The Effect of Stress Ratios on the Very High Cycle Fatigue Behavior of 9%Cr Turbine Steel at 630 °C

Abstract

Effects of the stress ratio on the very high cycle fatigue behaviors of 9%Cr turbine steel have been investigated at 630 °C. The experimental results show that the S–N curve has a continuous downward trend and has no fatigue limit with the increasing in the cycles at 630 °C. Meanwhile, according to the analysis of microstructure, there are two failure modes that were observed at different stress ratios (R = −1 and 0.1), including surface crack failure and internal crack failure, respectively. Besides, the theoretical threshold value of the crack growth is compared with the calculated value of the fracture surface. To decrease the difference between the threshold value of internal crack initiation and the corresponding theoretical value, a new model for the crack growth threshold of the interior-induced fracture at different stress ratios is proposed and discussed.

Feasibility Study of Fluidic Sealing in Turbine Shroud

“…In the most recent turbofan architectures, titanium-based alloys have been starting to take on a more and more important role in turbofan engines. The reason for this is weight reduction-the density of the titanium alloy (3.9 g/cm 3 ) is half that of nickel-based alloys (8.3 g/cm 3 ). At the same time, the maximum operating temperature of nickel-based alloys is 800 • C, compared to 1000 • C for titanium-based alloys; therefore, the former can only be used in colder parts of the engine.…”

“…They have to withstand significant heat (up to 1500 • C [2]) and high stresses from centrifugal and fluid forces. As a part of a rotating machine, they can be subject to vibration and fatigue damage [3]. Additionally, due to the combination of high temperature and stresses, creep can occur [4].…”