The Studies on Multiaxial Creep Behavior of Inconel 718 Notched Bar at 700 °C

This paper studies the multi-axial creep behavior of Inconel 718 alloy at 700 °C. The stress triaxiality was introduced into the sample by adding U-shaped notches of various sizes and shapes. Furthermore, the multi-axial creep test of notched round bar was completed, and the sample slice after the creep test was subjected to scanning electron microscopy (SEM) microscopic examination. The influence of notch radius on its creep behavior and damage evolution was studied through experimental and numerical investigation. The finite element method (FEM) implemented by Gurson–Tvergaard–Needleman (GTN) model is used to simulate the evolution of damage in multi-axial stress state. With the comparison between experimental and numerical data, the multi-axial creep behaviors are clearly revealed in terms of stress triaxiality, creep strain (stress), void volume fraction (VVF), and so on. It was found that the great agreement is achieved between FEM and test data, and multi-axial creep behavior is well predicted.

The Impact of Geometric Discontinuities on Alloy 617 Creep-Rupture Behavior

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.