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00:51 Jan-24-2006

Michael Moles †2014 *1948

,
Joined

Comments on paper on "Construction weld inspection procedures"

These criticisms of our paper “Construction weld inspection procedures using ultrasonic phased arrays” by Michael Moles and Jinchi Zhang, R/D Tech, were sent to ASNT. The criticisms and reply are below.

*******************************************************
Bonjour Mr. XXXXXXX – Mr. YYYYY ZZZZZZZZ told me to communicate with you on this matter.

I would like to submit a formal complaint about an article that has been published in different medias including ASNT’s. The article was declined in different flavors, but always include over-simplistic and false information about weld inspection technique. In essence, the article says that phased array ultrasound beams HAVE to be perpendicular to the wall (bevel) if one wants to detect lack of side-wall fusion defects. It should have been PROPOSING that perpendicularity provides stronger specular echoes, but still tip diffraction echoes of lack of fusion defect provide sufficient signal-to-noise ratio for most weld inspections.

You will find the articleflavors at:
http://www.ndt.net/abstract/wcndt2004/83.htm
http://www.npec.ca/Misc_Docs/Fri%20830am%20Moles.pdf
http://www.asnt.org/publications/materialseval/solution/jan05solution/jan05sol.htm

May I recommend you to talk with Mr. Greg Selby (EPRI, gselby@epri.com) and Mr. Jonathan Buttram, Level III (InterWAV, jonathanbuttram@hotmail.com) about this article. They seem to have the same opinion I have. EPRI also qualified a procedure recently which proves to some extent that Mr.Moles is wrong. If you are unsuccessful, I can propose you other names.

I think the technical revision commitee should have filtered this article or should have at least required a modification in the wording. I think an Errata should be published in the next ASNT Journal. But I also understand sometimes things slip through unfortunately...

Best regards,

*****************************************************
Reply by Michael Moles:

This article has received many comments and criticisms, invariably from people who didn’t read it properly. This author obviously didn’t read it correctly either, doesn’t appreciate the differences between construction and in-service inspections, and has compounded it with a lack of understanding on tip diffraction. Defining the article as “over simplistic and false” is itself raising the tempo beyond the normal realms of scientific discussions.

The article itself is primarily based on modeling, which has known limitations. In this instance, modeling is likely to overstate the issue of mis-orientation, but this is acknowledged in the text.

False is certainly an unnecessary and incorrect statement; there is no doubt that the response from a defect depends strongly on its orientation relative to the incident ultrasonic beam. This dependence has been known for years and has been the basis of codes such as ASME1 (“appropriate angles should be used”). To quote API, “A (second) limitation in the use of shear wave ultrasonic inspection is the failure to detect large two-dimensional (planar) discontinuities as a result of the inherent direction of the reflected beam.”2 A simple understanding of weld geometry and the position of the array shows that a single S-scan cannot have well oriented beam angles on the weld bevel, root and cap, at all locations. The thicker the component, the worse the problem becomes.

The modeling shows that location and orientation of the defect also matters, which will be no surprise to any operator. In the modeling, midwall defects were particularly difficult to detect, which mirrors real life pulse-echo ultrasonics. Again, thicker components presented more problems than thin components.

The article does not say that phased array beams (or any ultrasound beams for that matter) have to be perpendicular to the weld bevel; it says that the closer the angle is to the weld bevel incidence normal, the better. It is clear that current codes (e.g. ASME) do not demand normal incidence. How can they? Typically a 45o and/or 60o transducer is used for conventional inspections. With a 30o bevel, incident angles will be normal on the bevel, but root angles will be off. However, if a 37.5o bevel is used, the angles will be off-normal by 7.5o on both bevel and root, yet this is permitted by code.

Relying on tip back-diffraction alone would be a disaster. The author seems to have little understanding of diffraction, something which Olympus NDT (formerly R/D Tech) has been using for ~15 years. The standard diffraction technique is TOFD (Time-Of-Flight Diffraction Technique), which runs at least 20-40 dB above typical pulse-echo scanning levels because tip diffraction signals are so weak. Even with TOFD, we sometimes have trouble detecting crack tips, especially with noisy material. Using standard pulse-echo codes and looking for tip reflections would result in almost no defect detection; of course, some manufacturers would like that!

To address the issue of backward tip diffraction mentioned here (i.e. pulse echo), like everybody else we see diffracted signals quite often. They are a very good method of sizing, especially with phased arrays where the full image is available and coupling is a non-issue. However, the tip signals are mostly low amplitude (usually below recording threshold), and are identifiable primarily because of their association with nearby crack signals. Identifying, calling and characterizing tip echoes under normal pulse echo scanning is going to miss a lot of defects.

