|NDT.net - December 1999, Vol. 4 No. 12|
|TABLE OF CONTENTS|
Compared to the frequencies of 1 MHz and higher used in most contact ultrasonic applications, the 400 kHz of the air-coupled technique may be considered relatively low. Nevertheless, a lateral resolution of about 0.040 in. (1 mm) is achieved, due to the focusing effect of the air-coupled transducers. Such resolution has proven more than adequate for virtually all applications.
On the other hand, the aircraft industry is using more and more highly attenuative materials, such as foam sandwich structures and honeycombs. It is often impossible to penetrate most of these materials using frequencies of 400 kHz and higher. This has brought new challenges to the testing instrumentation. The main losses of foam material are very likely caused by beam scattering. Such losses are known to depend strongly on the frequency and increase with the fourth power of the frequency. It can therefore be expected that by using ultrasonic frequencies which are even lower than 400 kHz, it might be possible to penetrate through foam structures.
This paper demonstrates first results using an air-coupled ultrasonic testing technique at 50 kHz. C-scan images and lateral resolutions are compared to the 400 kHz and water coupled techniques. The 400 kHz air-coupled instrument  was modified to adapt to commercially available range finder transducers at 50 kHz for C-scanning. The benefits of using C-scans at two different wavelengths are demonstrated on porous materials. C-scans of foam blocks and of a foam sandwich are also shown, demonstrating the capability of the 50 kHz low frequency air coupled technique.
|Fig 1a: 50 kHz transducers in through-transmission||Fig 1b: 50 kHz transducers in 'displaced' through-transmission|
Various configurations of the 50 kHz transducers were investigated. In the through-transmission configuration, both transducers were mounted to produce a beam perpendicular to the surface and at a distance of 21/2 in. (Figure 1a.). In a quasi "focused" configuration the transducers in the through-transmission configuration were displaced laterally, so that that the ultrasonic beam was only marginally intersected by the receiver (Figure 1b.). For one-sided plate wave applications the transducers were placed at a distance of about 1/2 in. from the surface, separated from each other by a distance of 8 in., and produced a beam with an angle of 10 degrees from the vertical to the surface (Figure 2).
|Fig 2: Alignment of the 50 kHz transducers for one-sided plate wave inspection|
The first scan was used to establish a baseline and was performed using water squirters and 21/4 MHz transducers. The resulting C-scan image is shown in figure 3. The smallest inserts, measuring 1/16 in. and even slightly less, are still resolved in column 2, 3, and 4 of row 4.
Figure 4 shows a C-scan image performed with the resonant air coupled and focused transducers at 400 kHz in through transmission. Please note that despite the lower contrast and the lower frequency, the smallest Teflon inserts in row 4 can still be detected.
The C-scan image shown in figure 5 was performed with the 50 kHz transducers in through transmission, aligned as shown in figure 1a. As can be expected, due to the large diameter of 1.5 in. of these transducers and to the low frequency, the lateral resolution is significanly less than in the previous scan. The inserts of 1/2 in diameter in the top row are at the limit of resolution. The largest insert of 3/4 in diameter is clearly resolved, which is still only half the transducer diameter.
|Fig 3: C-scan image with water squirters at 2.25 MHz. Teflon inserts measuring 1/16 in to 3/4 in at 4 in. distances.||Fig 4: C-Scan image performed with 400 kHz air-coupled transducers. The smallest resolved Teflon inserts in the bottom row measure 1/16 in. in diameter.||Fig 5: C-Scan image performed with 50 kHz transducers in straight through-transmission. Teflon inserts with diameters less than 1/2 in. cannot be resolved.||Fig 6: C-Scan image performed with 50 kHz transducers, in laterally displaced, in through-transmission.Teflon inserts with diameters less than 3/8 in. cannot be resolved.|
For the C-scan of figure 6, the 50 kHz transducers were laterally displaced as shown in figure 1b, where the receiver captures only a small part of the transmitted ultrasonic beam. The result is improved resolution and higher contrast: 3/8 in. inserts in row 3 can be resolved.
Two carbon-carbon silicon carbide coated (SiC) panels having a thickness of 1/8 in., which were fabricated during an R&D stage, were tested ultrasonically with air-coupled transducers at two different frequencies. The C-scans were performed at a speed of 6 in./sec, with a step size of 0.030 in.
Figure 7 shows two C-scans from a 6 in by 8 in section of a Silicon carbide coated panel. The C-scan of figure 7a was produced using the 400 kHz focused transducers, while the C-scan of figure 7b was produced with the 50 kHz eletrostatic unfocused transducers.
