystal size at 2.25Mhz

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Conclusion

Results show a good agreement with generally less than 2dB difference for mono-element immersion probes.
The discrepancy may be a little higher in the near field. This is due to limitations of the model and is explained in details for rectangular contact probes.


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Side Drilled Holes at different depths and Contact probes

Global overview:

Configuration

Comparisons between measured and simulated data are also done with contact probes on the steel mock-up with Ø2mm SDH.
The measurements are performed upon a planar steel block containing Ø2mm SDH from 4 to 60mm depth with 4mm steps. As a reminder, the steel parameters are: density 7.9, P waves velocity: 5900m/s and SV waves velocity: 3230m/s. Since the SDH are inspected perpendicularly to their axis, the SOV interaction model is considered.


The following picture presents the mock-up that is used.

Different contact probes are used in Pulse Echo mode:

The results corresponding to each probe are available by clicking on the size of the probe.


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Results

For each probe, the simulated P or/and SV beam radiated in the specimen and in the incidence plane is displayed for at least one configuration. For each probe, the calibration defect is the closest SDH to the focusing depth.
The superimposition of measured and simulated curves of maximal relative amplitude of the P and SV specular echoes of the SDH versus the SDH depths are presented as the next figures.

Mono-element contact probe 2.0MHz, 20*22mm, SV45°

For the 20*22mm rectangular SV45° contact probe at 2MHz, the SV45° mode is used for inspection. The input signal frequency is 2.0MHz, with 41% bandwidth and 75° phase.
The acoustic focusing depth is 53mm, deduced from the simulated beam as illustrated below.

The results are calibrated versus the Ø2mm SDH at 52mm depth.

Mono-element contact probe 2.0MHz, 20*22mm, SV60°

For the 20*22mm rectangular SV45° contact probe at 2MHz, the SV60° mode is used for inspection. The input signal frequency is 2.0MHz, with 40% bandwidth and 0° phase.
The acoustic focusing depth is 32mm, deduced from the simulated beam as illustrated below.

The results are calibrated versus the Ø2mm SDH at 32mm depth.

For both probes, the curves show a good agreement between simulated and measured results in the far field zone and less than 2dB difference from the focal depth to the half of the focal depth.


The discrepancy is higher in the very near field.

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Discrepancies greater than 2dB occur in the very near field (up to 50% of the focal depth). In order to understand the behaviour of the defect responses for the smaller depths, the field radiated in the specimen has been computed in several zones perpendicular to the SV axis and along this axis.






These figures show the interferences between the planar wave and the edge waves of the probe leading to interferences patterns observed in the computation zones at the lower depths (within the near field distance of the probe). These interferences give rise to a lower amplitude of the field. Therefore the measured responses of the flaws seem to be coherent with this field computation. However the defect response module is based on a simplification of the radiated field. This simplification is less valid in the near field so that this additional phenomenon may cause this discrepancy with the model.

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Mono-element contact probe 2.25MHz, Ø12.7mm

For the Ø12.7mm circular contact probe at 2.25MHz, the P45°, P60° and SV45° waves are used for inspection. The input signal frequency is 2.25MHz, with 50% bandwidth and 280° phase for the P waves. The input signal is the inverse P45° experimental direct specular echo of a Ø3mm FBH, tilted at 45° and located at 30mm depth for the SV45° waves. P beams are associated with lower incidence SV beams: SV22° for P45° and SV26° for P60°.
The acoustic focusing depths are 8mm, 26mm, 3mm, 15mm and 17mm respectively for the P45°, SV22°, P60°, SV26° and SV45° modes, deduced from the simulated beam as illustrated below.

	P45°		P60°
	SV22°		SV26°
SV45°

The results are calibrated versus the Ø2mm SDH at 8mm, 4mm and 20mm depth for respectively P45°, P60° and SV45° waves.

The amplitudes of the echoes from P waves are well predicted, the maximum discrepancy being less than 2dB. The amplitudes of the echoes from SV waves show higher discrepancies in the near field zone (less than 5dB) but are often correctly estimated.

Mono-element contact probe 2.25MHz, Ø6.35mm

For the Ø6.35mm circular contact probe at 2.25MHz, the SV45° mode is used for inspection. The input signal frequency is 2.25MHz, with 44% bandwidth and 147° phase.

The acoustic focusing depth is 3mm, deduced from the simulated beam as illustrated below.

The results are calibrated versus the Ø2mm SDH at 4mm depth.

The discrepancy between simulated and measured results increases with the SDH depths but always less than 3dB, there is a good agreement.

Mono-element contact probe 4.35MHz, Ø12.7mm

For the Ø12.7mm circular contact probe at 4.35MHz, the SV45° mode is used for inspection. The input signal frequency is 4.3MHz, with 71% bandwidth and 330° phase.

