Andrew N. Norris
Dept. of Mech. Eng., Rutgers Univ., Piscataway, NJ 08855
Gerry V. Storch
Exxon Res. and Eng., Annandale, NJ 08801
A new method is proposed for attenuation of reflected energy at the edges of plates and bars, using a graded impedance interface. A standard approach is to embed the plate edges in sand. However, it has been verified that a graded impedance interface at the edges is far more effective in damping the radiating structural energy. It is well known that impedance mismatch causes complete reflection of energy at the free edge of a plate. Hence, it seems reasonable to expect a reduction in reflected energy when impedance is varied gradually. Experimental results for 1-in. plates indicate that at most 30% of the energy is damped for frequencies above 2 kHz when sand is used, whereas as much as 60%--80% of the energy is damped between 2 and 10 kHz using a graded impedance. Experiments on bars also give similar results. A theoretical formulation for a bar with graded impedance interface at the edges is presented, using both the Kirchhoff and Mindlin theories. The Mindlin theory predicts the level of energy damping that is observed in experiments, while the Kirchhoff theory predicts much lesser energy damping. The shear effects are modeled more accurately in the Mindlin theory and they seem to be important in this problem.