| Eddy current testing is the most commonly used Non-Destructive Evaluation (NDE) method for multi-layered aluminum aircraft structures, such as the CC-130 (Hercules) and the CP-140 (Aurora). This method is effective due to the high conductivity of the aluminum used in the aircraft structure. Specifically, conventional eddy current (EC) testing is used for detecting cracks in bolt holes in aluminum wing structures. EC testing is also used to detect surface breaking cracks in the vicinity of ferrous fasteners, but complete inspection of the bolt holes requires fastener removal since the magnetic properties of the fastener and depth at which cracks occur impedes their detection. In this thesis, the transient eddy current (TEC) method combined with probe development is used to investigate the detection of cracks in multi-layered aluminum structures with ferrous fasteners present.;Results demonstrate sensitivity to a notch 6.4 mm below the outer surface, a depth where second layer cracking in wing structures can occur.;In the basic physical principles of probe operation a central driver coil magnetizes its ferrite core and the ferrous fastener, allowing flux to be carried to greater depths within the multi-layered structure. Differential signals from pickup coils outside the drive coil at opposing 1800 angle and/or at right angles are examined as a function of angle relative to a subsurface notch. A change in the differential signal response is monitored for detection. |
