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Technical Papers
Terrance R Banach; PE, ASNT UT Level III
The Major Challenges of Inspecting Small Diameter UltraThin Wall Tubing with the Ultrasonic Testing Method
Abstract: For the purposes of this paper and presentation, small diame- ter, ultra-thin wall tubing will be considered as diameters less than 0.5 inch (12 mm) with wall thicknesses less than 40/1000 inch (0.5 mm). Ultrasonic inspection of this category of tubing requires very stable and precise mechanics and higher than usual ultrasonic frequencies, espe- cially to evaluate wall thickness. This paper discusses very important points to look at as you decide on the appropriate ultrasonic system supplier. Some currently available electronic techniques will also be presented as valuable system assets to exist in the systems offered by the potential suppliers. The presented electronic techniques have been evaluated, as well as applied to dif cult wall thickness tubular products
Part 2 Review
In Part 2 of this article, we discussed the second most critical element in the ultrasonic tube inspection system, the transducer. After insuring that the tube is centered with respect to the transducers inside the rotary system, we stated that the transducer beam parameters must be optimized to provide acceptable results during the tube inspection. To provide this insurance, we established that we needed to gener- ate higher transducer operating frequencies to measure such ultra-thin walls. The shorter wavelengths of these high frequencies will allow us to more easily distinguish the very short wall thickness distances (10 to 40 mils) found in ultra-thin wall tubing.
Transducer Parameters Requiring Improvement
Using a transducer frequency with a wavelength that is less than 3/4 of the wall thickness, the less the better, helps the instrumentation dis- criminate the short distances between the wall echo multiples. Refer- ring to Figure 1., the upper sine wave burst has a wavelength equal to the wall thickness. This wavelength makes it dif cult to discriminate the frequency from the thickness echoes. Whereas, at the bottom of the same gure, the shorter wavelength, higher frequency, single cycle sine burst pulse easily shows separation of the back wall echo from the front wall echo. This separation simpli es discrimination of the front and back surface echoes enabling the electronic system to make the required precise wall thickness measurements.
In addition, using highly damped, broad band transducers that produce single cycle transmission pulses further assists with better wall echo discrimination. The improved discrimination decreases changeover times, providing a major source of cost reduction.
Broad band transducers whose center resonant frequencies give this desired wavelength must be used. As thinner wall measurements are encountered, higher transducer frequencies are required. Frequently,
Fig. 1
ITAtube Journal No2/July 2018
42