Study On Mechanical Behavior Of MR Fluids And Dynamic Response Of MR Damper For Impact Mitigation Applications

Benefited from its outstanding properties of mechanical simplicity,high dynamic range,low power requirements,quick response and robustness,magnetorheological(MR)damper has drawn intensive attention during the last decades.Adaptive energy absorbers(EAs)utilizing magnetorheological fluids(MRFs)are currently being investigated for large force capacity and severe impact or shock mitigation problems because magnetorheological energy absorbers(MREAs)can adjust stroking load to account for severity of impact and payload mass.The severe impact has features with low frequency of occurrence,a short duration and intensive vibration energy.Considering the features of impact,the MR damper should have large stroke,large dynamic range and small response time.As for the MR fluids,it should have high suspension stability,high yield shear stress and low off-state viscosity.In order to address the problems mentioned above,theoretic analysis,numerical simulations,FEM simulations and experiments were conducted,thorough research on the mechanical behavior of the MR fluids and dynamic response characteristics of the MR energy absorber for impact mitigation applications were conducted.The main content of this thesis are:(1)Thorough research were conducted on the scanning measuring the particle concentration profile of MR fluids.A non-invasive inductance method of monitoring the characteristic sedimentation of MR fluids column was applied to continuously monitor the sedimentation process of various MR fluids.Two measures were introduced to address the problem of bad accuracy due to the averaging effect of inductance sensor.On one hand,a low aspect ratio solenoid(LARS)featured inductance sensor was developed to obtain higher measuring displacement resolution;on the other hand,a de-convolution based method of concentration profile reconstruction was proposed to obtain more convincing concentration profile.Furthermore,a concise and effective modification method was proposed to superess the Gibbs phenomenon,and then a series of numerical simulations and validation experiments were conducted to verify its efficiency.(2)A method of sedimentation zones identification based on concentration gradient profile was proposed,and numerical simulations of the sedimentation process were conducted based on the Kynch’s sedimentation theory and finite volume method(FVM).The concentration gradient profile based sedimentation zones identification method was proposed based on the fact that the boundaries between different sedimentation zones are always exist at the place where the concentration with a rapid change.The proposed method can be used to identify the time evolution of whole sedimentation zones accurately and with good physics understanding.Flux density function curves were obtained based on the experimental data and FVM was used to simulate the sedimentation process of MR fluids.The time evolution concentration profile of MR fluids with given initial particle concentration and height was achieved and the time-based evaluation parameters were used to predict the service life of MR fluids in the numerical simulations.(3)Rheological characterizations over a large shear rate range(up to 22,000 1/s)of various MR fluids were conducted using a custom Searle type magnetorheometer.For addressing the gap between the shear rate range of MR fluids characterization and the real practical condition in impact mitigation applications,a custom Searle type magnetorheometer was developed for generating high shear rate,and achieved experiments of MR fluids characterization over a large shear rate range(up to 22,0001/s).The experiments of characterization indicate the intensive shear thinning effect of a lab-made High Viscosity Linear Polysiloxane(HVLP)MR fluids.Besides,a revolving speed based varying parameter moving average filter was proposed for denoising the measured torque with noise of various frequency and amplitude effectively,and utilize high sampling frequency in recording to obtain sufficient data for characteristic curves,then the characteristic curve of MR fluids over a large shear rate can be obtained using the data of a single measurement,and needn’t to fit the data from different measurements.Then the characterizations of MR fluids based on Mason number were conducted,the apparent viscosity curves of different excitation currents collapse into a single master rheology curve.The Mason number characterization offers a path to measure the rheological behavior with a finite measurements.Besides,another useful function is to identify the undesired data from measured curves.(4)Thorough study on the formation time of magnetic field in MR damper,several schemes were proposed to shorten the response time by suppressing the generation of eddy current and then validated using transient FEM simulations.A Searle type MR damper was used as object of study,the formation time of magnetic field was experimentally determined as the dominated factor to compose the response time of MR damper.Then the analysis based on magnetic diffusion theory was conducted to demonstrate the step-field on an ideal conductor,and the expressions of response time constant for slab conductor and rod were extracted.Thus,several schemes of structure optimization and materials selection were proposed to shorten the magnetic field formation time.The FEM simulations validated that the proposed schemes can suppress the eddy current effectively.At last,the proposed schemes were validated using 3D transient FEM simulations with the excitations including rising edge and falling edge.(5)The Hardware-In-Loop experiments of constant torque control with impact stimulation were conducted to verify the fitness of MR fluids for use in impact mitigation applications.Then,comparative constant torque control experiments validate that the lab-made HVLP MRFs featured with intensive shear thinning effect is more suited for being used in impact mitigation applications than a same Fe particle concentration commercial MR fluids.The experiments indicate that how the shear thinning effect to influence the dynamic range of MR damper over a large shear rate range and achieve the high controllability in high shear rate.The experiments indicate that the lab-made HVLP MR fluids have better energy-dissipating capacity in low shear rate and better controllability in high shear rate.