Pulsed Eddy-Current Measurements of Corrosion and Cracking in Aging Aircraft
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ABSTRACT Pulsed, or transient eddy-current methods are an effective tool for quantitative characterization of hidden corrosion and cracking in multi-layer aircraft structures. Eddy currents are the method of choice for this task, since they penetrate multiple layers of metal, whether or not the layers are mechanically bonded. The pulsed eddy-current technique is an important advance over conventional eddy-current methods because it rapidly acquires data over a wide range of frequencies, thereby providing more information than a conventional, single-frequency eddy-current instrument. We have combined a pulsed eddy-current instrument with a portable two-axis scanner to produce an instrument capable of rapidly scanning aircraft lap splices in situ, producing pseudo-color images that reveal hidden corrosion or cracking. A unique feature of time-domain eddy-current data is the ability to selectively filter clutter from the image by time-gating the pulsed signal. Time-gating permits the user to select the inspection depth, thereby eliminating interference from upper layers, air gaps, lift-off variation and fasteners. By using a theoretical model of the pulsed eddy-current system, it is possible to interpret the data quantitatively, yielding quantitative maps of corrosion damage. Some of the same advantages of the pulsed eddycurrent technique apply to the characterization of hidden fatigue cracks as well, although the tieory for crack signals is less advanced. INTRODUCTION Hidden corrosion and wide spread-fatigue damage are the most important factors limiting
life-extension of aircraft in both commercial and military fleets. Non-destructive methods for characterizing damage caused by hidden corrosion and fatigue in layered structures such as aircraft lap-splices are, consequently, a high priority for commercial airlines and the Department of Defense. A considerable amount of effort is underway to develop new eddy-current techniques for detecting and characterizing hidden corrosion and cracking. Eddy-currents have the advantage of penetrating into subsurface layers and therefore being sensitive to their condition, whether or not the layers are mechanically bonded. In contrast, ultrasonic and thermal techniques require a mechanical bond between layers for the probing energy to penetrate to the second or third layers. Pulsed eddy-current systems have important advantages. One of the main advantages is that they can be constructed from simple, relatively inexpensive equipment. The information content of pulsed signals is inherently greater than conventional single- or dual-frequency signals, since they contain a broad range of frequencies. In this way, pulsed eddy-current is akin to sweptfrequency measurement methods, which sample a large number of frequencies. Yet the pulsed eddy-current technique is much faster - 100 milliseconds to acquire a pulsed eddy-current signal, compared to several minutes to complete a swept-frequency measurement. Pulsed eddy current systems are capable of providing better discrimination against i
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