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- since 1996 -
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Reza
Iran, Joined Jan 2014, 4

Reza

Iran,
Joined Jan 2014
4
20:59 Jan-14-2014
Defect sizing techniques on AUT

I have a question related to UT and especially on the field of AUT.
I wanted to know about applied methods of defect sizing on Automated UT machines.
for example we have a shaft including some defects with different sizes as listed below:
Defect #1: FBH 2mm
Defect #2: FBH 3mm
Defect #3: FBH 4mm
Defect #4: FBH 6mm

How is it possible to automatically detect and size these defects?
Many thanks,
Reza

    
 
 Reply 
 
andrew cunningham
NDT Inspector
Canada, Joined Jun 2008, 238

andrew cunningham

NDT Inspector
Canada,
Joined Jun 2008
238
02:40 Jan-15-2014
Re: Defect sizing techniques on AUT
In Reply to Reza at 20:59 Jan-14-2014 (Opening).

The amplitude of a reflector is governed by orientation before size. For example, a small reflector perfectly orientated to the angle of the probe, will give a strong signal, whereas a large defect poorly orientated to the probe will send back a weak to no signal. Some specs require the technician to size by amplitude, however, the more modern and enlightened specs are requiring cross-sectional sizing.

All the best

    
 
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Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1266

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1266
14:04 Jan-15-2014
Re: Defect sizing techniques on AUT
In Reply to Reza at 20:59 Jan-14-2014 (Opening).

Reza, you have asked 2 questions; how to detect the "defects" and how to size the "defects". As noted by Andrew, amplitude sizing has its limitations (many). Detection of flaws by NDT is considered a probabilistic event. Before you can size a flaw you must first detect it. In order to consider that a flaw has been "detected" UT requires not only that a flaw produce a response greater than some threshold amplitude, it must also provide a clear response well above the noise level. Experimenting with a variety of angles, modes, frequencies and focussing depths can provide a method of determining how well your technique is at "detecting" flaws. Several techniques are used for sizing. Tip diffraction, relative amplitude (DGS) and dB drop methods are the most common. The best method for you particular application could be determined by a programme of detection and then sizing followed by sectioning to see which one works the best. But be prepared to accept that you will not always find all of the flaws in a part and that your sizing method will probably have a large scatter of error.

    
 
 Reply 
 
Radoslaw
NDT Inspector, Materials Engineer
Poland, Joined Apr 2012, 51

Radoslaw

NDT Inspector, Materials Engineer
Poland,
Joined Apr 2012
51
18:55 Jan-15-2014
Re: Defect sizing techniques on AUT
In Reply to Reza at 20:59 Jan-14-2014 (Opening).

Hello,

Zonal dicrimination (AUT)

Defect size = %Amplitude x Zone Height

    
 
 Reply 
 
Reza
Iran, Joined Jan 2014, 4

Reza

Iran,
Joined Jan 2014
4
07:44 Jan-16-2014
Re: Defect sizing techniques on AUT
In Reply to andrew cunningham at 02:40 Jan-15-2014 .

Dear Andrew,
Thanks for your useful information on the influence of reflector orientation on the "amplitude sizing method". assuming that reflectors are perfectly oriented to the beam angle (perpendicular to the beam), what is the applicable method for sizing them? As I said on my first question, I am interested to know about applied method for sizing the reflector automatically. of course automatic inspection compared to human made inspectoin, has some limitations because in this case, a program wants to estimate the size. but I want to know the usual and more trusted method that UT cards are programmed in order to size the reflectors? As Ed Ginzel said, Tip diffraction, relative amplitude (DGS) and dB drop methods are the most common methods. Which is usually being used on the industrial AUT machines?
Many thanks,
Reza

    
 
 Reply 
 
Saam
Iran, Joined Jan 2014, 5

Saam

Iran,
Joined Jan 2014
5
07:47 Jan-16-2014
Re: Defect sizing techniques on AUT
In Reply to Ed Ginzel at 14:04 Jan-15-2014 .

