Development of Eddy Current Meander Probe Array, 15-9067

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Principal Investigators
Gary L. Burkhardt
Jay L. Fisher

Inclusive Dates: 01/01/98 - 12/31/99

Background - Commercial and military aircraft fleets are aging significantly, and the use of aircraft beyond their original design lifetimes has become necessary. Quality nondestructive evaluation (NDE) of aircraft structure is crucial to extending the lifetimes of aircraft. Reliable detection of cracks emanating from fastener holes in fuselage and wing skins is particularly important. Eddy current testing (ECT), the most routinely applied method for detecting fastener hole cracks in aircraft structure, has significant problems. Currently used ECT methods are very slow, difficult to set up and calibrate, or insensitive to long flaws connecting two fasteners. A method is needed that is fast, detects short flaws as well as those connecting fasteners, and provides data imaging for easy interpretation.

Approach - The objective of this project is to demonstrate the feasibility of an ECT probe array concept incorporating a meander excitation coil and separate sensors for detecting cracks in aircraft skin. This concept is designed to overcome major limitations of existing systems. The line-current excitation from the meander coil and the use of an array of sensors allow flaws of all lengths to be detected and will ultimately allow realtime data imaging to provide rapid inspection and easy interpretation.

Accomplishments - An ECT probe array consisting of a meander excitation coil and magnetic field sensors based on the giant magnetoresistive (GMR) effect was fabricated and tested. The probe design was guided by electromagnetic modeling, which was used to determine the induced current flow and associated magnetic fields around flaws. The meander coil consisted of conductors on a printed circuit board, and four GMR sensors were recessed into holes cut between the conductors, as shown in the illustration below. The probe was evaluated on specimens representative of aircraft structure with simulated cracks emanating from fastener holes. The probe was shown to be sensitive to short cracks, as well as long cracks that connected two fasteners. A concept for an array with a large number of sensors was also developed.

The eddy current array probe consists of meander excitation coil and giant magnetoresistive (GMR) effect sensors positioned in holes cut in the substrate between the meander coil conductors.

      Measurement & Nondestructive Evaluation of Materials & Structures Program
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