A unilateral magnetic resonance moisture sensor for aerospace composites

Magnetic resonance (MR) is a proven, nondestructive analytical technique in chemistry and medicine. The sensitivity of a MR experiment is principally dependent on the strength and homogeneity of a polarizing static magnetic field, termed B-0. This requirement has traditionally limited MR experiments to a 'closed' configuration of magnets or wires generating a highly uniform B-0, but limiting the size of the sample that can be investigated. To remove this size requirement, 'open' or unilateral sensors have been developed, in which a planar arrangement of magnets generates a suitable B-0 at a location displaced from the sensor. This configuration increases the range of samples that can be examined, at the expense of B-0 homogeneity. This paper presents the design of a portable, unilateral MR sensor suitable for nondestructive moisture detection. The sensor comprises an array of permanent magnets, designed using an inversion approach in which a target magnetic field is specified in a volume over the array. Using an analytical optimization, the magnet sizes, strengths, and positions are then determined to give the target field. The result is a large region of field homogeneity, displaced similar to 4.5cm from the array, suitable for MR. The sensor is demonstrated for the in situ detection of moisture within aircraft composites. The paper discusses the design and construction of the magnet array, and presents results of successful moisture detection.