UT Kindergarten

Date 07-18-2008 21:28

This technique is also referred to as pitch catch. I have not had the opportunity to use this in this application. I believe that the spacing would be determined by part thickness and location of probable defect. somewhat like the requirements of using table 6.7 in AWS D1.1 (06). You can get an idea of where the defect will most likely be and scan accordingly. Look at Commentary table 6.7 for more info. I'm sure other will have more experience and info for you.

edit:
not exactly kindergarten.

By js55

Date 07-24-2008 17:31

hogan,
Theoretical:
Ok, so if I am running a scan on a butt joint configuration similar to the double V shown per Table 6.7, on material
1 5/8" T, I first run a 70deg angle beam scan (in compliance with Procedure 1) per the Procedure Legend and then another one in compliance with F, if the alternative scan angle is more near perpendicular than the 70 deg scan?

And to clarify perpendicualrity, wouldn't the 70deg probe be closer to perpendicualr to all double V groove top portions and single V grooves, and for example the 45deg probe perfectly perpendicular to the bottom portion of a double V with 45deg angle or the opposite fusion surface of a 45deg single bevel?

Am I understanding this correctly?

And as a note, I don't see Procedure 2 listed anywhere in Table 6.7.

By hogan

Date 07-24-2008 18:04 Edited 07-24-2008 18:11

The requirement of F is to further evaluate an indication found by procedure 1, because of bevel angle 70 degrees might not give you the best evaluation of that indication

What you need is to draw out a sample sound path with a UT calculator (graph paper with joint drawn in and sound path). I don't have one handy at the moment. I will get that info for you. This should give a good visual representation for you.

By js55

Date 07-24-2008 18:40

Ah, the asterisk. thank you. I clearly had a case of cerebral rectumitis. I saw the thing in Table 6.7 and completely overlooked it in the Legend. And now I'm aware of the double asterisk. Have I made it to Level I yet? :)

By hogan

Date 07-24-2008 18:55

The D1 UT procedures and acceptance can be a little tough at first. I started in ASME and later went on to AWS work. The UT at first seemed overly complicated compared to a DAC curve. Now I feel AWS is by far simpler to work to.

By js55

Date 07-24-2008 19:10

Then I am assuming the purpose to acheiving greater perpendicularity is driven by the desire to verify pehaps a greater amplitude through greater discontinuity area verification.
Or that you are looking at a fusion defect planar to the fusion face as opposed to some other discontinuity?
Meaning that for example if it were a spherical porosity pocket the amplitude would not change with greater fusion face perpendicularity?

Also, looking at O, O, O, O, why would this not be an issue for thicknesses < 1 1/2"?

By hogan

Date 07-24-2008 22:03

With IP the signal will be large somewhat like a signal from a backing bar. It is difficult to miss with UT. With porosity you will have a distinct signal that typically will look the same regardless of angle. With non fusion areas the indication will fall on the bevel face, typically. Imagine this as a lamination easy to detect with straight beam from the top or btm of of the plate. But if you put your transducer on the side of the plate there is no reflective area. What the code is wanting is a scan that is perpendicular to the bevel face. That is why it is important for the UT person to see the weld detail.

By js55

Date 08-05-2008 15:19

OK, let me ask this as I work my way through the volume of info you guys have provided.
Realizing preliminary indications and then drawing out sound paths on graphic representations in order to calculate a better perpendicularity (I'm assuming) seems very time consuming. Is it safe to say that advances in UT are moving towards eliminating this procedure? And to engage Geralds post about UT replacing RT in many applications almost to obolescence, wouldn't computer radiographic tomography contribute to maintaining some nitch advantage for radiography?
So I suppose this is a two phase question. Reducing time is paramount. But the radiography guys aren't just sittin on their hands watching UT blow em away.

By hogan

Date 08-05-2008 15:44

I feel it is still important to write out your sound. It may seem out dated, but it can be a very useful tool in verification. Also, it helps keep your UT sharp and is a UT basic. I do agree with Gerald about UT. The main problem in the past with UT has been the recordabilty of testing. With x ray you have the film that can be reviewed at a later date. UT was left to the operator to interpret with no ability to review later. This has been an issue with ASME, DOT, and API 1104. The newer computer assisted UT now offers this ability. X ray has inherent difficulties, safety issues, and the need for at least two techs. The cost of the equipment is another issue, computer assisted UT is cheaper that computer assisted RT.

By CWI555

Date 08-05-2008 17:15

CR is a good thing, in effect, it allows layered radiography, but no matter what the latest bells and whistles are, the death nail of RT will not be in capability, but in security and cost. Ask yourself what kind of chaos would be created if some terrorist tapped up a Co60 source under a table at your local coffee shop, or tapped one to a pile of HE.
Security of those sources has and will become paramount as there are people actually thinking along those lines based on data I've read to date, and the change of rules between then and now. (01 to present)
That security is going to come at ever increasing cost. Radiography will probably never go away entirely, but it is losing it's prominence with every day that goes by, and will to continue to do so for the forseeable future.
The RT guys can jump up and down and throw a fit all they wish, but at the end of the day, the associated cost of security, extra training, licensing etc, will make it cost prohibitive for many of it's applications in the long run. Witness the move towards AUT for pipelines and tanks. They were once nearly the sole realm of RT, that is no longer the case.

