Flat Bottom Holes and Comparison FBH-SDH
In this part we consider echoes generated by Flat Bottom Holes (FBH) at different depths with different contact probes. The interaction between beam and FBH is simulated with the Kirchhoff model.
Then, the echoes from SDH and FBH have been compared at specular incidence, using SOV interaction model for SDH and Kirchhoff interaction model for FBH.
Flat Bottom Holes
While investigating Flat Bottom Holes (FBH) responses, simulated data is compared to Krautkrämer DGS curves in a first part and to experimental data from 45° tilted FBH in a second part.
DGS Curves
Global overview:
| CONTACT PROBES | 2.0MHz 20*22mm | 2.0MHz Ø24mm | 4.0MHz 8*9mm |
|---|---|---|---|
| P0° | Done | ||
| SV45° | Done | ||
| SV60° | Done | Done |
Configuration
DGS (Distance Gain Size) curves from manufacturer (Krautkrämer) are compared to simulation data for some probes. For each FBH diameter, this steel specimen with 11 FBH at different depths has been modeled:
For each hole the maximum amplitude of the specular echoes of the FBH is measured relatively to a calibration hole. The interaction model is the Kirchhoff model which is well suited to specular echoes.
The measurements have been made with the following contact probes:
| Frequency | Crystal | Mode | Calibration flaw | Calibration depth |
|---|---|---|---|---|
| 2.0MHz | 20*22mm | SV45° | Ø3mm FBH | 80mm |
| 20*22mm | SV60° | Ø1.5mm FBH | 200mm | |
| Ø24mm | P0° | infinite reflector | 100mm | |
| 4.0MHz | 8*9mm | SV60° | Ø0.5mm FBH | 30mm |
The results corresponding to each probe are available by clicking on the size of the probe.
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Results
Mono-element contact probe 2.0MHz, 20*22mm, SV45°
For the 20*22mm 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 results are calibrated versus the Ø3mm FBH at 100mm depth.
There is an overall good agreement with often less than a 2dB discrepancy.
Mono-element contact probe 2.0MHz, 20*22mm, SV60°
For the 20*22mm 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 36mm, deduced from the simulated beam as illustrated below, which corresponds to a 63mm distance.
The results are calibrated versus the Ø1.5mm FBH at 200mm distance at -44.5dB.
The curves show a good agreement between simulated and experimental data for FBH deeper than the focal depth. For FBH which depth is smaller than the acoustic focusing depth, CIVA over-estimates the echo: for example, for Ø6mm, Ø8mm or Ø12mm FBH, the discrepancy is around 5dB.
Mono-element contact probe 2.0MHz, 24mm, P0°
For the 24mm contact probe at 2MHz, the P0° mode is used for inspection. The input signal frequency is 2.0MHz, with 59% bandwidth and 0° phase.
The acoustic focusing depth is 51mm, deduced from the simulated beam as illustrated below.
The results are calibrated versus an infinite reflector at 100mm depth.
A very good agreement is obtained in P0° mode inspection. CIVA estimates the echo from FBH with less than 1dB difference with Krautkramer data. It can be noted than all the FBH of this experiment are deeper than the focal spot.
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Mono-element contact probe 4.0MHz, 8*9mm, SV60°
For the 8*9mm contact probe at 4MHz, the SV60° mode is used for inspection. The input signal frequency is 4.0MHz, with 42% bandwidth and 151° phase.
The acoustic focusing depth is 12mm, deduced from the simulated beam as illustrated below, which corresponds to a 21mm distance.
The results are calibrated versus the Ø0.5mm FBH at 30mm distance at -39dB.
CIVA simulation data are close to Krautkramer data with less than 2dB difference for FBH deeper than the focal depth. For FBH no deeper than the focal depth, CIVA over-estimates the echoes by 2dB for small (Ø0.5mm) FBH and large (Ø10mm) FBH and up to 8dB for medium (Ø4mm) FBH.
Conclusion
For each FBH diameter, there is a very good agreement for the FBHs echoes amplitudes in the far field where the specular echo amplitude linearly decreases with the depth on the figures, but there are discrepancies for the highest diameters at the smallest depths.
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Experimental data from 45° tilted FBH
Global overview:
| CONTACT PROBES | 2.0MHz 20*22mm | 2.0MHz Ø12.7mm | 2.25MHz Ø12.7mm |
|---|---|---|---|
| P45° | Done | ||
| SV45° | Done | Done |
Configuration
Some measurements have been carried out in SV45° and P45° modes upon a planar surface block containing a series of 45° tilted FBH (Ø:1mm, 3mm and 6mm) at different depths (from 5mm to 60mm with a step of 5mm and then at 80mm, 100mm, 125mm and 150mm). The specimen and flaws are represented in the following figure.
