# Performance Analysis of Magnetorheological Porous Fabric Composite
## 3 Discussion
### Shear Stress  
Figure 6 shows the variation curves of shear stress with the shear rate of each test sample under zero-field conditions. Because there is no rheological effect in MRF, its shear  
Figure 5: Variation of viscosity with shear rate under different magnetic flux densities. stress is mainly provided by the shear friction of the material itself. The maximum shear stress is about 42.8 Pa, which almost does not change with the shear rate, and the liquid shows the characteristics of Newton fluid. The untreated sample group (the lithium stearate content is 0 wt%) also changed similarly to the MRF group, but due to the porous structure of the non-woven fabrics, the mechanical properties of the untreated sample were improved to a certain extent, and the maximum shear stress is about 1.5 kPa. For the samples treated by immersion, the shear stress increases greatly under zero-field and increases with the increase of the mass fraction of lithium stearate. This is mainly because the samples are not affected by the magnetic field, and the shear stress is dominated by the friction between particles scattered inside the material. With the increase of the mass fraction of lithium stearate, the shear stress also increases. In order to further analyze the effect of the pretreatment for non-woven fabric on the composites, the output properties of each sample under different magnetic fields were tested. Figure 7 shows the variation curves of shear stress with the shear rate of four composite samples under different excitations, and the variation laws are roughly the same. At a low shear rate, the rheological effect occurs under the excitation of the magnetic field, and the shear stress of each sample increases greatly. The rheological effect makes the magnetic particles form a chain-like structure along the direction of the magnetic field, and the structure becomes more stable with the increase of the magnetic flux density. When the sample material is subjected to shear action, the chain-like structure formed by magnetic particles will undergo a dynamic \"break-recover\" process, thus showing the effect of shear stress. For the untreated sample, the maximum shear stress is about 61.1 kPa. As can be seen from Figure 7, when the non-woven fabric is soaked in lithium stearate solution, the mechanical properties of the composites do not always improve with the increase of the mass fraction of lithium stearate. Consistent with the apparent viscosity of the material, the shear stress of the 1 wt% sample is higher than that of the untreated sample, and the maximum shear stress is about 67.6 kPa. Under the same working condition, the shear yield strength of the 1 wt% sample is higher than that of the untreated sample, and the highest yield strength is increased by 52%. As the mass fraction of lithium stearate continued to increase, the mechanical properties of the samples began to decrease. The shear stress of the 1.5 wt% and 2 wt% samples are 64.7 kPa and 59.4 kPa, respectively. Figure 6: Variation of shear stress with shear rate under zero-field. It is obvious from the experimental results that the mechanical properties of the samples will not continue to improve with the increase of lithium stearate mass fraction of the non-woven fabric. When the mass fraction of lithium stearate increases to 2 wt%, the shear stress decreases compared with the 1 wt% sample. Mainly because with the increase of lithium stearate content, a part of the lithium stearate attached to the surface of non-woven fabric will be filled in the micro-porous structure of non-woven fabric, thus reducing the effect of non-woven fabric. Figure 8 shows the shear stress comparison curve of the MRF, untreated and 1 wt% samples. As can be seen from the figure, the shear stress of the untreated sample is enhanced compared with that of the MRF sample at zero-field due to the material properties of the non-woven fabric.