NDE Applications of Air-Coupled Ultrasonic Transducers - UFFC

Figures 10 and 11 show C-scans of two materials used inaemspace applications: a fiber-reinforced epoxy panel and ahoneycomb composite panel used to support solar cells.m. A SYSTEM FOR DEMONSTRATING THEFEASIBILITY OF IN-LINE INSPECTION OF NATURAL-GAS PIPELINESUltrasonic inspection of large-diameter, natural-gas pipelinespresents a great challenge to the gas-coupled transducertechnology. Natural gas exhibits a very low specific acousticimpedance (300 Rayls for methane at atmospheric pressure)compared to oil (1.5 Mrayls and higher). Consequently, evenat working pressures (6.9 MPa or 1000 psi), the ultrasonicsignalreflections from the pipe-walligas interfaces aresignificantly larger than the signals retuned from the outersurface of the pipe wall. To circumvent this obstacle, pastexploratoly developments included the use of a liquid-filledwheel", electromagnetic-acoustic-transducer (EMAT)" , andliquid-slug technologies7* . While prototypes of high-speed,in-line inspection systems employing such principles do exist,all exhibit serious operational shortcomings that preventwidespread commercial exploitation.Fig. IO. C-scan image from an uncured phenolicicarbon fiberlay-up sheet. Panel size: 20 x 20 inch x 0.1 inch wallthickness. The dark areas outline the presence ofdelaminations. Measurements were taken with 400 kHz noncontacttone-burst method.An alternative concept of an automated ultrasonic system fordetecting thickness variations and crack-like flaws in the pipewall is shown in Fig. 12. The concept shown in Fig. 12 woulduse the natural gas as an ultrasonic couplant and would befunctionally similar to a liquid-coupled system, using similarelectronics and transducers. As shown in Ref 55, operation inthe pulse-echo mode, which is very desirable but not currentlypossible at atmospheric pressures, can be achieved atpressures typical ofpipelines.lFig. 12.Concept ofan automated ultrasonic system for pipewall inspection.Fig. 11. C-scan image from a section of a solar honeycombpanel. This panel contains a reinforcement area which isinserted and bonded with foam adhesive into the surroundinghoneycomb core structure. Note the missing core cellsadjacent to the insert. Measurements were taken with 400 kHznon-contact tonebunt method.Figure 13 shows the functional block diagram of anexperimental system developed atNISTto demonstrate thefeasibility of the concept illustrated in Fig. 12. Theexperimental system uses a cylindrical pressure vessel, 305mm (12 in) in diameter and 610 mm (24 in) in length. Thepressure vessel can accommodate a variety of gases atpressures up to 10 MPa (l500 psi) and has appropriate feedthroughsfor sample and transducer-motion control, signalhandling, and pressure and temperature monitoring. Inside thevessel, a flexible stage with multiple degrees of freedommanipulates both the transducer and samples. Also, fourposition-adjustment motors are used to manipulate the Zcoordinate of the sample and the X, Y, and e coordinates of704 - 1995 IEEE ULTRASONICS SYMPOSIUM

the pulse-echo transducer. The coordinate 0 is the angle in thesagittal plane between the transducer symmetry axis and thenormal to the plate-surface.DiplexerTmmin). The notches have mouth widths of 0.3 mm (0.01-in) andare 44 mm (l .l5 in) long.In principle, the experimental arrangement shown in Fig. 13 isuseful for measuring the thickness of the plate, findingdelaminations in the plane of the plate, and detecting verticalcracks. Wall thickness is best measured using longitudinalwavesignals that propagate along the plate-surface normal.Such signals result fromcompressional-wave signals in thegas that propagate along the plate-surface normal direction (0,= Oo). On the other hand, vertical cracks are bestdetectedwith longitudinal- or shear-wave signals that propagate at anangle (0.) with respect to the plate-surface normal. Togenerate such signals, thesymmetry axis of thepulse-echotransducer must rotate in the sagittal plane to satisfy Snell'slaw for both longitudinal- and shear-wave signals.DitdodcilbAmp#fmPe-AmpliraFigure 14 shows an experimental configuration for detectingwall thickness variations. The ultrasonic signals are shown inFig. 15. The first signal in Fig. 15 is due to the directreflection off the hnt surface of the plate specimen (a). Thefollowing signals are multiple reverberations within the plate(b and c). Here, the transducer-plate separation distance is 38mm (1.5 in) and the pressure (nitrogen) is 6.9 MPa (1000 psi).The time-domain separation between the ultrasonicreverberations in the flat plate (b and c) is approximately 4 p,consistent with the nominal plate thickness of 12.7 mm (0.5in).NORMAL BEAMFig. 13. Functional block diagramofexperimental system forshowing feasibility of inspecting gas pipelines.The experimental system shown in Fig. 13 uses a flat, wideband,piezoelectric-ceramic transducer, 13 mm (0.5 in) indiameter to generate and receive the ultrasonic signals. Thetransducer, which is constructed as shown in Fig. Sf, exhibitsa center frequency of 2.25 MHz when operated in water. Togenerate, detect, and condition the ultrasonic signals, we use asquare-wave pulser with 8 kW available peak power at 400 V,a special diplexer circuit, and a receiver amplifier of highinputimpedance with 64 dB dynamic range and 60 MHzbandwidth. Manually stepped anenUators mntrul the outputpulsepower levels and receiver-amplifier gains. We also usean %bit digital storage oscilloscope (DSO) sampling at 400MHz to record the sip1 waveforms. A dedicated computercontrols the setup. As shown in Fig. 13, there is anothertransducer ooupled directly to the back surface of the flatplatespecimen. Specifically, a pin transducer, 1.4 mm (0.06in) in diameter provides ultrasonic-beam diagnostics and aidin initial alignment.In Figure 13, the specimens are two surface-ground flat steelplates. Each is 114 mm (4.5 in) long, 44 mm (1.75 in) wide,and 13 mm (0.5 in) thick. The two plates are typicallymounted side by side and contain thin, surface-breakingnotches made by standard electro-discharge machining(EDM) procedures. The notch depths are 20% and 40% of thenominal plate thickness, i.e., 2.5 mm (0.1 in) and 5.1 mm (0.2Fig. 14. Experimental configuration for detecting wallthickness variationsTime (full scale = 20 PS)Fig. I 5. Ultrasonic signal from the configuration of Fig. l41995 IEEE ULTRASONICS SYMPOSIUM - 705

