Development of Crack Damage Evaluation Method for Utilizing High Frequency A.C.Potenial Method

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Many cracks were initiated and propagated on the surface of gas turbine blades made by nickel base superalloys due to the thermal stresses during start up and shut down of turbines. Nondestructive crack detection method is necessary for the damage evaluation for blades. This study investigated the crack depth detection and the detection of the number of cracks simultaneously by a measurement using a.c.potential method.The material used was a solid bar with a rectangular cross section of 5 mm width, 10 mm height and 100 mm length. The artificial cracks with 50, 100, 150 200 200, 300 and 500 micron meter depth were made by a wire electric discharge machine. The number of artificial cracks was 1, 2, 3 and 5. The a.c.potentials were measured at the frequencies from 200 to 5000Hz using the four points probe method. The main conclusion obtained are as follows.1. A. c. potentials at all the frequencies increased with increasing the crack depth in all the specimens.2. For the same crack depth, a. c. potentials increased with increasing the number of cracks.3. The high frequency a. c. potentials were more sensitive to the shallow cracks while the low frequency a. c. potentials were sensitive to the deep cracks.4. By using the a. c. potentialst at many frequencies, the number of cracks and the crack depth are detectable at a measurement. The developed method in this study basically is extendable to the damage evaluation of actual gas turbine blades.5. The above experimental results agreed well with the finite element results of harmonic analysis of a.c.potentials.