Mechanical property change in the region of very high-cycle fatigue

超高サイクル疲労領域における力学特性の変化

Xiong, Z.; 直江 崇   ; Wan, T.; 二川 正敏  ; 前川 克廣*

Xiong, Z.; Naoe, Takashi; Wan, T.; Futakawa, Masatoshi; Maekawa, Katsuhiro*

核破砕中性子源の構造材料として用いられるオーステナイト系ステンレス鋼SUS316Lの超高サイクル疲労挙動について、ひずみ速度10sの超音波疲労試験により調べた。超高サイクルの繰返し応力負荷に伴う硬化及び軟化を調べるために、溶体化処理材及び冷間圧延材に対して、疲労試験後の試験片断面の硬度分布、引張り試験による残強度測定、非線形超音波による転位密度変化の計測を実施した。その結果、溶体化処理材では、10回以上の超高サイクル領域で繰返し硬化が観測された。冷間圧延材では、10回以下では、繰返し軟化が観測された一方、10回以上では、繰返し硬化が観測された。

Very high-cycle fatigue behaviour of SUS316L, which is used as the structural material of the spallation neutron sources, was investigated through the ultrasonic fatigue test with the strain rate of 10 s. Cross-sectional hardness distributions of the fatigue-failed specimens for solution annealed (SA) and cold worked (CW) 316L were measured to understand the cyclic hardening or softening. In addition, the tensile tests of the fatigue-failed specimens were performed at room temperature. Furthermore, the nonlinear ultrasonic system was used for evaluating the dislocation density variation. The results showed the cyclic hardening in the region of very high-cycle fatigue in the case of SA. In contrast, in the case of 10%CW, cyclic softening occurred when the number of cycles below 10 and followed by cyclic hardening. In the case of 20%CW, cyclic softening was observed when the number of cycles below 10, while cyclic hardening occurred subsequently.