Note the dark blue area in the center of figure 7a. Such an area could be indicative of intermittant voids. The same area, however, appears much lighter and green in figure 7b. It follows from the applied color code, that the dark blue area of figure 7a has a transmission of about 10% of the maximum amplitude (white spots), while the same area in figure 7b has a transmission amplitude of about 25% of the maximum transmitted amplitude (dark red spots). Such a frequency dependant transmssion is indicative of an increased porosity rather than of intermittant voids. The increased porosity was subsequently confirmed through destructive testing.
|Fig 7a: C-Scan image of a 6 in x 8 in. section of a SiC panel scanned with 400kHz air-coupled transducers.||Fig 7b: C-Scan image of a 6 in x 8-in. section of a SiC panel scanned with 50kHz air-coupled transducers.||Fig 8a: C-Scan image of a 6 in x 12 in. section of a SiC panel scanned with 400kHz air-coupled transducers.||Fig 8b: C-Scan image of a 6 in x 12 in. section of a SiC panel scanned with 50kHz air-coupled transducers.|
Another development panel, measuring 6 in. by 12 in., was also scanned twice, first at 400 kHz, and then at 50 kHz. The C-scans are shown in figure 8.
Note the similarities and the discrepencies between the above C-scans. The dark blue areas, which both scans have in common (transmission not frequency dependant), are interpreted as delaminations. The red areas in figure 8b, which correspond to dark blue spots in figure 8a (transmission frequency depandant) are interpreted as areas of increased porosity.
A rectangular block of foam, having a density of 75 kg/m3, and measuring x = 13 in. in length, with width and height y = z = 3.8 in. , was scanned with the 50 kHz electrostatic transducers (see figure 1a). The scans were performed in the two planes of the surfaces x-z, and x-y, which are perpendicular to each other. Both scans were performed with step sizes of 0.020 in. and at a speed of 6 in./sec.
Figure 9 shows the two scans in the two planes which are perpendicular to each other. The dark areas along the borders of the part are due to the mechanical support structure and have no further meaning. The dark line in the upper part of figure 9a is caused by a visible line of a delamination, laying in the x-y plane, and which is seen from the top. This same delamination is seen from the side in the scan in the x-y plane of figure 9b as a large dark area.
|Fig 9a: C-Scan of a 13 in. x 3.8 in. foam block at 50 kHz. Shown is the scan in the x-z. Note the dark line on top (arrows) and adjacent to the bright, white-red central line.|
|Fig 9b: C-Scan of a 13 in. x 3.8 in. foam block at 50 kHz. Shown is the scan in the x-y. Note the large dark area in the upper part of the scan.|
A second foam block of the same material, measuring 13 in. x 2.5 in. x 4.75 in. (x.y.z) was scanned in the x-z plane. The C-scan was performed with the 50 kHz electrostatic transducers in through-transmission ( see figure 1a), at a speed of 6 in./sec and with a step size of 0.020 in. and is shown in figure 9. Note the homogeneous through-transmitted amplitude. Again, the dark borders are due to the mechanical supporting structure.
|Fig 10: C-Scan of a 13 in. x 2.5 in. x 4.75 in. foam block at 50 kHz. Shown is the scan along the larger surface x-z. Note the homogeneous through-transmission amplitude.|
Testing from one side
A foam sandwich structure, consisting of a 2 in. thick, very low density foam layer bonded to an aluminum plate, was C-scanned with the 50 kHz transducers mounted on the foam side according to figure 1b. This configuration, often referred to as the "plate wave" configuration, is generating a plate wave in the aluminum plate after having penetrated the foam layer. This plate wave is received after traversing the foam layer a second time.
The C-scan of this one-sided inspection is shown in figure 11. The transducers were aligned along the long image side, which was the scan axis. The scanned area measures 12 in. x 6 in. and was scanned with a step size of 0.040 in., at a speed of 6 in./sec. The dark area is indicative of a nonbond.
|Fig 11: C-Scan performed from one side (in a plate wave configuration) of a 2 in. thick foam layer bonded to an aluminum substrate, using 50kHz air-coupled transducers.|
With the increasing application of foam and other very highly attenuative materials in the aerospace and infrastructure industries, the demand for a nondestructive testing method for these materials has grown dramatically. It is usually very hard or practically impossible to penetrate such materials with 400 kHz ultrasound. The 50 kHz electrostatic transducers provide a solution. It was shown that the lateral resolution of about 1/2 in. is well accepted by the manufacturers, and that one-sided testing of sandwich structures is an effective testing method. It was further demonstrated, that using both frequencies, increased porosity could be distinguished from delaminations and intermittant voids. Performing C-scans at 400 kHz as well as at 50 kHz provides a powerful tool in the evaluation of indications.
Further investigations about the application of air coupled ultrasound at different frequencies are in progress.
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