Using Mason number to predict MR damper performance from limited test data

Andrew C Becnel,Norman M Wereley

doi:10.1063/1.4977226

Abstract

The Mason number can be used to produce a single master curve which relates MR fluid stress versus strain rate behavior across a wide range of shear rates, temperatures, and applied magnetic fields. As applications of MR fluid energy absorbers expand to a variety of industries and operating environments, Mason number analysis offers a path to designing devices with desired performance from a minimal set of preliminary test data. Temperature strongly affects the off-state viscosity of the fluid, as the passive viscous force drops considerably at higher temperatures. Yield stress is not similarly affected, and stays relatively constant with changing temperature. In this study, a small model-scale MR fluid rotary energy absorber is used to measure the temperature correction factor of a commercially-available MR fluid from LORD Corporation. This temperature correction factor is identified from shear stress vs. shear rate data collected at four different temperatures. Measurements of the MR fluid yield stress are also obtained and related to a standard empirical formula. From these two MR fluid properties – temperature-dependent viscosity and yield stress – the temperature-corrected Mason number is shown to predict the force vs. velocity performance of a full-scale rotary MR fluid energy absorber. This analysis technique expands the design space of MR devices to high shear rates and allows for comprehensive predictions of overall performance across a wide range of operating conditions from knowledge only of the yield stress vs. applied magnetic field and a temperature-dependent viscosity correction factor.

Highlights

When combined with an appropriate feedback control system, magnetorheological energy absorbers (MREAs) adjust their energy absorption characteristics – especially stroking load – to variations in the system or its inputs, such as occupant weight or impact velocity
Linear stroking MREAs using pressure-driven flows have been shown to exhibit reduced controllability as impact speed increases.[1]. This has motivated the development of MREA designs that overcome the high speed limitations of linear stroke MREAs in the form of rotary, shear mode MREAs
Shear mode operation allows for higher dynamic range (DR ≥ 3) due to lower field-off stresses, since the force generated in shear mode operation is linearly proportional to the input velocity, or F ∝ v

Summary

Introduction

When combined with an appropriate feedback control system, magnetorheological energy absorbers (MREAs) adjust their energy absorption characteristics – especially stroking load – to variations in the system or its inputs, such as occupant weight or impact velocity. When normalized apparent viscosity of an MRF is plotted vs Mason number across the range of shear rates and magnetizations, the data collapse to a single curve. The Mason number requires the carrier fluid viscosity as an input parameter, but the specific formulation of the hydrocarbon carrier fluid used by LORD Corporation is proprietary.

Results

Conclusion