Development of a pulsed eddy current instrument and its application to detect deeply buried corrosion

摘要

Eddy current techniques have historically been valuable to nondestructive evaluation and testing. They allow detection of cracks, corrosion, and other inhomogeneities in conducting materials. However, due to the fact that traditionally eddy currents only use one frequency to excite the coil, limited information about the detected flaw is available. Swept frequency eddy current methods, which sweep the frequency of excitation over a specified range, allow much more information about the flaw to be extracted from the data. This method is much slower due to the amount of time required to sweep the frequency. The pulsed eddy current method was developed to decrease the time required for swept frequency measurements while retaining the ability to extract more information from the data.;The technique is a broadband measurement which requires the coil to be excited by a step function and the response of the coil monitored. Since the coil is excited with a spectrum of frequencies contained in the step function, similar information content to the swept frequency method is available. In this thesis, the electronic hardware for a pulsed eddy current system with the ability to operate a probe in the absolute or reflection mode using a step voltage drive waveform was developed and demonstrated. The system consists of a portable computer with three expansion boards to control the probe drive and signal amplification, digitize the signal, and control the scanner.;The system is controlled by custom designed software based on the Windows[Superscript TM] operating system. The capabilities of the pulsed eddy current system were then extended to allow for a step current drive waveform. The experimental results for corrosion detection in a two-layer structure of 1 mm thick aluminum plates simulating an aircraft lap joint are compared with theory and are found to be in good agreement. The ability to use the instrument with a magnetic sensor capable of sensing the magnetic field threading the coil was also added to the instrument. Using theory and experiment, it is demonstrated that a magnetic sensor is superior to the coil sensor at detecting deeply buried corrosion in aluminum.