| Vehicle crashes can cause a variety of serious personal injuries and great loss of property.Occupants are less likely to be injured through rational energy dissipation during the crash.The vehicle bumper system mainly includes crash box,beam,buffer material,front rail,outer plate,etc,in which the crash box has played a very important role in energy consumption during a crash.A large amount of literatures have shown that optimizing the material,size,shape,manufacturing process,etc.of crash boxes can improve the crashworthiness of bumper systems to a certain extent.However,their adaptivity is still poor in different working conditions.In order to improve the self-adaptability of bumper systems,domestic and foreign scholars have carried out the research on adjustable energy absorbers,in which the magnetorheological impact energy absorber(MREA)has provided a new solution with its excellent performance in time response.The traditional MREAs have some shortcomings using in vehicle crash buffer:(1)It is impossible to ensure the energy absorption capacity of the MREA after throttling failure only by controlling the flow throttling of the medium.(2)The effective damping channel with annular flow mode is too short,which results in low magnetic field utilization.(3)The control medium is magnetorheological fluid(MRF)which is prone to sedimentation.To solve these problems,an impact energy absorber with the combination of corrugation crush and radial flow throttling of MR cement is proposed in this paper,which can adapt their load-stroke profile to varying impact conditions in a very short time(milliseconds)accordingly to achieve the optimal buffer effect.The radial flow characteristics,pressure drop characteristics,effects of minor losses,impact characteristics and design strategy of the MREA are studied.The specific work of this paper is as follows:(1)The overall design of proposed MREA is studied.Three different working modes of the MR cement in the damping channel are introduced,then according to energy absorption characteristics of a vehicle crash,the flow mode is chosen as the working mode in the MREA.Then,the overall design concept of the MREA is proposed,the working principle of the buffer is described,and the total MREA force is analyzed.(2)The crush characteristics of the multi-layer corrugated tube are theoretically investigated under axial impact loading conditions.According to crush deformation characteristics,the deformation process of corrugated tube is divided into four stages: elastic stage,plastic stage,transitional stage and compact stage.Based on Kellogg model,a theoretical analysis is conducted for the four stages respectively.Expressions of the four deformation forces with respect to crush displacement at different drop heights are deduced.Further,evaluation parameters for the energy absorption performance of corrugated tube,i.e.,specific energy absorption,peak force,mean force,crush force efficiency,and stroke efficiency are obtained and analyzed.In order to verify the theoretical analysis,a drop tower test system is set up for experiments,requirements and technical indexes of its components are analyzed in detail.Then,the manufacturing method,size,and impact test condition of the corrugated tube are introduced.Finally,the experimental and theoretical results are analyzed.(3)The rheological properties of MR cement and the mechanical model of MREA are studied.Using commercial shear rheometer(MCR-301),the rheological properties of MR cement samples are tested and the effect of magnetic field on rheological properties is studied.According to shearing thinning behavior,a constitutive model(Herschel-Bulkley(HB)model)suitable for MR cement is established,and the model parameters are identified by experimental data.Based on the HB model,the total pressure drop in the flow channels is analyzed,and the flow characteristics of the MR cement in the radial flow channel are further analyzed.Using the theory of fluid mechanics,the continuity equation and the governing differential equation of the MR cement in the radial flow channel are established,and the expression of the radial velocity distribution is deduced.The minor losses in flow channels are described in detail,and the influence of the minor losses on the total pressure drop is analyzed based on the average fluid velocity.Finally,a mechanical model for MREA considering the minor losses is established,which is called HBM model.(4)An effective design strategy is presented for the MREA with a multi-stage radial flow mode,and is verified by experiments.Based on the HBM model,the overall design strategy of the MREA is presented,including the designs of flow channel geometry and the magnetic circuit.For the design of the flow channel geometry,optimal geometrical parameters,i.e.,radial stage number,radial damping length and gap size are obtained by gradually reducing the number of unknown variables via logical means according to the specific design objectives.For the magnetic circuit design,first,B-H curve is obtained by testing magnetization properties of the control medium using HH-15 vibrating sample magnetometer,then the finite element model of MREA has been formulated using the commercial software ANSYS.By analyzing the effect of thicknesses of the baffle and outer cylinder and number of coil turns on magnetic circuit,the distribution of magnetic flux in the effective region of MR valve is optimized.Further,a MREA prototype is fabricated and tested using a high-speed drop tower with the maximum nominal impact velocity of 4.2 m/s.The velocity distribution of the radial flow channel is analyzed,and the effect of drop height and excitation current on the radial flow velocity is also analyzed.Then,pressure drop characteristics in flow channels are analyzed,which includes the influence of the drop height and the excitation current on pressure drop in MR valve,radial pressure gradient and pressure drop arising from minor losses.The influence on the total pressure drop resulting from the minor loss regions is quantified,and the regions needed be optimized are founded for improving the controllability.According to the measured MREA force-displacement(time)curve,the results of the impact test are analyzed in detail.Comparison of HBM model with measured data is conducted via analysis of three parameters(peak force,dynamic range and maximum displacement)that indicate the dynamic behavior and performance of MREA,which validates the effectiveness of the design strategy of the MREA.Based on the Bernoulli equation,the pressure drop and the force generated by the inertial effect are quantitatively analyzed and compared with the HBM model.Finally,a conclusion is dropped that the inertial effect has a slight influence on the MREA force and the impact process.(5)The simplified model of MREA force is proposed as an analytic formula,and the model parameters are identified.The dynamic model of MREA and the governing equation are established.In order to verify the effectiveness of the model,the simulation platform of the MREA is established in Matlab/Simulink environment for the impact test.At last,the simulation results show that the simplified model curve is in good agreement with the HBM model,which proves that the simplified model is very effective in predicting the performance of MREA. |
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