Study On Dynamic Compression Properties Of NdFeB Magnetic Materials

This thesis mainly studies the dynamic compression response and failure characteristics of sintered NdFeB magnetic materials under different strain rates.Part of the specimens used in the test are magnetized(magnetic specimens),and some are not magnetized(non-magnetic specimens).Investigate the effect of magnetization on the mechanical properties of sintered NdFeB material,combine the test results to construct the dynamic constitutive equation of the material and verify its effectiveness,perform macro and micro analysis on the recovered specimen to reveal its fracture mechanism,etc.The contents are as follows:Using the MTS universal testing machine,the non-magnetic and magnetic N35 NdFeB specimens were subjected to compression test to obtain the average compressive strength of the non-magnetic and magnetic sintered NdFeB materials at a strain rate of 10^-3s^-1.And the compression failure process of the specimen is recorded by the camera.The results show that the average strength of quasi-static compression of magnetic and non-magnetic sintered NdFeB materials is approximately the same.The variance of the quasi-static compressive strength of magnetic specimens is smaller than that of non-magnetic specimens,and the quasi-static compressive strength test data of magnetic specimens is relatively stable.Using a Split Hopkinson pressure bar test device(SHPB),the N35 NdFeB cylindrical specimens were subjected to uniaxial compression tests at different strain rates to obtain the stress-strain curves of nonmagnetic and magnetic NdFeB materials at different strain rates.The results show that the average dynamic compressive strength under different strain rates.The dynamic average compressive strength and the average failure strain of non-magnetic and magnetic sintered NdFeB materials have certain strain rate correlation.Based on the results of compression tests at different strain rates,the HJC constitutive equation of sintered NdFeB materials was established.Using LS-DYNA and HJC constitutive equation,the numerical simulation of SHPB in the whole process of sintered NdFeB material was carried out.The incident wave is taken as the loading wave of numerical simulation.The transmitted wave and reflected wave of numerical simulation can reproduce the test wave,and the uniformity of stress and strain in the X direction of axial and circumferential direction of the specimen in the test process is checked,thus the validity of the constitutive equation of HJC is verified.The fragments of the typical recovered samples were analyzed,The results show that:In the case of quasi-static compression,the fragments after the destruction of non-magnetic and magnetic NdFeB are relatively large.Most of the non-magnetic NdFeB damage is caused by axial cracks that lead to material failure.Magnetic NdFeB damage is mostly caused by cracks in different directions leading to material failure.The relationship between the initial crack position,the failure behavior and the strain rate is found from the macroscopic photos.In the case of dynamic compression,the initial crack initiation position of the non-magnetic and magnetic NdFeB is located at the contact end of the specimen and the compression bar,and the direction of the crack penetration is different.Scanning electron microscopy(SEM)is used to microscopically observe the fracture morphology of NdFeB.From the microscopic perspective,the fracture failure of sintered NdFeB is mainly intergranular fracture due to strength failure of the neodymium-rich phase,and a small amount of transgranular fracture occurs on large grains of NdFeB main phase.With the increase of strain rate,the number of micro-cracks increases.At this time,the destruction of sintered NdFeB is controlled by intergranular fracture and transgranular fracture.