NOTCH EFFECT ON CREEP-FATIGUE BEHAVIOR OF ALLOY 617 AT ELEVATED TEMPERATURE

High-temperature reactor structural components are often under the complex multiaxial creep-fatigue (CF) loading conditions throughout the lifetime because of geometric and/or metallurgical discontinuities and complex loading paths. To assess the multiaxial CF deformation behavior and to evaluate the CF design rules in the ASME BPVC Section III, Division 5, Subsection HB, Subpart B, experimental and numerical studies are performed on Alloy 617 at 950°C using notch specimen geometries under CF loading in this study. Two types of notch specimen geometries, shallow-notch and sharp V-notch were designed for Alloy 617 per ASTM Standard E292-09. Experimental procedures were developed, and tests at 950°C were performed under CF loading with tension hold time using the two notch specimen geometries. The information collected from the experiments included the average axial stress relaxation histories, the changes in the relaxation behavior as a function of applied cycles, and failure information. Additionally, an inelastic constitutive model of Alloy 617 was used in the finite element analyses of the notch specimens to determine the stress triaxiality distributions, local and average stress relaxation behavior, and local elastic follow-up. The results from the experimental and numerical studies on the CF of notch specimens are compared with those on standard uniaxial smooth bar specimens. The notch effect on the CF deformation and failure of Alloy 617 is summarized and the assessment of the design rules regarding the treatment of multiaxial stress relaxation is discussed.