Analysis of impact loads in a magnetorheological energy absorber using a Bingham plastic model with refined minor loss factors accounting for turbulent transition

Abstract

This study presents the improvement of a Bingham-plastic damper model incorporated with refined minor losses (BPM damper model) by considering fluid friction models so as to predict the stroking load of a magnetorheological energy absorber (MREA) during high speed impact. MREAs produce a variable stroking load that adapts to a range of payload mass and/or impact velocity by applying a magnetic field. When used for impact protection, design goals are: (1) to provide optimal stroking load over a wide range of impact velocities without exceeding the maximum allowable force, and (2) maintain a high dynamic range, defined as the ratio of maximum field-on force to field-off force, at high impact speeds. Thus, it is critical to accurately predict the field-off damper force at high piston velocity. In order to more accurately capture the exponential increase in damper stroking load with piston velocity, various velocity squared dependent minor losses were added to the model. The increased roughness of the walls inside the fluid flow orifice produced by the electromagnetic coils was taken into consideration, and the combination of smooth and rough walls was adopted. A piecewise formula for predicting Darcy friction factor for fluid flowing through a rough pipe in the laminar, transition, and fully turbulent regimes was implemented. An MREA was fabricated and drop tested to obtain stroking load data in the field-off and field-on states. The refined damper model was shown to more accurately predict damper force in both the low and high speed ranges than previously tested BPM damper models.