Inspection of multi-layered riveted structures and detection of subsurface cracks under fastener head is critical to ensure safe operation of an aircraft. As defects are embedded deep in the structure, ultra-low excitation frequency is used to achieve larger penetration depth. Further, for rapid inspection of large areas, high scan velocity is required. Eddy current testing with ultra-low frequency exciting and high scan velocity poses additional challenges. This paper studies the effect of scan velocity on ultra-low frequency eddy current signals. Signal processing algorithms are developed and presented for compensating velocity effects.
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