Study On Fracture Characteristics Of Nd-Fe-B Under Impact Loading

Nd-Fe-B are widely used in all walks of life because of their excellent magnetic properties,it is of great significance to study the damage characteristics of Nd-Fe-B under impact load for its development and application.At present,the understanding of fracture characteristics of Nd-Fe-B rare earth alloys under impact loading is not clear.In this paper,the dynamic response and failure characteristics of Nd-Fe-B rare earth alloys under impact loading are studied.The specific work and research contents are as follows:The shock loading test of N42 Nd-Fe-B RE alloy is carried out by using one-stage gas gun and DISAR device.Through measuring the shock wave velocity of simple step sample and analyzing the error of plate impact test,it is found that the main reason for the measurement error of shock wave velocity of Nd-Fe-B RE alloy at low speed is the gradient of flyer when it impacts target plate,and the step test is improved.A symmetrical stepped specimen is designed to eliminate the error caused by flyer slanting impact on target plate by means of symmetrical probe measurement.It is found that the shock wave velocity of Nd-Fe-B RE alloy approached a constant in the low pressure range.The Hugoniot relationship of Nd-Fe-B RE alloy material is obtained by mass average method,and it is in good agreement with the experimental results of impact compression at high pressure.The spall failure of Nd-Fe-B RE alloy under one-dimensional strain is realized by plate impact test.The experimental results show that the spall strength increases first and then decreases with the increase of loading stress,and there is a critical stress threshold,when the impact pressure exceeds the threshold,Nd-Fe-B RE alloy will suffer compressive damage and reduce its spall strength.Through the observation of sample recovery,it is found that Nd-Fe-B RE alloy specimens can be damaged by three kinds of fragments under certain pressure spall failure.They are the fragments on one side of the impact surface,the fragments on the free surface and the fragments in the spall area.The reasons for this failure mode are analyzed theoretically.By measuring and analyzing the fragments of the recovered specimens,it is found that the size of the spall zone is related to the loading stress.With the increase of the loading stress,the thickness of the spall zone increases gradually.From Rosenberg's point of view,the Hugoniot elastic limit of Nd-Fe-B RE alloy is calculated by Griffth yield criterion,and the tensile strength under uniaxial stress is replaced by spall strength.The Hugoniot elastic limit of Nd-Fe-B RE alloy is obtained,and the approximate range of dynamic yield strength is calculated.The microstructures of Nd-Fe-B RE alloys are characterized,and the fracture mechanism of Nd-Fe-B RE alloys is analyzed from the microscopic point of view.At low strain rate,due to the low strength of Nd-rich phase,the main failure mode of materials is intergranular fracture.At high strain rate,the strain energy releases near the crack tip increases significantly,leading to the single grain splitting in front of the crack,transgranular will occur.It is found that the fracture mechanism of Nd-Fe-B RE alloy is controlled by both intergranular fracture and transgranular fracture under high strain rate loading.The spall failure of Nd-Fe-B RE alloy under plate impact is numerically simulated by using linear elastic model and maximum tensile stress failure criterion,and the results are compared with those of spall experiments using LS-DYNA finite element program.From the results of numerical simulation,the location and time of spall are in good agreement with the experimental data for the velocity-time curve of free surface particles.