DETECTION OF LEAKY-RAYLEIGH WA YES AT AIR-SOLID INTERFACES BY

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1 DETECTION OF LEAKY-RAYLEIGH WA YES AT AIR-SOLID INTERFACES BY LASER INTERFEROMETRY Laszlo Adler and Christophe Mattei Adler Consultants, Inc Kinnear Road Columbus, OH Michel de Billy and Gerard Quentin Groupe de Physique des Solides Universites Paris 6 and 7, CNRS U.R.A 17, Tour 23, Place lussieu, Paris cedex OS, France INTRODUCTION Leaky-Rayleigh waves have been studied extensively at liquid-solid interface in the last 3 years [1-4]. Both leaky-rayleigh and leaky-lamb waves have found many application in Nondestructive Evaluation, e.g. surface defect characterization, etc [5,6]. Because of the difficulties associated with ultrasonic wave generation and detection in air in the megaherz region air coupled non destructive evaluation has only recently been explored [7,8]. An optical method recently developed uses heterodyne interferometric probing of refractive index variation [9-1] proportional to the pressure of acoustic waves in the medium, allowed measurements of pulse propagation in air emitted by transducer in the megaherz region. In this paper, velocities and attenuation measurements of leaky Rayleigh waves from solids radiated into air is reported. EXPERIMENTAL SET-UP The schematic diagram of the experimental system is shown figure 1. The optical p r ois b a ~ Mach-Zender heterodyne interferometer (BMI type SH 13) used to measure the acoustical pressure of the leaky-rayleigh wave in air. The probe beam shifted in frequency by an acousto-optic Bragg cell (fb=7mhz) crossed a transparent medium where the index of refraction is modulated by the acoustic wave and is then reflected by a mirror. The phase modulated current measured at the output of the photodiode is given as: l(t)=lo cos[ 2n fb t + v(t)+ <Ds- <Dr]. (1) Review of Progress in Quantitative Nondestructive Evaluation. Vol. 18 Edited by Thompson and Chimenti, Kluwer Academic/Plenum Publishers,

where <l>s and <l>r are the phase constants corresponding to the reference and the probe beams of the interferometer, v(t) is the so-called Raman-Nath parameter [II], the optical phase change due to

2 where <l>s and <l>r are the phase constants corresponding to the reference and the probe beams of the interferometer, v(t) is the so-called Raman-Nath parameter [II], the optical phase change due to the ultrasonic waves is proportional to the integrated acoustic pressure across the ultrasonic beam of width : v(t) '" 21r )J(2L) P(t) AL (2) where ~ is the piezo-optic coefficient, P(t) is the pressure of the acoustic wave and AL is the wavelength of the laser beam (653nm). In order to detect the propagation of the ultrasonic pulses, a broad band electronic processing, similar to that used in the displacement measurements has been used in this experiment [12,13]. The ultrasonic Rayleigh waves were generated on the surface of the sample by a so-called direct generation technique [14]. A contact transducer is mounted directly over the edge of the specimen so that it can generate a bulk mode in the interior of the sample as well as surface modes along the surface. In this particular case, a I MHz shear transducer of vertical polarization, i.e. normal to the surface, is used. The samples are translated both in the vertical and horizontal position with 1 ~ m precision. The measurements were performed in air above the interface air-solid. This method measures the ultrasonic pressure integrated on the path that the light travels above the sample. EXPERIMENTAL RESULTS In figure 2, the optically detected signals are shown at.1, I and 2 mm below the surface inside a Plexiglass sample. The laser beam intersects the sound field at 15mm from the transducer surface. The first arrival signal is a longitudinal wave generated by the shear transducer. The second arrival signal is a Rayleigh wave showing amplitude decay with depth (z). The bulk transverse waves are not detected by the interferometer which detects dilatation only. When the probing laser is moved outside the sample to air, the signal amplitude drops significantly: nevertheless, after 5 times averaging a good measurable signal is obtained. In figure 3a, is shown the pressure field of the leaky-rayleigh wave measured Pulser-receiver E8 Oscilloscope Figure I: Experimental set-up. 1448

3 a Longitudinal wave ~ / Rayleigh wave b ~ - - ~ c Figure 2: Optically detected signals at O.lmm -a), lmm -b) and 2 mm -c) below the surface inside a Plexiglass sample. a b Figure 3: Optically detected signals: -a) the probe is in air (z=lmm) -b) the probe is in the plexiglass sample (z=lmm). in air above the interface I mm from the surface. The signal can be detected several centimeter away from the surface. For comparison, the ultrasonic field probed inside the Plexiglass at lmm below the surface is given in figure 3-b. Velocity measurements In order to measure the velocity of these waves, the field was probed parallel to the surface inside and outside the specimen. The detected signals recorded 2 mm apart 1449

4 a - ~ - - " ' \ b ' Figure 4: Optically detected signals recorded at 2 mm apart along the interface for velocity measurements. The probe is moved in the Plexiglass specimen. Figure 5: Optically detected signals recorded at 2 mm apart along the interface for velocity measurements. The probe is moved c) and d) in air above the plexiglass sample. in air and in the specimen are shown in figures 4 and 5 respectively. From these measurements, we obtained 1.28 km/s (Vr) and 1.26 km/s (VL,R) for the Rayleigh and the leaky-rayleigh wave velocities, respectively. The difference (less than 2%) is probably due to experimental error. In figure 6, the detected leaky-rayleigh wave signals in air are shown again 2 mm apart for a duraluminum sample. The leaky-rayleigh wave velocity (VL,R) for this specimen was 2.96 km/s which compares well with the Rayleigh wave velocity (VR) measurements of 2.88 kmls obtained by wedge technique measurements. Similar experiments were repeated for two other samples (glass and polyvinyl). Table I summarizes the velocity results for the four samples deduced from the time delay measurements. The measured values agree well with the Rayleigh velocities measurements obtained via the optical technique or a conventional technique and with the theoretical velocities. 145

