Delayed detection of cracks in a component adds considerably to the cost of repair of products in the aerospace and other industries. Under some circumstances early detection of a crack may enables not only avoidance of catastrophic failure but also the ability to repair the component, thus increasing the life of the component and delaying the component's replacement.
Cracks are likely to form around high stress areas of a component such as near or at fastener holes. If a crack near a fastener hole is detected early, the hole may be oversized to eliminate the crack and extend the life of the component.
Conventional non-destructive testing (NDT) techniques for inspection at fastener holes have had difficulty detecting cracks early enough so that repair is possible. Accordingly the component may have to be replaced at considerable expense. Ultrasonic testing (UT) which detects reflections of a sound wave from within the component under test have proven to have difficulty detecting cracks beyond the first layer of a component, particularly when the sealant is not uniform or geometric variations reduce the probability of detection (POD) significantly below 1.0.
Conventional eddy current sensing involves the excitation of a conducting winding (the primary) with an electric current source of prescribed frequency. The current in the conducting winding produces a time varying magnetic field at the same frequency. By Faraday's law of induction an electromotive force is induced in a sensing winding (the secondary). The electromotive force may then be measured as a voltage. The spatial distribution of the magnetic field which is measured by the secondary is influenced by the proximity and physical properties (e.g., conductivity and permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can sometimes be deduced from measurements of the transimpedance between the primary and secondary windings. Traditionally, scanning of eddy current sensors across the material surface is then used to detect flaws, such as cracks. Conventional eddy current testing (ET) has proven inadequate for detection of cracks near fastener holes in multiple layered structures. This is due to difficulty penetrating through thick outer layers and due to noise caused by geometric variations, fastener fit, and crack morphology variations.