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4.3. Influence of coating layer
Buried pipelines bear a resin coating to prevent corrosion. The coating
material is coupled to the metal surface and absorbs a large fraction of
ultrasonic energy, which has often made the guided-wave techniques
unpractical. The present technique was tested against the standard
triplelayer coating procedure, which consists of the adhesive undercoat,
the anti-corrosive polyethylene coating, and then the protective layer of
polyolefine. A 10-mm diameter hole of a v-shaped end was drilled through
the total coating thickness of 2.5 mm to give a minimum thickness of 4.0
mm of the steel wall. Fig. 7 shows that our PPM-EMAT technique is still
applicable to detect a defect even for a coated pipeline, although the
round-trip amplitude of the SH1 wave
becomes weaker by 15 dB compared with the uncoated flawless pipe of the
same dimension.
Fig. 7. Detection of a drilled defect
in a coated steel pipe (t = 5.4 mm).
5. Conclusion
The present EMAT technique has shown great potential for gas pipeline
inspection. The round-trip signals of SH0 and SH1 modes were proven to
respond uniquely to surface defects. The decrease in amplitude and phase
shift of SH1 mode were more sensitive to the presence of the defects than
those of the SH0 mode, but such changes did not show a correlation with
wall thinning. The phase shift of SH0 mode showed a good correlation with
the remaining thickness. It is possible to recommend a procedure for
corrosion monitoring by measuring the amplitude and phase of both modes.
If one detects a phase increase and a significant amplitude decrease of
SH1 mode, it will be a warning that the wall thinning has progressed below
the cut-off thickness. Then, the phase delay of the SH0 mode provides the
thickness information. Further research is necessary to determine the
multiple guided modes to be used, their frequencies related to the magnet
spacing (d ), and the size of EMAT for optimum performance. We emphasize
that this technique may also be useful for coated pipelines.
Acknowledgements
The authors are grateful to T. Kikuta and T. Nishizawa (Research &
Development Center, Osaka Gas Co., LTD.) for introducing this topic of an
industrial importance and supplying the sample pipes with defects. The
experiments were done with the assistance of A.Okuda.
References
[1] Thompson RB, Alers GA, Tennison MA. Application of direct
electromagnetic Lamb wave generation to gas pipeline inspection. New York:
Ultrasonic Symposium Proceedings, IEEE, 1972. p. 91–3.
[2] Thompson RB. Experiences in the use of guided ultrasonic waves to scan
structures. In: Thompson DO, Chimenti DE, editors. Review of progress in
quantitative NDE,NewYork: Plenum, 1997;16(A): 121–8.
[3] Alers GA, Huebschen G, Maxfield BW, Repplinger W, Salzburger HJ,
Thompson RB, Wilbrand A. Nondestructive testing handbook, 7, ultrasonic
testing. ASNT, Columbus, OH, 2nd edn, 1991. p. 326–40.
[4] Maxfield BW, Fortunko CM. The design and use of electromagnetic
acoustic wave transducers (EMATs). Materials Evaluation 1983;41:1399–408.
[5] Thompson RB. Physical principles of measurements with EMAT
transducers. In: Physical acoustics, New York: Academic Press,
1990;19:157–200.
[6] Ogi H. Field dependence of coupling efficiency between electromagnetic
field and ultrasonic bulk waves. J Appl Phys. 1997;82: 3940–9.
[7] Fortunko CM, Petersen GL, Chick BB, Renken MC, Preis AL. Absolute
measurement of elastic-wave phase and group velocities in lossy materials.
Rev Sci Instrum 1992;63:3477–86.
[8] Hirao M, Ogi H, Fukuoka H. Advanced ultrasonic method for measuring
rail axial stresses with electromagnetic acoustic transducer. Res Nondestr
Eval 1994;5:211–23.
[9] Petersen GL, Chick BB, Fortunko CM, Hirao M. Resonance techniques and
apparatus for elastic-wave velocity determination in thin metal plates.
Rev Sci Instrum 1994;65:192–8.
[10] Ogi H, Hirao M, Minoura K. Noncontact measurement of ultrasonic
attenuation during rotating fatigue test of steel. J Appl Phys.
1997;81:3677–84.
[11] Meeker TR, Meitzler AH. Guided wave propagation in elongated cylinder
and plates. In: Physical acoustics, New York: Academic Press,
1964;1(A):111–67.
[12] Auld BA. Acoustic fields and waves in solids, II, chap 10. New York:
Wiley-Interscience Press, 1973. |