Micromachined ultrasonic transducers for air-couplednon-destructive evaluation

Scan 'F. Hansen. F. Levent Degertekin. and Butrus '1'. Khuri-Yakuh

Edward L. Ginzton LaboratoryStanford University

Stanford. CA 94305-4085

ABSTRACTConventional methods of ultrasonic non-destructive evaluation (NDF.) use liquids to couple sound waves into the test

sample. This either requires immersion of the parts to he examined or the use of complex and bulky water squirting systemsthat must be scanned over the structure. Air-coupled ultrasonic systems eliminate these requirements if the losses at air-solidinterfaces are tolerable. Micromachined capacitive ultrasonic transducers (cMtJ'l's) have been shown to have more than 100dB dynamic range when used in the bistatic transmission mode. In this paper. we present results of a pitch-catch transmissionsystem using cMUTs that achieves a 103 d13 dynamic range. Each transducer consists of 10.000 silicon nitride membranes of100 l.lm diameter connected in parallel. This geometry results in transducers with a resonant frequency around 2.3 Mhz.These transducers can be used in through transmission experiments at normal incidence to the sample or to excite and detectguided waves in aluminum and composite plates. In this paper we present ultrasonic defict detection results from boththrough transmission and guided Lamb wave experiments in aluminum and composite plates. such as those used in aircratl.

Keywords: transducer, air, NDE, Lamb waves

1. INTRODUCTION

Due to the large impedance mismatch between common piezoelectric materials iid air, conventional piezoelectrictransducers are not very efficient sources of ultrasound in air. Therefore many conventional ultrasonic testing systemsrequire complicated liquid squirting systems or immersion of the sample to be examined, which may not he practical for largeaerospace structures. An alternative is to place the transducer in direct contact with the samples under test. I however. defect

detection or imaging can be time consuming. and fragile samples may he damaged by direct contact. In such situations, airboime ultrasound is a useful non-contact method of detecting and imaging defects in aging aircraft structures, given efficienttransducers for use as transmitters and receivers.

2. TRANSDUCERS

2.1. Transducer Description

Recently, several capacitive micromachined ultrasonic transducers (cMhJTs) have been developed, which are capable ofefficient excitation and detection of ultrasound in air4. As depicted in Fig. I. a single clement consists of a metalized 0.5-1p.m thick nitride membrane suspended above a silicon substrate. Several thousand such elements are electrically connected inparallel to make the transducer, shown in Fig. 2. When the membranes are biased with a [)(' bias voltage, the transducer iscapable of efficient excitation and detection of ultrasound in air. The resonant membrane structure results in a large dynamicrange. in excess of 100 dB. though the transducers are inherently narrow hand devices with fractional bandwidths of less than10%.

Part of the SPIE Conference on Nondestructive Evaluation of Aging Aircraft, Airports,310 and Aerospace Hardware Ill • Newport Beach, California • March 1999

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Fig. I. Schematic cross—section of a single cMf Ti Fig. 2. Magiiilied view of cMI II transducer with 50 p.mmembrane. membrane radii.

2.2. System Dynamic Range Simulations

Using an equivalent resonant circuit model for the impedance of the cMUT, we simulate the transducer in a pitch-catchtransmission configuration. Complete signal-to-noise analysis is possible using SPICE circuit simulation. This analysisaccounts for non-ideal losses through capacitances, insertion losses within the transducer, and for the thermal noise ofcomponents. For the simulations, we apply a 20 V peak-to-peak tone burst through a tuning inductor to the transmittingtransducer, which is under 50 V DC bias. The receiving transducer is connected through another tuning inductor to theAD600 low-noise amplifier by Analog Devices. This amplifier provides 40 dB of gain with an input noise voltage of I .4nV/'iHz. Using a generous noise bandwidth of 2 MHz around the transducer resonance, the ratio of the maximum receivedvoltage at the amplifier input to the noise voltage represents our dynamic range. For a transmit and receive system that usesinductors to tune out the transducer reactances on send and receive, simulations predict a dynamic range of 1 09 dB for the 2MHz noise bandwidth implemented in our system. If the noise bandwidth can be further reduced to 1 MHz, the dynamicrange increases to 116 dB.

