Acoustic emission leak testing of pipes for pressurized gas using active fiber composite elements as sensors.
Leak testing can be performed with various non-destructive test methods, among them acoustic emission (AE), on both, pressure vessels [1-4] and pipes or pipelines . Leak testing with AE can be performed to indicate the presence of leaks and/or to locate the leak position, as well . The references indicate that AE leak testing can be performed on pressure vessels made from metal [2, 3] or composites , containing either gaseous  or liquid media .
Based on a model experiment for detecting the presence of a leak in a pipe segment pressurized with fluid (water) and gaseous media (compressed air) using conventional AE sensors , the present contribution investigates the feasibility of performing leak testing with piezoelectric active-fiber-composite (AFC) elements instead of AE sensors. AFC are made with commercially available piezoelectric fibers that are embedded in an epoxy matrix and sandwiched between interdigitated electrodes. Originally developed as anisotropic piezoelectric actuators , AFC elements can also be used as piezoelectric sensors. The tests are monitored with the same AE sensors as in the previous experiment  for comparison. AFC sensors are thin, planar elements and offer an advantage compared with conventional AE sensors since they are conformable to curved surfaces . Mounting AFC sensors on pipes or pressure vessels does hence not require the use of waveguides, simultaneously reducing time required for mounting and attenuation.
The model pipe system for the laboratory tests was similar to that reported in , i.e. using an aluminum pipe segment, but with a length of 60 cm and a diameter of 50 mm, with a wall thickness of 2 mm (Fig. 1). Leaks were simulated by inserting screws with a hole into a borehole in the wall of the pipe segment, located at about one quarter length (Fig. 2). The diameters of the holes in the screws comprised about 0.1, 0.2, 0.35, 0.50, 0.80 and 1.2 mm (Fig. 3). The holes were manufactured by conventional drilling, except for diameters of 0.1 and 0.2 mm, which were made by laser drilling. For the screws with hole diameters below 0.5 mm, the nominal diameter extended about 1 mm into the screw, opening into a hole with a larger diameter (1mm diameter for the 0.1-and 0.2-mm leaks, 2 mm for the other leaks). The pipe segment was then equipped with fittings connected to the supply of the medium (pressurized air). Pressure in the pipe segment was read off a pressure gauge and noted for each test. The flow of the medium was not recorded and the pressure adjusted by the outlet valve mounted between pressure gauge and supply pipe. The pipe segment was put on wooden supports for the tests.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
AE data have been recorded with three channels using a system with transient recording capability (type AMS-3 from Vallen Systeme GmbH). One channel was used with the AFC sensor, the others with conventional AE sensors (type SE45-H from Dunegan Engineering Corp). Thresholds were set at 50 dBAE, preamplifier gain at 34 dB, the rearm time to 3.2 ms, and frequency band-pass filtering between 30 and 1,000 kHz was used in the preamplifiers and the data acquisition channels. Beside the AE signal parameter set, AE waveforms were recorded (5 MHz sampling rate) with the same threshold. AE data were analyzed with the equipment specific software (VisualAE and VisualTR from Vallen Systeme GmbH). The AFC sensor was mounted directly on the surface of the pipe using a commercial, thermosetting two-component epoxy-based adhesive, while the AE sensors were mounted on waveguides using a silicone-free vacuum grease as coupling agent and duct tape for applying the contact pressure. The AFC sensor was electrically shielded by a self-adhesive aluminum foil, simultaneously connected to its ground wire and the electrical ground of the AE equipment (Fig. 4). The position of the AFC sensor was about one quarter from the outlet end of the pipe segment on the top position (12 o'clock) that of the first AE sensor in the bottom position (6 o'clock) shifted towards the leak by about 6 cm from the center of the AFC. The second AE sensor (9 o'clock) was mounted 180[degrees] opposite the bore-hole simulating the leak (3 o'clock) one quarter length from the inlet end of the pipe segment.
[FIGURE 3 OMITTED]
The conventional AE sensors selected for this study and AFC elements do differ somewhat in their spectral sensitivity . In addition, AFC elements show a higher sensitivity along the direction of the piezoelectric fibers than normal to it . There are also indications that AFC elements may have a higher sensitivity to in-plane (shear) waves relative to that for out-of-plane waves than the conventional AE sensors. For the experiment, the AFC element was mounted with the fiber direction parallel to the axis of the pipe segment (compare Fig. 4, center).
