Abstract
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 heating due to energy generated via friction or crack propagation, boiling the surrounding liquid helium due to the resulting heat and potentially causing permanent, catastrophic damage to the magnet.This project aims to investigate the deformation and failure modes of an MRI magnet during operation by developing an experimental methodology for applying thermomechanical loads to test coupons, that mimic 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 spacers.
To investigate how this could lead to structural failure, a modified Arcan fixture (MAF) was implemented, using several loading hole pairs 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. 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 cryostat that isolates the region around the adhesive bond from the rest of the fixture. The implementation of this cryostat, which was iteratively improved throughout the test regime, facilitated the use of digital image correlation (DIC) to continually record the specimens’ full field response to the load at cryogenic temperatures.
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. The design process for the cryostat developed 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 materials´ load response.
| Date of Award | 30 Sept 2025 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Sponsors | UKRI EPSRC & Siemens Healthineers MR Magnet Technology |
| Supervisor | Janice M Barton (Supervisor), Ole Thomsen (Supervisor) & M'hamed Lakrimi (Supervisor) |
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