As to advances in UT, they are coming hand in hand with advances in computation. As an example; Those advances in computation that spawns the phased array units in use today.
The theory has been around as long as the wave mechanics has been around, but cost effective implementation was not up to speed.

Phased array utilized the principles of constructive and destructive interference,http://www.kettering.edu/~drussell/Demos/superposition/superposition.html
by pulsing (phasing) multiple beam components which are combined, steered or otherwise interacted with in such as manner as to produce meaningful results.
Add this with multiplexing of many elements, you get sectorial scans, and in many cases, equal or better imaging than radiography.
Once again, all of this is possible through advances in computational science. With the electronic data, comes the ability to record that data. Most PA units come with that ability these days. Hall effect/optical encoders, can attach to the inspection head to record x,y locational data, that is correlated to responses on the screen much like AUT. In fact, there are PAUT systems out there that have the best of all worlds.

EMAT, and others are also a viable option, as are AET options.

I believe you can see the commonality. That being computational speed. The faster that gets, the more it can do. The same cannot be said of radiography.

There is a group of people out there hell bent on making RT survive, but in my opinion, they are fighting a losing battle.

By g32141

Date 07-18-2008 23:16

These techniques are based on where defects are known to occur.

Depending on the wall thickness/bevel angle you'll need different angle transmitters/receivers to cover the thickness. The receiver angle is determined by the transmitter angle and other factors.

Moving the receiver back and forth optimizes the response of the reflector from the transmitter. You need a cal block designed for this and to get the spacing correct.

The sound from the transmitter can either hit before the receiver on target or after your receiver. So the answer to your question about the transmitter getting the reflected sound from a defect to the receiver is no.

Tandems are good for automatic welds that have steep bevel angles.

By js55

Date 07-21-2008 13:25

thanks guys. I'll be contemplating this awhile. And I look forward to more input.
And the reference to 'kindetarten' pertains to my current level of understanding of UT. :)
But I'm workin on it.

By dmilesdot (**)

Date 07-21-2008 18:42

When I used to do ISI inspections in Nuclear Plants, we had a "sled" that had a pitch-catch set up. The distance between the transducers was determined by thickness as we were only looking at the inner one third of the weld. The welds that we examined with this technique were vessle head welds. The material was 7 inches thick so we were only concerned with the bottom 2.5 inches or so. The calibration was pretty time consuming but otherwise seemed to be a good technique. I remember that you could see the interface of the internal cladding really well.
Dave

By CWI555

Date 07-22-2008 15:54 Edited 07-28-2008 11:10

Jeff,

RTT/UTT is used extensively in different fields and different manners. It had it's start in aerospace via the delta v technique (first true method of this) and it also gave rise to the TOFD method through the delta V.

Put simply, delta v technique is sending the energy in at angle interfacing within a part, and picking up wavelets put off by the crack tip if any.

The difference between RTT and UTT is that UTT utilizes two seperate transducers and is usually used on similar materials.
The RTT method is used for instance on A553 plate with inconel filler. Due to the grain size and other factors of the two materials, you can pass a shear wave through the A553, but you'll have near total lose of it in the weld material. This is due to the wavelength (wavelength=velocity/frequency) bigger grains, require larger wavelengths to pass through them put simply.
For RTT, a refracted longitudinal is used for that reason. Going back to the wavelength calc, and given that the velocity of a shearwave is approximately half that of a compressional/Lwave you inherently have a smaller wavelength when trying to utilize the shear/transverse mode of vibration.

Therefore, you shoot a longitudinal wave in through the A553, into the weld metal, with gates set up to read the centerline on this portion of the exam.
If there is centerline hot cracking etc, it will reflect back towards the backwall of the plate in a similar manner as a ball off the rail of a pool table.
When it gets to the back of the plate, it runs into another interface that being of A553 to air. Do to the velocity differences between the two and as required by snells law (sin1 over sin2 = velocity1 over velocity2) it mode converts into a shear wave (head wave) that reflects back to the area of the initial ping(transducer).
Care must be taken to use a proper cal block/reference block on this and to account for the velocity time differences between the head/shear wave and the Lwave.

