Abstract
Strain-controlled creep-fatigue crack growth tests were conducted using a circular sharp notched round bar specimen (CNS) for a nickel (Ni) base alloy, Alloy 617, which is a candidate material for the next-generation advanced ultra-supercritical (A-USC) power plant. The reduction ratio of stress range Δσ ( = σmax − σmin) was found to be a dominant factor for estimating crack initiation and growth lives under strain-controlled creep-fatigue conditions as compared with that of tensile peak stress σmax. Additionally, the direct current potential drop (DCPD) technique was adopted in this research for measuring the crack length. The developed equation that is applied for a CNS was used for converting the output voltage into crack length. With the use of these measurement methods, the crack length of a CNS was found to be well estimated under high-temperature strain-controlled conditions. From these results, fundamental knowledge was obtained concerning the method for estimating crack growth life using CNSs under strain-controlled creep-fatigue conditions and also concerning the criteria for crack initiation and growth lives. Furthermore, when experimental results for Alloy 617 obtained by this research were compared with previous results for high-chromium (Cr) steel, the proposed method using the reduction ratio of Δσ was found to be applicable for both cases.
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