885095990

technique, called laser ultrasonics, has the advantage of being noncontact. permits probing off curved surfaces and complex shapes and allows (he generation and detection of Iow frequencies as weil as high frequencics. In this presentation, the various physical principles used for laser generation and optical detection will be outlined. Several interferometrie detection Systems that have been developed in this laboratory will be reviewed and their merits and limitations for laboratory investigation or industrial inspection will be discussed. Several applications now being persued will bc presented, including various thickness gaging applications, the generation and detection of piąte waves and their use for evaluating piąte anisotropy, the determination of ultrasonic attenuation, and its use for evaluating Steel grain size.

8:30

4PA2. The generation of narrow-band and directed ultrasound using spatially and temporaliy modulated arrays. J. W. Wagner, J. B. Spicer, and J. B. Deaton, Jr. (The Johns Hopkins Univ. Ctr. for Nondestructive Evaluation, Maryland Hall 102, Baltimore, MD 21218)

Owing to their generally poor sensitivity relative to conventional contact ultrasonic methods, laser-based ultrasonic Systems havc proven to be cffectivc for only a limited rangę of applications, and only then with very careful and specific designs. Recent investigations at The Johns Hopkins University have been directed at the use of spatial arrays and temporal laser bcam modulation to generate ultrasonic signals which are both narrow band and may be directed over some angle within the test piece. The degrec to which sensitivity may be enhanced by these methods, however, is a strong function of the temporal and spatial naturę of the laser array source as well as the acoustic modę one wishes to excite. For example, variations in the dimension of each element in an array, array spacing, and array source rise time may all affect dramatically the degree to which one is able to generate narrow-band signals for high-sensitivity detection. Ultrasonic directivity issues are also somewhat morę complicated than they are represented in much of the current literaturę. In fact, the degree to which the ultrasonic energy may bc directed by laser array sources of any type is a strong function of the wave shape generated by each element in the array. Consequently, individual fcaturcs such as pulse height or pulse repetition, which may bc derived by superposition of the signals from elements of an array, may be directed over a rangę of angles in a solid materiał while total far-field energy directivity may remain unchanged.

8:55

4PA3. Photoacoustic monopole radiation from laser irradiated fluids. G. J. Diebold, M. I. Khan. T. Sun, and S. M. Park (Dept. of Chem., Brown Univ., Providence, RI 02912)

The temporal profile of photoacoustic waves generated by irradiation of fluid bodies with laser light is determined by the wave equation for pressure. Consider excitation where the heating is desenbed as a product of a spatial heating function multiplied by a delta function in time. Solution of the wave equation in one dimension shows that the deposition of heat in space is mapped directly into the time profile of the photoacoustic wave. In three dimensions, spherically symmetric deposition of heat gives a photoacoustic wave proportional to the product of the retarded time with a symmetrized spatial heating function. The wave equation also gives Solutions for photoacoustic waveforms generated by long light pulses. In one dimension the acoustic wave is proportional to exciting pulse, in three dimensions the emitted wave is proportional to the first derivative of the exciting radiation, whereas in two dimensions the acoustic wave is a complicated function of time. For long light pulses, the time profile of a photoacoustic wave depends only on the dimension of the wave and the time dependence of the exciting radiation Expressions for acoustic multipole radiation from irradiated bodies are derived as well. Acoustic waves generated by irradiation of fluids with the output of a Nd:YAG laser are compared with the theoretical results.

9:20

4PA4. Photoacoustic frequency-domain depth profiling of continuously inhomogeneous solids. Theory and quantitative profilomctry of octylcyano-biphenyl (8CB) liąuid crystals. Andreas Mandelis (Photoacoustic and Photothermal Sciences Lab. and Ontario Laser and Lightwave Res. Ctr., Dept. of Mech. Eng., 5 King’s College Rd., Univ. of Toronto, Toronto, Ontario M5S 1A4, Canada), Els Scoubs (Katholieke Univ. Leuven, Leuven B-3030, Belgium), Samuel B. Peralta (Univ. of Toronto, Toronto, Canada), and Jan Thoen (Katholieke Univ. Lcuven, Leuven, Belgium)

An application is presented of the Hamilton-Jacobi formulation of thermal-wave physics (A. Mandelis. J. Math. Phys. 26, 2676 (1985)] to the problem of photoacoustic depth profiling in inhomogeneous solids with arbitrary, continuously varying thermal diflfusivity profiles. Simple expressions for the modulation frequency dependence of the photoacoustic signal in the case of exponential thermal diffusivity profiles are obtained, and a working generał method for solving the inverse problem and obtaining arbitrary diflusivity depth profilem is dcmonstratcd through computcr simulations. The method wae found to possees excellent profile reconstruction fidclity. As a first experimental application of the theory, an observed change in the photoacoustic signal frequency response upon the application of a transverse magnetic field across octylcy-anobiphenyl (8CB) samples in the nematic phase at 37 *C is reported. The theory has given quantitative profiles of thermal diffusivity decreases extending to 20-30 fim below the liquid crystal surface. These decaying depth profiles are qualitatively consistent with earlier photoacoustic temperaturę seans of liquid

1909

J. Acoust. Soc. Am., Vol. 89, No. 4, Pt. 2. Aprił 1991

121 st Meeting: Acoustical Society of America

1909