Effect of stress ratio and mean stress on high cycle fatigue behavior of the superalloy IN718 at elevated temperatures

Smooth specimens of single crystal (SC) superalloy SRR99 with [001] orientation were subjected to high-cycle fatigue (HCF) loading at temperatures of 700°C, 760°C, 850°C and 900°C in air atmosphere. The results demonstrated that conditional fatigue strength reached the maximum at 760°C and decreases with increasing temperature. Analysis on fracture surface showed a trend for cleavage rupture at 850°C and 900°C and ductile rupture at 700°C and 760°C. Fatigue cracks initiated at the surface or subsurface were primarily responsible for the ultimate failure. The influence of testing temperature on fatigue lifetime was studied by examining evolution of the microstructure through SEM observation. With the process of cyclic loading at elevated temperatures, the primary cuboidal γ′ precipitates tended to agglomerate and spheroidized, meanwhile a larger number of secondary γ′ particles were formed in the γ matrix in specimens fatigue tested at 700°C, which would have a significant effect on the high temperature properties.

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Stress ratio effect on notched fatigue behavior of a Ti-8Al-1Mo-1V alloy in the very high cycle fatigue regime

International Journal of Fatigue ◽

10.1016/j.ijfatigue.2018.05.032 ◽

2018 ◽

Vol 116 ◽

pp. 80-89 ◽

Cited By ~ 9

Author(s):

Kun Yang ◽

Bin Zhong ◽

Qi Huang ◽

Chao He ◽

Zhi Yong Huang ◽

...

Keyword(s):

Stress Ratio ◽

High Cycle Fatigue ◽

Fatigue Behavior ◽

Cycle Fatigue ◽

Ratio Effect ◽

Very High Cycle Fatigue ◽

Very High

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High Cycle Fatigue Properties of Modified 9Cr-1Mo Steel at Elevated Temperatures

Volume 8: Mechanics of Solids, Structures and Fluids ◽

10.1115/imece2012-87329 ◽

2012 ◽

Author(s):

Yoshiaki Matsumori ◽

Jumpei Nemoto ◽

Yuji Ichikawa ◽

Isamu Nonaka ◽

Hideo Miura

Keyword(s):

Fatigue Strength ◽

Fatigue Limit ◽

Room Temperature ◽

Structural Reliability ◽

High Cycle Fatigue ◽

Elevated Temperatures ◽

Fatigue Behavior ◽

Cyclic Softening ◽

Fatigue Properties ◽

Cycle Fatigue

Since high-cycle fatigue loads is applied to the pipes in various energy and chemical plants due to the vibration and frequent temperature change of fluid in the pipes, the high-cycle fatigue behavior of the alloys used for pipes should be understood quantitatively in the structural reliability design of the pipes. The purpose of this study, therefore, is to clarify the high-cycle fatigue strength and fracture mechanism of the modified 9Cr-1Mo steel at temperatures higher than 400°C. This material is one of the effective candidates for the pipes in fast breeder demonstration reactor systems. A rotating bending fatigue test was applied to samples at 50 Hz in air. The stress waveform was sinusoidal and the stress ratio was fixed at −1. The fatigue limit was observed at room temperature and it was about 420 MPa. This value was lower than the 0.2% proof stress of this alloy by about 60 MPa. This decrease can be attributed to the cyclic softening of this material. The limited cycles at knee point was about 8×105 cycles. All fracture was initiated from a single surface crack and no inclusion-induced fracture was observed in the fracture surface by SEM. Thus, the high-cycle fatigue design based on the fatigue limit may be applicable to the modified 9Cr-1Mo steel at room temperature. The fatigue limit of about 350 MPa was also observed at 400°C, and it appeared at about 107 cycles, while it appeared at around 106 cycles at room temperature. Thus, it was confirmed that the fatigue strength of this alloy decrease with temperature. However, the fatigue limit didn’t appear at 550°C up to 108 cycles. The fatigue limit may disappear in this alloy at 550°C. It is very important, therefore, to evaluate the ultra-high cycle fatigue strength of this alloy at temperatures higher than 400°C.

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