Experimental study on fracture process of sintered Nd-Fe-B magnets during dynamic Brazilian tests

The deformation and fracture processes of the Nd-Fe-B magnets during dynamic Brazilian tests were investigated using a split Hopkinson pressure bar (SHPB) with a high-speed photography (HSP) and digital image correlation (DIC). The load and work applied to the specimens were determined with the strain signals of incident, reflected and transmitted wave of the SHPB tests. It was revealed with HSP and DIC that at a load time before the maximal load, some cracks along the loaded diameter on the specimen surface became visible. The localized fracture strains at representable points of the failed area were given by DIC. The tensile strength of the specimens was obtained based on the elasticity theory when the load reached the maximum. The widths of the failed area on the specimen surface at the maximal load were determined with DIC. Furthermore, the fracture work applied to the specimen per failed volume which corresponds to the maximal load was given and could be an energy criterion for the fracture of the Nd-Fe-B specimens. It was indicated that the tensile strength and fracture work of the Nd-Fe-B magnets increase with the increase of strain rate in the Brazilian tests. It was shown that the transgranular fracture became visible in the recovered specimens for the higher strain rate tests.