The acoustic focusing depth is 31mm, deduced from the simulated beam as illustrated below.

The results are calibrated versus the Ø2mm SDH at 36mm depth.

It can be observed that the simulated amplitudes are smoother than the experimental amplitudes which highlights probably a slight discrepancy in the measurements. There is an overall good agreement, the maximum discrepancy is less than 2dB, and from 20mm to 60mm depth, the discrepancy is always less than 1dB.

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Mono-element contact probe 4.8MHz, Ø6.35mm

For the Ø6.35mm circular contact probe at 4.8MHz, the SV45° mode is used for inspection. The input signal frequency is 4.8MHz, with 45% bandwidth and 90° phase.
The acoustic focusing depth is 8mm, deduced from the simulated beam as illustrated below.

The results are calibrated versus the Ø2mm SDH at 8mm depth.

The discrepancy is always less than 2dB, there is an overall good agreement.

Mono-element contact probe 5.0MHz, Ø6.35mm

For the Ø6.35mm circular contact probe at 5MHz, the P45° mode is used for inspection. The input signal is the inverse P45° experimental direct specular echo of a Ø2mm SDH at 8mm depth.
The acoustic focusing depths is 3mm and 12mm for respectively the P45° and SV22° modes, deduced from the simulated beam as illustrated below.

	P45°		SV22°

The results are calibrated versus the Ø2mm SDH at 4mm depth.

The discrepancy is less than 2dB for the 45° beam in the far field, there is an overall good agreement. In the near field zone, the SV beam associated with the P45° beam shows a discrepancy between 2 and 4dB, the simulation underestimates the echo.

At 32mm and 36mm depth, both SV22° and P45° experimental echoes are a little under-estimated since the SDH are damaged by rust. This block is no longer used for experimental validation.

Conclusion

Results shows a good agreement with generally less than 2dB difference in this case of SDH and mono-element contact probes. For a given probe, it can also be concluded that an SDH echo obtained for a given mode, can be used as calibration echo for all the other modes generated by the same probe with different settings.


The hypothesis of non distorted waveform on flaw, considered by Civa models, is generally valid in the far field zone or the focal zone but less in the near field zone, which leads to some discrepancies (less than 5dB) when considering flaws in the near or the very near field (from 0 to 50% of the focusing depth).



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Side Drilled Holes at different depths and Phased-Array probes

Comparisons between experimental and simulated data are also done with phased-array contact probes on the steel mock-up with Ø2mm SDH.


Global overview:

Configuration

This validation experiment also deals with Ø 2mm SDH at different depths. The measurements are performed upon the planar steel block containing Ø2mm SDH from 4 to 60mm depth with 4mm steps. As a reminder the steel parameters are: density 7.9, P waves velocity: 5900m/s and SV waves velocity: 3230m/s. Since the SDH are inspected perpendicularly to their axis, the SOV interaction model is considered.


The following picture presents the mock-up that is used.

The 2 phased array probes that are used in Pulse Echo mode are:

The results corresponding to each probe are available by clicking on the number of elements of the probe.

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Results

Phased Array contact probe of 28 elements with 0.7mm pitch at 5MHz

For the phased array contact probe at 5MHz, different focal laws are used with SV waves for inspection with 28 active elements out of 48. The first experiment uses a focusing at 20mm depth and a beam angle from 45° to 60°. The second one uses a beam steering of 45° and a focusing from 4 to 24mm depth.
The acoustic focusing depth is 20mm for the SV45° beam steering, deduced from the simulated beam as illustrated below.

The results from the first experiment show the influence of the beam orientation.
The results are calibrated versus the Ø2mm SDH at 20mm depth for SV45° waves focused at 20mm.

For all the angles, the amplitudes of the echoes are well estimated with less than 2dB discrepancies.

The results from the second experiment show the influence of the focusing depth.
The results are calibrated versus the Ø2mm SDH at 20mm depth for SV45° waves focused at 20mm.

With delay laws focusing at different depths, there is a good agreement. The maximum discrepancy is 3dB for SDH at 8mm depth and focusing at 20mm depth, and less than 2dB in all the other cases.

Phased Array contact probe of 20 elements with 0.7mm pitch at 5MHz

The phased-array probe is used to generate a P45° beam at 5MHz with 20 active elements out of 48. The acoustic focusing depth is 20mm, deduced from the simulated beam as illustrated below.

The results are calibrated versus the Ø2mm SDH at 20mm depth.

With less than 1dB discrepancy, there is a very good agreement between experiment and simulation.

Conclusion

Results shows a very good agreement with less than 2dB difference in the cases of SDH and contact phased-array probes.


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