TABLE 1: Rayleigh wave velocities measured for different materials Material Leaky Rayleigh Rayleigh wave velocity wave velocity measured with *the optical or measured in air +conventional method

5 TABLE 1: Rayleigh wave velocities measured for different materials Material Leaky Rayleigh Rayleigh wave velocity wave velocity measured with *the optical or measured in air +conventional method Duraluminum Polyvinyl Glass Plexiglas * 3.42 * Theoretical Rayleigh wave velocitiy Figure 6: Optically detected signals recorded in air at 2 mm apart along the interface airduraluminium for velocity measurements of the leaky-rayleigh wave. Attenuation measurements Attenuation measurements have also been performed. Amplitude of the propagating pulse has been measured every 5 millimeters along the surface for different materials. Figures 7 and 8 show the measured attenuation for four materials. Plexiglass and teflon present a rather high attenuation whereas leaky-rayleigh waves propagate in glass and duraluminum without too much attenuation. The attenuation of the leaky Rayleigh wave is mostly due to the intrinsic attenuation of the materials. 1.2,.;.8 Q. 5 ~ -< ".6 E.4 z.2 2 Distance (em) Plexiglas ---.-Tenon 3 4 Figure 7: Measured Attenuation of Leaky-Rayleigh wave propagating along air-teflon and air-plexiglass interface. 1451

1.2 ". ~.8 c. E : ".6. t! E.4 z.2. ~ --+-Duraluminum Glass, --f-----+- - +- 2 3 4 5 Distance (cm) Figure 8: Measured Attenuation of Leaky-Rayleigh wave propagating along air Duralurninurn and air-glass interface.

6 1.2 ". ~.8 c. E : ".6. t! E.4 z.2. ~ --+-Duraluminum Glass, --f Distance (cm) Figure 8: Measured Attenuation of Leaky-Rayleigh wave propagating along air Duralurninurn and air-glass interface. CONCLUSION The optical heterodyne interferometric method has been shown to be sensitive to detect leaky-rayleigh waves in the air. This method can be used for low optical reflectivity or rough solid surface and has potential applications for non-contact Nondestructive Evaluation problems. REFERENCES 1. H.L Bertoni and T. Tamir, "Unified theory of Rayleigh Angle phenomena for acoustic beams at liquid-solid interfaces", Appl. Phys (1973). 2. W. Neubauer, " Ultrasonic reflection of bounded beam at Rayleigh and critical angles for a plane liquid-solid interface". J. Appl. Phys. 44, (1973). 3. Breazale M.A., Adler L. and Flax I.," Reflection ofa gaussian ultrasonic beam from liquid-solid interface", J. Acoust. Soc. Am. 56, (1974). 4. T.Y Plona, L.E. Pitts and W.G. Mayer, "Ultrasonic bonded beam reflection and transmission effects at a liquid/solid plate/liquid interface", 1. Acoust. Soc. Am. 59, (1976). 5. G.S Scott and L. Adler, "The structure of ultrasonic leaky waves and their interaction with subsurface flows". Mater. Eval. 35, A.H. Nayfeh, D.E Chimenti, L.Adler and R.L Crane, " Ultrasonic leaky waves in the presence of a thin layer", 1. Appl. Phys. 52 (8), (1981). 7. B.T. Khuri Yakub, "Air Coupled Transducers", in N.D. T. Handbook: Ultrasonic Testing, P. McIntire, Ed ANST, (1991). 8. M. Castaings and P. Cawley, "The generation, propagation and detection of Lamb waves in plates using air coupled ultrasonic transducers", 1. Acoust. Soc. Am. 1, (1996). 9. X. Jia, G. Quentin and M. Lassoued, "Optical heterodyne detection of pulsed ultrasonic pressures", IEEE Trans. of Ultrasonics, Ferroelectrics and Frequency Controls 4, (1993). 1. Jia X., Mattei Ch. And Quentin G.," Analysis of optical interferometric measurements of guided acoustic waves in transparent solids", 1. Appl. Phys. 77, (1995) 1452

7 II. B.D. Cook, "Measurements of the optical nearfield of an ultrasonically produced phase grating". J. Acoust. Soc. Am. 6, (1976). 12. D. Royer and E. Dieulesaint, "Optical detection of sub-angstrom transient mechanical displacement", IEEE Ultrason. Symp. Proc. pp (1986). 13. J.P. Monchalin, "Optical detection of ultrasound", IEEE Trans. UFFC 33, ( 1986). 14. Nagy P.B. and Adler L.," A novel technique for interface wave generation" in Physical Acoustics, Eds. O. Leroy and M.A. Breazale (Plenum Press, NY, 1991) pp

ULTRASONIC FIELD RECONSTRUCTION FROM OPTICAL INTERFEROMETRIC

ULTRASONIC FIELD RECONSTRUCTION FROM OPTICAL INTERFEROMETRIC MEASUREMENTS C. Mattei 1 and L. Adler NDE Program, UHrasonie Laboratory Ohio State University 190 W 19th Avenue Columbus, OH 43210 INTRODUCTION