The Original PDF Version (290K) can be downloaded from the QNet Homepage.
Figure 1: Raster Scanning
This report presents an ultrasonic technique using guided plate waves propagating over large distances. In addition, the use of Electro-Magnetic Acoustical Transducers (EMAT) allows the technique to be applied on unprepared surfaces, if the scale on the coupling surface is no thicker than 1-2 mm.
|The detection of corrosion can be performed using guided and horizontally polarized shear wave modes. As depicted in Figure 2, this wave mode is transmitted and received without the need of a coupling medium when using EMAT search units generating SH-waves. Due to the guiding effect of the plate's inner and outer surface, SH-waves can propagate over distances of up to 1 m in plates having a thickness of several mm. Corroded areas on either plate surface are partially reflected due to local wall thickness variations. Instead of scanning the entire area with scanning offsets of 10 mm (or less) using straight beam search units, the guided wave technique requires only linear scanning of sections as long as 500 mm with a search unit offset of 20-30 mm, depending on the lateral sound field properties of the selected search unit. The economic benefits of EMAT are obvious.|
Figure 2: Linear Scanning
When corrosion has been detected and localized by evaluating the pulse arrival time, a precise measurement of the wall thickness can be performed with an EMAT search unit. The physical principle of the reflection of SH-wave modes for local wall thickness variations is explained in Figure 3. The "dispersion diagram" displays the sound velocity of the SH-waves as a function of the product of frequency f and plate thickness d. The different curves represent the dependency of the velocity of different SH-wave modes as a function of
The lowest mode (n=0) is of constant velocity independent from frequency and wall thick-ness and equal to the sound velocity of a bulk shear wave. The velocities of the other SH-wave modes (n>1) are dependent on the selected frequency and wall thickness of the component to be inspected. This means, that for a fixed frequency the n>1 SH-wave modes are reflected by local wall thickness reductions due to a change of the acoustical impedance caused by a local change of the sound velocity in accordance with the dispersion diagram. Other reflectors, such as pitting, cracking, etc., usually cause specular sound reflection or also sound diffraction.
Figure 4 shows a search unit optimized for SH-modes 1 and 2, with a coil periodicity of 8 mm used for plate thickness from 4 to 10 mm. A small electromagnet provides the necessary magnetic induction. Field inspections with this search unit type have been successfully performed.
Special electronic equipment was developed to operate the search units. For the validation of equipment and technology, field trials on actual tank bottoms have been performed.
30% through-wall ( t = 6mm)|
Figure 5: Wastage Detection
SH-wave search units with a coil periodicity of
8 mm were used for the above-mentioned
thickness range. Investigation of the
detectability of simulated wastage
demonstrated highest sensitivities for modes 1
and 2 as depicted in figures 5 through 7.
Figure 5 displays an A-scan (mode 1) and the test specimen with two corroded areas with diameters of about 100 mm, one on the inspection surface (OD), the other on the opposite surface (ID). In both cases the remaining ligament is 70%.
|The A-scan in Figure 6 shows the signal of wastage on the search unit surface (OD); the flaw on the opposite surface is shadowed by the first reflector and is therefore not detected. This flaw was detected in mode 2 with a signal-to-noise ratio of 26 dB as shown in Figure 6, the surface distance between search unit and reflector is 250 mm.||SH-Mode SS1|
Figure 6: Wastage Mode 2 (SS1)
Figure 7: Wastage Mode 1 (AS1)
|When flaws are smaller in diameter then the lateral width of the effective sound beam, several flaws located behind each other can be detected. As depicted in Figure 7, two flaws (dia 70 mm) on the near surface and on the far surface with 50 % remaining ligament and located behind each other at a distance of 30 mm, were detected with signal-to-noise ratios of 25 dB and 16 dB. Even at a surface dis-tance of 1 m, the reflector on the near surface was detected with a signal-to-noise ratio of about 20 dB.|
Figure 8: Pitting, Mode 1 (AS1)
|On another sample, pitting-type corrosion was simulated using 10mm diameter and 50% through-wall side-drilled holes (SDH). Three holes were located behind each other, spaced at a distance of 50 mm. The first and third SDH are located on the near surface, the centered SDH is located on the far (back) surface of the specimen. All three reflectors are separately detected (Figure 8) with signal-to-noise ratios of 18 dB and 11 dB respectively.|
|The last example (Figure 9) shows the signal of a simulated crack-type flaw (EDM notch) with a depth of 1 mm(10% TW) and a length of 20 mm in a 10 mm thick plate. The flaw was detected at a surface distance of 190 mm. The additional signals are reflections from the end (corner) of the specimen. For this larger thickness (10mm) mode 2 (SS1) was used.||
10% through-wall ( t = 10mm)|
Figure 9: Cracking Mode 2 (SS1)
|Flaw Type||S/N Ratio||Surface Distance|
Remaining Ligament 50-70%
|25 to 30 dB||200 to 500 mm|
|Pitting : |
Diameter 10 mm
|> 15 dB||200 to 300 mm|
Remaining Ligament 10-50%
Diameter 20 mm
|> 15 dB||200 to 300 mm|
|Table 1: Sensitivity and Surface Distance of SH-wave modes for the detection of corrosion.|
|| UTonline | |Top ||