| Impact is a common phenomenon,which often causes serious casualties and significant property losses due to its sporadic,unpredictable and high impact energy.Conventional impact isolators generally dissipate energy through metal deformation or friction or hydraulic oil consuming impact energy.These types of buffer devices only have a good cushioning effect for specific impact conditions,and their cushioning capacity will be greatly reduced when the impact conditions deviate from the set conditions.For this reason,experts and scholars are working on a buffer device that can adapt to different working conditions.Magnetorheological fluid is a very ideal new intelligent buffer material to replace the traditional material due to its unique characteristics and advantages of making instantaneous,continuous and stepless reversible changes to the applied magnetic field.In support by the Natural Science Foundation of China and Chongqing Basic Research and Frontier Exploration Project,and taking magnetorheological cemen as the magnetic control medium,this paper conducts a study on the characteristics of magnetorheological energy absorbers(MREA)combined with cylindrical corrugation crushing and radial flow throttling.The dynamic mechanical properties and anti-stability characteristics of cylindrical corrugated crusher under impact load are studied.The flow path structure of the central discharge orifice and the radial gradient gap is proposed to reduce the problem that the MREA controllability deteriorates due to the excessive flow rate of the magnetorheological cemen in the MR valve flow passage.The design method of MREA magnetic circuit in series with radial gradient gap flow channel and equally spaced gap flow channel is studied.The flow characteristics of magnetorheological cemen in the working channel are analyzed.Based on this,the calculation method of buffer force is obtained.The MREA of the central discharge orifice and the radial gradient gap flow channel structure was designed and fabricated,and the drop hammer yimpact test was carried out and compared with the theoretical prediction.The specific research contents are as follows:(1)Preparation and rheological properties of magnetorheological cemen.Two kinds of magnetorheological cemen samples were analyzed and prepared from three aspects,suspension stability,magnetic control range,particle volume fraction and carrier liquid viscosity respectively by using concentration moving velocity theory,particle mass flow theory and field induced dipole theory.The rheological properties of the samples were tested by a commercial rheometer(MCR-301)and the influence of magnetic field on the rheological properties of magnetorheological cemen was studied.The advantages and disadvantages of common constitutive models used to describe the rheological properties of magnetorheological cemen were compared and analyzed.Power-Law model was selected to describe the rheological properties,and the parameters of the model were identified by using sample test data.The magnetization characteristics of magnetorheological cemen were tested by HH-15 vibrating sample magnetometer.(2)The overall design of proposed MREA is studied.The ideal MREA dynamic buffer force-displacement curve has an approximate "plateau effect",while the traditional MREA dynamic buffer force-displacement curve has a peak.In addition,too high a flow rate of magnetically controlled media in the working channel also reduces MREA’s controllability.Therefore,the demand of MREA’s development for one-time collision buffering is put forward,i.e.the controllability of MREA needs to be improved while the platform effect is realized.The three different working modes of the magnetron medium in the damping channel are introduced,and the flow mode is selected as the working mode of the magnetorheological cemen in the MREA according to the mechanical characteristics.The overall design of MREA using cylindrical corrugated crushing elements in series with magneto-rheological throttling units is put forward.The dynamic characteristics of the cylindrical corrugated crushed parts under impact load are analyzed.In view of the deterioration of controllability caused by the high flow velocity of the magnetic control medium in the flow channel,a magnetorheological throttling unit in the form of a central bleed orifice and a radial gradient gap flow channel is proposed.According to the design requirements of external parameters such as crushing capacity and maximum impact load,MREA’s excitation coil,seal and magnetic control medium filling structure are designed.(3)Establishment of MREA magnetic circuit model and determination of parameters.The magnetic circuit analysis model of the gradual gap flow channel structure MREA is established by using the dynamic magnetic circuit theory and finite element method.According to the object-oriented scheme,the structure and parameters of MREA’s electromagnetic magnetic circuit are determined respectively,and then the functional relationship between magnetic induction intensity and excitation current in the working channel is established.The commercial simulation software ANSYS was used to analyze the influence of excitation current,radial gap gradient,radial gap height and other parameters on the magnetic field strength of the working gap.The magnetic induction strength of the working gap of the MR valve filled with magnetorheological cemen was detected and compared with the theoretical structure.(4)Flow characteristic analysis and dynamic buffer force calculation of magnetorheological cemen in flow channel.In view of the complexity of the MREA flow channel after adding the center bleed orifice and radial flow channel,the flow characteristics of magnetorheological cement in the corrugated crush element,MR valve inlet and outlet,annular flow channel and center bleed orifice were analyzed from the perspective of magnetic flow channel and non-magnetic flow channel,as well as the flow characteristics of magnetorheological cemen in the gradient gap flow channel and the equidistant flow channel,and the continuity equation and control differential equation of magnetorheological cemen flow in the gradient gap flow channel and the equidistant gap flow channel were established by using the relevant theories of fluid mechanics.The radial velocity distribution expression of magnetorheological cemen is derived.By introducing five parameters: radial velocity ratio,small loss ratio,dynamic range ratio,and platform effect angle ratio,the influence of center bleed orifice on MREA characteristics is analyzed.Taking the gradient of radial flow channel as a variable,a rheological model of magnetorheological cemen radial flow was established,and the influence of gradient value on MREA characteristics was further analyzed.The distribution of local loss factors in the MREA flow channel is analyzed in detail,and the pressure drop caused by local loss in the MR valve is established by using the average fluid velocity.(5)MREA fabricate and drop hammer impact test.According to the theoretical analysis,a set of MREA prototypes of three types of flow channel structures were designed and manufactured,and a drop hammer impact test platform was built to simulate the one-time impact buffering condition.A data acquisition system was constructed using piezoelectric force sensors and laser displacement sensors,and 12 different conditions were tested for each MREA prototype.The experimental results were compared with the theoretical results.The results showed that MREA rheological modeling was basically consistent with the experimental results,which verified the rationality of MREA rheological modeling and the main sources of errors were analyzed. |
