Volume 7, Number 2
April 2008
Q. Reader response to “Inbox” (Vol. 6, No. 2; Vol. 7, No. 1) SNT-TC-1A 2001, paragraph 8.2.2 (Color Contrast Differentiation) queries. A. I’ve noticed a few inquiries lately about Section 8.2.2 of SNT-TC-1A 2001 in the “Inbox”. Having run into this particular issue myself a couple of years ago during an audit, I have this to offer. Section 8.2.2 states in part “... distinguishing and differentiating contrast among colors or shades of gray used in the method... .” In MT and PT inspections, this usually means color vision capable of determining red powder and/or red dye. In radiography this would be shades of gray. The Ishihara color vision test is one of the accepted and standard tests used to meet the needs and requirements for color vision tests. A test standard does exist for gray scale — the Pelli-Robinson test. Called my ophthalmologist and he referred me to a supplier. S.C., Bakersfield, CA Q. Is there an ASNT standard for the visual inspection? M.M., Derby, UK A. ASNT only publishes standards regarding the certification of NDT personnel, so the answer to your questions is no, we don't have a standard for visual inspection. The AWS welding codes and ASME Section XI do discuss VT requirements within those documents. Q. Readers respond to “Inbox” yoke dragging query (Vol. 7, No. 1). A. The “Inbox” question in The NDT Technician for dragging of a yoke during inspection should be clarified further. This answer leads others to believe that this is allowable. It is not listed in the code or standard because it is not acceptable. If it were, there would be a travel speed for dragging. To drag a yoke, the legs would not be fully in contact with the part, which would lower the lines of flux. I believe this would lead to smaller or subsurface flaws being missed. D.H., Anchorage, AK A. A recent question in the TNT “Inbox” pertained to magnetic particle yoke inspection. While the answer is accurate, it occurred to me that there may be another aspect that should also be addressed. While I’ve never seen this dragging done during an inspection, I would presume that, in order to drag the yoke, it needs to be tilted away from perpendicular. There are a few technical papers that have looked at the coupling of a yoke as the angle is altered from ideal. As the yoke is tilted, there is less area in physical contact with the specimen. Authors have theorized that tilting decreases the inductance, increases the coil (within yoke handle) current, and decreases the amount of magnetic flux induced in the specimen. It is likely that a procedure qualified with perfect coupling would see a decrease in sensitivity when applied with an altered dragged yoke method, although it’s possible that this person’s application is seeking only very gross defects. Several articles that mention this tilting idea are below. Rick Lopez; Center for Nondestructive Evaluation, Iowa State University; Ames, IA
Walther, K.G., R.G. Walther and A.M. Watt. “Computer Monitoring of Coupling in Magnetic Particle Testing.” Materials Evaluation, Vol. 50, No. 10. Columbus, OH: American Society for Nondestructive Testing (1992): pp. 1237–1239, 1245.
Walther, K.G., R.G. Walther and A.M. Watt. “Need for Coupling Quality in Magnetic Particle Testing.” Materials Evaluation, Vol. 50, No. 10. Columbus, OH: American Society for Nondestructive Testing (1992): pp. 1210, 1212-1216.
“NDT Fundamentals, Magnetic Particle Testing.” [Part 3, The Magnetic Flow Method.], INSIGHT: Non-Destructive Testing and Condition Monitoring. Vol. 40, No. 3. Northampton, UK: British Institute of Non-Destructive Testing. (March 1998): p 220-221.
TNT
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hhumphries@asnt.org
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