For each probe an inspection of the surface of the block is realized and a C-scan obtained. In the C-scan image, the backwall echo has been removed in order to enhance the FBH echoes. However the B-scan extracted on one line of holes shows the backwall echo and the FBH echoes.
For each hole the maximal amplitude of the specular echoes of the FBH is measured relatively to a calibration hole. The interaction model is the Kirchhoff model which is adapted to specular echoes.
The measurements have been made with the following contact probes:
| Frequency | Crystal | Mode | Calibration flaw | Calibration depth |
|---|---|---|---|---|
| 2.0MHz | 20*22mm | SV45° | Ø3mm FBH | 80mm |
| Ø12.7mm | SV45° | Ø2mm SDH | 20mm | |
| 2.25MHz | Ø12.7mm | P45° | Ø2mm SDH | 8mm |
The results corresponding to each probe are available by clicking on the size of the probe.
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Results
The maximal amplitude of the specular echoes of the FBH are estimated relatively to a calibration reflector. The DGS curves are displayed for all the probes from simulated, measured or from litterature data in the next figures.
The results show a good agreement in most cases:
Mono-element contact probe 2.0MHz, 20*22mm, SV45°
For the 20*22mm 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 results are calibrated versus the Ø3mm FBH at 80mm depth.
There is an overall good agreement with often less than a 2dB discrepancy. For smaller holes at smaller depths, Civa slightly underestimated the amplitude of the echoes. Measured curves are less smooth than simulated curves, but show however a good agreement.
Mono-element contact probe 2.0MHz, Ø12.7mm
For the Ø12.7mm circular contact probe at 2.0MHz, the SV45° mode is used for inspection. The input signal is the inverse P45° experimental direct specular echo of a FBH F3mm, tilt 45° at 30mm depth.
The results are calibrated versus the Ø2mm SDH at 20mm depth in another block inspected with SV45° waves.
Even if the curves from Civa are smoother,there is a good agreement between measure and simulation for Ø1mm FBH and Ø6mm FBH. Civa overestimated the echo from the deep Ø3mm FBH, from 2dB at 60mm to 6dB at 150mm.
In addition to the good agreement in amplitude between simulated data and experimental data, the waveforms also show a very good agreement.
The following curves superimpose the direct specular echo from Ø6mm FBH at different depths with this contact Ø12.7mm probe at 2MHz generating refracted SV45° waves from experimental (black) and simulated (red) data.
Mono-element contact probe 2.25MHz, Ø12.7mm
For the Ø12.7mm circular contact probe at 2MHz, the P45° mode is used for inspection. The input signal frequency is 2.25MHz, with 50% bandwidth and 280° phase.
The results are calibrated versus the Ø2mm SDH at 8mm depth in another block inspected with SV45° waves.
The curves show a good agreement between measure and simulation with less than 2dB difference from the 3 FBH sizes. Experimental curves are less smooth than the curves from simulation data.
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Conclusion
An overall good agreement is observed between simulated results using Kirchhoff interaction model and experimental measurements. A very good agreement is also obtained in P0° mode inspection for the infinite reflector. However, some discrepancies with experimental results are observed in the near field.
It can also be seen that in some cases the measured echo-dynamic curves from FBH show irregularities. Those irregularities in the FBH measured curves are due to some anisotropy in the steel of which is made the specimen. Preliminary tests (not presented here) show that these irregularities increase with the frequency.
A simulation based study of the effect of non perfectly planar surface of the FBH, depending on the probe’s frequency, is in progress.
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Comparison Side Drilled Holes and Flat Bottom Holes
Configuration
In order to perform this comparison a 50mm thick mock-up contained three SDH (Ø2mm, Ø1.5mm, Ø1mm) and one FBH (Ø3mm tilt 45°) at 30mm depth.
- Amplitude analysis: for a given probe, the measured and simulated maximum amplitude of the specular echo of the three SDH and of the FBH are compared. The FBH is used for amplitude calibration. The aim of these measurements is to perform an experimental validation of the SDH simulation by using a FBH as calibration reflector.
- A-scan analysis: in some cases, the A-scans are also stored, which enlightens the creeping wave around the SDH.
The measurements are performed with contact mono-element and phased-array probes in pulse echo mode. The parameters of the probes are given in the following tables.
Mono-element probes:
| Frequency | Crystal | Mode | Calibration |
|---|---|---|---|
| 2.25MHz | Ø6.35mm | SV45° | Ø3mm FBH |
| Ø12.7mm | P45° | Ø3mm FBH | |
| Ø12.7mm | SV45° | Ø3mm FBH |
Phased-Array probe:
| Frequency | Elements | Pitch | Wedge angle (incidence) | Focal laws |
|---|---|---|---|---|
| 5MHz | 20 | 0.7mm | 21° (refracted P66° and SV30°) | Beam steering P45° |
The results corresponding to each probe are available by clicking on the size of the probe.
