Influence of key factors on Eddy current testing sensitivity and monotonicity on subsurface depth for ferromagnetic and non-ferromagnetic materials

Studies on key influencing factors (coil's size and excitation frequency) of eddy current testing (ECT) sensitivity, monotonicity and defect detectability have been carried out for detection of subsurface defects in ferromagnetic and non-ferromagnetic plate materials. A set of ECT finite element models and experiments have been accomplished and the detection performance has been analyzed and compared. The simulation and experiment results have indicated that for ferromagnetic material, the amplitude response of low frequency excitation has lower sensitivity but better monotonicity than high frequency. The phase response of low frequency excitation has higher sensitivity and better monotonicity than high frequency. For the non-ferromagnetic plate material, the lower the excitation frequency is, the smaller the detection sensitivity of the amplitude response is. Outside a defined excitation frequency, the defect monotonicity of the amplitude response reduces. For phase response, the lower the excitation frequency is, the smaller the detection sensitivity and the greater the monotonicity. The coil's size has a certain extent influence on the detection sensitivity. For ferromagnetic and non-ferromagnetic plate materials, it is effective to enhance the detection sensitivity of amplitude response by increasing the size of the exciter coil and the detector coil within a certain range; and it is effective to enhance the detection sensitivity of phase response by decreasing the size of the detector coil or increasing the size of exciter coil within a certain range.