The EPRI procedure referred to here is a manual S-scan code for reactor pressure vessels. (The procedure is “EPRI-proprietary” and not available.) However, EPRI states that the code is primarily for fatigue cracking, with the possibility of finding weld defects. (One seriously hopes that nuclear-quality reactor pressure vessels have very few unknown weld defects.) More to the point, fatigue cracks occur at known locations (the surfaces), and typically provide good corner reflectors. Weld defects occur at characteristic locations in the weld, and deterministic orientations, but often without corner reflectors; therefore correct incidence angles are important for detection. Different procedures are required for different types of defects. The work report in January 2005 was specifically for construction welds, as stated in the first word of the title – not for in-service inspections.

Incidentally, another EPRI-approved procedure for weld inspections from Zetec uses S-scans and linear scanning, but uses three S-scans as we recommend. Though Zetec ultrasonics is an offshoot of R/D Tech, this work was done independently, and provides support for the multi-S-scan approach. (Being nuclear, however, this procedure also requires extensive lateral scanning for mis-oriented defects; this is not normally required in the workmanship-type codes.)

References:

1. American Society of Mechanical Engineers, ASME Section V, Article 4, paragraph T-472.1.1, 2001.
2. American Petroleum Institute, “Recommended Practice for Ultrasonic and Magnetic Examination of Offshore Structural Fabrication and Guidelinesfor Qualification of Technicians”, RP 2X, Third Edition, Sept 1996, p. 7.

By Michael Moles
Olympus NDT (formerly R/D Tech)





 
03:20 Jan-25-2006

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1185
Re: Comments on paper on "Construction weld inspection procedures" Michael:
The writer of the complaint about your and Dr. Zhang's paper seems to have missed the real issues in weld inspection. Their wording that "..tip diffraction echoes of lack of fusion defect provide sufficient signal-to-noise ratio for most weld inspections" suggests a lack of experience in weld inspections and Code requirements. Harumi and others have pointed out that tip diffracted signals are typically 20dB lower than specular reflections.

Thinking that a "detection" scan would use diffracted signals is dreaming. The reference scan levels set by most Codes are based on the refelcted signals of perpendicular incidence on an ideal reflector (e.g. a side drilled hole or a flat bottom hole). Scanning levels are then set at reference level or a bit higher (e.g.20dB) and evaluation is usually required if the maximised signal exceeds reference level.

The writer of the complaint also seems to have missed the point that the concerns raised by the paper are for linear scanning (i.e. where the operator moves the probe parallel to the weld in a single pass. The paper clearly states; "In practice, S-scans are probably more reliable with manual than automated scans, since the operator can scan quickly around the weld as normal and quickly look for defects. "

Manual scanning provides the opportunity for the sweeping Sscan to locate the preferred angle.

But the concern for the "appropriate angle" is not limited to phased arrays. Even manual UT techniques using a single element probe are supposed to be designed with the optimum angle. When this is done it has a greater probability of detecting the lack of fusion flaws it is designed for, since the raster motion ensures the entire bevel is scanned with that same angle. The linear S-scan has only a limited region along the bevel where the angle approaches ideal when used in the linear scan (sometimes referred to as a "line-scn").

But for the writer of the complaint to state:
"The article was declined in different flavors, but always include over-simplistic and false information about weld inspection technique."
is a gross mis-read of the contents of the paper. It seems they do not have knowledge of the expectations of Code-imposed scanning "requirements" and they seem to have little actual weld inspection experience.

