by David Brearley
The advancements of magnets used in magnetic resonance imaging (MRI) have led to the incorporation of glass fibre reinforced polymer (GFRP) rings being adhesively bonded between epoxy-infused coils of superconducting wire. The two components respond differently to various loading conditions, potentially leading to structural failure. The magnet’s structure is integral to its’ performance as large electromagnetic forces (EMF) are induced due to the strong magnetic field present, under cryogenic temperatures. Unintentional quenches can be triggered by localised heat generated by friction or crack propagation. This results in Ohmic heating, boiling the surrounding liquid helium and potentially causing permanent, catastrophic damage to the magnet.
At the Bristol Composite Institute, the deformation and failure modes of an MRI magnet during operation were investigated by developing an experimental methodology for applying thermomechanical loads to test coupons, effectively mimicking MRI magnets in use. A simple finite element (FE) model of the magnet was constructed to assess the operational stress state and showed that high bi-axial stress concentrations were predicted around the adhesive bond between the coils and GFRP spaces.
To investigate how this could lead to structural failure, a Modified Arcan Fixture (MAF) as shown in Figure 1 was implemented. Several loading hole pairs were used to induce various compression-shear stress states in specimens that contained the adhesive joint cut from a full magnet. In the development of the experimental methodology, a more predictable bonded structure was manufactured using the coil infusion resin as an adhesive to simultaneously evaluate the epoxy’s isolated load response and refine the experiment. Tests were carried out at room and cryogenic temperatures so the thermomechanical load carrying capability of the bond could be evaluated.
Constant cryogenic cooling during quasi-static loading was achieved with the development of a novel modular cryostat that isolates the region around the adhesive bond from the rest of the fixture. The implementation of this cryostat facilitated the use of digital image correlation (DIC) to continually record the specimens’ full field response to the load at cryogenic temperatures, using just boiled pressurized nitrogen as the coolant. Based on a concept from NCC, a rapid prototyping approach was used to iteratively improve the design of the 3D printed cryostat, and in doing so, achieved test temperatures down to -150oC while maintaining the necessary optical clarity.
This research has improved the understanding of how adhesive bonds, of a similar geometry to those found within an MRI magnet, respond to various bi-axial stress states at room and cryogenic temperatures, see Figure 2. The design process for the developed cryostat opens the door to countless potential possibilities for mechanical testing under cryogenic conditions, where complex thermomechanical stress states require advanced measurement techniques to evaluate the material’s load response.
