21:03 Sep-11-2009 Manfred Richter Consultant, Richter & Tomasi Ltd., Brazil, Joined Jan 2008 29
AWS D1.1 versus Phased Array
Are the Acceptance Criteria of the table 6.2 and 6.3 ( statically and ciclically loaded) applicable to an inspection performed by PA system using sectorial or linear scan? The PA probes normally don't attempt to the characteristics established by AWS D1.1;
Thanks for any reply.
00:52 Sep-12-2009 Ed Ginzel R & D, - Materials Research Institute, Canada, Joined Nov 1998 1208
Re: AWS D1.1 versus Phased ArrayIn Reply to Manfred Richter at 21:03 Sep-11-2009 (Opening).
As I read the AWS D1.1, the rules are written for manual UT using the equipment and procedures described in 6.20 and 6.22. Clearly these rules belong to the old traditional methods using monoelement probes and the many restrictions AWS has maintained over the past 4-5 decades. The acceptance criteria methods it uses are similarly based on the old conservative workmanship standards.
For some time now, in paragraph 6.20.2, provision has existed to deviate from these old and restrictive methods. 6.20.2 states "Variations in testing procedure, equipment and acceptance standards not included in Part F of Section 6 may be used with the approval of the Engineer."
Using phased-array techniques is already a deviation from Part F. The Equipment does not conform to the AWS "standard" expectations for manual equipment so the Engineer needs to approve its use. The next issue comes with how to anayse data. Phased-array data is not "initially" evaluated like the old manual UT using A-scans. Instead, a colour display is used from which the operator will identify areas of concern. In order that it be treated with consistancy for the range of inspection there SHOULD be a TCG used to assure the operator uses the same colour identification method for all flaws of concern. This means that (at least for the initial assessment) the AWS Rating method is not practical. The guidance in Appendix S COULD apply. Table S1 is "recommended" acceptance criteria for the methods that would use TCG...but the entire Appendix S is not considered part of AWS D1.1 and is provided only for information. Again, the table S1 is based on conservative workmanship criteria. Even so, its use would requrie Engineer approval.
Alternatively, a full engineering critical assessment could be run and other acceptance criteria submitted to the Engineer for approval.
It has been my experience however, that many engineers do not want to do the engineering. This requirement for the Engineer to "approve" all deviations from the old methods is perhaps the greatest impediment to advanced UT being used on AWS work. (IMHO)
Perhaps a way around the the problem of trying to asses indications to the criteria specified in AWS D1.1 while at the same time performing a Phased Array examination as may be allowed in Clause 6.20.2, would be to conduct a Phased Array examination to an ASME like procedure and to measure the length based upon the PA data, but to manually rate the defects using a standard AWS D1.1 rating procedure. Final defect assessment would be based on data from both methods, and when the location of the defects to be rated are known, the actual time spent rating them would not be that much extra.
As usual, Mr. Ginzel has cut to the heart of the problem. One possible suggestion, though:
You can point out to the structural Engineer that manual UT per D1.1 compares flaws to a 0.060 inch calibration hole, and that you can perform your PA calibration using the same 0.060" holes to build your TCG curve. This will give better correlation of deeper flaws to the calibration standard, instead of "add 2 dB for each inch of soundpath". A multipoint calibration with at least one point closer and one point farther away than the 'area of interest' will always be a more accurate calibration than a single-point calibration.
Further, if you show the Engineer the color pictures that a C-scan scope like the Omniscan can capture, you should have a customer for life. Personally, I have gotten tired of the scepticism I get at the end of a manual examination exam. I have nothing to show the Engineer but a chart listing indications, locations, and ratings. Some believe me, some remain sceptical until they get reports back from their welder that the flaws really were where I said they were, and what I said they were. "One picture vs 100 words"
The picture and data file is especially valuable to prove later that the indications judged as 'Acceptable" were indeed within the Code standards. On Bridge work especially, there is a lot of resistance to accepting any known defects, even when well within the Code limits.
I do agree with you, but it is not only in the bridge industry that acceptable indication are rejected by the client. We have the same situation with some of our clients. After seeing a few accidents happening 40 - 50 years after construction, can we blame the engineers?