Each leak diameter was tested for a range of pressures between about 400 and 800 kPa. The upper pressure was limited by the supply of compressed air available in the laboratory and showed some variation (typically about 20 to 30 kPa). Before recording AE signals and waveforms at a selected pressure corresponding to a setting of the outlet valve, the pressure was allowed to equilibrate for about one to two minutes. Nevertheless, for some tests, pressure variations during data acquisition were observed. This variation, however, was less than the pressure change between different tests. For each setting, data were recorded for about 5 to 10 seconds. Power spectra of selected AE waveforms within the frequency range between 0 and 400 kHz were visually compared for the two types of sensor for a given leak and pressure, as well as for increasing pressure for a given leak diameter and for increasing leak diameter at constant pressure, respectively. This is analogous to an investigation reported for leaks with other geometry and size .
[FIGURE 4 OMITTED]
Figure 5 shows the power spectra for the pipe without a leak (screw without hole inserted into the bore-hole of the pipe) indicating frequency contributions between about 30 kHz (limit of high-pass filter) and about 150 kHz for both types of sensor, essentially independent of the pressure of the air (varied between about 400 and 800 kPa). As expected, the power spectra for AE sensors and AFC elements are somewhat different for comparable pressure values, and the dominant contributions appear at frequencies around 50 kHz. There are no contributions significantly exceeding the noise level at frequencies above about 160 kHz. For a pressure of 800 kPa both sensors (AE and AFC) show a distinct peak with comparable amplitude just above 150 kHz (note the difference in scale of the graphs for the AE and AFC sensors).
Figure 6 shows power spectra for one leak diameter (0.35 mm) at different pressures between about 400 and 800 kPa. Compared with the spectra shown in Fig. 5, there are clear contributions at frequencies above 150 kHz for all pressures that have been tested. These can be observed for both types of sensor (AE and AFC element). It can be noted that a noticeable contribution to the power spectra above about 160 kHz appears for the chosen leak diameter of 0.35 mm at about 165 kHz. This contribution can be noted in the power spectra of both types of sensors (AE and AFC element). A further contribution appears at a frequency around 275 kHz for the AE sensors and between about 280 and 300 kHz for the AFC element. The latter also shows indications of another contribution at about 375 kHz. Indications of that are also observed for the AE sensor, but are less apparent in the graphs because of the difference in scale. Further, the intensity (amplitude relative to maximum peak in power spectrum) of these contributions is changing with the pressure of the medium (compressed air). Compared with the peak contribution between about 30 and 100 kHz, the peaks at frequencies above 160 kHz are increasing with increasing pressure. This increase is more pronounced for the AE sensor than for the AFC element.
Figure 7 shows power spectra for selected leak diameters at a constant pressure of about 600 kPa. The graphs in Fig. 7 can be complemented with the center part of Fig. 6 showing the power spectra for a leak diameter of 0.35 mm at a pressure of 600 kPa. Compared with the spectra shown in Fig. 5, there are again clear contributions at frequencies above 160 kHz. These can be observed for both types of sensor (AE and AFC element). Contrary to the case of constant leak diameter tested with different pressures, the frequencies are now changing with leak diameter (if tested at constant pressure of about 600 kPa). For a leak diameter of 0.2 mm, a small, but distinct contribution occurs at about 195 kHz (observed with both sensors), complemented by a weak contribution around about 330 kHz. For the leaks with diameters of 0.35 and 0.5 mm the frequency contributions shift to about 165 and 275 to 280 kHz, respectively, and an additional contribution around 370 kHz appears. For the leak with 0.8 mm the frequency contributions shift again to lower frequencies, namely 130 and 265 kHz. The AE sensor indicates additional contributions centered around 265 and 340 to 350 kHz and the AFC sensor around 230 and 270 kHz, respectively.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Independent of leak diameter (or more general, leak geometry) and operating pressure of the simulated pipe, leaks yielded additional contributions in the power spectra of recorded AE signals at frequencies above about 150 to 160 kHz for both types of AE sensor. Of course, this statement holds for the range of leak diameters (0.1 to 1.2 mm) and pressures (about 400 to 800 kPa) that have been investigated to date.
[FIGURE 7 OMITTED]
A specific leak diameter (geometry) does seem to yield power spectra contributions at the same frequency, independent of the operating pressure. However, the intensity (amplitude) of the power spectra contributions indicating the leak is increasing with increasing pressure. At an intermediate, but roughly constant pressure (600 kPa) the power spectra contributions do seem to shift to lower frequencies with increasing leak diameter (geometry). These observations imply lower bounds for the size of leaks that can be detected within specific operating conditions. For small leaks (below 0.1 mm diameter) the indication of the leak may not differ sufficiently from the noise in the power spectrum.
