Quantification of damping mechanisms of active constrained layer treatments

In active constrained layer (ACL) treatments, a layer of viscoelastic material is bonded to the host structure and constrained by an actuator. Since several mechanisms simultaneously damp vibrations in ACL treatments, quantifying these mechanisms is essential to understand and optimise these treatments. Several approaches in the literature quantify ACL damping mechanisms. However, these approaches do not take into account the reduction of input power into the structure that can be induced by the actuator, although it is known that this reduction is an important mechanism in treatments based on active control. This paper proposes an approach that allows quantification of all the damping mechanisms of ACL treatments. In this approach, three indices quantify the efficiency of the three identified damping mechanisms: the shearing of the viscoelastic layer in open loop, the increase of shearing in the viscoelastic layer due to the motion of the actuator, and the decrease of total input power into the structure due to the forces applied by the actuator through the viscoelastic-layer. The sum of these two last indices quantifies the efficiency, of the active actions, and the sum of all three indices quantifies the total efficiency of the ACL treatment. In order to implement the approach, a wave approach model of beams treated with ACL is selected from the literature and is experimentally validated. The model is used to find numerically the control voltage optimising the efficiency of the active action, and to calculate the resulting quantification indices for the six first modes of partially or fully treated beam. Results indicate that with a high shear modulus of the viscoelastic material, the optimum control voltage is lower and less sensitive to changes of the stiffness that might occur with temperature variations. A high shear modulus also leads to a better complementarity between the active and passive frequency of the ACL treatment, resulting in a broader effective frequency range. The effect of the forces applied to the structure is to minimise the total input power rather than to maximise the absorption of power by the actuator, suggesting that control laws ensuring absorption of energy by the actuator (and thus the stability of the control) might be suboptimal in terms of vibration reduction. It can be concluded that the quantification indices presented in this work are a powerful tool to get insight into the damping mechanisms of ACL. (C) 2004 Elsevier Ltd. All rights reserved.