内圧および軸力を負荷した多軸応力状態における疲労寿命および強度特性評価

抄録

This study discusses an effect of loading path on fatigue life for type 316 stainless steel. The testing machine for multiaxial fatigue test is equipped with three actuators. This testing machine can apply push-pull, reversed torsion and inner pressure to the specimen by controlling the three actuators. Load and pressure controlled fatigue tests at room temperature were carried out using a hollow cylinder specimen under multiaxial loading. The fatigue life N_f is determined as the cycle at which the maximum inner pressure was reduced by the oil leak due to an initiation of through crack or rupture of the specimen. Loading conditions were 5 types: they are a cyclic pull loading, a cyclic inner pressure loading, a cyclic equi-biaxial loading and two combination loadings by cyclic inner pressure and cyclic pull loading. The two combination loadings are a Square-shape loading and a L-shape loading. In the Square-shape test, axial stress σ_z and hoop stress σ_θ are applied by trapezoidal waveforms with 90° degree phase difference so that the loading path is shown by square shape in a plane stress state. In the L-shape test, axial stress σ_z and hoop stress σ_θ are applied alternately so that the shape of loading path becomes the shape of the letter “L”. Fatigue lives of the latter two combination loadings were reduced due to increase in cyclic ratchetting deformation. In addition, the cyclic deformation behavior was discussed through simple finite element analysis. The analysis results shows that the strain energy density W under the Square-shape and the L-shape tests were larger than those under the other tests.