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Technical Discussions
Sai Kumar
India, Joined Dec 2019, 3

Sai Kumar

India,
Joined Dec 2019
3
18:19 Dec-05-2019
45 degree probe calibration

When we do calibration for 45 degree probe why we get second hole relfection amplitude is higher then frist hole amplitude. But we cannot see why calibrating 70 r 60.,Is there any reason for that please give me explanation my dear respectable Friends.

 
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Pedro a. Serruto gamboa
Director, Level II UT, PT
Red Code Quality, Peru, Joined Sep 2013, 88

Pedro a. Serruto gamboa

Director, Level II UT, PT
Red Code Quality,
Peru,
Joined Sep 2013
88
01:31 Dec-06-2019
Re: 45 degree probe calibration
In Reply to Sai Kumar at 18:19 Dec-05-2019 (Opening).

What kind of holes you are talking about, tell me the dimensions of your calibration block

 
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Creed
NDT Inspector,
Vietnam, Joined Jul 2009, 37

Creed

NDT Inspector,
Vietnam,
Joined Jul 2009
37
01:32 Dec-06-2019
Re: 45 degree probe calibration
In Reply to Sai Kumar at 18:19 Dec-05-2019 (Opening).

Because the SDHs drop into near zone which is the zone where the sound wave pressures at each point is not uniform so the echo amplitudes are not drop gradually like in far zone.

 
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Michel Couture
NDT Inspector,
consultant, Canada, Joined Sep 2006, 897

Michel Couture

NDT Inspector,
consultant,
Canada,
Joined Sep 2006
897
02:01 Dec-06-2019
Re: 45 degree probe calibration
In Reply to Sai Kumar at 18:19 Dec-05-2019 (Opening).

If I understand your question properly you are telling us that you are doing a DAC curve?
What you have to remember with ultrasound is that your soundbeam is divided in 3 sections.
The first one is the "Dead Zone" which is located directly under the transducer. In this part of the soundbeam, no discontinuity can be detected.
The second section is called the "Near Field". In this section, discontinuities can be detected, but cannot be evaluated. You should also know that the Dead Zone is included in the Near Field.
The last portion of the soundbeam is called the "Farfield". This is the portion of the soundbeam where everything happen. First, any discontinuity can be evaluated. As well, the soundbeam will spread (Beamspread) and the "Signal Amplitude" will decrease as the distance between the transducer and the discontinuity increases.
When you make a DAC curve, you always start by finding which hole will give you the highest amplitude. That is the hole that you will use to set the dB to 80% "Full Screen Height" (FSH). This will become your reference dB. Afterward, you go to the shallowest hole and mark the location on your screen. You will proceed to the second hole, than to the third hole and do the same. Your DAC curve is done.
Now, we can look at your problem. With the 45 degree wedge, the first hole amplitude is lower than the second hole because it is located in the Nearfield of your soundbeam. So you can detect it, but cannot properly evaluated. For the 60 and 70 degree transducers or shoes, the soundbeam angle is such that the distance between the transducer and the hole is located in the Farfield and therefore can be properly evaluated.
With the appropriate formulas, the Nearfield and Farfield can be calculated and you can draw the soundbeam on a sheet of paper to visualise your results.

1
 
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Sai Kumar
India, Joined Dec 2019, 3

Sai Kumar

India,
Joined Dec 2019
3
02:57 Dec-06-2019
Re: 45 degree probe calibration
In Reply to Pedro a. Serruto gamboa at 01:31 Dec-06-2019 .

19 mm sdh block 1st hole 4.5mm, 2nd 9mm, 3rd hole 13.5mm

 
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Sai Kumar
India, Joined Dec 2019, 3

Sai Kumar

India,
Joined Dec 2019
3
03:14 Dec-06-2019
Re: 45 degree probe calibration
In Reply to Michel Couture at 02:01 Dec-06-2019 .