Ed,
I reckon the DGS method is used to size the defects in Portable DFD's. Please advise which method is suitable to size the defects (related to known FBH dia) in Automatic UT Systems (in-line inspection)

As you are aware data acquisition and data processing rate in in-line application is completely different from portable/manual inspection. (e.g. in-line inspection with 50m/s test speed)

    
 
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Reza
Iran, Joined Jan 2014, 4

Reza

Iran,
Joined Jan 2014
4
08:15 Jan-16-2014
Re: Defect sizing techniques on AUT
In Reply to Ed Ginzel at 14:04 Jan-15-2014 .

Dear Ed, many Thanks for your answer. I got some useful information from you and Andrew. a very useful fact that you mentioned is the probabilistic nature of the UT estimation that I should always keep that in my mind. I am familiar with some AUT concepts like Gate, Threshold and am trying to understand more. You mentioned some more common methods like Tip diffraction, DGS and dB drop methods. Now I am interested to know about industrial UT machines which are used for inspection of bars, plates and etc. assuming that we have always a level of uncertainty on those machines, how do they really inspect (detect and size) the parts. the methods you mentioned are usually performed by human and are used on manual inspection. for example DGS is a defined method that user should perform it according to some specific steps like changing the gain continuously and reading the gain drops and some other calculations. so I do not know is it applicable on Automatic inspection? because it seems to be complicate for the fast inspection used on AUT machines. So I want to know which methods are applicable for automatic UT inspection? maybe those machines can not size the defects and they just detect that a defect is larger than a known reflector or not. in this case we have to say that AUT do not do the sizing and it seems strange to me. otherwise, if they do the sizing what is the method?
Many thanks, Reza

    
 
 Reply 
 
Reza
Iran, Joined Jan 2014, 4

Reza

Iran,
Joined Jan 2014
4
08:24 Jan-16-2014
Re: Defect sizing techniques on AUT
In Reply to Radoslaw at 18:55 Jan-15-2014 .

Dear Radoslaw,
can you explain a little more about the technique, Zonal dicrimination? what is the Zone height on the formula and how is it obtained during UT test?
many thanks,
Reza

    
 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1266

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1266
13:41 Jan-16-2014
Re: Defect sizing techniques on AUT
In Reply to Reza at 08:15 Jan-16-2014 .

In-line production testing of regular shaped components is indeed different than the manual option. Generally automated systems simply configure the settings with a gated region of time in the component and set sensitivity levels based on the response from the reference targets (like FBH). The system is then checked for its ability to consistently alarm on these targets by sending a calibration piece through the system and ensuring that all of the relevant reference targets are detected. For things like round bars or tubing the scanning can be accomplished by spinning the stock while it advances past the beam(s) and the rotational speed and rate of movement of the stock past the probes is adjusted to ensure you meet the specification.
Several of the equipment manufacturers you see on the exhibitors list provide both the inspection equipment as well as the mechanical apparatus and electronics to handle these applications. These systems can include amplitude recording per channel, position of the component during the scan, audio and visual alarms, automatic stopping upon alarm (so you can manually confirm locations of indications) and even paint markers to spray the component at the point the alarm triggered.
Go to the "Exhibition" tab at the top of the NDT.net webpage and search for Manufactures in All Countries and use the search for the term Automate
This will give you a good selection of those that might be able to provide the system you need.

    
 
 Reply 
 
K.S. Walton
Consultant,
USA, Joined Feb 2011, 7

K.S. Walton

Consultant,
USA,
Joined Feb 2011
7
06:09 Jan-20-2014
Re: Defect sizing techniques on AUT
In Reply to Reza at 20:59 Jan-14-2014 (Opening).

