In order to enhance the sensitivity of the d-c potential drop technique, the principle of a technique using the closely coupled probes is proposed. A calibration equation for this technique is derived, where the path-independent integral je for electric crack problems is utilized. Experiments on laboratory specimens are discussed which validate the use of the calibration equation for the nondestructive evaluation of cracks. It is shown that the sensitivity of the present method using closely coupled probes is higher than that of the usual method using a uniform current flow in the region far from the crack.

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