The shortcomings of the 6dB drop method for ultrasonic flaw length sizing
In manual ultrasonic inspection, the most commonly understood practice for length sizing discontinuities larger than the beam width is the 6 dB drop method. However, peaking the signal then moving laterally to 50% signal height can often under size the length. The result is repairing just the middle "really bad part" of a flaw, leaving two smaller discontinuities at the ends.
The 6 dB drop method is best suited for ideal reflectors like side drilled holes in reference blocks and tends to fall apart when applied to real,
irregularly shaped discontinuities. Unfortunately, the 6 dB drop method is often followed with blind faith regardless of discontinuity type, without an understanding of it's limitations or acceptance criteria limits below "the peak" area. As such, discontinuities are easily undersized, resulting in multiple repairs, delays, and the question "how did we miss that the first time?". Not good for the optics of ultrasonic inspections.
As an example, consider a flaw (above) in a 3/4" thick butt weld (non-tubular, cyclic service) with a "d" rating of -8 dB. This corresponds to a discontinuity of Class A, which for a 3/4" weld has a "d' limit of +5 dB and lower.
To find the extremities, the probe would be moved laterally until the "d" rating was reduced not 6 dB below the peak of -8 dB, but to 6 dB below the Class A limit of +5 dB (which would be +5 dB minus 6 dB = 11 dB). If to you, +5 minus 6 seems like it should be negative 1, keep in mind that we're in the wonderful world of attenuation calculations where everything is backwards (i.e. "low score wins"!). A rating of +11 dB is well outside the boundaries for all possible classes and ensures that the entire flaw is captured.
Interestingly, the AWS code accounts (somewhat) for the possibility of under sizing flaws by requiring the 6 dB to be subtracted not from the peak signal, but from the "rating for the applicable discontinuity classification" (see 6.29.2 in the 2015 edition of AWS D1.1). However, even this enhanced method fails when the span between the maximum and minimum classes exceeds 6 dB. Take for instance the same flaw in a 2-1/2" thick butt weld. At 70 degrees, there is an 8 dB span between Class A and Class D. Subtracting 6 dB from the peak level of a Class A may leave some flaw remaining at the extremities if it exceeds the allowable length for the discontinuity class. Again, all you did was cut the middle out and make one big discontinuity into two little ones.
I teach technicians to recognize the limitations of the method and recommend chasing the flaw extremities all the way out to a Class D. The only exception would be if leaving some small, allowable length of a Class B or Class C discontinuity is necessary due to access limitations for repair or similar.
Technical Manager at Libertad Communications.
4 年Thanks Paul - well laid out - sorry I didn't see this before - totally agree.
ASNT Level III ET, MT, PTLevel II in UT. OSHA 30 General Industry
6 年In AWS D1.1 and D1.8, the 6dB drop is used to determine whether an indication is rejectable or not. Some steel companies only want that length marked for removal taking the chance the rest of the defect will not grow to rejectable dimensions after the heat stress of repairing the area. In my experience, they loose a lot of these battles. However, chasing the defect until it completely disappears ultrasonically can quadruple the linear foot of weld you need to remove.
Instructor at SAIT Polytechnic
6 年In the example above the so called "max amplitude " as described by other posters (sub peaks as you move to edges are considered new maximums as the defect is traced to the edge) procedure will get you much closer, to the real size of the defect without having to resort to TOFD or other sizing methods and equipment. Still not ideal, but better than the simplest version of the 6 dB drop. Another point of clarification on the limits of the 6 dB drop is that it over-sizes reflectors smaller than the sound beam...even ideal ones. In any case the 6 dB drop, or any other so called sizing technique (with the possible exception of TOFD) used in ultrasonics, is not so much about the actual physical size of the defects, rather the severity. A good explanation with regards to sizing was written by Ed Ginzel ..."In many cases the "sizing" we do in UT is not so much sizing as an evaluation tool whereby we estimate if the flaw is critical or not and record it because the Code or Specification has put a limit on amplitude for a flaw. "
Great explanation. It's important also to check aws comments. Item C6.25.7 states some requirements to avoid this kind of undersizing.
PAUT specialist.
6 年Great read Paul, thanks for sharing! This is an issue I've delt with quite a bit, flaws found in welds are typically non-symmetrical, you can really gauge the experience of an inspector if they can understand the inconsistency of this flaw sizing technique. I try to teach my students to “trace” the flaw beyond the 6dB drop for good measure. In addition to asking the welder to clean out the entire flaw when found.