"How is it possible to automatically detect and size these defects?" That's a simple, but complex question!
Gating and focusing strategies are going to have a lot to do with detection in AUT. Transducer frequency will come into play. Scanning lateral resolution will also be VERY important. As previously stated by others, flaw size and orientation is a major factor. Sizing will take more user precision and experience during analysis. As far as 0- degree AUT C-scans go, employing multiple gating strategies simultaneously, when possible, will generally improve your detection results. Please excuse my measurement units in the examples, I normally work in inches, but am trying to get everything in terms of mm.
For 0 degree AUT scanning of plate or pipe for corrosion/ damage detection and mapping, I like to run a Peak gate and an Edge gate simultaneously, set in the exact same positions. The system I use allows you to calibrate TOF within each Gate mode independently. (Some other machines like the Omniscan do not. You either have a Peak or edge A-scan. Depth readings with be inconsistent between the 2 gates. Edge will always read less than Peak. The new 4.1 OmniScan software does allow for a Max Peak reading of the 1st peak to cross the Gate, which helps.)
Example: Say you are examining a part that is 19 mm thick. I set a Peak Gate at 20% detection threshold. I start the gate as close to the left side of the A-scan as possible, without getting it interfered with by any entry surface/ wedge/ water interface signals you may have near 0 mm. So maybe it starts at 5 mm. I then extend that gate to about 20% past the anticipated nominal backwall thickness, maybe 24 mm. Gives you a gate width of 19 mm. I then set an Edge gate in the exact same position.
I display a position(thickness/ depth) C-scan for each gate. I then use a C-scan color palette that will show a good deal of color contrast across various thicknesses/ depths. For scanning and detection, I generally set the color- depth palette scale to the start/ stop positions of the gates. During analysis of detected signals, I adjust the color- depth palette as needed to bring out the color contrast at various indication depths.
The Peak- gate C-scan will monitor the highest peak, which will usually be the backwall thickness. It will also show any macro damage, such as gross wall loss, and sharp laminar indications with amplitude greater than the backwall, or that obscure the backwall all together.
The Edge- gate C-scan will also monitor the backwall, and, in addition will display indications from small, low amplitude isolated pits, and any low amplitude micro- damage indications such as inclusions or small FBHs, even stepwise cracking.
Sizing on each C-scan is different:
The Peak C-scan is less susceptible to beam spread concerns, as it only starts displaying the edge of an indication once its amplitude just overtakes that of the backwall, just beyond reaching the amplitude equalization point. At 20% detection threshold, the gate represents a 12 dB drop from a backwall reference amplitude of 80%. It is usually pretty accurate, maybe slightly under- sizing the lateral extents of an indication.
The Edge C-scan will show the indication as soon as it crosses the gate detection threshold. It can slightly or greatly over size the lateral extents of an indication.
Focusing depth (or lack thereof) can have significant effects on your C-scan displays and sizing as well.
An unfocused beam should react as you would think, with the maximum amplitude of a reflector of a certain size coming at the very end of the fresnel zone, before attenuation and divergence occurs, at a location depending on your probe and part. Add in a wedge or water path, and the nearfield point shifts within your part thickness.
A beam focused near the backwall (immersion or PAUT) can cause near- ID signals, such as isolated pits, inclusions, or FBH, to have a high amplitude return, maybe even enough to diminish the backwall amplitude and show up on the Peak gate C-scan. If your part thickness is great enough, the ID focal depth can also have the effect of returning low- amplitude responses from midwall indications, those outside of your focal zone. The existence of waterpath or wedge nearfield loss is going to affect the true focal depth.
On the other hand, a midwall focused beam could return high, exaggerated amplitude returns from midwall located indications, and rather low amplitude responses from ID proximity indications, especially if the ID is past the focal zone. Again, The existence of waterpath or wedge nearfield loss is going to affect the true focal depth.
The standard calibration block I use for my AUT 0 degree system is a large step wedge type. It has thickness steps of .250"(6.35mm), 0.500"(12.7mm), 0.750"(19mm), 1.00"(25.4mm), 1.25"(31.75mm), and 1.5"(38.1mm.) Each step has two FBHs drilled into the bottom. The large FBH in each step is 0.100" deep and 0.125" in diameter. The small FBH in each step is also 0.100" deep, but 0.065" in diameter.
Regardless of where I focus, if my lateral C-scan resolution is tight enough, both holes on each step are usually detected on my Edge C-scan. Sometimes the 0.150" deep FBH (0.100" on 0.250" step) may not be due to low amplitude near- field effect.
On my Peak C-scan, depending on focal depth, the larger diameter hole will usually be detected on all steps. The smaller FBH may not be. My 10MHz probe usually does a better job of seeing the small hole then the 5MHz probe does.
FBH diameter sizing on the Peak C-scan is usually more accurate then diameter sizing on the edge C-scan. The 20% Edge gate basically detects at a 12 dB drop from 80% amplitude. As I have seen, for indications smaller then your beam spread, 6dB drop will often oversize indications smaller then your beam spread, often sizing the beam spread itself. 12 dB sizing drop is even more drastic. But you have to detect an indication before you can size it. These gating strategies work well for detecting both large and small indications. You just have to establish a reliable sizing method for certain sized indications at certain depths. I trust my Peak C-scan for sizing in most cases, in the regard that it starts measuring an indication at the equalization amplitude between it and the backwall. I can simply use X, Y reference/ measurement cursors and take measurements that are equivalent to S(m-r) and I(m-r), and use those as my sizing dimensions, unless special conditions arise.
There is no one- size fits all solution. Detection is a factor of focusing, probe selection, scanning resolution and speed, PRF, visual aids like color palettes, color palette scale, etc. An indication might be detected, but not showing on your C-scan due to the color palette being too broad, not shallow enough, or not deep enough.
Sizing methods are particular upon certain circumstances, and must be tested, proven, and calibrated for across various conditions. My analysis software has auto- analysis features, but unless I, as the operator, set the system up correctly and know how to get the most out of it, using those auto analysis functions can prove to be useless and severely inaccurate. Even after setting everything precisely, the auto feature can still be incorrect due to various anomalies (industrial electrical noise, poor grounding, non- relevant indications, transducer lift- off, etc.) Use all of your tools, and all of your display views. C-scans, A-scans, B and D scans, S- scans. One scan may detect and display, the other may not. You may have to adjust gates during analysis to include or exclude certain indications.
When time alllows for it, practice, tinker, experiement, fail, succeed as much as possible. At my permanent onsite location, the customer and workload are very demanding and sometimes it is hard to find the time for developing and improving techniques. But I am a tinkerer, an experimenter, and like you, a questioner. Currently, I am on night shift, and our client's onsite NDE lab has a few million dollars worth of testing equipment and flaw samples, to which I have been given almost unlimited access. My nighttime workload is small, so I am in the lab every night trying to take over the world.
Best of luck!!!!!!!!

