Improving on the 6 dB Drop
Technique for Determination
of Flaw Length
Publication: Publication Date: 1 September 2016Testing Method:
This paper details how applying a 6 dB drop from
only the signal peak may fail to correctly identify a
flaw’s extent and also discusses some of the relevant
variables and limitations of the UT method. The
concluding section presents an improved approach to
length sizing to assist practicing technicians in accurately
sizing flaws and avoiding subsequent repeated
attempts at repairs. This improved approach is not
new, and in actuality is the basic intent of many
welding inspection codes. However, it is explained
here in detail and is contrasted with the way the 6 dB
drop technique is traditionally taught to highlight the
fundamental errors incurred when oversimplifying the
technique. The presumption is not that length sizing is
always performed incorrectly, but that a significant
proportion of practicing technicians are instructed to
use a simplified approach that will likely produce
undersized estimates of flaw lengths.
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