Damping in Sandwich Beams With Shear-Flexible Cores

This paper is a theoretical and experimental study of the effect of core shear flexibility on the lowest natural frequency, node locations, and damping in sandwich beams with cores of high shear flexibility as exemplified by honeycomb-type cores. A new method of analysis is presented for predicting the logarithmic decrement for damping in sandwich beams undergoing free vibration, provided that the beam geometry and constitutent material properties are known. Natural frequency, modal shape, and logarithmic decrement are all dependent upon the dynamic shear coefficient. Two new simplified derivations for this coefficient are presented in this paper. Flexural vibration experiments were conducted on free-free sandwich beam strips at frequencies from 300 to 700 cps. Facings were glass-epoxy laminates and cores were hexagonal-cell honeycomb of either aluminum or glass-phenolic. For each beam, lowest natural frequency, associated node locations, and logarithmic decrement in free vibration were measured and compared with those predicted by applying four different theories.