Gigacycle fatigue behavior of structural material for the mercury target vessel

Naoe, Takashi   ; Xiong, Z.*; Futakawa, Masatoshi  

At J-PARC, a mercury target for neutron source (made of 316L SS) suffers not only proton and neutron radiation damage by proton and neutron but also cyclic impact stress caused by pressure waves. The number of impact stresses exceeds in the design operating period for 2500 hours at 1 MW. Recently, it was reported that the fatigue failure occurs below the fatigue limit which was determined by a test up to 10 cycles. In this study, we carried out an ultrasonic fatigue test to investigate the gigacycle fatigue strength of 316L SS and its welded material, concluding that an obvious fatigue limit was not observed below 10 cycles for 316L SS. Smoothed weld material which removed weld bead showed higher fatigue strength than that of base metal. By contrast, the fatigue strength of the weld material with bead decreased below that of base metal due to the stress concentration at the weld toe.