Field-Induced Dynamic Impedance Characteristics of a Magnetorheological Damper

A key challenge in evaluating magnetorheological (MR) dampers for rotor systems is that magnetic-field-induced force enhancement does not directly indicate useful energy dissipation. The dissipative effectiveness of the increased force depends on the coupling between dynamic impedance reconfiguration and motion-dependent energy exchange. To address this issue, this work develops a dynamic impedance formulation for MR dampers based on a proposed smooth-transition dynamic model. The formulation identifies current-induced reconfiguration of local stiffness and damping, traces its hydrodynamic origin through pressure sensitivity fields and contribution density maps, and evaluates the conversion of reconfigured force into dissipated energy using an orbit energy measure. Dynamic characteristic experiments on a damper prototype further verify the measurable evolution of dynamic impedance under current excitation. The results reveal that current excitation does not merely increase the damper response, but reorganizes the distribution of storage and dissipation within the dynamic impedance. This reorganization is strongly direction dependent, making the increased force more effective for certain motion patterns than for others. The force enhancement and useful energy dissipation are not intrinsically equivalent. The same increase in controllable oil-film force can contribute to net energy dissipation when it is compatible with the dominant impedance channel, or be redirected into reversible stiffness-related energy exchange when this compatibility is weakened. A phase-resolved energy decomposition further indicates that the reduction in force-normalized dissipative effectiveness originates from stiffness-induced energy return and phase-wise offsetting between dissipative and reversible energy components, rather than from negative damping. These findings redefine MR dampers as field-reconfigurable dynamic impedance elements and provide a physics-based criterion for selecting current, tailoring impedance channels, and improving vibration suppression reliability in adaptive rotating machinery.