18:43 Apr-06-2009 Amy NDT Inspector, - - USA, Joined Jan 2009 87
UT-Exit Point Verification
I recently purchased an Aluminum IIW Type 2 Block and 2 Aluminum wedges, 45 and 60 degrees. I was unable to verify the exit point and beam angle was off from the measured point by 3-7 degrees, about 3-5 from the marked point. Another vendor's wedges were compared and they showed similar results. Both vendor's wedges were also checked with another aluminum IIW to be sure that it wasn't the block that was off. One of the manufacturer's has said this is a common problem.
My question is, has anyone else run into this problem?
If so, what caused the variation?
Amy...this situation has indeed been seen before. It is a result of anisotropy of the aluminium. I observed the same effects that you describe when I was doing my Level 2 practical exam over 30 years ago. The exit point seemed to be outside of the probe in my case. It was not until I had discussions with Dr. Fred Hotchkiss (then with Panametrics) that it was explained to me the problems of forming aluminium plate into the well known calibration blocks we use in UT. Not only does the general rolling direction effect the perceived velocities, the grain may also have a change in direction along the sound path due to cooling effects from the ingot.
Although the requirements for steel used in IIW blocks is generally well detailed in the International Standards, I suspect this is not the case for other metals.
Does anyone know of specific metallurgical requirements for aluminium in the fabrication of ultrasonic reference blocks like the IIW styles?
Nick...thanks for the references. I looked them up and could not find reference to IIW. E127 relates to the ALCOA style of blocks and E1158 describes a similar fabrication of blocks with flat bottom holes but as used for setting acceptance criteria. E-428 also addresses fabrication of reference blocks but is again limited to flat bottom hole targets and materials other than aluminium.
I located an old Australian standard (AS-2083-1981) that describes calibration blocks. But it seems specific to steel. That standard requires use of normalised plain carbon steel of grain size No. 5 or smaller. Perhaps a similar set of instructions exists for aluminium IIW blocks.
With the issues that Amy re-discovered I had hoped by now that industry had a method of standardising the material to minimise the effect.
Thanks for the great responses.... Just a few more facts..The IIW is 7075 T6 from a reputable vendor. The block is constructed to USAF T.O. 33B-1-1 ( I couldn't get my hands on a copy so I'm not sure what it is) and certified to multiple other standards. The wedges are supposed to be 45 and 60 shoes for aluminum, from two different reputable vendors. The exit point I'm measuring is, as Ed said almost out of the wedge, and using the "new" mesured exit point the angle is measured at 48 and 64. If I were to use the original marked exit point I get angles of 46 and 58 respectively. I've never run into a case where I can't verify my equipment to a standard block and can't get a real answer on what the standardizaton process is for manufacture and verification of the aluminum blocks and wedges. If I can't verify my equipment, how can I confidently use it.
Michel...your approach is essentially correct. The indicated exitpoint on a "new" wedge is simply the point where the normal to the centre of the element intersects with the test surface. A steel IIW block could indicate the resultant refracted angle for the "new" incident angle that the manufacturer calculated for aluminium and the exit point determination should be just as effectively determined using steel. Working backwards you can then "calculate" the angle that SHOULD occur in the aluminium.
That is a good practical solution. But that defeats the purpose of the reference block as a direct verification tool.
But this situation also indicates that there could be uncertainties in the test piece itself. If velocities are not constant throughout the reference piece they might not be in the tested component either! One of the functions of the determination of exit point and actual refracted angle is to accurately plot flaw locations. If the beam does not follow a straight line this function cannot be accomplished.
I agree with your thinking, but I was on the impression that all Amy wanted to do was to verify the exit point and the angle of the transducer for quality control purposes? My reasoning is that if the blocks have these inherent problems, the stock that we are inspecting will have the same problems as well.
Amy doesn't mention who she is working for except that it is in the aircraft industry. From my experience with Boeing and Airbus the IIW Block is not a widely use calibration block. The aircraft industry has a habit of using a test piece that is identical to what you will be inspecting. They will also write the procedure detailing all that you have to do. Exemple: Position the 0.020" EDM Nothc at 6 on the time base. Increase your gain to obtain a 80% FSH and set your gate at 40% FSH from 4 to 8 on your time base. Anything that break the gate between 4.5 and 7.5 is rejectable. Simple as that.