The behaviour of magnetorheological fluids in squeeze mode

Magnetorheological (MR) fluids possess rheological properties, which can be changed in a controlled way. These rheological changes are reversible and dependent on the strength of an excitation magnetic field. MR fluids have potentially beneficial applications when placed in various applied loading (shear, valve and squeeze) modes. The squeeze mode is a geometric arrangement where an MR fluid is sandwiched between two flat parallel solid surfaces facing each other. The distance between these two parallel surfaces is called the gap size. These surfaces are either pushed towards or pulled apart from each other by orthogonal magnetic-induced forces. In this study, a test rig was designed and built to perform the experiments with three different types of MR fluids. One type of water-based and two types of hydrocarbon-based MR fluids were activated by a magnetic field generated by a coil using different magnitudes of DC electrical current. To finalize the design, a Finite Element Method Magnetics (FEMM) was used to predict the magnetic field strength throughout the MR fluids. For each trial, combination of three process parameters were experimented in both compression and tension modes on each type of MR fluid. The three process parameters were the electric current applied to the coil, the initial gap size and the compressive or tensile speed. In every test, the speed and the current in the coil were kept constant, while the instantaneous compressive and tensile forces were recorded. Experimental results showed that MR fluids have distinct unique behaviour during the compression and tension processes. The behaviour of MR fluids was dependent on the relative movement between the solid magnetic particles and the carrier fluid in both squeeze modes. A high ratio of solid particles to carrier liquid in the MR fluid is an indication of high magnetic properties. The water-based MR fluid had a relatively large solids-to-liquid ratio. At a given applied current, significant increases in compressive and tensile stresses were obtained in this fluid type. On the other hand, the hydrocarbon-based MR fluids had relatively lower solids-to-liquid ratios, whereby, less significant increases in compressive and tensile stresses were obtained. The magnetic field strength was proportional to the applied current. Consequently, the MR effect, in terms of resulting stresses, was directly proportional to the current. When plotting stress against strain for each experiment, the slopes of the curves were found to be larger in general when the initial gap sizes were smaller. This was due to higher magnetic fields generated in smaller initial gap sizes. However, the stress-strain relationships were slightly affected by changing the compressive or tensile speeds. In general, the compressive stresses were much higher than the tensile stresses for the same experimental parameters throughout this study