As part of an emerging effort in what is now termed the area of mechamatronics (Browne et al., 2004), an effort was begun to assess the suitability of magnetorheological (MR) material-based devices for impact energy management applications. A fundamental property of MR materials is that their yield stress alters almost instantaneously (and proportionally) to changes in the strength of an applied magnetic field. Based on this property, MR-based devices, if found suitable, would be desirable for impact energy management applications because of attendant response tailorability. However, it was identified that prior to adopting MR-based devices for impact energy management applications several key issues needed to be addressed. The present study focuses on one of the most significant of these, the verification of the tunability of the response of such devices at stroking velocities representative of vehicular crashes. Impact tests using a free-flight drop tower facility were conducted on an MR-based energy absorber (shock absorber) for a range of impact velocities and magnetic field strengths. Results demonstrated that over the range of impact velocities tested — 1.0—10 m/s — the stroking force/energy absorption exhibited by the device remained dependent on, and thus could be modified by, changes in the strength of the applied magnetic field.
SynopsisAn experimental investigation of the rheological response of magnetorheological suspensions subjected to step changes in applied magnetic field strength at fixed shear rate is reported. For small applied field strengths, the shear stress increases rapidly to a steady value. Above a critical field strength, the rapid initial increase in shear stress is followed by a slow, transient increase in stress. The critical Mason number corresponding to the critical magnetic field strength at the onset of this transient depends on the particle volume fraction as well as the shear rate. This is in contrast to a previous analysis where the critical Mason number was predicted to depend on only the particle volume fraction. The discrepancy is attributed to colloidal forces that are significant in our experimental system, but were not included in the analysis. Further comparison with the previous analysis requires either including the effects of colloidal forces, or performing experiments with systems in which colloidal forces are not important.
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