Very high-cycle fatigue behavior in mercury target vessel for high-power pulsed spallation neutron source

Xiong, Z.; Naoe, Takashi   ; Futakawa, Masatoshi  ; Wakui, Takashi  ; Kogawa, Hiroyuki  ; Haga, Katsuhiro  ; Maekawa, Katsuhiro*

An enclosure vessel of liquid mercury spallation target, which is made of 316L stainless steel, is suffered from the cyclic loading due to the proton beam-induced pressure waves. The number of loading cycles will be beyond 10 to giga-cycle region throughout the expected service life. In and around giga-cycle region, the fatigue failure occurs under the conventionally defined fatigue limit. In addition to the cyclic loading, the strain rate at the beam window of the target vessel reaches to 50 1/s at the maximum, which is much higher than the conventional fatigues. In this work, ultrasonic fatigue tests were conducted with controlling specimen surface temperature to investigate effects of the temperature and the work hardening that simulates the irradiation embrittlement on very high-cycle fatigue strength of 316L SS. The experimental result showed that the obvious fatigue limit was not observed up to 10 cycles, and the fatigue strength was reduced about 75% of R.T. around 250 C regardless of the work hardening, which was the conceivable maximum temperature of the beam window at the 1 MW operation.