Measurements of surface-breaking flaws in ferromagnetic plates by means of an imaging SQUID susceptometer

We detected sub-millimeter surface-breaking flaws in ferromagnetic plates using an imaging SQUID susceptometer. The normal component of the field was imaged with an axial, first order gradiometer with 3 mm diameter coils and a 4 cm baseline. We mapped the field above mild steel test samples, detecting field distortions due to surface flaws with volumes as small as 0.1 cubic millimeter. A comparison is made with conventional flux leakage methods. Our method of observing distortions in a perpendicularly applied field may be more sensitive for detecting surface-breaking flaws. Image enhancement techniques successfully removed background signals with low spatial frequencies.<
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