Study On Dynamic Compression And Fracture Characteristics Of Sintered Nd-Fe-B

In this paper,the dynamic compression and fracture characteristics of sintered Nd-Fe-B were studied.The quasi-static uniaxial compression tests of sintered Nd-Fe-B specimens with different types of magnetized and non-magnetized states were carried out by MTS universal testing machine.The experimental results show that the compressive strength of sintered Nd-Fe-B specimens decreases with the increase of Nd content,and the failure strain of specimens decreases gradually.The addition of a heavy rare earth element has a higher compressive strength than a specimen of a light rare earth element added and breaks down the former to absorb less energy under the same loading conditions,and the fracture strain at the time of breaking the specimen increases.Due to the magnetostrictive anisotropy of the magnetite particles,the compressive strength of the specimen in the magnetizing state is weaker than the compressive strength in the non-magnetizing state,and the fracture strain at the former fracture is smaller than the fracture strain at the latter fracture.The dynamic uniaxial compression tests of sintered Nd-Fe-B specimens at different loading rates were carried out using a split Hopkinson pressure bar.The experimental results show that the failure process of sintered Nd-Fe-B specimens under dynamic compressive load has strain rate effect.As the strain rate increases,the peak stress of the specimen gradually increases and the peak strain decreases gradually,which indicates that the compressive strength of the specimen gradually increases with the increase of strain rate.The area surrounded by the stress-strain curve of the specimen also gradually increases,indicating that the impact compression toughness of the specimen also gradually increases with the increase of the strain rate.DIC analysis shows that when the axial compressive load acting on the specimen is close to the maximum value,some locations on the surface of the specimen randomly form cracks and expand to the periphery in a certain direction.In fact,before the damage occurs on the specimen surface,the uniaxial stress state will deviate due to the boundary effect.The multi-axis stress state of tensile hoop stress,axial stress and shear stress acting on the specimen is the cause of cracks on the specimen surface.When the axial compressive load acting on the specimen reaches a maximum,the work per unit volume applied to the specimen at this time can be used to characterize the impact stability of the specimen or the strain energy of the specimen during deformation and fracture process can be used as a criterion for specimen fracture.The three-point bending tests of sintered Nd-Fe-B specimens at different loading rates were carried out by MTS universal material testing machine.The experimental results show that the flexural strength of sintered Nd-Fe-B specimens is related to the loading speed.The greater the loading speed,the greater the flexural strength and the smaller the deflection.The elastic modulus obtained by DIC method at different loading speeds is similar to the elastic modulus obtained under quasi-static conditions and is consistent with the elastic modulus recorded in the relevant literature,which verifies the validity of DIC method for this test.The DIC method shows the variation of the strain of sintered Nd-Fe-B specimen under three-point bending,and the specimen shows obvious pull-down condition.The strain distribution curve shows that the strain of sintered Nd-Fe-B specimen is not uniform deformation under bending load,and the inhomogeneity is caused by the complex crystal structure and less slip system.Sintered Nd-Fe-B specimens show obvious brittle fracture characteristics at room temperature,the fracture mode is mainly intergranular fracture.Only the radial fracture mode exists on the fracture surface of the magnet.The fracture mainly consists of grains of "rock candy",and the grain surface is smooth and without features.