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Improving on the 6 dB Drop Technique for Determination of Flaw Length

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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