Volume 6, Number 1
January 2007
Q. For Ultrasonic Testing, much of the work done to ASME, API and other similar codes and specifications is done using a 2.25 MHz transducer. What is the logic behind using this specific frequency as opposed to a 2 or 3 MHz transducer? P.K., Canada A. Most of the standards you have referenced do not specifically require a 2.25 MHz transducer but usually give a range of frequencies that can be used. The ASTM Standard E 164, Standard Practice for Ultrasonic Contact Examination of Weldments, states that frequencies in the range from 1.0 5.0 MHz are generally used but does not mandate a specific frequency within that range. Article 5 of Section V of the 2004 ASME Boiler & Pressure Vessel Code requires a frequency in the same range, 1.0-5.0 MHz, but leaves the actual frequency up to the user. In the 2002 AWS D1.1 Structural Welding Code — Steel, paragraph 6.22.7.1 requires a frequency of 2.0 2.5 MHz, and also states that this applies only to ferrous and aluminum materials with thicknesses of 5/16 in. (8 mm) or greater. When discussing this we need to keep in mind the relationship between codes, standards and NDT procedures. Codes and standards are generally written to be applied to a wide range of product forms and materials and as a result often give a range of values for certain requirements (as shown above). A company NDT procedure is written to address specific applications within the range of such standards so the Level III must take into account the specific requirements and limitations of the task in question. These include the differences in sound velocity and grain size in the material to be inspected and transducer penetration ability, resolution, wedge angle and near field length. Since 2.25 MHz transducers are widely available and fall near the midpoint of most required frequency ranges, it does seem to be the frequency of choice for many applications. Q. Eddy current testing is sometimes called electromagnetic testing. Is there a difference between the two terms? P.M., Columbus, Ohio A. The Electromagnetic Committee of ASNT’s Technical and Education Council has officially recognized four electromagnetic test techniques for purposes of personnel qualification: eddy current testing, remote field testing, magnetic flux leakage testing, and alternating current field measurement. All four share the same physical principles but differ according to equipment and applications. Each of the four gets a chapter of its own in the NDT Handbook, third edition: Vol. 5, Electromagnetic Testing. The next edition of ASNT Recommended Practice No. SNT-TC-1A will make these four techniques official, and training aids for each may follow. Magnetic particle testing is also an electromagnetic technique but over time has established testing regimens and a body of knowledge all its own. Here too, the classification of methods is based on industry practices as much as on their science. TNT
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