| Eddy current damper is a braking device based on the law of electromagnetic induction.In the working process,magnetic field is used as the energy transmission medium,and there is no direct contact between relative moving structures.Therefore,it has the advantages of low noise,long life,simple maintenance and high reliability that traditional contact damper does not have.As an important foundation of modern information industry,the amount of rare earth permanent magnet material has become one of the important symbols to measure the comprehensive national strength and national economic development of a country.Compared with the previous two generations of rare earth permanent magnets,the third generation of rare earth permanent magnets,NdFeB,has incomparable advantages in terms of magnetic energy product and cost.China is rich in rare earth resources and occupies the development advantage of NdFeB.Vigorously developing and applying NdFeB is an important strategic direction in line with national conditions.An important application of high-performance NdFeB alloys is permanent magnet eddy current dampers(PMECDs)and the excellent magnetic properties of NdFeB greatly broaden its application fields.Considering that the PMECD can well match the the recoil situation of artillery,the basic principle of the eddy current damping is applied to the design of the counterrecoil mechanism.This can give full play to the advantages of PMECDs such as low noise,long life,simple maintenance and high reliability,and effectively overcome the defects of traditional hydraulic brakes,such as liquid leakage,wear and sealing.However,there are also many technical challenges in the development of permanent magnet eddy current recoil brakes(PMECRBs).At present,there is a lack of sintered NdFeB hysteresis model with clear physical meaning,simple form and convenient engineering application.Meanwhile,there is still a lack of in-depth understanding of dynamic mechanics and demagnetization behavior of sintered NdFeB under high transient and intensive impact.In addition,the calculation method of sintered NdFeB life considering material brittleness and parameter randomness also needs further study.Regarding the issue above,the magnetomechanical characteristics of sintered NdFeB in PMECRB are focused on,and a systematic study on the basic scientific problems that affect the further development of PMECRB and hinder the application of NdFeB in places with high energy consumption density are made.The main research contents are as follows.The advantages of the permanent magnet cylindrical eddy current brake in magnetic field utilization are analyzed.Magnetic shoes guide and converge the magnetic field generated by the permanent magnets,and improves the radial component of the air gap magnetic field.The composite cylinder reduces the leakage of the magnetic field and increases the utilization rate of the magnetic field.The cylindrical structure reduces the lateral end effect.Then the prototype of the permanent magnet eddy current recoil brake is designed and manufactured.According to the recoil motion process,the force conditions of the recoil part are analyzed and the recoil motion equation is established,which provided the model basis for the subsequent numerical simulation calculation.Meanwhile,the existing magnetomechanical characteristics of sintered NdFeB is described,and the applicable scenarios and limitations of the current magnetomechanical model are pointed out.The nonlinear hysteresis behavior of sintered NdFeB and the magneto-thermal coupling modeling method of PMECRB are discussed.The hysteresis curves of sintered NdFeB under different temperatures are tested by using NIM-2000 and TC-200.Based on the experimental data,the nonlinear hysteresis behavior of NdFeB under the combined action of external magnetic field and temperature field is theoretically modeled.The established recoil motion equation and the hysteresis behavior of sintered NdFeB are coupled with the magneto-thermal model,and the model of PMECRB is constructed.The model is verified by comparing the simulation and experimental values.To fully reflect the advantages of the hysteresis model considering the temperature effect in the simulation calculation,the PMCERB model is further used under other working conditions such as different ambient temperatures and operating frequencies.The temperature distribution and braking force under these conditions are explored.The dynamic damage constitutive model of NdFeB is studied.The dynamic stress-strain curves of sintered NdFeB at different strain rates are obtained by processing the SHPB experimental data.Combined with high-speed photography,it is found that the dynamic mechanical properties of the material are related to the strain rates.Meanwhile,the fracture of the sample is recovered and analyzed by Scanning electron microscopy.The grain surface is very smooth and the fracture is relatively flat,which is an obvious feature of brittle fracture.Based on the uniform distribution hypothesis of microcracks and crack propagation criterion,the damage evolution equation of brittle materials is established.Combining the damage propagation equation with the ZWT constitutive model,DZWT constitutive model for brittle materials is established.The model is applied to sintered NdFeB,and the model parameters are identified according to the experimental results.According to the structure and mechanism topological relationship of the prototype,a finite element dynamic model is established.The dynamic characteristics of the PMECRB and sintered NdFeB are studied.Based on the study of dynamic mechanical response of sintered NdFeB,the problem of material demagnetization caused by impact is further discussed.An experimental platform for impact demagnetization is built,and the corresponding impact load is calibrated by a thin-film piezoelectric sensor.Magneto-electric transformation is used to transform the magnetic field change that is difficult to capture in the dynamic process into electrical signal output that is easier to observe.The variation curve of induced electromotive force and magnetic flux is obtained.On the basis of the reversible and irreversible demagnetization phenomena in the experimental process,a new impact demagnetization model is proposed,which is constructed by stress knee hypothesis and irreversible aggregation hypothesis.The effectiveness of the model is verified by co-simulation.The identification problem of key parameter in the cosimulation model is transformed into a problem of finding functional extreme values.The neural network surrogate model is used to simplify the calculation process,and the genetic algorithm is used to solve the problem.The verified impact demagnetization model is applied to the artillery launch process,and the reversible demagnetization domain and irreversible demagnetization domain appearing in the working process are explored.Finally,the decline of braking force caused by magnetic field degradation is discussed.A probabilistic fatigue life calculation method for sintered NdFeB considering brittleness and randomness is proposed.To reflect the dispersion characteristics of fatigue life,the distribution of parameters in brittle fatigue model is adopted to reflect the intrinsic dispersion of material properties and the external dispersion of service load.Combining the probability density evolution theory with fatigue life calculation,the corresponding probability density evolution equation of fatigue life is established.The obtained probabilistic fatigue life distribution combines the brittleness of materials and the randomness of fatigue life,which has a broad application prospect in anti-fatigue design of structures. |
PDF Full Text Request