The Nuclear Regulatory Commission (NRC) is currently reviewing Section III, Division 5 of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code for potential endorsement. A prior NRC-sponsored review on the application of Section III, Division 5 design rules for elevated-temperature reactors have identified issues currently not in the code that need to be addressed. These include an insufficient knowledge of the effect of notches, structural discontinuities, and multiaxial stress. This work qualitatively evaluates Alloy 617 as notch strengthening or notch weakening and investigates the impact of multiaxial stress on Alloy 617 creep behavior.

Multiaxial stress does not degrade Alloy 617’s short-term creep-rupture properties. This was concluded from the following observations: 1) multiaxial creep resided on or to the right of the Larson-Miller curve for uniaxial creep-rupture data, 2) Alloy 617 exhibited notch strengthening, and 3) creep-rupture life increased with a stronger multiaxial stress. Long-term and intermediate creep-rupture tests are in progress in order to determine whether notch behavior will cross from strengthening to weakening at lower stresses and longer rupture lifetimes. A nondestructive characterization methodology using X-ray computed tomography is explored as a means to identify the failure location on the specimen prior to rupture.

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