RT interpretation when using edge blocks

Date 05-19-2008 12:39

I've been a radiographer for 7 years now and I know I have NOT seen it all but I have been around. I'm not familiar with the term "edge blocks." Could you expand on that? Enlighten me. ;)

By Bill M

Date 05-19-2008 13:05

Edge blocks are blocks of steel that are positioned at the edge of a weld, to help maintain consistant film density right to the edge of a butt weld. See AWS D1.1 Section 6.17.13 (2006 edition) for description and requirements for using edge blocks.

I guess not knowing the indication depth you have to assume to be more critical and measure the indication to the nearest edge line (shortest dimension).

By jmdugan10 (*)

Date 05-29-2008 12:36

Thanks for the information, I primarily work with B31.3 and section VIII so I don't look at D1.1 much.

It would seem to me that the distance the indication is from the edge is not a factor for evaluation. Only the actual size of the indication is relevant. I don't see how the viewing or measuring the lines of the edge blocks (the geometric distortion due to the offset of the source) is involved with the indication evaluation. As I see it the edge blocks are only used to avoid burn off of the weld at the ends. If the distortion is causing the indication to be longer than acceptable, I would have it re-shot with the source centered over the indication to limit this factor. On second thought double check the radiographers set up to make sure he had the stand off distance to meet Ug in the first place.

Maybe I'm way off, somebody correct me if I'm wrong.

By Bill M

Date 05-29-2008 16:44

Take a look at fig 6.5 -There are limits to indication size depending on the distance to the edge of the plate.

By jmdugan10 (*)

Date 05-29-2008 18:26

Hey... cool! Then... yes mesurement should be taken from the edge closest to the film. I still think there may be a question to the amount of geometric distortion. I'm wondering if the actual veiw length is to big to give an accurate interpritation of this indication?

Thanks Bill, I have learned somthing new!

By TMU Date 05-29-2008 18:44

Without know the depth of the defect I would guess you would have to interpret to the closest edge but you would think there would be something that would dicuss this to know for sure?

By CWI555

Date 06-02-2008 12:53

Typical GU allowances, you will have to find the code of record for your application for specific info.

Material Thickness (in.) Ug Maximum (in.)
Under 2    0.020
2 through 3    0.030
   Over 3 through 4    0.040
Greater than    4    0.070

Ug = Ft/D where:

Ug = geometrical unsharpness
F = effective source size in inches--the maximum dimension of the radioactive source
or x-ray focal spot as viewed from the film location. For an uncollimated cylindrical
source or a rectangular focal spot, the effective size is not the diameter or length of the
longest side, but is the diagonal as projected on the film.
D = distance in inches from source of radiation to the source side of the weld or object
being evaluated.
t = distance from source side of weld or object being radiographed (evaluated) to the
film.

If it's within the GU limits, it is measured as is, where is for most codes.

Not sure what you mean by "actual view length". The characterization of this flaw will
be independant of sizing assuming again, that it is a code shot.

Regards,
Gerald

By jmdugan10 (*)

Date 06-03-2008 13:45 Edited 06-03-2008 14:05

This is what I mean by to "long of a view length"; if a view on a plate shot, like we are talking about, is long enough the ends of the view could be distorted, not out of Ug. We haven't been told specific dimensions, so I'm just guessing here, but if you're using edge blocks the RT crew would most likely want to cut down on the number of exposures by backing off and using a longer view length. If they didn't back off far enough to get the Ug any indication at the extremity will be elongated. Kind of like a bad elliptical shot. At those extreme ends you are actually shooting through more material than your base thickness because you are shooting at a diagonal so this would change your Ug calculations (Object to film distance).

Is that clear has mud or what? I know what I mean! If that doesn't expain it I'll try to upload a picture.

Edit: Does that do it?

By CWI555

Date 06-03-2008 16:36

There are a few things required of a code shot.

A. Have the correct station markers and area of interest been identified on the radiograph? Yes=got to B No=reshoot
B. Has the required density throughout the area of interest meet the limitations of the code? Yes=Go to C No=reshoot
C. Has the required geometric unsharpness limititations been met? Yes=Go to D No=reshoot
D. Has the required sensitivity been achieved as demonstrated by viewing the designated IQI wire or hole? Yes=Go to E No=Reshoot.
E. Evauluate the film for artifacts, scratches or other film related potentials for degredation of image quality in the areas of interest. Yes = go to F No=reshoot
F. Evaluate the radiographed object to the referenced code.

If it meets step A through E it doesn't matter how long the "view" is. If you could show me in the code where this "view length" is designated I would be much appreciative. I've never seen that in Section V, or any other document of that nature.

To use your train of thought; every single indication found in "any" radiograph that is not directly under the source when shot would have to be reshot dead on. Moving the source to a different angle is usually only done when diffraction is suspected or other unusual condition. However; I can move the source around taking 3 and 4 shots and various angles and unless it's gross, end up buying off nearly any indication except for very large volumous ones from one of those films.

If you suspect the shot was taken at to short of a distance, and the station markers have been accurately stamped or written on the object, you can get a rough idea of how close it was by backcalculation the station marker distortion.

All in all, if conditions A through E are meet, code wise it is what it is. No where in the code does it allow reshooting to make it go away if those conditions are first meet.

Regards,
Gerald

By TMU Date 06-03-2008 17:06

My original question was not about goemetric unsharpness or is this piece of film acceptable per code. I was just curious as to what your opinion was based on if you had a piece of porosity 1\16" in diameter and it was located 1\2"(maximum allowable distance for 1\16" diameter is 5\8" per tension criteria D1.5) from the first line which would represent the bottom edge of the weld as seen on the film utilizing edge blocks. If you wanted to accurately interpret if this was acceptable or rejectable would you take this to another step and UT this to find the depth?

By jmdugan10 (*)

Date 06-03-2008 17:37

You're right of course Gerald. I am not suggesting "every single indication found in "any" radiograph that is not directly under the source," be reshot. That is ridiculous, besides RT is not the end all be all of NDT. I was just trying to illustrate that if this was a borderline call a reshot should be considered for my previously posted reasons.

The question of whether to use the top edge or the bottom edge for measurement made me think this was a borderline call. It also made me wonder about the source placement if there was a wide gap between the top and bottom wall on the radiograph. If the base metal is at the minimum requirement for edge blocks were talking about 0.500". If the top wall is...say...0.250" from the bottom wall on the radiograph there is probably a source placement issue.

In the end I am just guessing here since we have not been given the specifics.
Thank you for your posts Gerald. I value your knowledge.

I don't know ya... but you seem to know what your talking about. ;)

By CWI555

Date 06-05-2008 10:55

I avoid using the word "borderline" in regards to RT calls. If the code states maximum length is .750" and it's .750" it's good. If it's .751" it's bad. Somebody wants to argue it, or an owner wants to over ride it, Not my problem. It is what it is when I lay the comparators on it, I don't back off of RT calls for that reason. Someone may interpret something a different way, someone may not like my interpretation, but if there is one thing I don't trust, and don't believe anyone should trust, is an weak kneed interpreter, if they are no more sure than that, they should not be interpreting film. Make a call and stand by it. My opinion.

Regards,
Gerald