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09:02 Apr-21-1998
Jeff Phillips
Weld Flaw Sizing Methods

With regard to weld testing and sizing flaws.
I am aware of the following ultrasonic sizing methods eg.
* 6db Drop
* 20db Drop
* Maximum Amplitude Method
* Tip Diffraction Method

Would it be possible to get a general comment from the UT experts that covers the following points:
- What are the main sizing methods used for manual UT scanning of welds.
- From a practical point of view, are a combination of sizing methods generally used to obtain flaw dimensions.
- Do sizing methods differ from country to country.
- What are the accepted practical tolerance for flaw sizing (eg +-1mm ?).
- What are the main advantages & disadvantages of each sizing method.

Thank you,

Jeff Phillips
jeffreyp@deakin.edu.au


 
05:10 Apr-23-1998

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1191
Re: Weld Flaw Sizing Methods : With regard to weld testing and sizing flaws.
: I am aware of the following ultrasonic sizing methods eg.
: * 6db Drop
: * 20db Drop
: * Maximum Amplitude Method
: * Tip Diffraction Method

: Would it be possible to get a general comment from the UT experts that covers the following points:
: - What are the main sizing methods used for manual UT scanning of welds.
: - From a practical point of view, are a combination of sizing methods generally used to obtain flaw dimensions.
: - Do sizing methods differ from country to country.
: - What are the accepted practical tolerance for flaw sizing (eg +-1mm ?).
: - What are the main advantages & disadvantages of each sizing method.

: Thank you,

: Jeff Phillips
: jeffreyp@deakin.edu.au

Mr. Phillips: You have broached a much debated topic! The 4 options for sizing that you mentioned are now probably the most common options and I suspect we could add things like ALOK and AVG the latter being more area than linear based. Let me address the 5 questions you asked and qualify this as opinion, since the feeling and practices established on this matter are quite entrenched in some quarters.


1- What are the main sizing methods used for manual UT scanning of welds.

Roughly speaking we group sizing methods into 2 types, either amplitude based or time based. Since the earliest inspections it has been easiest to relate large amplitude signals to flaw sizes. A direct relationship relating amplitude and defect "size" was developed by Krautkramer (AVG or DGS in English). Other amplitude based sizing techniques relate size to amplitude drop from a maximum as related to a change in probe position. These include your Maximum Amplitude, 6 and 20 dB Drop and ALOK methods.

We have more recently recogonised Time based methods as useful sizing options (forward scattered technique TOFD and back scattered TOF methods using both single and bi-modal probe configurations). TOFD might be better considered a mechanised rather than manual UT scanning technique. Similarly SAFT, used for lateral sizing, is more likely to be used in mechanised systems where B-scans are being evaluated.


2.- From a practical point of view, are a combination of sizing methods generally used to obtain flaw dimensions.

This depends on your point of perspective. If you are a diligent technician or researcher you might want to evaluate every indication in detail. If you are a manufacturer you want to just meet the code requirements so too much detail is not desirable. In nuclear in-service inspections flaws are often located by manual scanning. Occasionally flaws are located that the manufacturer missed and the unit has been in service for some time. Had the flaw been detected in the shop it would have been repaired. In-service repair is VERY inconvenient so now the owner looks for rationale to avoid it. Elaborate sizing options are then used to explain to regulatory bodies why the flaw is not critical. Then a combination of methods is used to allow more precise engineering of the failure analysis calculations.

Similarly, defect length is usually a significant criteria for accept/reject of a weld. Since most codes' acceptance criteria for UT are conversions of Radiographic acceptance criteria, the defect lengths are based on "workmanship" therefore they are conservative. Typically 10-12mm would be the maximum allowable length for some flaw types. However, for short flaws, the normal dB drop method measures only beam width not defect length. If this became a problem then someone would look to a combination of methods to look at size flaws when it appeared it would be costly to repair the weld. This is not always just an excuse by the manufacturer to cut cost (although it might be one reason). If a repair is made on many small flaws because the Code is very conservative, the weld metal structure could be degraded and a loss of strength result (even with all unacceptable Code defects removed).


3.- Do sizing methods differ from country to country.