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Amplitude analysis
Mono-element contact probe 2.25MHz, Ø6.35mm
For the Ø6.35mm 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 Ø3mm FBH at 30mm depth.
| Measured (dB) | Simulated (dB) | Difference (dB) | |
|---|---|---|---|
| FBH Ø3mm | 0 | 0 | 0 |
| SDH Ø2mm | -3.1 | -3.5 | 0.4 |
| SDH Ø1.5mm | -5.0 | -5.8 | 0.8 |
| SDH Ø1mm | -7.7 | -7.7 | 0.0 |
Mono-element contact probe 2.25MHz, Ø12.7mm, P45°
For the Ø12.7mm contact probe at 2.25MHz, the P45° mode is used for inspection. The input signal frequency is 2.25MHz, with 50% bandwidth and 280° phase.
The acoustic focusing depth is 8mm, deduced from the simulated beam as illustrated below.
The results are calibrated versus the Ø3mm FBH at 30mm depth.
| Measured (dB) | Simulated (dB) | Difference (dB) | |
|---|---|---|---|
| FBH Ø3mm | 0 | 0 | 0 |
| SDH Ø2mm | 2.2 | 2.3 | -0.1 |
| SDH Ø1.5mm | 1.3 | 1.4 | -0.1 |
| SDH Ø1mm | 0.3 | -0.6 | 0.9 |
Mono-element contact probe 2.25MHz, Ø12.7mm, SV45°
For the Ø12.7mm contact probe at 2MHz, the SV45° mode is used for inspection. The input signal is the inverse P45° experimental direct specular echo of a Ø3mm FBH, tilt 45° at 30mm depth.
The acoustic focusing depth is 17mm, deduced from the simulated beam as illustrated below.
The results are calibrated versus the Ø3mm FBH at 30mm depth.
| Measured (dB) | Simulated (dB) | Difference (dB) | |
|---|---|---|---|
| FBH Ø3mm | 0 | 0 | 0 |
| SDH Ø2mm | -2.7 | -2.4 | -0.3 |
| SDH Ø1.5mm | -3.9 | -4.4 | 0.5 |
| SDH Ø1mm | -5.6 | -6.6 | 1.0 |
Phased array contact probe of 20 elements with 0.7mm pitch
For the phased array contact probe of 20 active elements (out of 48) with 0.7mm pitch at 5MHz, the P45° mode is used for inspection. The focal law is a P45° beam steering.
The acoustic focusing depth is 20mm, deduced from the simulated beam as illustrated below.
The results are calibrated versus the Ø3mm FBH at 30mm depth.
| Measured (dB) | Simulated (dB) | Difference (dB) | |
|---|---|---|---|
| FBH Ø3mm | 0 | 0 | 0 |
| SDH Ø2mm | -3.2 | -2.5 | -0.7 |
| SDH Ø1.5mm | -4.5 | -3.8 | -0.7 |
| SDH Ø1mm | -5.8 | -5.3 | -0.5 |
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A-scan analysis
An additional study is carried out concerning the A-scans of the signals with one contact probe and the phased-array probe with the previous configuration.
Mono-element contact probe 2.25MHz, Ø6.35mm
For the Ø6.35mm contact probe, the echoes from all reflectors are well estimated.
All the A-scans show a good agreement between experiment and simulation.
Phased array contact probe of 20 elements with 0.7mm pitch at 5MHz
For the contact phased-array probe, the echoes from all reflectors are well estimated.
The A-scans of the specular echoes generated by the holes are well estimated by Civa.
Moreover, the A-scans enlighten the fact that a creeping wave is also present for associated SV mode echo. This creeping wave propagates around the circumference of a SDH and create an additional echo, which is also correctly predicted.
Conclusion
A good agreement is obtained between the measured and simulated amplitudes of the specular responses of SDH of different diameters relatively to the specular response of a FBH (discrepancy less than 1dB).
Depending on the probe and the mode (P or SV), the experimental direct echoes of the three SDH are stronger or weaker than the experimental FBH direct echo. This relation is well reproduced in simulation. In most cases, the shapes of the simulated FBH and SDH echoes are very close to the experimental one for both SV45° and P45° modes (once the input signal is well calibrated).
The creeping wave (see the SDH Ascans obtained for SV modes) is well simulated in all cases (good shape, position in time and amplitude relatively to the specular contribution), although some discrepancies occur on the smallest SDH.
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