Experimental studies on a structure of eddy current probe for detection of magnetic flux distorted by a flaw

Bobbin‐coil‐type eddy current probes, which are conventionally used for nondestructive inspection of steam generator tubes in pressurized‐water‐type nuclear power plants, have poor detection sensitivity to circumferential flaws. Hence a new type of eddy current probe was proposed to effectively detect the magnetic flux component that is indicative of a flaw and thus to eliminate the dependency on sensitivity in flaw detection. In the course of development of the proposed method, probe structures were investigated based on the measurement of magnetic fields induced by variously‐shaped excitation flat coils. The new type of probe proposed here consists of differential pick‐up coils detecting magnetic flux and excitation coils having a parallelogrammic shape. The structure of the coils was fabricated in order to detect flaws independently of their directions in trial. Nondestructive flaw detection tests were then conducted with the probe. The results obtained concrete evidence showing that both the horizontal, the vertical slit‐flaws and even the small hole of 0.5 mm in diameter can be detected successfully.