The feasibility of integrating the enhanced active constrained layer (EACL) and active}passive hybrid constrained layer (HCL) treatments to achieve a better combination of the system's closed-loop damping and open-loop (fail-safe) damping (without active action) is investigated in this research. Given a uniform strain "eld in the host structure, the EACL with sti! and equal edge elements (symmetric EACL) has been shown to provide high closed-loop damping by signi"cantly increasing the direct active control authority of the cover sheet. The open-loop damping of the system, however, could be low. On the other hand, the HCL has been demonstrated to o!er more balanced open-loop and closed-loop damping actions, although the HCL closed-loop damping is not as high as that of the EACL. The idea here is therefore to combine the two approaches and develop an integrated HCL}EACL treatment. The focus is to maximize the system closed-loop damping while maintaining an open-loop damping margin for fail-safe reasons. For a given strain "eld in the host structure, optimization routines are used to search for the best design parameters: the optimal control gain, the sti!ness of the edge elements and the active material coverage ratio in the constraining layer. It is found that integrating the EACL and the HCL will introduce more #exibility in the design of constrained layer damping treatments with actively enhanced actions. Higher open-loop damping can be achieved for the same closed-loop damping requirement and vice versa. The hybrid cover sheet is found to create signi"cant shear in the viscoelastic layer while the edge elements are used to provide strong direct active control authority for the constraining layer. A better mixture of the open-loop and closed-loop damping can generally be obtained with the integrated system.
2002 Elsevier Science Ltd. All rights reserved.
1. BACKGROUND
Active constrained layer (ACL) is a treatment that can provide both closed-loop and open-loop damping (fail-safe damping with no active e!ects) actions [1}3]. Such a system generally comprises a layer of viscoelastic material (VEM) sandwiched between a host structure and an active cover sheet, such as a piezoelectric layer. The active constraining layer can create active shear in the VEM as well as apply direct control action to the host structure. When the active action fails, the constraining layer behaves passively and can still