Abstract
Japanese utilities are increasing their use of gas turbines for generation of electricity. In order to maintain competitive electricity rates, the utilities need to find ways to reduce maintenance costs. There is a need to develop nondestructive evaluation (NDE) methods that can be applied to gas turbine blades in situ during periodic turbine maintenance, and to be able to use the NDE results and operating history to predict future degradation. This paper describes the development of methods for detecting cracks right under MCrAlY overlay coatings, and entimating crack depths from the results of eddy current testing. For these purposes, an eddy-current measurement system has been developed, and Ni-base superalloy specimens with the CoCrAlY coatings have been prepared for the eddy-current measurements. Notches simulating the cracks right under the coatings can be detected, discriminating from surface-breaking notches in the phase angles of their eddy-current responses. The proposed flaw-sizing approach can provide more accurately the estimated depths of the cracks, irrespective of their lengths.
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