Development of SDT sensor based eddy current probe for detection of deep fatigue cracks in multi-layer structure

The detection and characterization of deeply buried fatigue damage in thick, multi-layer airframe components pose significant technical challenges to the aviation safety community. Currently, no nondestructive evaluation technique is available to reliably detect such potential damage from the exterior of the airframe, which is highly desirable in light of inspection cost as well as avoidance of structure damage. Recent technological advances in high-sensitivity magnetic sensors, i.e., spin-dependent tunneling (SDT) sensors, make it feasible to employ electromagnetic inspection techniques for deep fatigue crack inspection. In this work, we report on the development and fabrication of a low frequency eddy current probe based on a magnetically shielded SDT pickup sensor concentrically located in the interior of an induction drive coil to enable localized deep diffusion of the electromagnetic field into the part under test. Simulation studies were conducted to demonstrate the deep penetration capability of this probe configuration and to understand inspection sensitivity based on magnetic field perturbation due to subsurface cracking. Experimental results obtained using this SDT sensor on samples with induced flaws demonstrate its potential for practical application.