Crack damage detection by ultra-high frequency A. C. potential method for coated single crystal superalloys

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This research project investigated two main topics by using A. C. potential method. The both topics are related with the damage assessment of nickel base superalloy gas turbine blades with anti-corrosive and thermal barrier coatings. One was to measure the number of cracks and the crack depth formed on the turbine blade due to thermal stressing. The other was to measure the size and the depth of defects occurred at the interface of the coating and base superalloy.The gradient of the A. C. potential - frequency relationship was deferent depending on the number and the depth of cracks. This means that the number of cracks and the crack depth are determinable by utilizing the frequency dependence of A. C. potentials. The both are actually determinable by comparing the relationships of the target specimen to those in the database where the relationships of many cracked specimens are stored.The maximum value of A. C. potential was corresponded with the size of defects while the gradient of the A. C. potential-frequency relationships reflected the depth of defects. The A. C. potential method was concluded to be capable to measure the size and the depth of defects by measuring the maximum value and the gradient.Harmonic electro-magnetic finite element analyses were carried out to confirm the experimental results obtained. Absolute values obtained in the analyses were not rigorously agreed with the experimental results but the potential ratio normalized by the potential away from the defects agreed well with the experimental results. This clearly indicates that the experimental results obtained in this study had a physical background of electro-magnetic theory.