I mention the RTT variation as many new UTT techs often run into this situation when there is some irregularity in place (wrong filler, enlarged grain structure for various reasons etc) and they will often mistake that head wave conversion to be irrelevant because it's not showing up where it is susposed to be, or in the case of a butter layer of say 309 or other similar condition, mistake the reflection as an actual flaw without having qualified the response level. (simply noting what phase it's in would normally eliminate such concerns, but reading echodynamics is simply not taught in this day and time anymore to my knowledge)

Regards,
Gerald

By js55

Date 07-25-2008 14:33

Gerald,
Can you reference any pages or articles with diagrams so I can visualize what you're discussing?

By CWI555

Date 07-28-2008 11:08

I believe Materials Evaluation had a piece on it not to long back. I'll have to check which one and get back to you. If I can't find it, I'll put together a rough sketch.

By js55

Date 08-05-2008 16:39

Gerald,
Got any handy links to kindergarten graphics and explanations of Delta V technique?

By CWI555

Date 08-05-2008 17:52

http://www.ndt.net/ndtaz/ndtaz.php

By js55

Date 08-05-2008 18:04

Most excellent.
It appears as though the reason for the "Delta" name is also clear now.

By CWI555

Date 08-05-2008 19:51

It's the predecessor of the current TOFD technique. The radial waves coming off the top tip are 180 out of phase with the impinging energy, and the receiving transducer is positioned at the optimal location to pick them up in an immersion bath. Basically it's old school TOFD. It's been a long time, but this was what I learned on. Most of the "TOFD" techs these days don't even know what this is. For that matter, the RF waveform, they now call a "TOFD" wave in some cases. They generally have no clue how to read the RF wave, the only thing they know is the signal is susposed to corospond the Tofd window. (the one with all the white/grey lines) Ask them what those lines represent and they can't tell you. Not all of them are like that, but a disturbing number are. Of the disturbing number, they are no different than the manual hands who auto cal and use the machine to tell them whats there and where. Both are lost when the part is not what it's dimensioned to be, or when the machine does something it's not susposed to do.
I have and will run the later type off my jobsite. However a good UT technician I'll do anything in my power to help them out.

By js55

Date 07-24-2008 19:25

When you say tandems are good for steep bevel angles I am assuming it is once again based upon perpendicularity just as with changing angles on single probes. And is exactly the same as the intent of the drawings I posted with cracks transverse to the scanning surface. And that without this perpendicularity you end up with a very small amplitude (A Scan) as the echo bounces off the planar discontinuity tip, or pehaps maybe even some angular surface roughness of the discontinuity, which I would assume would be even far less evident on the screen than the planar tip.

By g32141

Date 07-27-2008 06:08

Yes defects in an orientation like that. The tandem probes I am used to have a farther standoff and hit the backwall first and skip up into the bevel face.
They are tandems for automatic welding.

There must be other diagrams explaining how that is a transverse crack. That picture just shows a near vertical defect without knowing anything about the bevel angel or how it was welded. That information helps believe it or not.

I have never hand scanned a weld with a tandem and I would be reluctant to call that a crack. That is something that has vertical height for sure.

Cracks may give you almost zilch as far as pulse echo response.

By ndeguy

Date 08-05-2008 13:43

To set-up pitch-catch tandems in an offshore environment on automatic welds (typically used by the Shaw InFocus system), a calibration block of project material with side-drilled hole (SDH) reflectors at various depths is provided, e.g. upstream and downstream fill 1 (F1) through F5. To optimise tandem set-up from its relevant calibration block SDH reflector (assuming probe radiusing and all other mechanical aspects are correctly set-up), first try using the set-up given on the appropriate data sheet provided by the office (Tx to centreline distance, probe angles and probe separation). If you are lucky and you have a response just work on maximising the response. If not then do the practical method of obtaining and maximising a response from the appropriate SDH using the Tx probe in PE mode with high gain, then switch to dual probe set-up and, maintaining the Tx probe stand-off distance, tap the Rx probe in and out until the maximum response is achieved. There are a few set-up variations possible, 3-leg, 5-leg, top-down (direct), bottom-up (indirect), Tx/Rx line-up in the jig or reversed to Rx/Tx according to the test circumstances. Plus this is complicated by needing balance upstream/downstream set-ups within +/- 6dBand having 20/80/20% FSH, where the 80% is the channel of interest and the 20% are the neighbouring channels. Phased Array cuts out a lot of this fiddly stuff. But the set-up can be incredibly sensitive to a few millimetre of probe separation. They can be fiddly to set up, but once done properly unless something goes wrong mechanically they usually calibrate consistently during production.

I have used tandems also in "shadow" mode where you look for a reduction in the received signal - however I have to admit that this was in my early days as an UT tech and I did not realise all the parameters such as probe size and frequency significantly affecting test efficiency.

By g32141

Date 08-08-2008 06:50 Edited 08-08-2008 06:59

I remember setting them up in Azerbaijan using an Epoch 3.

Where are you at these days? The last time I heard from you you were in Georgia or Khazikstan.

I have had some experience on a PA job. They save a lot of backache for the scanner tech thats for sure. Give me a PM if you can.

Good to hear from you.