A practical method is proposed for in-situ fatigue crack detection in actual notched structures. This method is available for cases when the initiation position and propagation direction of a crack are uncertain and the load direction or load mode changes. In the present method, the stress fields near a notch root are estimated, using the general stress function in a polar coordinate system and the stress values obtained from strain gages around the notch root. The stress fields are characterized by the residual stress due to plastic deformation around a fatigue crack. Therefore, not only a propagating crack but also a non-propagating crack can be detected from the characteristic stress field. Moreover, the detection limitation is subject to the relative strength of the residual stress field to an elastic stress field due to loads. The fatigue experiment of a notched specimen of steel was carried out, in which a 0.33mm long crack was detected in a notch whose radius was 12.5mm.

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This paper proposes an innovative excitation system for eddy current testing. The key idea of the system is the suppression of eddy currents on the surface of specimens. A new eddy current probe consisting of four coaxial rectangular tangential exciter coils driven by phase-shifted AC currents and a pickup coil is fabricated as the basis of the system; then numerical simulations and experimental verifications are performed. It is demonstrated that the new probe provides 20% difference between signals due to a 15 mm deep notch and those due to a 20 mm deep one although an excitation frequency of 10 kHz is applied. In contrast, a conventional eddy current probe driven with the same frequency showed as little as 1% difference between signals due to the two notches.

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