1    
 
 Reply 
 
Saam
Iran, Joined Jan 2014, 5

Saam

Iran,
Joined Jan 2014
5
08:12 Jan-23-2014
Re: Defect sizing techniques on AUT
In Reply to K.S. Walton at 06:09 Jan-20-2014 .

Dear Walton,

I would appreciate it if you please explain the difference between SDH and FBH?

    
 
 Reply 
 
K. S. Walton
Consultant,
USA, Joined Feb 2011, 7

K. S. Walton

Consultant,
USA,
Joined Feb 2011
7
17:03 Jan-23-2014
Re: Defect sizing techniques on AUT
In Reply to Saam at 08:12 Jan-23-2014 .

Saam, I'm not positive that I follow your question. The side- drilled holes (SDH) and flat- bottomed holes (FBH) are two complete different reflectors. Generally FBH at varying depths and with different diameters are inserted in plates or shafts as reflectors for 0 degree beams, basically for sensitivity and resolution tests, and checked once an acceptable TOF (thickness or soundpath) calibration has been established. If you have a 50mm thickness with a 20mm deep FBH drilled into the bottom, you should get a reflector at 30mm. If you put two 20mm deep FBH in a 50mm plate, one FBH having a diameter of 4mm, the other a diameter of 2mm, the FBH with the larger diameter should produce a higher amplitude indication than the smaller diameter hole.

