Eddy-current imaging of cracks.

As a consequence of metal fatigue, cracks can develop and grow in operational aircraft. Periodic inspections must be made in order to detect and repair them before they reach a dangerous length. Cracks which grow from holes are a significant problem for aircraft since the wings and fuselage can contain many thousands of fasteners, or rivets. Since it is impractical to remove them all, inspection must be made with them installed. Research into the application of eddy currents to this problem has led to the development of a scanning procedure in which a small coil is moved around the circumference of the fastener while its impedance is repeatedly measured at different positions. This set of data constitutes an image which can be analvsed using pattern recognition techniques to identify the presence of a crack. A self-contained automated instrument has been built on these principles. It incorporates a microprocessor which controls all aspects of the systems operation, including analysis and display of results. Tests show that it can detect the presence of simulated radial cracks as small as 0.2 mm long beneath the heads of fasteners. The natural extension of these ideas leads to the concept of eddy-current imaging in which a 3D picture of a defect is reconstructed from measurements of the surface magnetic field. The feasibility of implementing this, using techniques such as tomography, is discussed