Conventional lag dampers use passive materials, such as elastomers, to dissipate energy and provide stiffness, but their damping and stiffness levels diminish markedly as amplitude of damper motion increases. Magnetorheological (MR) fluids based dampers have controllable damping with little or no stiffness. In order to combine the advantages of both elastomeric materials and MR fluids, semi-active magnetorheological fluid elastomeric (MRFE) lag dampers are developed in this thesis. In such a damper configuration, magnetic valves are incorporated into the chamber enclosed by elastomeric layers. Preliminary MRFE damper design analysis was conducted using quasi-steady Bingham-plastic MR flow mode analysis, and MRFE damper performance was evaluated analytically. To investigate the feasibility of using a combination of magnetorheological (MR) fluids and elastomeric materials for augmentation of lag mode damping in helicopters, a semi-active linear stroke MRFE lag damper was developed as a retrofit to an existing elastomeric helicopter lag damper. Consistent with sinusoidal loading conditions for a helicopter lag damper, single frequency (lag/rev) and dual frequency (lag/rev and 1/rev) sinusoidal loadings were applied to the MRFE damper. Complex modulus and equivalent damping were used to compare the characteristics of the MRFE damper with the passive elastomeric damper. The experimental damping characteristics of the MRFE damper were consistent with the analytical results obtained from the Bingham plastic analysis of the MR valve. Based on measurements, the Field-OFF MRFE characteristics are similar to the passive elastomeric damping, and controllable damping as a function of different flight conditions is also feasible as the applied current is varied in the MR valve.

A second key objective of the present research is to develop an analytical model to describe the nonlinear behavior demonstrated by an MRFE damper. Since the damping behavior of both elastomers and MR fluids is dominated by friction mechanisms, a rate-dependent elasto-slide element is developed to describe the friction characteristics. An MR model developed from a single elasto-slide element successfully emulated the yield behavior of the MR damper, and this model captured nonlinear amplitude and frequency dependent behavior of MR dampers using constant model parameters. Meanwhile, using a distributed elasto-slide structure, an elastomeric model was developed to describe the stiffness and damping behavior of the elastomer as the amplitude of excitation increases. The fidelity of this five parameters time domain model is demonstrated by good correlation between modeling and experimental results for both the complex modulus and steady-state hysteresis cycles. Since an MRFE damper was shown to be a linear combination of the elastomeric and MR component, a time domain MRFE damper model was constructed based on the linear combination of the MR and elastomer models to describe the nonlinear behavior of the MRFE damper. Good correlation between the model and experimental data demonstrates the feasibility of the MRFE model for future MRFE damper applications.