Demonstration of an OPM flux guide scanner for high-resolution magnetic non-destructive testing

Non-destructive testing (NDT) techniques are important for the evaluation of properties in materials, such as crack initiation or heat stresses induced by weld seams, without causing further damage to the component itself. Optically pumped magnetometers (OPM) offer a high sensitivity in the range of 15 fT/√Hz to detect resulting localized changes of the magnetization in ferromagnetic materials. Here, the sensitivity of OPMs allows the detection of small variations in magnetic stray fields on the surface of the material originating from such stresses in a volume only 0.1 mm3. However, to measure stresses in larger devices for NDT applications, the volume in which the magnetization is measured must be controlled by special devices like flux guides. They are fabricated from soft magnetic ferrites to adapt its magnetic characteristics. This paper presents first results from a novel OPM flux guide design to effectively pick up local magnetic stray fields on the surface of specimen with an increased spatial resolution and directing them to the sensing cell of a commercially available OPM. The scanning system is shielded against environmental magnetic perturbations to exploit the OPM sensitivity even in industrial environments. It is demonstrated how the two components, OPM and flux guide, can be combined to build a system for high resolution non-destructive testing of ferromagnetic steel samples.