The Committee on Aging of U.S. Air Force Aircraft, of the National Research Council (1), identifies as a primary technical need a method for nondestructive evaluation sensitive enough to detect small cracks in multi-layered structure of airframes. Research proposed here can meet the primary technical need for high sensitivity and takes an integrated approach to developing a quantitative capability for a new method using pulsed eddy currents with superconductive probes. A superconductive probe offers high sensitivity and resolution at low frequency making it possible to find small cracks and corrosion hidden deep in multi-layered structures. Superconductive probes, operating with pulsed eddy currents, enable imaging a crack and superimposing the image on a cross sectional view of a part, showing, for example, fatigue crack at a fastener hole. The image gives depth of a crack below the surface as well as its length and area. A color code for the image gives the probability of detection (POD) of a fatigue crack. Inspectors can readily interpret and evaluate the display. Similar signal analysis can be carried out to indicate regions of corrosion. Research proposed here develops a measurement model f3r pulsed eddy currents, with superconductive probes, and applies it to assess reliability in terms of probabilities of true and false detection. The reliability assessment integrates measurements of crack depth and size with a display of a crack outline on a section drawing of a part, using a color map to mark reliability of a measurement. The research leads to development of superconductive probes operating with pulsed eddy currents to give a new method of quantitatively evaluating depth and size of small cracks deep in multi-layered structure of airframes.
Keywords: NONDESTRUCTIVE EVALUATION, PULSED EDDY CURRENTS, SQUID, SUPERCONDUCTIVE PROBE, PROBABILITY OF DETECT
