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Proceedings of the International Conference on 205
Advances and New Challenges in Earthquake
Engineering Research, Hong Kong Volume
CRITICAL FACTORS FOR MAGNETORHEOLOGICAL
FLUIDS IN CIVIL STRUCTURES
J. David Carlson
Lord Corporation, Materials Division
406 Gregson Drive, Cary, North Carolina 27511, USA
E-mail: jdcarlson@lord com
ABSTRACT
Magnetorheological (MR) fluid technology has progressed to the point where it is now
routinely used on a large, commercial scale to enable semi-active control devices,
particularly for automotive application. Experience in developing MR fluids for successful
commercial application has shown that the greatest barriers to success of MR fluid
technology are not the factors originally presumed to be most important in the laboratory and
early development stages. While the most common responses to the question of what makes
a good MR fluid are likely to be "high yield strength" or "non-settling", other factors have
actually proven to be more critical. The present paper looks at conditions found in MR fluid
devices operating in real-world applications where shear rates may exceed 10 5 sec" 1 and
devices are called upon to operate for very long periods of time. The problem of "in-usethickening"
wherein a MR fluid subjected to long-term use progressively thickens until it
eventually becomes an unworkable paste is presented. The search for a solution to this
heretofore unrecognized problem delayed the successful introduction of commercial MR
fluid dampers for heavy-duty trucks for several years. Today, good MR fluids are able to
operate for long periods with minimum in-use-thickening and settling.