885095925

THURSDAY MORNING, 2 MAY 1991

CARROLL, 8:30 TO 11:45 A.M.

Session 6PAa

Physical Acoustics: Scattering I

Michael R. Stinson, Chair

National Institute for Microstructural Sciences, National Research Council. Ottawa. Ontario KIA 0R6, Canada

Contributed Papers

8:30

6PAal. Ray synthesis of backscattering by thin cylindrical shells. N.

H. Sun and P. L. Marscon (Dept. of Physics, Washington State Univ., Pullman, WA 99164-2814)

In previous research [P. L. Marston, J. Acoust. Soc. Am. 83, 25-37 (1988)] the leaky Lamb wave contributions f, to the form function for backscattering from right circular cylindrical shells were expressed in terms of the phase velocity rafio c/c, a radiation damping parameter 0, in Np/rad, and a coupling coefficicnt G, with |ćr,| zi%n0/{irka)^xn. It was subsequently shown that the phase arg(ć?/) ~rr/4. The present research concems the exten$ion to contributions termed “crceping and trapped" waves from the far-field limit of an analysis (when corrected) by Felsen et al. (J. Acoust. Soc. Am. 87, 554-569 (1990)]. Such con-tributions are especially important Tor understanding scattering from thin shells. The extension of Marston’s original cquations rcquires that when c,<c, the effective angle of incidence 0, for launching a guided wave bccomes rr/2 instead of the previous expression sin 0i=c/cf, also, arg(G/) Łs rcvised when c,źc. For example, for trapped waves arg((7/) becomes z: — rr/4. The expression for f, allows the caustic radius b,=ac/ci to be larger than the radius a of the shell. Comparisons with cxact computations for 2.5% thick hollow Steel cylindcrs support the approximations for the leaky and trapped wave contributions. The ka dependence of leaky, creeping, and trapped wave parameters are exam-ined. [Work supported by ONR.]

8:45

6PAa2. Ray synthesis of the angular dependence and backscattering by thick cylindrical shells. P. L. Marston and N. H. Sun (Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814)

Previous research has confirmed ray models of leaky Lamb wave contributions f are applicable to the forward and baekward scattering from thick spherical shells [S. G. Kargl and P. L. Marston, J. Acoust. Soc. Am. 88. 1103-1113 (1990); and work in press]. The present research concerns the extension of the ray models to leaky Lamb waves that tend to dominate the elastic scattering contributions of thick cylindrical shells. The extension is important because the cylinder model should be generalizable to noncircular-shapes where Solutions for the scattering may not be available. A prcvious analysis found that for cylinders, the coupling coefficient becomes 1^7,1 ~%Tr0/(nka)^xn with the phase arg(G/)^n-/4, where 0_f is the radiation damping parameter. The present computations show that tor a 16.2% thick stainless-stccl cylinder, a superposition of leaky and specular contributions synthesizes the exactly computed scattering for ka as smali as 7. The scattering as a function of angle was synthesized at ka = 20 for the fuli 180* rangę. The synthesis combined leaky Lamb wave contributions with a back-ground contribulion of a rigid cylinder. The agreement with exact computations was good except near ray transition regions where paths changed abruplly with changes in the scattering angle. (Work supported by ONR.]

9:00

6PAa3. Scattering by thick cylindrical shells: Ray methods for shell reverberations and expcriments. P. L. Marston, N. H. Sun, S. P. Parry, and D. H. Hughes (Dept. of Physics, Washington State Univ., Pullman, W A 99164-2814)

A prerious analysis of internat reverberations [S. G. Kargl and P. L. Marston, J. Acoust. Soc. Am. 88, 1114-1122 (1990)] gave the cur-vature correction to the specular contribulion J,^ to the form function for backscattering from spherical shells. This was extended to the case of circular cylinders at normal incidence. The result for Ls R esp( — 2ix) + f_cc where the curvature correction is

/V"
(l +nB)	^IJj

)(^)

with x=ka and the other symbols are as defined for Eq. (8) of the previous analysis for fluid spherical shells. Comparisons with exact partial-wave scries computations for fluid shells show good agreement. They exhibit a dip in [/^j when x is smali and when x approaches the thickness resonance. The features are generally similar to those for spheres. The relevance of f^ to the calibration of tank experiments on the scattering of short tonę bursts from elastic shells rs examined. The experiments separate leaky Lamb wave contributions from specular contributions. [Work supported by ONR.]

9:15

6PAa4. Analysis of the acoustic scattering from large aspect ratio, axisymmetric shells. Roger H. Hackman and T. Fang (Pało Alto Res. Labs., Orgn. 91-50, Bldg. 251, 3251 Hanover St., Pało Alto, CA 94304-1191)

The development of theorctically sound, ^,‘exacl" numerical Solutions to the scattering from spheroidal shells is important both to es-tablish a nontrwial benchmark and to establish those salient features that may be characteristic of a larger class of morę complicated, but related geometries. In prcvious meetings [J. Acoust. Soc. Am. Suppl. I 83, S94 (1988); 82. S49 (1987)]. the prediełions of the spheroidal-coordinate based transition matrix formalism was validated for such targets by a direct comparison with experimental measurements of the backscattered form function of a copper prołate spheroidal shell for generał angle of incidence. In these studies, a number of open questions emerged pertaining to the prominent elastic features of the form func-

1949 J. Acoust. Soc. Am., Vol. 89, No. 4, Pt. 2. April 1991

121 st Meetirtg: Acoustical Society of America 1949