Validation of the applicability of the best-fit fatigue curves for 550 °C in Mod.9Cr-1Mo steel to 1×1011 cycles

Abstract

In order to design fast reactors, it is necessary to consider high cycle fatigue of structural materials up to 1×109 cycles. Since the fatigue usage factor (UF) is calculated down to the order of 0.01 in fast reactor design, it is necessary to develop fatigue curves applicable up to 1×1011 cycles to evaluate high cycle fatigue at 1×109 cycles. In this study, the applicability of the best-fit fatigue curve of JSME up to 1×1011 cycles was verified in order to develop a high cycle fatigue evaluation method for Mod.9Cr-1Mo steel, which is a candidate material for fast reactor structural materials. In the applicability verification, high cycle fatigue tests were first conducted at 500 °C, 550 °C, and 600 °C under strain-controlled conditions, and ultrasonic fatigue tests were conducted at 550 °C. The test results were then evaluated and showed that there was no rate dependence and no difference by product types for high cycle fatigue strength of Mod.9Cr-1Mo steel at high temperatures. It was also clarified that there was no difference in high cycle fatigue strength of Mod.9Cr-1Mo steel at high temperatures even if the failure mode changed from surface origin to internal origin. A best-fit fatigue curve was developed from test data obtained in high cycle fatigue tests up to 1×109 cycles. And based on the high cycle fatigue mechanism discussed from the ultrasonic fatigue test results, the best-fit fatigue curve of JSME was confirmed to be applicable to Mod.9Cr-1Mo steel up to 1×1011 cycles regardless of product types.

Keywords

• high cycle fatigue
• mod.9cr-1mo steel
• sodium-cooled fast reactor
• ultrasonic fatigue test
• best-fit fatigue curve