Through Thickness Inspection of Layered Magnetic Material Using Pulsed Eddy-Current Testing

Steel microstructure determines the mechanical properties of steel such as hardness and tensile strength. To monitor steel microstructure evolution during thermal processing such as hot rolling, online measurement of microstructure is vital to steel quality control. Previous studies using multifrequency eddy-current testing to monitor the change in electromagnetic properties and infer steel microstructures have achieved remarkable success, yet due to the high-frequency excitation signal, the inspected thickness of steel microstructures is constrained on the surface due to the skin effect. This study introduces the pulsed eddy-current (PEC) method to characterize the electromagnetic property profile along the depth of thick steel plates. The low-frequency square-wave signal enables PEC to reflect multilayer characteristics in a single measurement with good penetration ability. An explicit analytical equation for calculating the time-domain PEC response is derived for the first time which shows advantages in terms of speed and gaining in-depth physical insights. A custom PEC system has been designed and built with a tunnel magneto-resistance (TMR) sensor as the receiver. Theoretical and numerical analyses are performed. A time-domain intersection point on the Tau curve is found to be a useful indication of the properties of a planar structure. Experimental results on layered planar structures of various electromagnetic properties are analyzed and validated.