In the September issue of Materials Evaluation, the American Society of Nondestructive Testing’s (ASNT) monthly journal, an article was published which highlights the shortcomings of the most widely used ultrasonic testing (UT) measurement technique. In the article “Improving on the 6 dB Drop Technique for Determination of Flaw Length,” the author identifies the shortcomings of the 6 dB Drop Technique and all amplitude-based flaw sizing techniques used in UT. The author Paul Holloway points out that rejectable defects can be significantly undersized because the technique assumes a uniform flaw, but if the flaw varies in size, instances could occur where only the “really bad part” of the flaw is rejected. This is a well-known consequence of amplitude-based flaw sizing methods and as the journal editor states in a forward to the article, “the [inspection] code writers for AWS, ASME, API, ASNT, and so on have examined the process…as of today, the 6 dB technique is the standard of tolerability.”
EWI will use the 6 dB technique when it is required by customers, but in other instances, our preferred method of flaw measurement is a high amplitude time based technique using phased array ultrasonic testing (PAUT) or time of flight diffraction (ToFD). Using this technique and equipment, we are able to differentiate crack tip signals and subtle microstructure changes providing comprehensive data for 2D cross sections of a weld profile.
In addition to using the 6 dB sizing technique, the author also assumes that a manual, conventional UT technique is used. This technique uses a single sound source, a single inspection angle and a scrubbing motion to inspect the entirety of the weld. This conventional approach can inaccurately size a defect for any of the following reasons listed by author Paul Holloway:
- Eyeballing a straight projection out from the front of the wedge
- The width of the marker, chalk, etc. used to mark the flaw location
- Beam shape irregularities
- Non-parallel inspection and reflection surfaces
- Unintentional probe movement while measuring
- Variations in coupling/surface conditions.
When EWI uses PAUT with a semi-automated inspection technique, we can use multiple angles and focal depths in a single pass. We also use a scanner to move the probe and an encoder to track position. This semi-automated approach eliminates any eyeballing, chalk markings and unintentional probe movements. However, non-parallel inspection and reflection surfaces and variations in coupling/surface conditions are still a concern and will always be variables to be aware of when performing contact inspections.
At EWI we combine the latest inspection technologies and novel scanning approaches to reduce errors inherent in any inspection method. If you would like to know how EWI can improve your inspection process or if you would like to learn about our NDE training classes please contact Ruth Sunderman at email@example.com or (614)688-5271.