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.
- API, API RP 2X: Recommended Practice for Ultrasonic and Magnetic Examination of Offshore Structural Fabrication and Guidelines for Qualification of Technicians, American Petroleum Institute, Washington, D.C., 2004.
- AWS, AWS D1.1: Structural Welding Code – Steel, American Welding Society, Miami, Florida, 2015.
- Birring, A.S., “Sizing Discontinuities by Ultrasonics,” Materials Evaluation, Vol. 68, No. 11, 2010, pp. 1208–1215.
- Ciorau, P., J. Coulas, and T. Armitt, “A Contribution to Length Sizing of Weld Flaws Using Conventional, PAUT and TOFD Ultrasonic Techniques,” NDT.net, No. 11, 2011.
- CSA, CSA W59, Welded Steel Construction (Metal Arc Welding), CSA Group, Mississauga, Ontario, Canada, 2013.
- DNV, DNVGL-CG-0051: Non-destructive Testing, Class Guideline, DNV GL, Baerum, Norway, December 2015.
- Drury, J.C., Ultrasonic Flaw Detection for Technicians, Dinefwr Press, Llandybïe, Wales, 1992.
- Gardner, W.E. (ed.), Improving the Effectiveness and Reliability of Non-Destructive Testing, first edition, Northern Research Laboratories, United Kingdom Atomic Energy Authority, Pergamon Press, Ltd., Oxford, England, 1992.
- Halmshaw, R., Introduction to the Non-Destructive Testing of Welded Joints, second edition, Abington Publishing, Cambridge, England, 2006.
- Houf, J. “Practical Contact Ultrasonics – Angle Beam Scan Patterns and Defect Location,” The NDT Technician, Vol. 4, No. 1, 2005, pp. 4–6.
- Mathers, G., “Ultrasonic Examination Part 2: Job Knowledge 128,” www.twi-global.com/technical-knowledge/job-knowledge/ultrasonic-examination-part-2-128, accessed January 2016.
- Murphy, R.V., “Ultrasonic Defect-sizing using Decibel Drop Methods,” Vols. 1–2, project No. 85.1.9, Atomic Energy Control Board Ottawa, Ontario, Canada, 1987.
607 Page Views
0 PDF Downloads
0 Facebook Shares