Electromagnetic Attenuation Factor Based NDE Approach for Depth Detection of Hidden Defects Using HTS rf-SQUID

We present a new approach for non-destructive evaluation (NDE) in homogeneous and isotropic metallic objects which contain defects at unknown depths based on single scan and multi frequency excitation. As known, there is an optimum frequency for each depth of defect. Finding the depth of an unknown defect requires us to find the optimal frequency. In conventional single frequency methods, the optimal frequency is obtained by applying a wide range of frequencies to the system separately and comparing the corresponding results in a time-consuming process. Conventional multi frequency inspections were introduced to obtain more information about test specimens. There are two ways to apply multiple excitation frequencies to the test sample. First way is to apply them sequentially to a single excitation coil which is still time consuming and the second method is to apply them simultaneously to multiple coils. In simultaneous excitation the testing time is shorter, but its processing of phase and amplitude signals is still challenging. We propose and examine the capability of the multi frequency eddy current (MFEC) method to detect the depth of hidden cracks in a shorter time period; using our HTS SQUID gradiometer-based system. The measurements are performed in a noisy environment, and a planar double-D shaped printed-circuit-board coil is used as the excitation coil. Comparing with the results of the consecutive single frequency excitation measurements, the obtained depths of the flaws using MFEC method was confirmed.