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Publication Date: 2010-Jul-23
Document File: 4 page(s) / 78K

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The present invention relates to the detection of cracks in metals. More particularly, this invention relates to a method for using eddy current to inspect metal aircraft components for cracks. The present invention utilizes eddy current inspection to improve the accuracy of depth measurements for cracks located below an aircraft's surface. As a result, the scatter of the acquired POD data measurements is reduced resulting in a more effective inspection method for the aircraft industry.

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The present invention relates generally to the detection of cracks in metals, and more particularly to a method for using eddy current to inspect metal aircraft components for cracks.

The safety and structural integrity of aircraft continues to be of concern to manufacturers as well as consumers. As the average age of aircraft increases, reliable and accurate inspection of all aircraft components becomes increasingly important. This is especially true for the aircraft surface and surface fasteners. It is thus necessary to detect cracks or other flaws located below the aircraft surface as such flaws may eventually lead to system failure.

Current methods of detecting the degree of cracks and other flaws on aircraft surfaces include replication and the removal of metal to expose a cracked area. Replication involves pouring a liquid into the crack, allowing it to harden, and then extracting the hardened product from the crack. Although the technique is nondestructive, it is prone to errors and does not provide accurate results. Removing the metal to expose a cracked area allows for close inspection of a physical crack and is more accurate than replication. However, the removal technique is destructive and precludes future use of the part being examined for cracks.

Another method is characterized by an estimation of a crack’s area and is illustrated in Figure 1. The United States Air Force (USAF) Engine Structural Integrity Program (ESIP) has defined a standard crack as a circular segment whose length is the length of a chord (c) and whose depth (d) is the maximum distance from the chord (c) to the center of the circle. The area of the circular segment (shaded portion of Figure 1), which can be determined by geometric calculations known in the art, is considered to be equivalent to the area of the physical crack.

The problems with the methods discussed above are realized when analyzed using a method called Probability of Detection (POD).  POD is a method of analysis for quantifying the performance of a particular non-destructive inspection technique or for quantifying the performance of a particular operator. When analyzed using a conventional POD method, the measurements acquired using the above-mentioned inspection methods are widely scattered, or show a large standard deviation, and indicate the ineffectiveness of the methods to accurately detect cracks in an aircraft and to estimate the area of detected cracks. In particular, the methods have been proven to be inaccurate in determining the area of cracks that have a length of 0.050 inches or less.

To improve the precision of the POD calculations, accurate knowledge of the area of the crack is required. Therefore, the efficacy of the inspection technique would be improved by a method that could provide an accurate measurement of the length, as well as the depth, of a crack.       


Figure 1...