Effect of Loading Rate on Mode-I Crack Initiation Fracture Toughness of a NdFeB Rare Earth Permanent Magnet

Rare earth, NdFeB (Neodynium Iron Boron), permanent magnets now have a wide range of applications due, in part, to their low cost and high magnetic energy product. As such, the mechanical properties of the NdFeB magnets need to be characterized to understand the range of compatible applications. Fracture toughness is an important material parameter to represent material reliability. In this study, four-point-bending tests were conducted with a 3 ''-diameter Kolsky compression bar and an actuator, implemented with high-speed imaging, for characterizing Mode-I crack initiation fracture toughness of a NdFeB magnet from high to low rates of loading. Pulse shaping was employed in the Kolsky bar tests to ensure nearly constant rates of loading. The crack initiation fracture toughness of the NdFeB magnet was determined at different loading rates ranging from similar to 10 to 700,000 MPa . root m/s. The NdFeB magnet shows significant effect of loading rate on crack initiation fracture toughness. The crack initiation fracture toughness increased by over four times (from 3 to 13 MPa . root m) within the range of loading rate investigated in this study.