~ ~~~ ~~ ~~ ~~Figure 16 shows the experimental configuration for detectinginternal crack-like flaws. In principle, both longitudinal- andshear-wave signals are useful in a pulse-echo configurationfor detecting flaws. In watercoupled systems, longitudinalwavesignals are preferable. Although both longitudinal- andshear-wave flaw signals are observed experimentally, thelatter clearly separate from the front-surface reflection signals.The A-scan in Fig. 17 illustrates this effect. The longitudinalwaveflaw signal occurs at only 6 p after the first observablefront-surface reflection. Although it is possible to detect thepresence of the vertical notch (40% of wall thickness) bymonitoring the behavior of the interferences between the twosignals, the process is not reliable. On the other hand, theshear-wave reflections, arriving at approximately 11 pfollowing the beginning of the front-surface reflection, areclearly discernible.will be needed to explore the impact of surFdce roughness,pipe-wall curvature, and gas composition and temperature.WI. PULSE-ECHO ACOUSTIC MICROSCOPYWe have also used the system as shown in Fig. 13 with aspherically focused instead of a flat transducer. More detailedinformation on this system can be found in Ref. 51. Both,polymer-piemlecmc and piezoelectric-ceramic transducers,can be used, depending on the frequency of operation. Thesupport electronics is essentially identical to that used in theexperiment designed to demonstrate the feasibility of in-lineinspection of natural-gas pipelines. Figure 18 shows anominally 2.25.- pulse-echo C-scan of a 25-cent coinembedded in a PMMA plate. Argon, at 30 atmospheres, isused as the coupling gas and the coin is located approximately7.5 mm below the front surface of the PMMA plate. Whilethe image shown in Fig. 18 demonstrates the feasibility of agas-coupled microscope, muchwork remains to be done inorder to show practicality of the concept. Potentialapplications may include the inspection of electronic chipsmounted on ceramic substrates".SIDdRsb-%-m18.. '\0, '....,'\'..',N&hFig.16. Experimental configuration for detecting internal,crack-like flawsI Time (full = scale 20 us) IFig. 17. A-scan of the experimental configuration of Fig. 16.The above experimental results clearly demonstrate that itmay be feasible to use pressurized natural gas as theultrasonic wuplant in a pulse-echo system to detect flaws andmeasure thickness in natural-@ pipclines. Because soundpropagates much more slowly in a gas than in water, (300-500mis vs. 1500 m's), the incidence angles of the ultrasonicprobes are correspondingly smaller. Furthermore, sensitivityto misalignment is expected to be greater for gas-coupledsystems than for liquid-coupled systems. Furtherdevelopments in this area will be needed to show thepracticality of the concept. In particular, additional studiesFig. 18. C-scan of a 25-cent coin embedded in PMMAWII. FUTURE POSSIBILITIESIn light of the recent advances in air transducer technologies,it is interesting to compare the sensitivity limits of variousultrasonic transducers with ideal and micromachined airtransducers. Figure 19, which is obtained from a companionpaper'4, shows the calculated sensitivity limits, in terms of atime-averaged, minimumdetectable displacement, in a I-Hzbandwidth, of optical interferometers, EMATs, direct-contact(piexelectric), and capacitive air transducers. The absolutelimit, as calculated using the fluctuation-dissipation theoremas assuming an ideal (loss-less and mass-less) transducer, isalso shown.706 - 1995 IEEE ULTRASONICS SYMPOSIUM

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