For 0 degree AUT, you want to be sure that your scanning and probe resolution is tight enough to see the holes. I have clients advise me of what the smallest acceptable critical flaw size is that I need to detect. I set my scanning resolution to at least 1/2 of that diameter. If they tell me 6mm, I make sure my scanning resolution is AT LEAST 3mm x 3mm, generally much finer. I scan a cal block with a a 1.5mm diameter and a 3mm diameter FBH, both 1.5mm deep, and make sure that the scanner will detect them. Probe diameter (or aperature size with Phased Array) also comes into play. I generally like at least a 1/3rd overlap of my probe when scanning. But the dimensions of your beam have to be considered. Also codes and procedures. If they say I must have a 1mm resolution, that's what I do. A 10mm diameter immersion transducer will not necessarily have a 10mm diameter beam. The beam diameter at the focal spot will be less then the beam diameter near the probe or well into the far field. I use a 10mm (actually 0.375 inch) 10MHz spherically focused probe on a regular basis. I usually run a 0.200 inch x 0.050 inch scanning resolution with it for detection in thicknesses ranging from 0.3 - 3 inches thick, using adjustable waterpaths for focusing, or sometimes purposefully out of focus. If need be, I can increase the resolution to see better detail. The same thing can be achieved with PAUT 0 degree probes. Usually higher resolution along a Linear 0 active set of VPA's, and then the side to side passive resolution can be adjusted based on critical flaw size or client specs. Generally these are not code inspections, so, again, resolution is up to the inspector, the job, and the client. Again, focusing, or the need not to, come into play, and the resulting beam profile at depths of interest will have to be considered. Focusing can be your friend, or it can get you in trouble by weakening your beam away from the point of focus. The near- field absorbing effects of wedges and water paths have to be taken into account as well. The portion of the near field consumed within the coupling medium (water or wedge) at the velocity of that medium, will reduce the nearfield available for true focusing within your test part.

Basically, for focusing, just calculate your nearfield using whatever math problem is suitable for your probe and circumstance. Figure out first how much nearfield is used in any wedge, water, or coupling path you are using, at the velocity of that medium. Take what's left, and figure out where the focus REALLY is in your test part. Example: I have an immersion probe with a 3 inch focus in water. I am using a 1 inch water path before my beam enters my part, in this case, carbon steel. That leaves 2" of focal length in the part, right? WRONG! The remaining 2 inches of focus, I must then divide by the velocity ratio of steel/ water (.232 inches per microsecond/.0584 inches per microsecond= 3.97) 2"/ 3.97= 0.503, or about 1/2 inch is where I am focused in this case. Mr. Ed Ginzel has a few REALLY INFORMATIVE papers on the subject of focusing and wedge nearfield on this website, just search for phased array nearfield, or similar keywords.

To the subject of side drilled holes SDH, they are another matter. They are great for establishing sensitivity, and getting an idea of depth. I dislike performing depth calibrations (or wedge delays) on them, because the official depth is usually measured to the center of the hole. The top of a 1 inch deep SDH that is 0.046 inch in diameter will actually measure at about 0.977" deep. If you tell a probe that it is 1" deep, then your depth readings will be slightly deeper then what they should be. Also for angle beams, as your angles increase, the depth of maximum reflection from the hole changes, but if you are telling the machine it is the same across all angles, the indication will get skewed as your angles increase. Also, the larger the hole, the more error is introduced. In phased array, I perform wedge delay calibrations for angled beam probes on a radius, and on a large stepwedge for 0 degree. I will perform a sensitivity calibration on a side drilled hole, and also a TCG or DAC. But the time of flight calibrations need to be more precise. I'd rather not accept 0.23" of slop if not necessary.

    
 
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