Recent advancements in Lorentz force eddy current testing
Lorentz Force Eddy Current Testing (LET) is a non-destructive testing technique based on induced eddy currents due to relative motion between a permanent magnet and a conductive, non-ferromagnetic device under test which has been recently introduced. The Lorentz force acting on the magnet is measured and perturbations in conductivity change the Lorentz force profile along the conductor. The permanent magnet is placed in a lift-off distance above the specimen moving with a constant velocity relative to the magnet. The design of the magnet is the most crucial element to improve the technique. Therefore, we present new developments in LET: the optimization of an innovative magnetic structure enhancing the Lorentz force and an uncertainty analysis to identify most important sources of variance. Futhermore, in a defect depth study a detection limit for LET was determined. A new cylindrical magnetic Halbach structure has been designed to concentrate and magnify the magnetic field below the structure. For internal defects a multi-objective, non-linear optimization to maximize the defect response of the drag force is performed. The optimized magnet shape depends on the geometrical parameters of the experimental setup and therefore the optimal shape is highly problem-specific. Secondly, we investigate the uncertainties in our existing experimental setup quantified by a non-intrusive polynomial chaos expansion to determine the impact of multiple unknown input parameters. The experimentally determined statistics of velocity, magnetic remanence of the permanent magnet, conductivity of the specimen and the lift-off distance are modeled as uniform and beta-distributed random variables. The numerically predicted force profiles were validated by experiments. The included analysis of variance of the Lorentz force enables the enhancement of defect detection capability. Finally, experiments with a specimen containing a quasi-infinite crack were performed. By variation of the defect depth a detection limit for LET for drag- and lift-force components of the Lorentz force was determined. It showed the compatibility of LET compared with traditional eddy current testing.