On the relation between the Mason number and the durability of MR fluids

Abstract

The durability of magnetorheological (MR) fluid is a fundamental aspect of any MR device, as MR fluid is submitted to shear stresses that cause its degradation. While various studies have identified MR fluid degradation behaviors and proposed improvements, there exists no generalized failure theory to help understand the in-use degradation of MR fluids. In this regard, this study suggests that a relation exists between the Mason number and the maximum achievable lifetime dissipated energy (LDE) of MR fluids. To validate this hypothesis, custom (perfluoropolyether) and commercial (hydrocarbon) MR fluids are aged in various Mason number conditions using a clutch-type durability device. Experimental results show that under a critical Mason number of ∼1, LDE of the MR fluids can be related to the Mason Number using an analogy to metal fatigue failure. Above this critical Mason number, the fundamental degradation mechanism appears to change from a dominant mechanical wear of the particles (i.e. particle-limited) to another mechanism (i.e. fluid-limited) which is not yet fully understood. Since MR generally operate well below the critical Mason number of ∼1, the relation between the Mason number and the particle-limited regime durability of MR fluids can be very useful to design novel MR fluids or MR devices, as it provides a way to estimate the impact of various design strategies on the total expected life of the device.