ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

5aPA4. Relation between the damping coefficient and the excitation coefficient of leaking Lamb waves.

M. Ech Cherif El Kettani

F. Luppe

J. M. Conoir

J. Ripoche

L.A.U.E., U.R.A. C.N.R.S. 1373, Univ. du Havre, pl. R. Schuman, 76610 Le Havre, France

The relation between the leaking ability and the excitation coefficient of a propagating Lamb wave on an immersed plate is studied. The more the Lamb wave radiates during its propagation, the more strongly it is expected to be excited from the incidence of an ultrasonic beam at the appropriate angle. The excitation coefficient of a given Lamb wave at a given frequency--thickness product is obtained from a residue calculation. It is numerically achieved from the exact formulation of the reflection coefficient, and analytically performed with the help of the resonance scattering theory approximations. Both methods exhibit clearly the proportionality of the excitation coefficient with the normalized damping coefficient. The direct measurement of the excitation coefficient is not possible because of the interference phenomenon of the Lamb wave radiation with the specularly reflected beam. Consequently, the measurements are performed far away from the interference area. The linearity of the pressure amplitude (in dB) with the propagation distance allows the direct determination of the damping coefficient, as well as the extension of the experimental data to the interference area. The excitation coefficient is herefrom deduced. The results confirm the theoretical ones. [Work supported by D. R. E. T., France.]