This block, cut from an MRI magnet, consists of an epoxy-infused, insulated copper wire (embedded with filaments of Nb-Ti) spool. The epoxy binds the wires to maintain the structure’s shape, and the copper is used as an electromechanical support for the filaments, which gain superconductive properties when cooled with liquid helium to 4K (-269oC).
The combination of high operational current (500A, similar to 50 kettles) and a strong magnetic field result in large electromagnetic forces, equivalent to the maximum take-off weight of Boeing 747!
These forces have the potential to cause crack propagation in the epoxy, releasing energy that could significantly increase the local temperature of the wire, meaning it is no longer superconductive. If this happens, the stored current is released, resulting in a rapid chain reaction where the entire magnet undergoes a “quench”.
During the quench, the massive amount of stored electrical energy transforms into heat, causing rapid boil off of the surrounding liquid helium, which is very expensive and, if not vented properly, potentially dangerous to the patient receiving the MRI.
My project aims to get a better understanding of the composite material’s failure initiation, post manufacture, due to operational cryogenic exposure and mechanical loads. My next steps are to examine the combined shear-compression loading effects on the material under cryogenic conditions and use this to inform a model that can predict quench-initiating crack propagation loads.
~ David Brearley, PhD, Aerospace Engineering