Development of a high sensitive magnetic flux leakage instrument for imaging of localised flaws in small diameter ferromagnetic steel tubes
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The authors report the development of a high sensitive, 16 channel magnetic flux leakage (MFL) instrument for imaging of localised flaws in small diameter ferromagnetic steel tubes. The instrument consists of a magnetising unit of two bobbin coils wound on a ferrite core, a sensing unit of flexible giant magneto-resistive (GMR) sensor array and an image visualisation unit. 3D-non-linear finite-element modelling has been carried out to optimise the inter-coil spacing of the bobbin coils and to identify the number of GMR elements and their locations between the bobbin coils. A series of experiments has been carried out to evaluate the flaw detection performance of the MFL instrument. Studies reveal that the instrument can image as small as 1.1 mm diameter hole and 0.46 mm deep shallow outer diameter circumferential notch in 17.2 mm diameter Mod. 9Cr-1Mo steel tubes used as steam generator material in prototype fast breeder reactor, with a signal-to-noise ratio better than 8 dB. Studies also indicate that intensity of MFL signal measured by the instrument increases with increase in hole diameter and notch depth. The instrument is portable and easy to use for field inspection of ferromagnetic steel components.