Ｖ形環状切欠材の回転曲げ疲労強度に及ぼす析出硬化の影響

Abstract

The effect of precipitation hardening on rotary bending fatigue strength of circumferential V-notched specimens was investigated at room temperature by using two commercial steels, i.e., 18 Ni maraging steel and an austenitic heat resisting steel, JIS SUH38. The rotary bending fatigue tests were carried out on the as solution-treated, peak-aged and over-aged specimens.The fatigue limits of the smooth specimens as well as all of the notched specimens were not influenced by precipitation hardening in 18 Ni maraging steel. Non-propagating cracks were found in all the notched specimens having the stress concerntration factor, α, of 6.2 except the over-aged specimens. It was also found that the stress concentration factor at the branching point, α0, is nearly 4. In SUH38, the fatigue limits of the smooth specimen and the notched specimen with α of 2.1 were found to increase with precipitation hardening, while the fatigue limits for the other notched pecimens having α larger than 2.1 did not change in spite of precipitation hardening. The value of α0 was nearly equal to 2 in the as solution-treated specimens and 4 in the other aged specimens. Furthermore, nonpropagating cracks were easier to be formed in SUH38 than in 18 Ni maraging steel and in the as solution-treated specimens than in the aged specimens.Discussions were finally made on the fatigue limits by dividing them into the fatigue limit for crack initiation, σw1 and the fatigue limit for crack propagation, σw2. Especially the behavior of nonpropagating cracks was discussed on the basis of roughness-induced crack closure.