β型チタン合金のねじり疲労特性

抄録

Torsional fatigue tests were conducted for circumferentially notched round bar specimens and smooth specimens of Ti-22V-4Al alloy (Ti22) to evaluate the torsional fatigue characteristics. The notch sensitivity was studied by using sharp (stress concentration factor Kt=4.0) notched specimens. The results of fatigue tests showed that the torsional fatigue strengths of STDA is higher than that of STA on both smooth and notched specimens. This is caused by the difference of the ratio of alpha and beta grains. The more the material has alpha grain, the harder the materials are. Crack length were calculated using the formula (5). The crack propagation rate of STA is slower than that of STDA. It is assumed that crack propagation rate is decreased with increasing the ratio of alpha grain. On STDA Kt4, when the stress amplitude is low the crack propagated towards the longitudinal direction of the specimen then 45° direction of the longitudinal, while under high stress amplitude the shear crack propagated in the circumferential direction and it forms flat fracture surface. On STA Kt4, under all of the stress amplitude which was tested the fracture surfaces showed not flat. Therefore, the ratio of alpha and beta grain effects the torsional fatigue characteristics on Ti22.