Based on the leak testing performed with gaseous and fluid media using conventional AE sensors on a similar system with the same type of leaks , it is expected that AFC elements would also yield indications of leaks, if the pipe were operated with a fluid medium (water) in a similar pressure range. Since the pipe used in the previous experiments  was of a different diameter (60 mm instead of 50 mm) and shorter, it is expected that changes in the pipe geometry would not affect the results significantly. Changing to pipes made of a different material has not been explored yet. Pipes made of another metal are expected to yield similar results, while for composite or polymeric pressure pipes, the pronounced frequency-dependent attenuation in the material may affect the sensitivity for leak detection. The dependence of the sensitivity on the distance between leak and sensor has not been investigated (fixed leak and sensor position in the tests reported here). Depending on the pipe material and the distance of the sensor from the leak, attenuation effects may also reduce the sensitivity. Noise interference from operating conditions or environment may further affect the sensitivity for leak detection.
As a last remark, it can be noted that the AFC elements permanently mounted on the pipe can also be used for other types of nondestructive testing or structural health monitoring; for example, electrical impedance measurements  by simply connecting the wires to another measurement system.
Experiments with a laboratory-scale model system for leak testing of pipes first indicates that piezoelectric active-fiber-composite (AFC) elements yield acoustic emission signatures (power spectra) comparable to those recorded with conventional AE sensors. Conformable AFC elements, however, can be mounted directly on pipes without the use of special waveguides. Simulated leaks yield additional frequency contributions mainly above about 160 kHz, while tests without leaks do not show significant contributions above this limit. Varying leak diameter at constant test pressure shifts the additional frequency contributions to lower frequencies, while varying the test pressure for constant leak diameter changes the intensity of the additional frequency contributions relative to the dominant peaks in the power spectrum at frequencies below 150 kHz. This indicates that there may be a lower limit of the leak diameter (size) that can reliably be detected.
The manufacturing of the model pipe system including the simulated leaks, its setup, as well as technical support during operation by Mr. K. Ruf is gratefully acknowledged.
 M.A. Goodman, R.K. Miller "Acoustic Leak Testing", in R.K. Miller, E.v.K. Hill, P.O. Moore (Eds.), Nondestructive Testing Handbook, Vol. 6 Acoustic Emission Testing, 3rd ed., American Society for Nondestructive Testing, Columbus, 181-226, 2005.
 ASTM Standard Test Method for Examination of Seamless, Gas-Filled, Pressure Vessels Using Acoustic Emission, E1419-02b, American Society for Testing and Materials International, Book of Standards, Vol. 03.03 (annual edition).
 ASTM Standard Test Method for Examination of Liquid Filled Atmospheric and Low Pressure Metal Storage Tanks Using Acoustic Emission, E1930-02, American Society for Testing and Materials International, Book of Standards, Vol. 03.03 (annual edition).
 ASTM Standard Test Method for Acoustic Emission Examination of Pressurized Containers Made of Fiberglass Reinforced Plastic with Balsa Wood Cores, E1888/E1888M-02, American Society for Testing and Materials International, Book of Standards, Vol. 03.03 (annual edition).
 R.K. Miller, A.A: Pollock, D.J. Watts, J.M. Carlyle, A.N. Tafuri, J.J. Yezzi Jr. "A reference standard for the development of acoustic emission pipeline leak detection techniques", NDT&E International, 32(1), 1-8, 1999.
 ASTM Standard Practice for Leak Detection and Location Using Surface-Mounted Acoustic Emission Sensors, E1211-02, American Society for Testing and Materials, International, Book of Standards Vol. 03.03 (annual edition).
 A.J. Brunner, M. Barbezat "Acoustic Emission Monitoring of Leaks in Pipes for Transport of Liquid and Gaseous Media: A Model Experiment", Proceedings 27th European Conference on Acoustic Emission, Advanced Materials Research, 13-14, 351-356, 2006.
 A.A. Bent, N.W. Hagood "Piezoelectric fibre composites with interdigitated electrodes", Journal of Intelligent Material Systems and Structures, 8(11), 903-919, 1997.
 A.J. Brunner, M. Barbezat, P. Flueler, Ch. Huber: "Composites from piezoelectric fibers as sensors and emitters for acoustic applications", Journal of Acoustic Emission, 22, 127-137, 2004.
 M. Barbezat, A.J. Brunner, Ch. Huber, P. Flueler: "Integrated Active Fiber Composite Elements: Characterisation for acoustic emission and acousto-ultrasonics", Journal of Intelligent Material Systems and Structures, 18(5), 515-525, 2007.
 K. Yoshida, H. Kawano, Y. Akematsu, H. Nishino "Frequency Characteristic of Acoustic Emission Waveforms during Gas Leak", Proceedings 26th European Conference on Acoustic Emission Testing, German Society for Nondestructive Testing, BB 90-CD, 321-327, 2004.
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ANDREAS J. BRUNNER and MICHEL BARBEZAT
Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Mechanical Systems Engineering, Uberlandstrasse 129, CH-8600 Dubendorf, Switzerland
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|Author:||Brunner, Andreas J.; Barbezat, Michel|
|Publication:||Journal of Acoustic Emission|
|Date:||Jan 1, 2007|
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