| Magnetorheological fluid(MRF)is a suspension composed of magnetic particles,additives and base fluid.The magnetic particles will form an ordered magnetic chain structure under the action of an external magnetic field,which will change with the strength of external magnetic field.Therefore,its mechanical properties,especially the yield stress,will react in millisecond-level and reversibly with the control of external magnetic field,which is also the premise of wide application of MR fluid in many fields.Because of its wide application prospect,MRF has become one of the most potential intelligent materials in recent years.At present,the research on the stability characteristics of MRF settlement mostly focuses on the characterization of settlement stability and how to improve its settlement stability.Based on the characteristics of shear yield stress caused by the change of physical properties during the settling process of MRF,this paper proposes a new idea to characterize the stability of MRF settlement based on changes in shear yield stress is to study the settlement stability of MRF during long-term standing,that is,to evaluate the stability characteristics of MRF settlement using shear yield stress changes.First of all,based on the analysis of the current situation of the research on the characterization methods and shear yield stress testing devices of the existing MRFs,this paper puts forward an improved method for the shortcomings of the existing characterization methods and shear yield stress measuring devices,and according to this improved method,a shear yield stress test bench based on the pull-out shear principle is proposed for shear yield stress measurement.The shear yield stress in the process of settlement was used to characterize the settlement stability of MRF.Secondly,based on the lifting method,the shear yield stress testing device of MR fluid is designed.The lifting block with 1 mm deep copper rectangular groove wall feature is selected.The magnetic circuit is made of neodymium iron boron permanent magnet,and the shear yield stress when yielding is recorded by the pull pressure sensor.According to different HLB values,sodium dodecylbenzene xanthate,polyethylene glycol and oleic acid were selected as surfactants,molybdenum disulfide,boron nitride,graphite,hydrogenated castor oil and polytetrafluoroethylene were selected as lubricants according to the safety of materials,and 9 kinds of MRF formulations were designed and prepared.Using the designed shear yield stress test device,the shear yield stress test experiment was carried out for different surfactants and lubricants.The experimental period was 36 days,which was used to study the relationship between shear yield stress and settlement stability of MRF.In addition,the least square method is used to fit the settling stability curve of MRF,which can be used to predict the change of settling stability in the next 90 days,and the experimental value of the 60 th to 75 th days is compared with the theoretical value for verification.Finally,the recovery experiment of MRF was carried out by mechanical agitation.On the one hand,taking Formula 1 as an example,the recovery ability of MRF samples was observed after 90 days standing.On the other hand,MRF-122 EG,MRF-132 DG,and MRF-140 CG,which have been settled for 587 days,were recovered and compared with the original technical parameters.Through the recovery experiments of self-made MRF,and Lord's MR fluid,the recovery ability of MR fluid under the two processes was compared. |
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