3.1. Transmission Through Composite Plate

3. TRANSMISSION IMAGING RESULTS

Simulations of an electronic system using cMUTs suggest that the dynamic range of the system is sufficient to transmitultrasound through materials such as metal and composites in air, without the need for coupling fluids. Figure 3 shows thereceived tone from bistatic transmission through a four-layer carbon fiber composite plate at normal incidence. By varyingthe amplitude of the received signal with and without the sample present, we estimate that the composite plate and itsinterfaces with air attenuate the ultrasound signal by 68 dB. A 0.7 cm air gap further attenuates the signal by 6 dB at 2.3MHz. Since the received signal without averaging has a signal-to-noise ratio of 29 dB, Fig. 3 demonstrates a dynamic rangeof 1 03 dB, 6 dB less than the electronic simulation prediction. However, other sources of noise such as electromagneticinterference and supply voltage noise sources were not accounted for in the SPICE circuit simulation. Nonetheless, thesystem's dynamic range is more than adequate to image impact damage in a section ofthe composite sample.

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Fig. 3. Received voltage waveform through composite plate.

Figure 4 shows an image produced by linearly gray-scaling the amplitude of the received signal over a section ofcomposite. Density variations in the plate are evident by regions of light and dark. Figure 5 is produced by scanning thesame section ofthe plate after it was struck in two places, which appear in the scanned image as dark spots. The damage spoton the left is not visible optically from the surface, but is clearly present in the ultrasound image.

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20 22 24 26 28 30 32

o0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Fig. 4.damage.

Amplitude image of composite prior to Impact Fig. 5. Amplitude image ol conipo ite alter impact damagein t0 places.

3.2. Transmission Through Aluminum Plate

The image in Fig. 6 shows the amplitude variations in a 3 mm thick aluminum l)late that has a 0.5 mm deep pattern.shown in Fig. 7, milled on the underside of the plate. The dynamic range in this transmission experiment is also 103 d13. withabout 82 dB of loss through the aluminum and air interfaces at 2.3 Ml—lz.

0.5 cm

Fig. 6. Amplitude image ofmilled pattern in aluminum plate.lig. 7. Mit led pattern on underside of

plate. ((.5 mm deep.

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4. LAMB WAVE DEFECT DETECTION

The ultrasound transmission experiments of the previous section are useful for scanning small-sized samples that haveboth sides of the material exposed to air. Generation and detection of Lamb waves from air into plates is a more promisingmethod for inspecting materials and structures in which only one side of the sample is accessible. Furthermore, large areasmay be scanned rapidly since Lamb waves are capable of traveling relatively long distances once coupled into the solid. Anyinteraction of the Lamb wave with a defect can be detected by the receiving transducer, which may be near or far from thetransmitter as long as it is not in the path of a specular reflection. In the experiments that follow, we present calculations andexperimental results ofthe symmetric s mode propagating in an aluminum plate that is 1.2 mm thick.

4.1 Lamb Wave Propagation Calculations

To excite a Lamb wave in a solid from an acoustic wave in fluid, the acoustic wave in air must strike the surface of thesolid at specific incidence angles for the supported mode. The plane wave reflection coefficient calculation shown in Fig. 8for a 1 .2 mm thick aluminum plate and 2.3 MHz ultrasound suggests that three propagating modes are possible, shown bythree dips in the reflection coefficient ofthe aluminum. The mode at 5.8 degrees corresponds to the lowest order symmetricso mode.

IA1 Mode

S0 A0 Mode

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Angle of Incidence (degrees)

Fig. 8. Reflection coefficient from 1 .2 mm thick aluminum plate for range of incident angles.

To estimate the conversion efficiency of acoustic power from air into the s mode, we must consider the size of thetransmitting and receiving transducers. The expression for conversion efficiency r, for one-sided excitation or reception, isgiven by

= (1—e)2 (1)

where ct is the leak rate of the mode and 1 is the projected transducer length in the direction of propagation. 6 For ourtransducer length of 1 cm, projected length ofO.995 cm for a 5.8° angle ofincidence, and a leak rate ofO.085 Np/m for the smode, the calculated conversion efficiency is 8.45x104. Squaring this efficiency represents conversion from air to solid, andback to air again, yielding the two-way conversion efficiency. The optimal sized transducer for efficient coupling for thisattenuation constant satisfies al=l.26 and is about 12 m in length. For a more practical 1cm transducer, we expect 61.5 dB

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for the two-way coupling loss to and from the solid plate into air for this incident angle and mode. Ifwe again assume a 103dB dynamic range for our system, we expect our received signal-to-noise ratio to be 41 .5 dB minus any attenuation in the airgap between the solid and transducer.

4.2 Lamb wave Transmission Results

The 2.3 MHz transducers are set in the configuration shown in Fig. 9, with incidence angles near 5.8 degrees to exciteand detect a Lamb wave in the 1 .2 mm aluminum plate. An amplified voltage signal from the receiver is shown in Fig. 10.The peak signal, which occurs about 48 ts after the transmit tone burst, represents the s Lamb wave which travels about 3cm in the aluminum prior to detection by the receiver. Received signals at 295 ts and again at 538 ts are due to multiplereflections of the Lamb wave from the plate edges, one of which is still detected after traveling distance of more than 1 m inthe plate. The group velocity calculations using these multiple echoes confirm that the Lamb wave is indeed the s0 mode.

