Triple-frequency eddy current estimation of nonmagnetic metal clad thicknesses over nonmagnetic metals

Apparent eddy current conductivity (AECC) spectroscopy recently established its potential capabilities as a practical approach to assess nonmagnetic metal coating thicknesses over nonmagnetic metals while eliminating the measurement sensitivity to lift-off in a +/- 25.4 mu m range. It reduces the electromagnetic problem to three unknown parameters, namely the clad thickness of interest along with clad and substrate conductivities. This study exploits the use of triple-frequency AECC measurements to retrieve these unknowns following the planewave approximation and the Nelder-Mead optimization approach. The three frequencies, their incremental factor and operating range selection criteria is investigated to achieve the desired accuracy in clad thickness estimation. The proposed approach is validated for different Al 1230 clad thicknesses over Al 2024 substrates. It is demonstrated that selecting any three frequencies operating between 0.1 and 15 MHz with a 10 incremental factor delivers a 3% uncertainty in a clad thickness estimation of interest.