Notched Low Cycle Fatigue of Alloy 718

Abstract

Low Cycle Fatigue (LCF) damage is the life-limiting factor for rotating components in advanced gas turbines. LCF tests in laboratory are generally conducted in strain control using smooth cylindrical specimens. Smooth geometry may not represent actual component, as LCF life can be significantly influenced by notches present in components and the latter is not accounted by the data based on smooth specimens. Most of the data available in literature is generated using circumferential or single or double edge notch. Use of circumferential notch may not be entirely appropriate for comparison with smooth fatigue specimens, because of difference in their diameters, as there is a decrease in volume or surface area of the specimen as the diameter decreases, and this is of significance, since fatigue failures usually start at the surface. In the present study tangential slot notch was used to investigate the effect of slot notch on LCF behavior of Alloy 718 as it removes a very small percentage of the specimen cross-sectional area and does not cover entire circumference. So comparison can be made between the smooth and tangential notch specimens without much variation in their surface area or volume. The studies are conducted at ambient temperature and 650°C and a comparison is made with smooth specimen data. Results indicate that tangential slot notch also reduces significantly the fatigue life of alloy 718.