It would be more accurate to say that "preferred" or "more frequently used" methods have national or regional differences. In North America we are having a difficult time making changes to the rules that regulate how we do UT. Codes dictate what methods will be used. Pressure vessel code, structural steel construction codes, etc. made and used in North America still base sizing on dB drop methods. ASME now permits (Section XI) other sizing methods to be used in certain cases. In North America the 6 dB drop is preferred. It was only when I met technicians from the UK did I learn of the 20dB Drop and MaxAmp methods. I suspect AVG is more commonly required in continental Europe. Asia has many codes using a 6dB drop method too. In Australia I think the 20dB drop is preferred.

Tip diffraction methods of sizing are not usually a "first line" option. For convenience of recording, the amplitude drop methods are probably the first choice. The Japanese have developed tip diffraction methods to areliable level since the modelling algorithms by Harumi explained many of the signals on the UT scope but it normally involves a defect be detected and assessed by "traditional" methods before tip diffraction is used.


4.- What are the accepted practical tolerance for flaw sizing (eg +-1mm ?).

I doubt we can reasonably expect to assign such a quantity to amplitude based methods considering the variables involved. For Tip diffraction methods a "practical" tolerance is often a percentage of wall thickness and a lower limit (under good conditions) might be +/- 0.5-1mm. Frequency and probe characteristics and actual flaw size will dictate a lower limit of the flaw size that can be sized. I.e. when looking to size a surface braking flaw with a 2mm high profile, the frequency and probe damping may not be adequate to see a time difference between the tip and "corner" reflection. Type of flaw will also dictate accuracy since planar defects usually provide a single set of signals to assess and multi-faceted flaws will provide several signals which might detract from the overall accuracy.


5.- What are the main advantages & disadvantages of each sizing method.

Here I will again be general in my reply.
Any amplitude drop based method is subject to the normal variables such as reflectivity, angle of incidence, coupling effects, surface conditions, geometry of part, frequency, etc. On the ideal reflectors such as flat bottom holes and side drilled holes the theory is fine, but for real defects the theory quickly breaks down as we try relating the change of amplitude with change of position in the beam,.

For time based methods the variables are not quite so bad but assumptions must be made and these should be reasonable based on good information from echo dynamics to characterise the flaw. Orientation of the flaw with respect to the beam will still be a major factor.

Amplitude based options are fast and relatively convenient methods to position and rank a flaw but generally have poor sizing accuracy. TOF options are usually better for accuracy but time consuming and difficult to incorporate as part of a general pre-service inspection.

Two gentlemen with the Australian DND, H.R.Chin Quan and I.G.Scott, did a study on the NDT operator and factors affecting their performance. They came up with 8 conclusions in their study. The last 4 of their conclusions relate in one way or another to your general concerns for flaw sizing.

1. The statement of a minimum detectable defect size is of use only in fixing the lower bound of detection process; it is not an efficient assessment of either operator or technique. Valid statements can only be made in terms of probabilities and confidence levels, for defect sizes relevant to the task envisaged. These can thereafter be sensibly applied to real problems concerned with the reliability of finding defects.
2. An entirely different set of problems arise when the operator is asked to make measurements. Present NDT equipment is not designed for this purpose and the operators' training is unsuitable. The requirement arises when improvements in structural reliability are sought using NDT and fracture mechanics.
3. Caution is needed in assessing various comparisons. Tests on operators in the laboratory can be extrapolated to the field but not necessarily without qualification. Comparison techniques should be made with care because changes in the conditions may produce entirely false indications and some comparisons are invalid, e.g. when attempts are made to compare unsuitable techniques. No universal solution to these problems has been found and, of necessity, workers need to examine their own situation with care.
4. The need to improve the overall results obtained by NDT is apparent-if improvement is not achieved then some replacement process is likely to be developed.
(From: Operator Performance and Reliability, by H.R.Chin Quan and I.G.Scott of the Department of National Defence, Australia, in series Research Techniques in NondestructiveTesting, ed. R.S.Sharpe)


If you would like more information the literature on this subject is very extensive.

Ed Ginzel
98.04.23



 
06:19 Apr-23-1998

Rolf Diederichs

Director, Editor, Publisher, Internet, PHP MySQL
NDT.net,
Germany,
Joined Nov 1998
600
Re: Weld Flaw Sizing Methods Thanks to Ed Ginzel for his comprehensive explanation.

Ed Ginzel mentioned the ALOK method as an option for flaw sizing.

