Stand-off free layer damping is a vibration reduction treatment established on the traditional free layer damping structure. Based on the theory of stand-off free layer damping, polyurethane-rubber stand-off layer damping material was prepared by using rigid polyurethane foam as stand-off layer and rubber as damping layer for suppressing structural vibration. With considering the shear deformation of damping layer, the equation of motion for cantilever beam treated with the composite material was derived by the principle of modal superposition and Lagrange equation. The composite damping material with the thickness ratio of 1 to 3 and segmented stand-off layer was applied to the cantilever steel beam. The amplitude-frequency curves, loss factors, and mode frequencies were analyzed by the hammering test. The results showed that when the thickness ratio of the stand-off layer to the damping layer is increased from 1 to 3, the vibration response peaks of the model are reduced by 13%～62% and the loss factors are also increased obviously. When the ratio exceeds 1.5, the damping performance of the material on the low-order mode of the beams continues to increase, while the damping effect of the higher-order mode tends to be stable. As the results of the segmented stand-off layer, its bending stiffness is reduced and the processing area of the damping layer gets larger. The vibration response peaks of the models are reduced by more than 16%, while the additional mass of damping treatment is not significantly increased. It is concluded that the vibration reduction performance of the composite damping material could be better by increasing the thickness ratio of the stand-off layer to the damping layer, especially at low frequency. The design of segmental stand-off layer is helpful to further improve the damping performance based on composite materials.