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Fig. 9. Configuration and geometry for Lamb wave coupling into aluminum plate.

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Fig. 10. Received voltage signal showing multiple reflections ofs0 mode Lamb wave.

An expanded view of the detected Lamb wave is shown in Fig. 1 1 , averaged 1 6 times to reduce the noise level. Withaveraging, the peak signal is 15.41 mV rms while the noise level is 147 j.tV mis, yielding a signal-to-noise ratio of 40.4 dB.Therefore, we expect a factor of 4, or 12 dB, decrease in signal-to-noise ratio for a signal without averaging. Accounting for8.7 dB attenuation due to the air gaps, our actual signal-to-noise ratio of 28.4 dB falls about 4.4 dB short of the prediction.

17cm

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Receiver

Lamb Wave ------------------ 1 I1 i .2 mm

200 300 400TimeAfter Transmit Burst (1.tS)

Any unaccounted signal loss due to diffraction and Lamb wave attenuation should be negligible for the Lamb wavelength andtransducer spacing of 3 cm. Therefore we attribute the 4.4 dB discrepancy between the predicted and actual signal to noise toimperfect angular alignment of the transducers, noise due to additional amplification, and the finite size of the transducer intwo dimensions. In the future, use of phase delayed arrays of the membranes to control the desired incidence angle couldimprove the control over mechanical alignment.

Fig. 11. Received voltage signal from air-coupled s0 Lamb wave, averaged 16 times.

Fig. 12. Configuration for line scan of defect using air Lamb wave.

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4.2 Defect Detection Using Lamb Waves

A measured signal-to-noise ratio of 29 dB is sufficient to detect the presence of a thickness change in the aluminumplate, representing a defect. Figure 12 schematically shows the configuration and the transducer with respect to a 0.6 mmdeep rectangular 1 cm by 0.3 cm rectangular milled pattern on the underside of the plate. A single 5cm line scan shown inFig. 13 shows the drop in received signal amplitude as the Lamb wave crosses the defect. The defect effectively reduces theplate thickness and changes the propagation efficiency of the s0 mode. The slight increase in received signal when the defectis directly centered between the transducers is likely due to edge effects around the defect, since its size is comparable to thetransducer's field of view. By taking similar line scans from a variety of angles one can use Lamb waves to locate andreconstruct the defect shape using tomography.7

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Fig. 13. Received signal from s, Lamb wave line scan over defect in aluminum plate.

TFig. 14. Configuration for horizontal and vertical scan of defect inAluminum plate.

0 1 2 3 4 5Scan Distance (cm)

A Defect

X-YScan, Lj9 x 9 cm

Receiver Transmitter

Aluminum Plate

Figure 14 shows the configuration for a Cartesian coordinate scan, in which the transducers scan in both horizontal andertical directions to create an image. For the best resolution and replication of defect geometry. it is advantateous toosition the transducers as close as possible to one another. As shown in the defect image of Fig. 15, even a 3 cm separationith 1 cm wide transducers results in an image that expands the 0.3 cm defect to nearly 4 m in the horizontal direction.

lowever. the approximate defect size and location can still he ascertained if the geometry ol the scan is known.

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lig IS. Scaled amplitude image of defect in aluminum plaie using air—coupled s9 I anib wave.

5. CONCLUSION

Electronic circuit simulation suggests that cMUTs are capable of dynaniic ranges of up to 116 dB with the use of filtersreduce noise and tuning inductors to improve power delivery to and from the transducer. In transmission experiments

wough aluminum and carbon tuber samples. cMtJTs have demonstrated a dynamic range of 103 dB. This dynamic range.long with cMUTs ability to transmit ultrasound into air, makes them ideally suited for air-coupled NDE applications. Forne-sided inspection of aircraft materials, the transducers ma also excite and detect Lamb waves coupled from air. Weemonstrate the feasibility of Lamb wave defect detection for the 5 mode in a 1.2 mm thick aluminum plate, with a receivedignal-to-noise ratio of approximately 28 dB. Future work includes characterization (if detects in aircraft structures forifferent Lamb wave propagation modes in aircraft structures. 13v phase delaying arrays of membranes in the transducer, weope to be able to electron ical Iv select the incidence angle and desired propagating mode of 1.amh waves.

ACKNOWLEDGMENTS

his work is supported by WPAFB and the US Office of Naval Research. We also wish to thank Igal I.adabauni tCribricating the transducers.

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