What is ALOK?
Amplituden Laufzeit Ortskurven (ALOK = Amplitude Time Locus Curves)

Ultrasonic Testing Encyclopedia:
http://www.ndt.net/article/ut_az/ut_a/ut_a.htm

ALOK : Is a method, the German term for Amplituden Laufzeit Orts-Kurven, for fast data-acquisition and preprocessing (essential data reduction) achieve noise and ghost elimination as well as defect imaging and reconstruction. The data acquisition by ALOK works independent of time gates, that means, each maximum within the RF-signal, satisfying a probe dependent filter function, is recorded. C-scans of these raw data are used to detect welding defects; the defect characteristics result from further signal processing of the same data by a geometric or iterative reconstruction method. A variation of ALOK is the so-called SAFT (synthetic aperture focusing technique).

I know about ALOK mainly in conjunction with data reduction methods.

Next month NDTnet issue will present the article
'Specifications of Digital Ultrasonic Instruments
in In-Service Inspection of Nuclear Power Plants'
G. Csapo, T. Just

It is stated:
A high data reduction rate can be achieved with the proven ALOK - Algorithm. This method is based on the determination of the half wave maximums of the RF signal. The precise determination of the half wave maximum is achieved by using a high sampling rate (usually 50 MHz).
The ALOK - Algorithm recognizes only those amplitude and time of flight values, which lie within a time gate, and are greater or equal to the 'i' preceding half wave value maximums and are higher than 'k' successive half wave maximums. The measurement resolution of this technique can be adjusted with the 'reduction parameter' i and k.

ALOK may be presents images of UT data which can be used for flaw sizing. But I am not sure, can anybody confirm the use of ALOK for flaw sizing?

Finally I attached below two Internet resources of Flaw Sizing Methods.

Rolf Diederichs

At NDTnet:
http://www.ndt.net/article/wsho0597/schleng2/schleng2.htm
Determination of crack depth using ultrasonics - An overlook
By Udo Schlengermann Krautkramer Germany
Presented on the UTonline Application Workshop in May '97

At Panametrics Web Site:
Detection and Sizing of ID Connected Cracking
http://www.panametrics.com/ndt/theory/detsize/index.shtml





 
03:01 Apr-23-1998
Jeff Phillips
Re: Weld Flaw Sizing Methods With regard to Mr.Ed Ginzel's comments on Weld Flaw Sizing, posted on the Ultrasonic Forum on April 23, 1998 at 15:10:42:.

Thank you for your comprehensive and very informative comments on this subject. I found them to be most enlightening!

I did a search on NDTnet for flaw sizing and found many references, I have included the www address of one article that I consider may be of interest to others.

This article was also written by Mr. Ginzel.

http://www.ndt.net/article/ut_az/ut_s/info/sizing.htm




 
05:44 May-12-1998
Jeffrey Bragg
Re: Weld Flaw Sizing Methods : With regard to weld testing and sizing flaws.
: I am aware of the following ultrasonic sizing methods eg.
: * 6db Drop
: * 20db Drop
: * Maximum Amplitude Method
: * Tip Diffraction Method

: Would it be possible to get a general comment from the UT experts that covers the following points:
: - What are the main sizing methods used for manual UT scanning of welds.
: - From a practical point of view, are a combination of sizing methods generally used to obtain flaw dimensions.
: - Do sizing methods differ from country to country.
: - What are the accepted practical tolerance for flaw sizing (eg +-1mm ?).
: - What are the main advantages & disadvantages of each sizing method.

: Thank you,

: Jeff Phillips
: jeffreyp@deakin.edu.au

Good day mate

I am a shearwave Ultrasonic inspector for the US navy

I frequently preform Hull survaliences on submarines using both compressional
and shear wave on T-welds from the back side using only single skip however I
can set up a machine for multiple skips using a distance amplitude correction
set up(DAC)

In general I monitor known defects of frame welds found in previous inspection
to check for expansion or worsening The method I use is to peak ont the
reflection at my calibrated dB the reduce the gain on the machine to show the
signal @100% screen height
Then I slowly work fwd and the rev the shoe (45 degree) to note where the
signal drops off 'sharply' this method leaves for some disagreement in where
the signal drops, It could drop down to 80% and then disapear or sooner.
On an A-scan presentation the screen is set up to resresent depth of defect
I must be within .100" or about 3mm to certify in this method.
My calibration is a 1.5mm hole at a depth equal to the thickness of the
through member +/- .25" set at 20% screen height, +12dB

I would reccomend trying to find TM 271 under US gov publications (TECH MAN)
or call me in the US (Pacific time) MON-WED 360-697-4347



 


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