How can you say the frist hole is in near zone and 2 nd one is in far zone, how can you calculate the dead zone near zone for angle probes? I tell you one example calculation for normal probe 10mm dia and 4 mhz formula D2.f / 4v and u will get 16.8mm what is 16.8mm is near zone value and we cannot interpretate defects upto this thickness or what? can you explain it please

 
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Paul Holloway
Consultant,
Holloway NDT & Engineering Inc , Canada, Joined Apr 2010, 255

Paul Holloway

Consultant,
Holloway NDT & Engineering Inc ,
Canada,
Joined Apr 2010
255
14:54 Dec-06-2019
Re: 45 degree probe calibration
In Reply to Sai Kumar at 03:14 Dec-06-2019 .




Sai, the D^2*f/4v equation assumes no wedge. The actual near zone sound path will be less than the calculated value once you account for wedge material.

Also, for a 10mm diameter, 4MHz transducer without wedge, I get a NZ of 31 mm, not 16.8. See ESBeamTool plot attached.

You would then have to correct for refracted angle to get the actual depth of the NZ.

But in any case, Michel and Creed are correct... your first hole is in the NZ. For 45 degrees, however, I wonder why are you trying to hit the 1/4T hole in an ASME basic block? I don't think it's realistic (although I don't know your weld geometry) to expect to get close enough to the weld to pick anything up at that short soundpath.
 
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Michel Couture
NDT Inspector,
consultant, Canada, Joined Sep 2006, 897

Michel Couture

NDT Inspector,
consultant,
Canada,
Joined Sep 2006
897
03:52 Dec-07-2019
Re: 45 degree probe calibration
In Reply to Sai Kumar at 03:14 Dec-06-2019 .

In the Nearfield of a T/R probe (Transmit/Receive), the pressure in the soundbeam and at the side lobe are uneven. This region of the soundbeam is so close to the transducer, that the transmitted impulse and the received impulse are interfering with each other. This is why you cannot evaluate an indication. Remember there is a difference between evaluate and detect.
Detection is seeing something on your screen. Evaluation is to assess the size of your indication against your calibration hole.
I suggest that any time you have a situation like this, you use your formulas and draw your set up on a sheet of paper. you will have a better understanding of the situation.

 
 Reply 
 
Mario Talarico
NDT Inspector,
Italy, Joined May 2010, 423

Mario Talarico

NDT Inspector,
Italy,
Joined May 2010
423
19:51 Dec-07-2019
Re: 45 degree probe calibration
In Reply to Sai Kumar at 18:19 Dec-05-2019 (Opening).

Sai,
I have often noticed this too. The DAC curve forms a bell with a focal point. In the case of a single emitter, the maximum path of the DAC is associated to the end near flied (natural focus of a plane reflector, ie infinite radius). If we have to calculate the exact point in steel, I think we have to start from the emitter in plexi and the passage from one medium to the other. Paul's calculation considers a 'virtual' emitter in shear wave starting from the interface, but something has already happened and loss in the plexi in terms of acoustic pressure: so there may be an overestimation in the indicated 31 mm.
Regarding the effect on the low angles rather than the higher ones, trivially, it could be right due to the angle: a 30 mm near field in steel is detected in this way at various angles:
- Probe 70°, near field on a hole at 10 mm depth (10 mm in the direction of the thickness): starting from 10 mm we could have a regular descending curve
- Probe 60°, near field at 15 mm deep hole (in the thickness direction): starting from 15 mm we could have a descending curve. The previous reflector remains to the left of maximum, in the near field zone, even if only 5 mm, and perhaps in an influential way (in practice the maximum is an area, not a mere point).
- Probe 45 °, near field on a hole 21 mm deep (in the thickness direction): ONLY starting from 21 mm we could have a descending curve. Both the two reflectors remain in the near field zone as calculated, therefore in the ascending, irregular, part. Considering the maximum response at 21 mm depth, I would not be surprised by some strange effect at 7-10 mm depth…… where instead we would still be roughly around the area of maximum reflection with a 70 ° probe.
Keeping to the practical, when an abnormal reflection is noticed, it is good to detect the anomalous reflector also in rebond and insert it in DAC in constructing the DAC also in rebond. This will allow a correct evaluation. I don't remember any major differences but this helps the quiet life, avoiding possible discussions with the verifiers, who have memorized these things, very well covered in basic courses. Alternatively, double-beam E-R probes, but with detection problems already noted in other posts.
Greetings
mario

 
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