Long Term Behavior of Semi-Active Magnetorheological Dampers

The adoption of semi-active (SA) structural control systems based on magnetorheological (MR) dampers in earthquake engineering is at the moment highly questionable due to lack of experimental information about the long term behavior of MR fluids and MR dampers. As a matter of a fact, MR fluids adopted in smart dampers are essentially made up of magnetizable particles suspended in a fluid carrier. The MR effect is then associated to the uniform distribution of such particles in the fluid. Due to gravity, magnetizable particles tend to settle down in time, making the damper not able to perform as expected. This phenomenon has little or no importance in applications where SA MR dampers are continuously excited. For instance, in smart car dampers, the almost continuous stroke activity shakes the fluid and counteract the settling effect. In earthquake applications, SA MR dampers could stay still for decades before starting to work and, therefore, particles settling could make the dampers useless at the very moment they are needed. In the present paper an experimental activity on SA MR dampers is presented. On a purposefully made testing apparatus, the SA MR dampers have been tested and fully characterized. The devices have been then stored on a shelf of the laboratory for about 5 years, after that one of the original test (harmonic cycles at imposed frequency and displacement amplitude) has been repeated. The comparison of the results may give an interesting insight on the long term behavior of SA MR dampers.