THE SHORTCOMINGS OF THE 6DB DROP METHOD FOR ULTRASONIC FLAW LENGTH SIZING

By: Publisher Team | Nov 28, 2020 16:34 PM
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.

Source: The shortcomings of the 6dB drop method for ultrasonic flaw length sizing — Holloway NDT & Engineering

 



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