Regards
Ed

----------- Start Original Message -----------
: These criticisms of our paper “Construction weld inspection procedures using ultrasonic phased arrays” by Michael Moles and Jinchi Zhang, R/D Tech, were sent to ASNT. The criticisms and reply are below.
: *******************************************************
: Bonjour Mr. XXXXXXX – Mr. YYYYY ZZZZZZZZ told me to communicate with you on this matter.
: I would like to submit a formal complaint about an article that has been published in different medias including ASNT’s. The article was declined in different flavors, but always include over-simplistic and false information about weld inspection technique. In essence, the article says that phased array ultrasound beams HAVE to be perpendicular to the wall (bevel) if one wants to detect lack of side-wall fusion defects. It should have been PROPOSING that perpendicularity provides stronger specular echoes, but still tip diffraction echoes of lack of fusion defect provide sufficient signal-to-noise ratio for most weld inspections.
: You will find the article flavors at:
: http://www.ndt.net/abstract/wcndt2004/83.htm
: http://www.npec.ca/Misc_Docs/Fri%20830am%20Moles.pdf
: http://www.asnt.org/publications/materialseval/solution/jan05solution/jan05sol.htm
: May I recommend you to talk with Mr. Greg Selby (EPRI, gselby@epri.com) and Mr. Jonathan Buttram, Level III (InterWAV, jonathanbuttram@hotmail.com) about this article. They seem to have the same opinion I have. EPRI also qualified a procedure recently which proves to some extent that Mr.Moles is wrong. If you are unsuccessful, I can propose you other names.
: I think the technical revision commitee should have filtered this article or should have at least required a modification in the wording. I think an Errata should be published in the next ASNT Journal. But I also understand sometimes things slip through unfortunately...
: Best regards,
: *****************************************************
: Reply by Michael Moles:
: This article has received many comments and criticisms, invariably from people who didn’t read it properly. This author obviously didn’t read it correctly either, doesn’t appreciate the differences between construction and in-service inspections, and has compounded it with a lack of understanding on tip diffraction. Defining the article as “over simplistic and false” is itself raising the tempo beyond the normal realms of scientific discussions.
: The article itself is primarily based on modeling, which has known limitations. In this instance, modeling is likely to overstate the issue of mis-orientation, but this is acknowledged in the text.
: False is certainly an unnecessary and incorrectstatement; there is no doubt that the response from a defect depends strongly on its orientation relative to the incident ultrasonic beam. This dependence has been known for years and has been the basis of codes such as ASME1 (“appropriate angles should be used”). To quote API, “A (second) limitation in the use of shear wave ultrasonic inspection is the failure to detect large two-dimensional (planar) discontinuities as a result of the inherent direction of the reflected beam.”2 A simple understanding of weld geometry and the position of the array shows that a single S-scan cannot have well oriented beam angles on the weld bevel, root and cap, at all locations. The thicker the component, the worse the problem becomes.
: The modeling shows that location and orientation of the defect also matters, which will be no surprise to any operator. In the modeling, midwall defects were particularly difficult to detect, which mirrors real life pulse-echo ultrasonics. Again, thicker components presented more problemsthan thin components.
: The article does not say that phased array beams (or any ultrasound beams for that matter) have to be perpendicular to the weld bevel; it says that the closer the angle is to the weld bevel incidence normal, the better. It is clear that current codes (e.g. ASME) do not demand normal incidence. How can they? Typically a 45o and/or 60o transducer is used for conventional inspections. With a 30o bevel, incident angles will be normal on the bevel, but root angles will be off. However, if a 37.5o bevel is used, the angles will be off-normal by 7.5o on both bevel and root, yet this is permitted by code.
: Relying on tip back-diffraction alone would be a disaster. The author seems to have little understanding of diffraction, something which Olympus NDT (formerly R/D Tech) has been using for ~15 years. The standard diffraction technique is TOFD (Time-Of-Flight Diffraction Technique), which runs at least 20-40 dB above typical pulse-echo scanning levels because tip diffraction signals are so weak. Even with TOFD, we sometimes have trouble detecting crack tips, especially with noisy material. Using standard pulse-echo codes and looking for tip reflections would result in almost no defect detection; of course, some manufacturers would like that!
: To address the issue of backward tip diffraction mentioned here (i.e. pulse echo), like everybody else we see diffracted signals quite often. They are a very good method of sizing, especially with phased arrays where the full image is available and coupling is a non-issue. However, the tip signals are mostly low amplitude (usually below recording threshold), and are identifiable primarily because of their association with nearby crack signals. Identifying, calling and characterizing tip echoes under normal pulse echo scanning is going to miss a lot of defects.
: The EPRI procedure referred to here is a manual S-scan code for reactor pressure vessels. (The procedure is “EPRI-proprietary” and not available.) However, EPRI states that the code is primarily for fatigue cracking, with the possibility of finding weld defects. (One seriously hopes that nuclear-quality reactor pressure vessels have very few unknown weld defects.) More to the point, fatigue cracks occur at known locations (the surfaces), and typically provide good corner reflectors. Weld defects occur at characteristic locations in the weld, and deterministic orientations, but often without corner reflectors; therefore correct incidence angles are important for detection. Different procedures are required for different types of defects. The work report in January 2005 was specifically for construction welds, as stated in the first word of the title – not for in-service inspections.
: Incidentally, another EPRI-approved procedure for weld inspections from Zetec uses S-scans and linear scanning, but uses three S-scans as we recommend. Though Zetec ultrasonics is an offshoot of R/D Tech, this work was done independently, and provides support for the multi-S-scan approach. (Being nuclear, however, this procedure also requires extensive lateral scanning for mis-oriented defects; this is not normally required in the workmanship-type codes.)
: References:
: 1. American Society of Mechanical Engineers, ASME Section V, Article 4, paragraph T-472.1.1, 2001.
: 2. American Petroleum Institute, “Recommended Practice for Ultrasonic and Magnetic Examination of Offshore Structural Fabrication and Guidelines for Qualification of Technicians”, RP 2X, Third Edition, Sept 1996, p. 7.
: By Michael Moles
: Olympus NDT (formerly R/D Tech)
------------ End Original Message ------------




 
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