Saka et al [1] has recently proposed a new technique for NDE of a 2-D surface crack, which uses closely coupled probes. The technique is called CCPPD (closely coupled probes potential drop) technique. They have utilized a small sensor composed of the closely coupled four probes, where the distance, f, between the two probes for measuring potential drop was 5 mm and the distance, d, between the two probes for current input and output was 6 mm (Sensor A). It has been shown that the CCPPD. technique can evaluate smaller crack by the same amount of current as the usual technique, in other words that the CCPPD. technique can evaluate the same crack as the usual technique by the use of the smaller current. Namely the CCPPD. technique has been proved to enhance the sensitivity for sizing a crack.

The purpose of the present paper is to explore higher sensitivity achieved by the CCPPD. technique. A sensor composed of the more closely coupled four probes with decreased to 0.5 mm and d decreased to 1.5 mm (Sensor B) is examined. The calibration equation relating the potential drop to the crack depth and the crack length is derived for the problem in which Sensor B is used, where a finite element method for electric crack problem is utilized. It is shown that increase in the potential drop due to crack in the case of using Sensor B is enhanced significantly in comparison with the case of using Sensor A, especially for smaller crack. For example, while increase in the potential drop due to crack in the case of using Sensor B is about 40 times as much as that in the case of using Sensor A when a semicircular crack with the length 4 mm and the depth 2 mm is employed, it is enhanced up to nearly 80 times when a semielliptical crack with the length 4 mm and the depth 1 mm is employed. One can evaluate sensitively not only a large crack but also an extremely small crack by using the present technique.

REFERENCE

1. M. Saka, A. Oouchi and H. Abe, "NBE of a Crack by Using Closely Coupled Probes for DCPD Technique", Trans. ASME, J. Pressure Vessel Technology, (1996), in press